High-performance single-photon detectors and applications in quantum communication

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Thesis

Reference

High-performance single-photon detectors and applications in quantum communication

KORZH, Boris

Abstract

Two approaches to single-photon detection at telecom wavelengths have been investigated, using semiconductor and superconductor technologies. The low temperature operation of free-running InGaAs/InP negative feedback avalanche diodes (NFADs) was studied, demonstrating that they can operate with an extremely low dark count rate, which opens up a host of new applications. Secondly, superconducting nanowire single-photon detectors (SNSPDs) based on amorphous materials were developed, fabricated and characterized.

These detectors are highly attractive because they can achieve excellent attributes simultaneously, such as high efficiency, low noise, good temporal resolution, no afterpulsing and high count rates. Finally, it was demonstrated that the use of NFADs can enable record distance quantum key distribution (QKD) over 307 km of optical fiber. A next generation QKD system has been proposed and numerically analyzed, aiming at extending the record distance by an additional 100 km and increasing the state-of-the-art secret key rate by over an order of magnitude.

KORZH, Boris. High-performance single-photon detectors and applications in quantum communication . Thèse de doctorat : Univ. Genève, 2016, no. Sc. 4995

URN : urn:nbn:ch:unige-889792

DOI : 10.13097/archive-ouverte/unige:88979

Available at:

http://archive-ouverte.unige.ch/unige:88979

Disclaimer: layout of this document may differ from the published version.

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U

NIVERSITÉ DE

G

ENÈVE

F

ACULTÉ DES

S

CIENCES

Groupe de Physique Appliquée Professeur Hugo Z

BINDEN

High-performance single-photon detectors and applications in quantum

communication

Thèse

présentée à la Faculté des sciences de l’Université de Genève pour obtenir le grade de Docteur ès sciences, mention Physique

par

Boris Alexander K

ORZH

de la Grande Bretagne

Thèse N° 4995

G

ENÈVE

Centre d’impression de l’Université de Genève

30 Septembre 2016

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I dedicate this thesis to my grandfather, Boris Matantsev.

An inspirational inventor.

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Acknowledgements

Firstly I would like to thank my supervisor Prof. Hugo Zbinden for giving me the opportunity of undertaking this thesis. I am really greatful for the flexibility he allowed me in participating in a vast array of projects as well as for always being available for discussions. I would also like to thank Prof. Nicolas Gisin, for I have thoroughly enjoyed the wonderful scientific atmosphere atGAP Optique, which he has created.

Special thanks goes to Nino Walenta, Tommaso Lunghi and Charles Ci Wen Lim, my mentors at the beginning of the thesis, who introduced me to the world of single photon detectors and quantum cryptography. I am also very greatful to Félix Bussières who lead the launch of the superconducting nanowire detector (SNSPD) activities at the group and together with whom I dived into thenew worldof nanofabrication of these detectors. It has been wonderful to be at the heart of the of a new activity for the group, from the beginning. I’d also like to thank Anthony Martin, the go-to expert of quantum optics, who was key to several cryptography projects. I have been very fortunate to work together with Gianluca Boso, my office neighbor.

I cannot think of many projects where we did not work together, which has been a truly enjoyable and fruitful experience. It has been great to work together with Misael Caloz, Alberto Boaron and Emna Amri, who are progressing with the continuation of the work in this thesis. I also enjoyed working with Nuala Timoney, even if only for a short time. I’d also like to thank Rob Thew and Mikael Afzelius for always being available for scientific discussions.

Progress in experimental physics is difficult without outstanding technical support. For this I must especially thank Raphael Houlmann, our hardware programming specialist, Claudio Barreiro, our electronics and mechanics specialist and Olivier Guinnard, who laid the foun- dations of most of the detector and cryptography prototype implementations used in this thesis. I would also like to thank Roland Pellet, manager of the mechanical workshop which fabricated all of the mechanical prototypes with unprecedented quality and precision. Huge gratitude goes to the staff of the Centre of MicroNanofabrication (CMi) at EPFL, who were always at hand to aid with any nanofabrication queries, in particular Cyrille Hilbert, Zdenek Benes, Joffrey Pernollet, Giancarlo Corradini, Julien Dorsaz, Rémy Juttin, Kaspar Sutter, Guy Clerc and Phillipe Langlet.

I would like to aknowledge all of the collaborators that I have been fortunate to work with.

Varun Verma and Sae Woo Nam of NIST as well as Francesco Marsili and Matt Shaw of JPL have been influential on a large part of the work carried out during this thesis. The launch of the SNSPD fabrication would not have been possible without the participation of our colleagues at

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Acknowledgements

Sacker. I am also grateful to Jérémie Teyssier and Stefano Gariglio from the Department of Quantum Matter Physics for their support with ellipsometry and crystallography, respectively.

I’d like to thank Daniel Nolan and Ming-Jun Li from Corning Inc. for supplying us with ultra low loss optical fiber, used in the record distance quantum cryptography experiments. I have thoroughly enjoyed collaborating with members of ID Quantique, who have supported a number of projects during this thesis, in particular Matthieu Legré, Damien Stucki, Bruno Sanguinetti, Patrick Trinkler, Sylvain Chuard, Yacine Felk and Mathilde Soucarros.

The best feeling a scientist experiences is when they witness the result of their work being utilized by others. For this I have to thank a number of members of the group, including Alexey Tiranov, Christoph Clausen, Fernando Monteiro, Natalia Bruno and Thiago Guerreiro.

I am greatful to all of the members of my jury, in particular the external participants, Prof.

Richard Warburton and Prof. Gerald Buller.

Throughout my thesis I have made many friends atGAP Optique, who have contributed to a wonderful experience. As well as all of the colleagues that I have already mentioned, they include Pierre Jobez, Cyril Laplane, Tomer Barnea, Ephanielle Verbanis, Marc Olivier Renou, Jonathan Lavoie, Florian Flöwis, Valentina Caprara, Emmanuel Zambrini Cruzeiro, Peter Strassmann, Jean Etesse and Gilles Pütz. I cannot forget the first friend I made upon arrival to the Geneva area, Carole Herve, to whom I owe a great deal. Finally, I would like to thank my mum and David for being so supportive throughout my studies and life endeavors.

Geneva, 15 August 2016 B. Korzh.

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Abstract

During this thesis two approaches to single-photon detection at telecom wavelengths have been investigated, using semiconductor and superconductor technologies. On the application side, quantum key distribution (QKD) was implemented.

In the first part of the thesis I studied the operation of free-running InGaAs/InP negative feedback avalanche diodes (NFADs). Through extensive study of the low temperature behavior of these detectors, insights into the fundamental characteristics such as the efficiency, dark count rate (DCR), afterpulsing and jitter, were found. The key finding is that these detectors can operate with an extremely low dark count rate, which opens up a host of new applications.

The second part of the thesis concentrated on the development, fabrication and characterization of superconducting nanowire single-photon detectors (SNSPDs). These detectors are highly attractive because they can achieve excellent attributes simultaneously, such as high efficiency, low noise, good temporal resolution, no afterpulsing and high count rates. The main attribute of the developed devices is the material of choice, being amorphous MoSi.

This is a relatively new material to be used for SNSPDs and shows great promise due to its highly uniform nature, which yields good fabrication yields and favorable attributes such as saturated internal efficiency.

The final part of the thesis looks at the application of these detectors to QKD. It is demonstrated that the use of NFADs can enable record distance QKD over 307 km of optical fiber.

Crucially, this experiment is the first long distance QKD demonstration which could provide a quantitative measure of the security parameter. Finally, a proposal and numerical analysis of a QKD experiment which can further improve the performance of QKD through the use of SNSPDs, is outlined. In particular two scenarios are analyzed, namely long distance operation which could extend the record distance by an additional 100 km and high rate operation which could increase the state-of-the-art secret key rate by over an order of magnitude.

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Résumé

Au cours de cette thèse, j’ai étudié et approfondi deux approches pour la détection de photons uniques aux longueurs d’ondes télécoms. La première est fondée sur la technologie des semi- conducteurs, la seconde sur les supraconducteurs. J’ai ensuite appliqué ces développements à des expériences de distribution clés quantiques (QKD).

Dans la première partie de la thèse, j’ai observé le fonctionnement des photodiodes à avalanche InGaAs/InP. Grâce à une étude poussée du comportement à basse température de ces dé- tecteurs, j’ai acquis une compréhension approfondie de leurs caractéristiques fondamentales telles que l’efficacité, le bruit, et le résolution temporelle. Nous avons pu établir que ces détecteurs peuvent fonctionner avec un bruit très bas, ce qui ouvre la voie à de nouvelles applications.

La deuxième partie est axée sur le développement, la fabrication et caractérisation de nanofils supraconducteurs (SNSPD) pour la détection de photons uniques. En effet, ces détecteurs présentent de nombreux avantages, comme une très haute efficacité, un bruit faible, une bonne résolution temporelle, de hauts taux de détection et aucune afterpulsing. La propriété principale de nos détecteurs est le choix du matériau de fabrication, le MoSi amorphe. Ce matériau encore peu utilisé pour la fabrication de SNSPD présente une structure très uniforme, ce qui en fait un candidat très prometteur. En effet cette caractéristique intrinsèque permet un bon rendement de fabrication et des attributs favorables, telle qu’une efficacité interne saturée.

Dans la section finale de cette thèse, j’ai appliqué les technologies développées au cours de cette thèse pour la distribution de clés quantiques. Nous avons ainsi pu démontrer la distribution de clés quantiques par fibre optique sur une distance record de 307 km. Cette expérience est la première QKD sur longue distance qui donne une mesure quantitative du paramètre de sécurité. Enfin je tiens à souligner l’intérêt d’une proposition d’expérience de QKD, complétée d’une analyse numérique, qui permettra d’améliorer la performance de distribution de clés quantiques par le recours aux SNSPD. En particulier, deux scénarios sont analysés: une opération longue distance, qui étendrait la distance record de communication de 100 km, et une opération à taux d’échange rapide, qui accroîtrait les taux de clés secrètes d’un ordre de grandeur comparé à l’état de l’art.

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1 Introduction

Single-photon detectors at telecom wavelengths play an important role in many applications.

Among these are, quantum key distribution (QKD) [1, 2], singlet-oxygen dosimetry for photodynamic therapy [3, 4], photon counting optical communication [5], optical time domain reflectometry [6], eye-safe laser ranging [7, 8], testing of integrated circuits [9], biomedical imaging [10] and general quantum optics experiments.

Many single-photon detection schemes exist [11, 12, 13], however, in the last decade the most progress has been made in InGaAs/InP based single-photon avalanche diodes (SPAD) as well as superconducting nanowire single-photon detectors (SNSPD). These two detector technologies will form the basis of study in this thesis. A specific focus will be made on free-running detectors, which are useful for asynchronous tasks.

The dark count rate (DCR) is one of the most important metrics for a single-photon detector.

Often, this can be the limiting factor in many applications. For example, in QKD, the maximum achievable distance is limited by the DCR of the detectors that are used, since as soon as the signal-to-noise ratio drops below a certain limit, the increasing error rates will no longer permit the generation of a secret key. Detection efficiency is also a crucial metric, sometimes even of fundamental importance for certain experiments, such as Bell-like tests of quantum correlations [14].

Prior to this thesis, the DCR of free-running InGaAs/InP detectors was typically limited to around 100-1,000 counts per second (cps), for efficiencies of around 10-20% [15, 16]. SNSPDs however, could reach almost 60% efficiency [12], whilst during the period of the completion of this thesis, the record increased to 93% [17], which showed the remarkable potential of these detectors. Since the SNSPD technology is relatively new, only a few laboratories were capable of producing high-performance devices, making their availability limited. Two of the main aims of this thesis were to improve the DCR performance of InGaAs/InP SPADs and to develop, in-house fabricated, SNSPDs with a high detection efficiency so as to increase their availability and contribute to the understanding of their fabrication and operation.

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Chapter 1. Introduction

The final part of the thesis concentrated on applying the advancements achieved in the detector technology to further the progress of QKD. Primarily, this involved the increase of the maximum operational distance.

During the course of this thesis a number of peer-reviewed publications were published, which are listed at the end of this Introduction. Where the publication forms an integral part of the discussion in the thesis, it is included in the Appendix. Note that these publications are directly referred to by their Appendix number. Where the publication is outside the scope of this thesis, usually when it was an application (other than QKD) of the developed detectors, it is was not included in the Appendix, and is cited with a Roman numeral in this Introduction.

External citations use decimal digits and are listed in the Bibliography at the end of the thesis.

A list of the main projects carried out is listed below, together with a reference to the relevant Section or Appendix, where it is discussed in detail.

1. Study of the DCR performance of free-running InGaAs/InP negative feedback avalanche diodes (NFAD) whilst operating at low temperatures, down to 140 K. Record low DCR of 1 cps was achieved. Please see Sec. 2.2 and App. A.3. This work lead to a successful technology transfer toID Quantique SAwhich resulted in the ID230 module [18].

2. Study of the low temperature afterpulsing behavior of NFADs over an extended temporal range of 300 ns to 1 ms. This lead to a new afterpulse model to be proposed, which is characterized by a broad spectrum of charge trap sites. Please see Sec. 2.3 and App. A.7.

3. Characterization of the temporal jitter of NFADs, which showed that values as low as 50 ps could be achieved with moderate excess bias voltages. Please see Sec. 2.4 and App. B.1.

4. WSi SNSPDs fabricated at NIST (Colorado, USA) were characterized in all of the main parameters and operation at a temperature of 2.3 K was demonstrated for the first time.

This extended the usability of these detectors since convenient cryogenic systems exist at these temperatures. Please see App. A.4.

5. MoSi SNSPDs fabricated at NIST were characterized, which showed performance near the state-of-the-art in terms of the system detection efficiency, as high as 87%. Please see App. A.8.

6. An in-house fabrication process was launched for MoSi based SNSPDs. Within two generations of devices the system detection efficiency was increased up to 65%. Please see Sec. 3.1 and Chap. 3 more generally.

7. Material properties for superconducting MoSi thin films were studied. In particular this included the optical properties, critical temperature and crystallography. Please see Sec. 3.1.1.

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8. The energy-current relation for MoSi SNSPDs was studied, which lead to interesting insights in terms of the detection mechanism in these detectors. Please see Sec. 3.2.3 and App. B.2.

9. The NFAD detectors were applied to a high-speed QKD system based on the coherent one-way protocol (COW) [A.2]. This lead to a new record distance QKD demonstration, over 307 km of ultra-low loss optical fiber. Particular attention was made to ensure that a quantifiable security parameter could be given for the demonstration. Please see Sec. 4.1 and App. A.6.

10. A newly proposed QKD protocol known as detector-device-independent QKD was studied both in a proof-of-principle [A.5] and a high-speed [A.9] experiment. This protocol was developed to overcome situations where detector side-channels may exist in a system. Please see Sec. 4.2 for further discussion.

11. A multi-protocol QKD transmitter was developed, on the basis of a dual-drive electro- optical modulator. This transmitter can be used for the COW, DPS and BB84 protocols.

Please see App. A.1.

12. Numerical analysis was carried out for a decoy-state BB84 QKD protocol implementation with SNSPDs, which could provide further advancements in terms of the maximum achievable distance and secret key rate. This work is discussed in Sec. 4.3.

As with any technological advance, improvements in single-photon detection systems lead to improved performance in applications, as well as opening up completely new ones. The single- photon detection prototypes, which resulted from the completion of this thesis, have been used for multiple experiments, ranging from quantum optics and metrology, to biomedical applications. These applications are outside the scope of this thesis work, hence doesn’t merit a detailed discussion, however the following list gives a short description with references to further information. This goes some way to highlight the potential impact of the developed technologies, especially in the field of quantum communication.

1. Thanks to the excellent signal-to-noise ratio and ease of use of the free-running NFAD detector [A.3], it became a popular tool for characterization of novel single-photon sources. Most notably, it was used to characterize a narrowband photon pair source developed for quantum networks [19] as well as an integrated AlGaAs source of highly indistinguishable and energy-time entangled photons [20].

2. The free-running NFAD detector [A.3] was used as a detector under test in a proof-of- principle demonstration of the absolute calibration of a single-photon detector [I]. This approach is based on an Erbium-Doped-Fiber-Amplifier (EDFA) radiometer as a primary measurement standard for optical power, and on an ultra-stable source of spontaneous emission. This approach is suitable for frequent characterizations of high-efficiency

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3. Quantum teleportation is a cornerstone of quantum information science due to its essential role in important tasks such as the long-distance transmission of quantum information using quantum repeaters. This requires the efficient distribution of entanglement between remote nodes of a network. Reference [II] describes a demonstration of quantum teleportation of the polarization state of a telecom-wavelength photon onto the state of a solid-state quantum memory. Entanglement is established between a rare-earth-ion-doped crystal storing a single-photon that is polarization-entangled with a flying telecom wavelength photon. The latter is jointly measured with another flying polarization qubit to be teleported, which heralds the teleportation. The fidelity of the qubit retrieved from the memory was shown to be greater than the maximum fidelity achievable without entanglement, even when the combined distances traveled by the two flying qubits is 25 km, in standard optical fiber. This demonstration took advantage of high-efficiency WSi SNSPDs optimized for 1340 nm [A.4], which drastically increased the success rate of the experiment, since it relies on triple-coincidence detection. These results demonstrated the possibility of long-distance quantum networks with solid-state resources.

4. In clinical applications, such as photodynamic therapy of cancer, direct singlet-oxygen detection through its luminescence in the near-infrared range (1270 nm) has been a challenging task due to its low emission probability and the lack of suitable single- photon detectors. By using a low noise NFAD [A.3], a practical setup was proposed as a viable alternative to the current state-of-the art for different clinical scenarios, especially where geometric collection efficiency is limited (e.g. fiber-based systems, confocal microscopy, scanning systems etc.) [III]. The proposed setup was characterized with a standard photosensitizer and was also used to measure the singlet-oxygen quantum yield of a new set of photosensitizers, developed for site-selective photodynamic therapy.

5. Reference [IV] describes a demonstration of postselection free heralded qubit amplification for time-bin qubits and single-photon states in an all-fibre, telecom wavelength, scheme that highlights the simplicity, stability and potential for fully integrated photonic solutions. This experiment exploited high-efficiency MoSi SNSPDs [A.8]. This provides a significant advance towards demonstrating device-independent quantum key distribution as well as fundamental tests of quantum mechanics over extended distances.

6. By using high-efficiency MoSi SNSPDs [A.8], it was possible to demonstrate the vio- lation of an EPR steering inequality developed for single-photon path entanglement with displacement-based detection [V]. Thanks to the high efficiency of the SNSPDs and the use of a high-rate source of heralded single-photon path-entangled states, the scheme was free of any post-selection and thus immune to the detection loop- hole. This result conclusively demonstrated single-photon entanglement in a one-sided device-independent scenario, and opened the way towards implementations of device- independent quantum technologies within the paradigm of path entanglement.

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Peer reviewed articles

Below is a list of peer reviewed articles published during the course of this thesis. Where the publication forms an internal part of the thesis, it is included in the appendix and cited with the appendix number, otherwise, roman numerals are used.

A.1 B. Korzh, N. Walenta, R. Houlmann, and H. Zbinden. A high-speed multi-protocol quantum key distribution transmitter based on a dual-drive modulator. Opt. Express, 21(17):19579–19592, 2013.

A.2 N. Walenta, A. Burg, D. Caselunghe, J. Constantin, N. Gisin, O. Guinnard, R. Houlmann, P. Junod, B. Korzh, N. Kulesza, M. Legré, C. C. W. Lim, T. Lunghi, L. Monat, C. Portmann, M. Soucarros, R. T. Thew, P. Trinkler, G. Trolliet, F. Vannel, and H. Zbinden. A fast and versatile quantum key distribution system with hardware key distillation and wavelength multiplexing. New J. Phys., 16(1):013047, 2014.

A.3 B. Korzh, N. Walenta, T. Lunghi, N. Gisin, and H. Zbinden. Free-running InGaAs single photon detector with 1 dark count per second at 10% efficiency. Appl. Phys. Lett., 104(8):081108, 2014.

A.4 V. B. Verma, B. Korzh, F. Bussières, R. D. Horansky, A. E. Lita, F. Marsili, M. D. Shaw, H. Zbinden, R. P. Mirin, and S. W. Nam. High-efficiency WSi superconducting nanowire single-photon detectors operating at 2.5 K.Appl. Phys. Lett., 105(12):122601, 2014.

A.5 C. C. W. Lim, B. Korzh, A. Martin, F. Bussières, R. T. Thew, and H. Zbinden. Detector- device-independent quantum key distribution. Appl. Phys. Lett., 105(22):221112, 2014.

A.6 B. Korzh, C. C. W. Lim, R. Houlmann, N. Gisin, M. J. Li, D. Nolan, B. Sanguinetti, R. Thew, and H. Zbinden. Provably secure and practical quantum key distribution over 307 km of optical fibre. Nat. Photonics, 9(3):163–168, 2015.

A.7 B. Korzh, T. Lunghi, K. Kuzmenko, G. Boso, and H. Zbinden. Afterpulsing studies of low-noise InGaAs/InP single-photon negative-feedback avalanche diodes. J. Mod. Opt., 62(14):1151–1157, 2015.

A.8 V. B. Verma, B. Korzh, F. Bussières, R. D. Horansky, S. D. Dyer, A. E. Lita, I. Vayshenker, F. Marsili, M. D. Shaw, H. Zbinden, R. P. Mirin, and S. W. Nam. High-efficiency superconducting nanowire single-photon detectors fabricated from MoSi thin-films. Opt.

Express, 23(26):33792–33801, 2015.

A.9 Alberto Boaron, Boris Korzh, Raphael Houlmann, Gianluca Boso, Charles Ci Wen Lim, Anthony Martin, and Hugo Zbinden. Detector-device-independent quantum key distribution: Security analysis and fast implementation. Journal of Applied Physics,

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I T. Lunghi, B. Korzh, B. Sanguinetti, and H. Zbinden. Absolute calibration of fiber- coupled single-photon detector. Opt. Express, 22(15):18078–18092, 2014.

II F. Bussières, C. Clausen, A. Tiranov, B. Korzh, V. B. Verma, S. W. Nam, F. Marsili, A. Ferrier, P. Goldner, H. Herrmann, C. Silberhorn, W. Sohler, M. Afzelius, and N. Gisin. Quantum teleportation from a telecom-wavelength photon to a solid-state quantum memory.

Nat. Photonics, 8(10):775–778, 2014.

III G. Boso, D. Ke, B. Korzh, J. Bouilloux, N. Lange, and H. Zbinden. Time-resolved singlet- oxygen luminescence detection with an efficient and practical semiconductor single- photon detector. Biomed. Opt. Express, 7(1):211–224, 2016.

IV N. Bruno, V. Pini, A. Martin, V. B. Verma, S. W. Nam, R. Mirin, A. Lita, F. Marsili, B. Korzh, F. Bussières, N. Sangouard, H. Zbinden, N. Gisin, and R. Thew. Heralded amplification of photonic qubits. Opt. Express, 24(1):125–133, 2016.

V T. Guerreiro, F. Monteiro, A. Martin, J. B. Brask, T. Vértesi, B. Korzh, M. Caloz, F. Bussières, V. B. Verma, A. E. Lita, R. P. Mirin, S. W. Nam, F. Marsilli, M. D. Shaw, N. Gisin, N. Brunner, H. Zbinden, and R. T. Thew. Demonstration of Einstein-Podolsky-Rosen steering using single-photon path entanglement and displacement-based detection. Phys. Rev. Lett., 117:070404, 2016.

Conference Proceedings

Below is a list of the conference proceedings published during the course of this thesis.

• B. Korzh and H. Zbinden. Low temperature performance of free-running InGaAs/InP single-photon negative feedback avalanche diodes. Proc. SPIE, 9114:91140O–91140O–9, 2014.

• B. Korzh, M. Legre, N. Walenta, T. Lunghi, H. Zbinden, and B. Sanguinetti. Free-running single photon detector based on an ingaas negative feedback avalanche photodiode with an extremely low dark count rate. InCLEO: 2014, page AM2L.7. Optical Society of America, 2014.

• G. Boso, B. Korzh, T. Lunghi, B. Sanguinetti, and H. Zbinden. Low noise InGaAs/InP single-photon detector for singlet oxygen detection. Proc. SPIE, 9370:93701S–93701S–8, 2015.

• G. Boso, B. Korzh, T. Lunghi, and H. Zbinden. Low noise InGaAs/InP single-photon negative feedback avalanche diodes: characterization and applications. InProc. SPIE, volume 9492, pages 94920Q–94920Q–11, 2015.

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Patents

One patent was published during the course of this work.

• S. Chuard, F. Bussières, B. Korzh. Apparatus and method for cryocooled devices ther- malization with RF electrical signals. EU and US pending, 2015.

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2 Single-photon detectors based on InGaAs/InP negative-feedback avalanche diodes

It is convenient to use semiconductor technology for single-photon detection due to its ease of use and relatively mature fabrication techniques. At telecom wavelengths, an appropriate material is InGaAs since it possesses a sufficiently small band-gap. Upon the absorption of a single photon within a semiconductor, a single electron-hole pair is generated. In order to create a detectable signal from this event, it is possible to make use of an impact ionization process, whereupon one of the charge carriers is accelerated in a high electric field such that it can ionize atoms in its path, creating further electron-hole pairs. Such devices are typically called single-photon avalanche diodes (SPAD).

Unfortunately, InGaAs is not compatible with operation in high electric fields. This is because it exhibits large dark currents, due to trap-assisted tunneling (TAT) [21]. By using a semiconductor with a larger band-gap, the TAT is significantly reduced. In this case the material of choice is InP. Therefore, the devices are fabricated with a separate absorption and multiplication (SAM) regions [22]. InGaAs, kept at low bias field, is used for the absorption region and InP, at high bias field, forms the multiplication region. The band-gaps of the two materials define the spectral response of the device. At room temperature, InGaAs has Eg ≈0.75 eV, which corresponds to a cut-off wavelength of≈1.67µm, whilst InP has Eg≈1.35 eV which means photons with a wavelength of less than≈0.92µm will get absorbed in the substrate, before getting to the absorption region [21].

In order to achieve single-photon sensitivity, InGaAs/InP SPADs are operated in the Geiger mode. This mode is achieved by increasing the bias voltage above the breakdown voltage in InP. This breakdown occurs because both holes and electrons can cause impact ionization in InP, where the k-ratio [23] (the ratio of electron to hole ionization rates) is around 0.3-0.5 [24].

Due to this, above the breakdown voltage, the SPAD effectively has infinite gain, meaning that a single-photon detection can lead to a self-sustaining avalanche. In order to reset the APD, the current must be quenched.

The simplest way to quench the avalanche is to gate the SPAD, where the use of smaller and smaller gates effectively reduces the avalanche duration [25, 26, 27, 28]. The main reason

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Chapter 2. Single-photon detectors based on InGaAs/InP negative-feedback avalanche diodes

to reduce the avalanche duration is to minimize an effect known as afterpulsing, where spontaneous dark detections can occur shortly after previous photon detections, due to trapping phenomena [21]. By reducing the avalanche current, the probability that a trap gets filled in the first place is also reduced [29]. Unfortunately, not all applications are compatible with gated operation. For such asynchronous applications it is necessary to operate the SPAD in the free-running regime, which requires the avalanche to be either passively or actively quenched [30, 31, 32]. Even synchronous applications, such as QKD, can benefit from free-running operation, since in this case, the clock rate of the system is defined not by the maximum gating frequency, but by the intrinsic temporal resolution of the detector.

The work in this chapter focused on the development of a free-running InGaAs/InP single- photon detector. This was based on a passively quenched SPAD, which is achieved by connect- ing a series resistance between the SPAD and the ground terminal. As the current starts to flow inside the SPAD, a voltage is induced across the resistor, meaning that the bias voltage across the SPAD effectively drops, which achieves the quenching. The benefit of passive quenching is that the device can be operated under a DC bias voltage, making free-running operation possible

The work involved the use of commercial device known as a negative-feedback avalanche diode (NFAD). This is a SPAD with a monolithic, thin-film feedback resistor integrated directly on its surface [33]. Such integration reduces the parasitic capacitance, resulting in very fast passive quenching of the avalanche current, which subsequently reduces the charge flow and therefore the afterpulse probability. In the following I shall outline the characterization of NFADs operating at low temperatures, in order to achieve extremely low noise levels, which is key for demanding applications such as long distance QKD. The effect of such low temperature operation on the other characteristics, such as the afterpulsing and temporal resolution, shall also be covered.

2.1 Free-running operation of negative-feedback avalanche diodes

As discussed previously, the NFAD is an avalanche photodiode with a monolithic, thin-film feedback resistor integrated directly on its surface [33]. The value of this resistor is a very important parameter for the correct operation of the NFAD, where a large enough resistance is required to prevent a persistent current flow prior to avalanche quench [34]. The devices studied during the thesis had typical values in the range of 500-1700 kΩ, which was sufficient for the desired operation. The devices were produced by Princeton Lightwave.

The readout electronics used for NFADs is described in Ref. [16]. An important element is the adjustable active hold-off time which enables further reduction of the afterpulse probability.

The speed at which this active hold-off is applied, however, does not have to be as fast as the passive quenching achieved by the NFAD itself.

In order to measure the avalanche flow duration in an NFAD, which characterizes the quality 10

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2.1. Free-running operation of negative-feedback avalanche diodes

of the quenching process, we carried out a measure of the electroluminescence [29]. During an avalanche, hot carriers are generated inside the multiplication region, which can emit infrared photons during their relaxation. There is evidence that the number of emitted photons is proportional to the number of charge carriers flowing in the multiplication region, which means that a time correlated measurement of the electroluminescence should yield the waveform of the avalanche current. Such a measurement can be carried out through the use of two single-photon detectors, as described in Ref. [29]. In our case we used two free-running NFADs, connected together with an optical fiber. Hence, if an electroluminescence photon from one of the detectors (detector under test, DUT) is coupled back into the optical fiber, it can be detected by the other detector (probe detector). Figure 2.1 shows the electroluminescence for two NFADs with different series resistances. It’s evident that the avalanche duration is less than 1 ns (full-width-half-max), which is faster than active quenching schemes [29].

This demonstrates that NFADs are very effective in reducing the avalanche flow, which subsequently reduces the afterpulse probability. The NFAD with the 1700 kΩseries resistor seems to have a slightly smaller average current flow compared to the 500 kΩNFAD, which is especially visible in the falling tail.

In order to achieve efficient cooling of the detectors, we use a free-piston sterling cooler (FPSC) (Twinbird SC-UE15R or SC-UD08), as described in App. A.3. These coolers are cheap, maintenance-free and enable cooling of the NFAD detectors down to temperatures as low as around 140 K. Figure 2.2 shows the second-generation detector module which two fiber- coupled NFADs inside. This allowed us to investigate the performance of these detectors over a larger temperature range compared to those previously reported [33, 34, 35, 16, 15]. Note that significantly smaller Stirling coolers are available for the same temperature ranges [36, 37], which could lead to very compact detection modules.

1 0 1 2 3 4 5

Time (ns) 10^-4

10^-3 10^-2 10^-1 10⁰

Normalised Counts

Rs=500 k Rs=1700 k

Figure 2.1: Time correlated electoluminescence detection for two NFADs with different series resistors. This represents the current flow in the diode.

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Figure 2.2: Second generation FPSC-based single-photon detection module with two NFAD detectors inside.

2.2 Detection efficiency and dark count rate

The NFADs used in this work have a SAM structure, where InGaAs is used for the absorption region and InP with its larger bandgap is used for the high-field multiplication region. Dark- carrier generation can occur due to either, a field-dependent tunneling process, specifically TAT in the multiplication region (InP), or a thermally driven process in the absorption region (InGaAs) [21]. Typically, SPADs have a relatively large TAT contribution, hence, it is often sufficient to cool the devices to rather moderate temperatures, around 220 K, to reduce the thermal contribution well below the former effect. However, recent improvements to InGaAs material quality and careful consideration of the SPAD device structure [38], have effectively reduced the contribution of the dark count generation through TAT. One of the main parameters dictating the TAT, is the multiplication region thickness [39]. Since these improvements have been incorporated by Princeton Lightwave in their NFAD devices, the possibility of operating them at lower temperatures has opened up, in order to achieve reduced DCR.

The system detection efficiency (SDE) is a product of several parameters, SDE =ηcηqP_iP_a, whereηc is the coupling efficiency from the optical fiber to the detector active area, ηq

is the quantum efficiency of the carrier creation in InGaAs,Pi is the probability that the photogenerated carrier is injected into the multiplication region andP_a is the probability of generating a detectable avalanche. The excess bias, which is the difference between the bias voltage and the breakdown voltage, is the main parameter affectingPa. Whilstηq is temperature and wavelength dependent [39].

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2.3. Afterpulsing

(a) Dark count rate. (b) Afterpulse probability.

Figure 2.3: Dark count rate and afterpulsing for a free-running NFAD, as a function of operating temperature, for four different detection efficiencies. The deadtime was kept constant at 20µs for all measurements.

The detection efficiency and DCR were measured between the temperatures of 160 K and 220 K. The characterization setup is discussed in App. A.3. Figure 2.3a shows the DCR as a function of operating temperature for four different detection efficiencies (achieved by varying the excess bias), at a wavelength of 1550 nm. A DCR of 1.2 cps was obtained for a detection efficiency of 11.6%, which is nearly two orders of magnitude better compared to the previously demonstrated record for free-running InGaAs/InP detectors [15]. Even operation at high efficiencies gives excellent performance, with the DCR at 27% efficiency being only 15 cps. One can see that at temperatures above 200 K, the slope of the curve seems to increase. This could be partially due to a transition between different dominant dark carrier generation mechanisms, namely TAT in the multiplication region at temperatures below 200 K and thermally generated carriers in the absorption region above 200 K [39]. At temperatures where TAT is the dominant effect, the temperature dependent decrease in DCR arises due to the reduction in the breakdown voltage, leading to a reduction of the electric field required for a given detection efficiency. In addition, the slope will be affected by the variation of theηq

with temperature, due to the shift of cut-off wavelength [39]. Figure 1 in App. B.1 shows the PDE as a function of excess bias, for different temperatures, which shows that below 200 K, the efficiency starts reduce. It should be noted that the the latter data was measured using a different batch of devices, compared to those presented in Fig. 2.3a, however, the nominal design structure is identical.

2.3 Afterpulsing

Characterization of the aftepulsing in SPADs involves the measurement of the correlation of dark count events occurring after a photon detection. When SPADs are operated in the gated mode, characterization of the afterpulsing is usually done by the well-known double-window method [40]. In this case, straightforward variation of the hold-off time between two gates

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complex. We employ a field-programmable gate array (FPGA)-based test procedure outlined in detail in Ref. [16]. The FPGA first imposes a cycle where it waits for no detection to occur in a user defined time, which in this work was set between 75 and 150µs, to ensure that the detector is in a well defined initial condition. Once this condition is fulfilled, a pulsed laser is triggered and the probability of a photon detection in the corresponding time-bin provides a direct measure of the efficiency. Conditioned on a detection of the laser pulse, the FPGA looks for an afterpulse and subsequently updates a temporal histogram. Unlike the normal double-window method, this procedure allows the higher-order afterpulse contributions to be easily characterized, i.e., afterpulse of afterpulse. During the characterization, the user can choose to plot only the first afterpulse detection or to include all of detections. This distinction is crucial because to calculate the total afterpulse probability (AP), all of the detections should be included, whilst when the afterpulse decay curve is being characterized, only the first afterpulse should to be plotted.

Figure 2.3b shows the dependence of the AP on the temperature, for the same range as in the previous section. The hold-off time was set to 20µs for all of the measurements. The AP decreases exponentially with temperature; however even at the lowest temperature the AP was only 2.2% at 11.5% efficiency, which is acceptable for many applications. This demonstrates the feasibility of operating this detector with extremely low noise.

The dependence of the AP on the detector hold-off time is an important parameter since it determines the amount of deadtime that has to be applied to the detector following a detection event, and hence the maximum count rate. Moreover, there is still no consensus on the full physical description of the microscopic origins of this phenomenon, the enlightening of which may be enabled by the study of its temporal behavior. The main open question is the description of the trap sites that are responsible for the effect. It has recently been proposed that the legacy belief, that one or just a few defects with exponential carrier detrapping rates dictate the afterpulsing behavior, is in not the true description since it does not yield physically meaningful results for detrapping time constants [41]. Instead, it was shown that the experimental data is described by a power law. The suggested physical significance of this is that there could be a broad spectrum of trap levels with a specific distribution of detrapping rates. In App. A.7 a detailed study of the afterpulse probability density (per ns) as a function of time after an initial detection event is detailed. The study was carried out for temperatures from 143 K to 223 K and a temporal range of 300 ns to about 1 ms. The most suitable description of the data as indeed a broad spectrum of traps and in addition we were able to estimate the shallowest and deepest energy boundaries of this trap spectrum, being 0.05 eV and 0.22 eV, respectively.

2.4 Temporal jitter

Timing jitter is a very important figure of merit for a single-photon detector, especially for high-speed applications, such as QKD, where high temporal resolution is crucial. It is defined

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2.5. Summary

as the temporal uncertainty of the detection announcement with fixed arrival time of photons.

As outlined in App. B.1, we characterized the temporal jitter as a function of excess bias and temperature for several NFAD batches. The dominant cause of the timing jitter is the stochastic nature of the impact ionization process that produces the detection avalanche [21]. Once the avalanche is triggered, a fundamental build-up time is required for the avalanche amplitude to reach a predetermined threshold level. Crossing this threshold signals the detection event via the readout electronics. The build-up time and its standard deviation, here referred to as the timing jitter, both decrease with increasing detection efficiency. As shown in App. B.1, Fig. 3(c), a temporal jitter as low as about 50 ps (FWHM) can be achieved at an excess bias of 3.5 V, which is comparable to many SNSPD systems [12]. In the temperature range from 210 K to 170 K, the jitter reduces at lower temperatures for all efficiencies, by approximately 10%. This temperature dependence is expected to originate from the increase of the electron and hole ionization coefficients in the InP multiplication region with the reduction of temperature [42], which reduces the avalanche build-up time. Interestingly, some devices show a sharp increase in the temporal jitter at low temperatures and low excess bias (see App. B.1, Fig. 3). This effect is believed to be due to carrier trapping in the hetero-interface between the InGaAs and InP [43]. Devices which have a higher breakdown voltage are not susceptible to this effect, since the electric field in the hetero-interface remains sufficiency high to maintain a negligible trapping barrier.

2.5 Summary

This chapter has been dedicated to studying the low temperature (down to 140 K) performance of free-running InGaAs/InP negative feedback avalanche detectors. These are commercially available devices, however, such low temperature characterization have not been carried out prior to this work.

The most important finding was that these devices can operate with extremely low dark count rates, as low as a few counts per second at moderate single-photon detection efficiencies. This opens up a host of new applications and one which will be discussed in particular in Chapter 4 is long distance quantum key distribution. As an example of a different application, this type of detector has also been applied to singlet-oxygen detection in the context of photodynamic therapy [44]. For a a discussion of other applications which can benefit from this technology, please see the Introduction.

Fruitful investigations were also carried out into the afterpulsing in NFADs, which has enabled a proposition of a new model characterized by a broad spectrum of trap sites, which moves away from the legacy modeling of afterpulsing. In addition, we were able to quantitatively estimate the edges of this spectrum, which may lead to insights relating to their physical origin.

The temporal jitter of these devices also depends on the operating temperature and we have

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To conclude, these detectors are ideal for applications where very high signal-to-noise ratios are required. In addition, they operate in the free-running regime, supplemented by a satisfac- tory temporal resolution, making them ideal for asynchronous tasks. One of the main benefits of these detectors is that the operation temperatures are easily achievable with compact Stir- ling coolers, which makes the system implementation significantly less complex compared to SNSPDs. However, these detectors are far from ideal, mainly due to the limited detection efficiency and, above all, the significant effects of afterpulsing, which limit the maximum count rate to around 10-100 kHz, depending on the operating temperature (see Sec. 4.1 for further discussion of this dependence).

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3 Superconducting nanowire single- photon detectors

Superconducting nanowire single-photon detectors (SNSPDs) were first proposed in 2001 [45]

and have made fast progress to become a very competitive alternative to semiconductor based single-photon detectors [11, 12]. The most notable developments have been made very recently [46], especially in terms of the system detection efficiency (SDE), which has increased the popularity of such detectors, especially for challenging quantum optics experiments [47, 14, 48]. The current state of the art detectors have demonstrated detection efficiencies as high as 93% [17], recovery times of<10 ns [49, 5, 50, 51], temporal jitter of<100 ps [52, 53] and dark count rate (DCR) of<1 cps, both intrinsic [17] and background [54].

The operating principle of SNSPDs is to form a nanowire from superconducting material, the absorption of a single photon within which leads to a superconducting-to-normal state transition, which is detected thanks to a flow of current. The superconducting material is selected such that its energy gap, which is the energy required to break a single Cooper pair, is much smaller than the energy of the photon to be detected. The absorption of such a photon results in an excitation of single electron, which subsequently relaxes by breaking multiple Cooper pairs in its vicinity. By making the nanowire cross-section sufficiently small, (typical nanowire thickness and width is<10 nm and<100 nm, respectively), this disturbance of the superconducting-equilibrium state is sufficient to form an initial normal-conducting domain across the nanowire, leading to a detection event. The exact mechanism leading to the normal-conducting domain is still under discussion within the community. At the beginning, a simple phenomenological model was commonly cited, which predicted the formation of a normal-core within the nanowire, leading to a current redistribution and the surpassing of the critical current density [55]. However, it was eventually recognized that this model failed to quantitatively reproduce experimental results [56], which has reinforced the development of new theoretical models. So far, no single model can reproduce all experimental observations simultaneously. For a review of the latest theoretical developments, please see Ref. [57]. One of the main experimental observations which a detection model is expected to reproduce, is the energy-current relation. This relation defines the photon energy required to cause a detection event, for a fixed current flowing in the nanowire. An experimental measurement of

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Chapter 3. Superconducting nanowire single-photon detectors

(a) Completed wafer with snap-out chips. (b) Fully packaged detectors. The white ferrule is the receptor for an optical fiber, which passively aligns the optical mode onto the centre of the SNSPD.

Figure 3.1: Packaging of detectors using the self-aligned design.

this relation for a typical SNSPD is demonstrated in Sec. 3.2.3.

The events following the formation of the normal-conducting domain play an important role in the correct operation of the SNSPD and have a large influence on the temporal performance [49, 50, 58]. The initial normal-conducting domain starts to extend rapidly due to Joule heating created by the flowing current. Once the resistance is sufficiently large, the current is forced out of the nanowire and detected by the readout electronics. During the absence of current in the nanowire, it is cooled and hence recovers back to the superconducting state. Since SNSPDs have a large kinetic inductance (Lk), the recovery time is limited byτR=Lk/RL, where R_Lis the load resistance of the readout electronics [49]. In the hope of increasing the count rate of SNSPDs, one might try to reduceLkor increaseRL, however, this can only be done to a certain extent due to an electro-thermal feedback mechanism [58]. This mechanism dictates that if theτR becomes too small, the normal-conducting domain does not have enough time to grow sufficiently, due to the current exiting the nanowire too quickly, which leads to a stable, self-heating hotspot. This is know as the “latched” state, from which the detector cannot recover. In addition, even if the current exits the nanowire successfully, the rate at which the current can be redirected back into the nanowire is limited by the thermal recovery time. If the electrical recovery time becomes faster than the thermal recovery time, this can lead to afterpulsing [51]. Ultimately, the temporal performance of SNSPDs is limited by the 18

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thermal properties governing the process. It has already been shown that devices fabricated on substrates with a higher thermal conductivity lead to improved temporal performance [59].

Many different materials have been investigated for the fabrication of SNSPDs, including Nb [60, 61], NbN [62, 63], NbTiN [64], NbSi [65], TaN [66], WSi [17], MoGe [67] and MoSi [68, 69].

Even high temperature superconductors have been investigated, although single-photon sensitivity is yet to be demonstrated [70, 71]. The most widely investigated material in the literature is NbN and more recently NbTiN. Such detectors have benefited from extensive materials research which has led to improved deposition techniques for these polycrystalline materials. By embedding the resulting nanowires within an optical stack, to increase the absorption, an SDE as high as 80% has been demonstrated [62, 63, 64, 72]. An open question within the community is whether it is possible to produce devices with a high yield using these materials. Some investigations have pointed to a possible intrinsic inhomogeneity within such materials [73], which might be responsible for limited yields, possibly due to the polycrystalline nature of the materials. In this scope, some groups began to work with amorphous superconducting films [74], to take advantage of the highly uniform nature of the films. Due to the lack of a well-defined crystal structure, amorphous superconductors can in principle be deposited on any smooth substrate without significant degradation in material properties, as well as enabling the addition of various dielectric materials on top of the films in order to form an optical stack. Indeed, investigations with MoSi have shown a critical temperature independent of the substrate used [75]. Rapid progress with WSi has led to record breaking SDE [17], the ability to produce complex 3D architectures [76] and the feasibility of producing large SNSPD arrays thanks to the high yield [77, 78].

Given the promising recent progress made with amorphous superconductor SNSPDs, this chapter will focus on such devices. One significant characteristic that should be taken into consideration when choosing a superconducting material, is the critical transition temperature (Tc), which will define the requirements on the cryogenic system. Amorphous superconductors posses a lowerTccompared to common crystalline materials. For example, theTcfor WSi, MoSi and NbN, is approximately 5 K, 8 K, and 15 K, respectively, in bulk materials. In turn, a lowerTcmeans that the cross-section of the nanowires can be larger, for the same photon energy [66], relaxing the nanofabrication constraints. In order to strike a balance between these constraints, MoSi was selected as the material of choice, since it has the highestTcout of the amorphous superconductors tested for SNSPD fabrication so far (WSi, MoGe, MoSi) [79].

Moreover, if required, theTccan be tuned through the variation of the stoichiometry [80, 75].

This chapter will outline the fabrication procedure of SNSPDs based on amorphous MoSi.

These detectors, as well as devices fabricated by collaborating groups were characterized in all of the key characteristics. These include SDE, DCR, temperature dependence, spectral response, temporal jitter and maximum count rate. The final part of this chapter will analyze the main fabrication imperfections limiting the current system performance, as well as outlin- ing the progress and outlook towards overcoming these imperfections. Where applicable, the detector design and imperfections will be analyzed with finite-element modeling, in order to

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gain a quantitative measure of the impact of these factors.

3.1 SNSPD Fabrication

The fabrication of the SNSPDs is carried out on 4 inch silicon wafers, with one wafer producing around 150 devices. Figure 3.1a shows a complete wafer. Since each detector is defined using electron beam lithography, it means that each device design can be unique, which allows one to vary several design parameters on each wafer. In this study, the main parameters which will be varied are the nanowire width (w) and the fill-factor (FF) which is the ratio of the nanowire width to the nanowire pitch. The transverse design of the SNSPDs is kept constant for every detector on a given wafer. The main objective of the transverse SNSPD structuring is the enhancement of the absorption in the nanowire by the creation of an optical micro-cavity [72].

Two transverse designs shall be discussed in this chapter. The first is a “simple” nanowire structure, which consists of the nanowire, embedded within SiO2, for passivation. The second is “micro-cavity-enhanced” structure, where the nanowire is embedded into an optical stack consisting of a metallic mirror below and a series of dielectric layers, optimized for maximum absorption within the nanowire.

The fabrication process-flow for a micro-cavity-enhanced design is outlined in App. C. The simple nanowire design follows the same process-flow, whilst omitting steps 5, 6, 7, 8, 20 and 22. The nanofabrication is carried out a the CMi cleanroom facility at EPFL. An SEM image of a nanowire meander is shown in App. B.2, Fig. 1. In order to facilitate fast, simple and reproducible packaging of the devices, we employ the self-aligned detector design [81]. This achieves passive alignment between the SNSPD and the input optical fiber with an alignment precision of several microns and allows for plug-and-play installation of detectors. Figure 3.1b shows the packaged devices.

In the following subsections, two critical steps of the fabrication procedure shall be discussed, namely the characterization of the superconducting films and the design of the optical stack.

Later on in the chapter, in Sec. 3.2, the characterization of the two different detector designs shall be presented, whilst in Sec. 3.3, the most important fabrication imperfections of the current process-flow shall be discussed, namely the etching of the nanowires and the SiO2

deposition.

3.1.1 Amorphous molybdenum silicide superconducting thin films

Relatively little information is know about amorphous superconducting films, since the major- ity of SNSPDs are usually fabricated using polycrystalline superconductors [55]. In addition, the material properties are heavily dependent on the deposition conditions, hence it is crucial to characterize the the films thoroughly. One of the main characteristics is the critical temperature since it reflects the film quality.

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3.1. SNSPD Fabrication

4 5 6 7 8

0 5 10 15 20

R ()

T (K) down up

Sample MoSi1 Mo_0.75Si_0.25 T_c=5.69K

(a) Mo0.75Si0.25, thickness=40 nm.

6 7 8 9 1 0

02468

1 0 1 2 1 4 1 6 1 8 2 0

R (Ω)

T ( K ) u p

d o w n

M o_{0 . 8}S i_{0 . 2}

d = 4 0 n m

(b) Mo0.8Si0.2, thickness=40 nm.

4 . 0 4 . 2 4 . 4 4 . 6 4 . 8 5 . 0 5 . 2 5 . 4 5 . 6 5 . 8 6 . 0

0

1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0

R (Ω)

T ( K ) d o w n

u p

M o_{0 . 8}S i_{0 . 2}

d = 5 n m

(c) Mo0.8Si0.2, thickness=5 nm. (d) X-Ray diffraction from a 40 nm thick Mo0.8Si0.2 film.

Figure 3.2: Critical temperature of MoSi thin films for different stoichiometries and thicknesses as well as X-Ray diffraction verification that the films are amorphous.

200 400 600 800 1000 1200 1400 1600 1800 2000 Wavelength (nm)

2 3 4 5 6

n, k

nk

Figure 3.3: Optical constants of Mo0.8Si0.2thin films.

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The stoichiometry of MoSi films has a large influence on both the critical temperature and the material phase [80]. Molybdenum itself, has a transition temperature of 0.92 K, however, with the addition of silicon, this increases to approximately 7-8 K at a silicon content of about 20% [79], for bulk material. In addition, a crystalline-to-amorphous phase transition occurs at approximately 18% silicon content, which also depends on the deposition conditions, such at the substrate temperature [80], with the amorphous phase favored at lower substrate temperatures.

The deposition of MoSi was carried out at the University of Basel (collaboration with the groups of Richard Warburton and Christian Schönenberger), using magnetron co-sputtering of separate Mo and Si targets. The substrate temperature was kept at ambient temperature. The stoichiometry was controlled by varying the relative deposition rates of Mo and Si. Figures 3.2a and 3.2b show the transition temperatures of Mo0.75Si0.25 and Mo0.8Si0.2, which are 5.7 K and 8.3 K, respectively. These values compare favorably with values reported previously in literature for bulk materials [80, 75]. The film thickness in this case was 40 nm. With reducing film thickness the transition temperature reduces and Fig. 3.2c shows that for a 5 nm Mo0.8Si0.2

film, it becomes about 5 K. Given these results, Mo0.8Si0.2was selected at the stoichiometry of choice.

The MoSi films were always protected with a capping layer of amorphous silicon. This capping layer was deposited by simply leaving the Si target illuminated after the required MoSi thickness was deposited. The capping layer thickness was 3 nm. It was found that omitting this capping layer reduced the critical temperature by about 7% on the day of the deposition.

Hence, the capping layer was utilized to prevent any degradation of the MoSi. The long term effects of the cap layer omission were not studied.

To ensure that the material remains within the amorphous phase, X-Ray diffraction was carried out on the 40 nm sample and Fig. 3.2d shows the resulting scan. The only structural peaks correspond to the silicon substrate and the broad distribution around the main peak indicate the amorphous nature of the main film.

Since we are concerned with maximizing the optical absorption within the SNSPD, in order to achieve an efficient micro-cavity-enhanced structure, it is crucial to characterize the optical constants of the MoSi films. This enables accurate modeling of the micro-cavity, which will be the topic of discussion in the following section. This characterization was carried using a spectroscopic ellipsometer. In order to improve the robustness of the material model, the measurement was done at two different angles, for three samples of different thicknesses (6.5, 7.3, 8 nm), which is commonly done when characterizing absorptive films [82]. The material model fitting was carried out using the open-source RefFit software, developed by Alexey Kuzmenko. The optical constants from the resulting model, consisting of the a Drude-Lorentz model with one oscillator, are shown in Fig. 3.3.

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3.1. SNSPD Fabrication

(a) Fill-factor of 0.5. (b) Fill-factor of 0.3.

Figure 3.4: Unit cell of detector. Materials and thicknesses are (from top to bottom): vacuum, TiO2(59 nm), SiO2(401 nm), aSi/MoSi nanowire (3/6.5 nm), SiO2(200 nm), Al2O3(15 nm), Ag (50 nm). The colour scale shows the electric field amplitude. In this simulation, the dielectric stack was optimized for the 0.5 fill-factor, which is why larger electric field beating is visible outside of the detector stack for the 0.3 fill-factor detector, since more electromagnetic energy escapes the optical stack.

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(a) TE polarization, electric field parallel to the nanowire.

(b) TM polarization, electric field perpendicular to the nanowire.

(c) Poynting vector diagram around the nanowire.

TE polarization, FF = 0.5.

(d) Poynting vector diagram around the nanowire.

TM polarization, FF = 0.5.

Figure 3.5: Absorption in the nanowire versus the fill-factor for TE and TM polarization as well as the time averaged Poynting vector diagram in the vicinity of the nanowire for the two polarizations. The color scale on the Poynting vector diagrams represents the amplitude of the Poynting vector, whilst the arrows represent the vector at the head of the arrow. It is possible to see that the absorption for the case of TE polarization is greater, since there is a larger energy flow towards the nanowire.

24

High-performance single-photon detectors and applications in quantum communication

Thesis

Reference

High-performance single-photon detectors and applications in quantum communication

KORZH, Boris

KORZH, Boris. High-performance single-photon detectors and applications in quantum communication . Thèse de doctorat : Univ. Genève, 2016, no. Sc. 4995

URN : urn:nbn:ch:unige-889792

DOI : 10.13097/archive-ouverte/unige:88979

U

G

F

S

Groupe de Physique Appliquée Professeur Hugo Z

High-performance single-photon detectors and applications in quantum

communication

Thèse

présentée à la Faculté des sciences de l’Université de Genève pour obtenir le grade de Docteur ès sciences, mention Physique

par

Boris Alexander K

de la Grande Bretagne

Thèse N° 4995

G

Centre d’impression de l’Université de Genève

30 Septembre 2016

Acknowledgements

Abstract

Résumé

Contents

1 Introduction

Peer reviewed articles

Conference Proceedings

Patents

2 Single-photon detectors based on InGaAs/InP negative-feedback avalanche diodes

2.1 Free-running operation of negative-feedback avalanche diodes

2.2 Detection efficiency and dark count rate

2.3 Afterpulsing

2.4 Temporal jitter

2.5 Summary

3 Superconducting nanowire single- photon detectors

3.1 SNSPD Fabrication