Ronan Gourgues
Promotoren: prof. dr. ir. Z. Hens, prof. dr. ir. D. Van Thourhout, prof. dr. V. Zwiller Proefschrift ingediend tot het behalen van de graad van Doctor in de ingenieurswetenschappen: fotonica Vakgroep Chemie Voorzitter: prof. dr. P. Bultinck Faculteit Wetenschappen Vakgroep Informatietechnologie Voorzitter: prof. dr. ir. B. Dhoedt Faculteit Ingenieurswetenschappen en Architectuur Academiejaar 2020 - 2021
Wettelijk depot: D/2020/10.500/89
Vakgroep Informatietechnologie
Promotoren:
Prof. dr. ir. Zeger Hens
Prof. dr. ir. Dries Van Thourhout Prof. dr. Val Zwiller
Examencommissie:
Prof. dr. ir. Dani¨el De Zutter (voorzitter) Universiteit Gent Prof. dr. St´ephane Clemmen Universiteit Gent Prof. dr. ir. Pieter Geiregat Universiteit Gent Prof. dr. Wolfram Pernice University of M¨unter
Prof. dr. Jean-Michel G´erard CEA and Universit´e Grenoble Alpes
Universiteit Gent
Faculteit Ingenieurswetenschappen en Architectuur Vakgroep Informatietechnologie
Technologiepark-Zwijnaarde 126 iGent B-9052 Gent, Belgi¨e Tel.: +32 (0) 9264 3316
Fax.: +32 (0) 9264 3593
My PhD adventure started with an email from Julien send to our master’s promotion saying that Single Quantum had an open PhD position to work on SNSPDs. Surprisingly I was the only one to answer and with my very bad English decided to apply for it. I thought that this position was perfect for me since it allows to have a good trade off between industry and academia.
I went to Delft for the interview and I had a really good impression about SQ as well as for the city. The interview went very well and few months later I started my PhD. Now its the time to thank people who contributed from near or far to the smooth running of my PhD.
First of all, my sincere thanks go to the founders of SQ, professor Val Zwiller and Dr. Sander Dorenbos for the opportunity to join and be part of the big family that is Single Quantum. I really appreciate your trust, your support and the freedom that you gave me to work on different projects related to SNSPDs. Thank you also for the excellent idea to create Single Quantum.
Val and Sander, you are the kind of bosses that we have all dreamed of ha- ving. Then I would like also to express my sincere thanks to my supervisors from UGent, professor Zeger Hens and professor Dries Van Through. Then I would thank Sander and Val (again)I have had a lot of fun to work with SQ and I spent a lot of great moments. Also, it allows me to learn a lot of new cool things and to arouse my curiosity to the highest.
I would like to thanks colleagues and friends that are/ have been part of Sin- gle Quantum and contribute to make this company so awesome! Gabriele, to have you while starting my PhD has been a great thing, you placed me in the good condition to improve my English and to learn the fundamental of SNSPDs. I really appreciate your experts advices and your encouragements to carry out all these projects. I really enjoyed all these trips; we had a lot fun and the time was going really fast! What a memory playing beach volley until night on the beach volley fields of Tel Haviv. Also, once I recover fully from my surgery, I hope to do sport with you as before, the tennis field of TuDelft are waiting for the return of Rafa and Roger. I am deeply grateful to Iman, who made an enormous contribution to this PhD! Thank you for
your trust and your support, always in a good mood with unlimited power of energy and never running out of great ideas! Niels, thank you so much for sharing your expertise on SNSPDs! This guy is the one that can fix and answer to every issue encountered in the lab, I have learnt so much by working with you! Jessie, thank you for always being in a positive mood, your kindness and taking care so well of Single Quantum and its employees.
Andreas, I am so impressed by your general scientific knowledge and by the way you can fixe problem, it looks like that with there is none with you!
Thank you for your help with Attocube, the great discussions on a lot of different subjects from physic to linguistics and all the very good laughs!
Antonio (Vivaldi) you were (still now) my office mate, we had a lot fun together! Im glad that we share a mutual passion for football and music but also about the stupidities of human nature. Thank you for helping me with my different projects! Jin (Tonic), it was nice from you to share your Chinese culture and wisdom. Thank you for helping me with my projects and in particular for bringing me in the clean room the weekend to bond my chips when the bonder was off! Misha, Monique, Jan, Gijs, Pei-Yi, Nima, Henry, Yves, Joeren, Caterin, its nice to have you around guys, I had a great time and lot of fun with you whether it was at work or for drinking beers on Friday evening (also later at party). We still need to go in that way! I would like to thank some former colleagues from SQ: Serguy, Victor and my red big Dutch kroket Ren´e, thank you for your technical support and for being a great Dutch teacher. I want to thank the people from KTH and a special thanks for Ali for your help and your support with my first project.
I was part of a European project called Phonsi which allows me to travel a lot and to meet every 6 months great scientists. For sur, the travel in Israel was amazing and will remain a great souvenir. I would like to offer my special thank to Lukas, it was nice to work with you. It was not easy; we had many failures (Murphy law) but at the end we succeed to carry out this project. I would like to thank Joanna and Adam for their pleasant company and the fun that we had together during my secondment in Haifa. Annalisa, it is nice that you have joined SQ, Niall, Federico, Michael, Darius, Bogdan, Renu, Maryam, I wish you good luck with your future scientific career.
I would like to thanks all my different flatmates: Sebastian (never live with a person thats long), Kris, Tito for all the good vibes, the fun and that ma- kes Gerards house an awesome place to live in. Julien and Francisca thank you for being around at the beginning of my PhD, your kindness and the good times spent together. Paul, Im amazed every time you speak about planes, it was great to have you around, in a sense you helped me to bear the homesickness.
I would like to thanks all my friends from France: Marie, Sol`ene, Apolline, Nico, Alex, Toto, Raphy, Vinz, Fabien, Benjamin, Tom, et tout ceux que jai oubli´e du 77! Thank you so much for still being around after all these years, for visiting me in Delft and your happy, nice reunions when I come
back to France. Thank you for the unforgettable moments that we spend together! No worries guys, it is not over and I am convinced that there will be plenty of more! Last but not the least, I would like to thank my whole family! Marino, Mumu, Francky, Yanou, les couzs, C´ecile, Marine, merci pour tout At least, I would like to thank my parents and my sister for all your support and your unlimited love that I received when I was far away from you!
Dankwoord i
Samenvatting xxiii
Summary xxvii
1 Introduction 1-1
1.1 Superconducting Nanowire Single Photon Detector . . . 1-3 1.2 Photonic integrated circuit . . . 1-6 1.3 Quantum dots . . . 1-8 1.4 Scope of the thesis . . . 1-10 1.5 Publications . . . 1-11 1.6 Conferences contribution . . . 1-12 2 Single photon detection: overview and theory 2-1
2.1 Key parameters for single photon detectors . . . 2-2 2.1.1 Quantum efficiency . . . 2-2 2.1.2 Dark counts . . . 2-2 2.1.3 Dead time . . . 2-3 2.1.4 Timing jitter . . . 2-3 2.1.5 Photon number resolution . . . 2-3 2.2 Main available technologies for single photons detection . . . 2-4 2.2.1 Photomutiplier tube . . . 2-4 2.2.2 Single photon avalanche diode . . . 2-4 2.2.3 Transition-edge sensor . . . 2-5 2.3 Superconducting nanowire single photon detectors . . . 2-6 2.3.1 Superconductivity . . . 2-7 2.3.2 General operating detection principle . . . 2-9 2.3.3 Electrical operation of the SNSPDs . . . 2-11 2.3.4 Key parameters in the context of SNSPDs . . . 2-12 2.3.4.1 Quantum efficiency . . . 2-13 2.3.4.2 Dark Count . . . 2-16 2.3.4.3 Dead time . . . 2-17
2.3.4.4 Timing jitter . . . 2-18 2.3.4.5 Photon number resolution . . . 2-20 3 Single photon detector combining high efficiency, detection
rates and ultra high timing resolution 3-1 3.1 Introduction . . . 3-1 3.2 Device design and fabrication . . . 3-2 3.3 Experimental set up . . . 3-5 3.4 System detection efficiency and dark count rate . . . 3-7 3.5 High count rate . . . 3-9 3.6 Timing jitter . . . 3-12 3.7 Conclusion . . . 3-14 4 High performance single nanowire single photon detectors
operating at temperature from 4 to 7 K 4-1 4.1 Introduction . . . 4-1 4.2 SNSPDs fabrication . . . 4-3 4.3 Measurements of SNSPDs critical current and dark count
rate as a function of temperature . . . 4-5 4.3.1 Critical current . . . 4-5 4.3.2 Dark count rate . . . 4-6 4.4 SNSPDs efficiencies and timing jitter characterization as a
function of temperature . . . 4-7 4.4.1 Internal efficiencies up to 785 nm . . . 4-7 4.4.2 System detection efficiency at 785 nm and 1550 nm . 4-8 4.4.3 Timing jitter . . . 4-11 4.5 Conclusion . . . 4-12 5 Integration of superconducting single photon detectors with
quantum dots in photonic integrated ciruits 5-1 5.1 Introduction . . . 5-1 5.2 Controlled integration of selected SNSPDs and emitters on a
silicon nitride platform . . . 5-3 5.2.1 Design and fabrication of the superconducting nanowire 5-3 5.2.2 Deterministic integration of the detectors . . . 5-5 5.2.3 Realisation of the complete photonic integrated circuit 5-7 5.2.3.1 Optical absorption of the SNSPD . . . 5-7 5.2.3.2 Layout of the photonic integrated circuit . . 5-9 5.2.3.3 On-chip measurements . . . 5-10 5.3 Integration of colloidal PbS/CdS quantum dots with plas-
monic antennas and SNSPDs on a silicon based platform . . . 5-11 5.3.1 Design . . . 5-11 5.3.2 Fabrication . . . 5-13 5.3.3 SNSPD characterization . . . 5-14 5.3.3.1 Detection efficiency . . . 5-14
5.3.3.2 Timing jitter and dead time . . . 5-17 5.3.4 Experimental set-up for on-chip measurements at cryo-
genic temperature . . . 5-18 5.3.5 On-chip measurements . . . 5-20 5.3.6 On-chip lifetime spectroscopy . . . 5-23 5.4 Conclusion . . . 5-26
6 Conclusion and outlook 6-1
6.1 Conclusion . . . 6-1 6.2 Outlook . . . 6-3
1.1 (a) SEM picture of a meander shape SNSPD. (b) SEM picture of a traveling-wave detector. . . 1-3 1.2 (a) Optical picture of a 300 mm wafer containing multiple
PICs. (b) Optical microscope image of a photonic integrated chip including an emitter, several micro-optical guiding com- ponents and single photon detectors. (c) Scanning electron microscope picture of an optical waveguide fabricated on top of a superconducting single photon detector. . . 1-6 1.3 (a) Energy band structure as function of density of states for
a bulk and QDs with different size. The energy band gap (Eg) of the QD increases while its size decreases, Ref adapted from [38]. (b) General schematic of energy diagram for a quantum dot. An exciton is generated after absorption of a photon. . 1-8 1.4 (a) Photoluminescence spectra of CdSe/ZnS and PbS/CdS
core/shell colloidal QDs with different size as function of the energy. The QDs present a size-dependence of the emitted photons. The inset shows a schematic of a typical core/shell colloidal QD surrounded by ligands. Adapted from [40] (b) Photoluminescence intensity of single exciton (X), biexciton (XX) and trion (X−) of a single nanowire QD as function of the wavelength. The measurements were performed at a temperature of 4.2 K. The inset shows a SEM modified picture of a InAsP quantum dot (red) embedded in a InP tapered nanowire. Scale bar is 0.5µm. Adapted from [41]. . 1-9 2.1 Illustration of the variation of the delay time between the
optical input pulse and the generation of a SNSPD electrical output pulse. . . 2-3 2.2 Basic schematic of a PMT with an illustration of the electron
multiplication process. . . 2-4 2.3 Basic schematic of a SAPD with a reverse bias voltage and
showing the electron multiplication process in the intrinsic junction. . . 2-5
2.4 Behaviour of the resistance as function of the temperature for a typical TES upon the absorption of a photon. . . 2-6 2.5 Magnetic phase diagram for type-I and type-II superconductor. 2-8 2.6 Schematic illustrating the detection mechanism for SNSPD
in the context of the hotspot model. . . 2-10 2.7 Schematic of the SNSPD equivalent electrical circuit. In (a)
SNSPD is in the superconducting state and the current (red arrow) can flow through while the absorption of a photon switch the state to resistive (b) and the current is diverted to the load resistance. . . 2-12 2.8 (a) for 4.2 K of base temperature, detection efficiency vs bias
current for 100 nm and 70 nm nanowire width at 881 nm wavelength. (b) Detection efficiency vs bias current for dif- ferent photon energy at a base temperature of 4.2 K for a 100 nm wide nanowire. . . 2-13 2.9 (a) Absorption efficiency as a function of wavelength for an
SNSPD with and without gold mirror. (b)) Polarization de- pendence measurement of a meander detector (green dot) and a fractal detector (blue dot) at 1550 nm. Red curve shows sine fitting for the meander type detector. The insets are SEM pictures of fractal detector. . . 2-15 2.10 Dark count measurement as a function of the bias current
with the different contributions. . . 2-16 2.11 Electrical output pulses of a 10 µm and a 16 µm diameter
SNSPD. The pulses are fitted with an exponential to extract the dead time of the SNSPDs. . . 2-17 2.12 Histogram measurement of the instrument response function
of a NbTiN SNSPD, illuminated with a 1064 nm pulsed laser.
The Full Width at Hall Maximum (FWHM) of the Gaussian distribution gives the temporal resolution of the detector. . . 2-18 2.13 Dependence of jitter on bias current for the cases of a room
temperature and a cryogenic (located at 40 K stage) ampli- fier. . . 2-19 3.1 (a) Cross-section schematic of the detector with the dimen-
sions and used materials. The light is coupled from the top of the detector. The orange dashed rectangle corresponds to the simulation unit. (b) A 3D FDTD simulation of optical absorption of SNSPD versus wavelength and film thickness. . 3-3 3.2 A scanning electron microscope image of a fabricated detec-
tor. The top inset is a zoomed picture of the device. The bottom inset is an optical microscope photo demonstrating a detector on a gold mirror. . . 3-4
3.3 (a) Complete device after mounting in a FC mating sleeve, gluing to the printed circuit board, and wire bonding to the transmission lines. (b) Left: the cold finger with mounted detectors and right: a rendered picture of the cryostat. . . . 3-4 3.4 Histograms of critical currents for different film thicknesses.
For the case of an 8.4 nm film, fabrication of narrower nanowires was required to achieve saturation of internal efficiency. . . . 3-5 3.5 Schematic of the setup used to characterize the detectors.
The performance of our detectors is evaluated for both con- tinuous and pulsed excitations. . . 3-6 3.6 Schematic of the SNSPD driver. The bias current is applied
via the dc port of a bias tee. The SSPD RF signal is amplified and detected by a pulse counter. . . 3-6 3.7 (a) Dark count rate versus bias current for a fiber-coupled
detector with and without spooling. For lower bias currents, most dark counts are due to blackbody radiation. At higher currents, intrinsic dark counts are dominant so the effect of spooling on the dark count rate becomes minimal. (b) The system detection efficiency (for O-band photons) and dark count versus current. . . 3-7 3.8 Measured versus simulated normalized detection efficiency for
wavelengths of 1310 nm, 1490 nm, 1550 nm, and 1625 nm.
The inset is a cut of the intensity profile of light for the simulated structure. . . 3-8 3.9 (a) Timing jitter measurement of the SNSPD. The data are
fitted with a Gaussian function and the timing jitter FWHM
= 48.83±0.22 ps. (b) Detection pulse from the SNSPD. The fitted data are an exponential with a decay constant of 20.31
±0.17 ns. . . 3-9 3.10 Oscilloscope traces of 25, 75, and 150 MHz excitation pulses
and their corresponding SNSPD detection events. For each pulse, there is only one detection event. . . 3-10 3.11 Efficiency versus detector count-rate measured with pulsed
(a) and CW mode (b) for different wavelengths. Under CW mode, the efficiency drops as the count-rate increases due to Poisson distribution of the laser and finite dead time of the detector. . . 3-11
3.12 (a) An illustration of the dependence of the detector internal efficiency on the separation between optical pulses. A detec- tor with strong saturation (darker curve) can achieve higher count rates without a significant drop in efficiency. (b) Simu- lation of the readout capacitor voltage. If multiple detection events take place within a short time, the capacitor charges up to higher values. When the values of these charges are random, to avoid the SNSPD from latching, the detector has to be underbiased. . . 3-11 3.13 Gain (S21) versus frequency for our cryogenic amplifier. Inset
shows a photo of a cryogenic amplifier. . . 3-13 3.14 (a) Timing jitter measurement of fabricated SNSPD after ad-
dition of resistive network. (b) Same measurement as (a) with reduced amplifier bandwidth (from∼1GHz to∼550MHz), the jitter is improved by more than 17 ps. (c) Measurement of timing jitter after addition of cryogenic amplifier (immersed in liquid nitrogen), the fit yields: FWHM = 48.83±0.22 ps. 3-13 3.15 (a) An optimized SNSPD with a timing jitter of FWHM =
14.80 ± 0.05 ps. (b) Measured timing jitter for the photo- diode and oscilloscope. . . 3-14 3.16 The efficiency versus current for the same device. . . 3-14 4.1 (a) Measurements of the superconducting transition temper-
ature for 8 nm and 13 nm film thickness. The table in the inset summarizes the Tc for the studied films. The error is estimated to be ±0.1 K. (b) False-color SEM image of the superconducting single photon detector meander. The inset shows the 70 nm nanowire width of the superconducting de- vice. . . 4-3 4.2 (a) Free-space holder with the sample wire-bonded in the cen-
ter of the PCB. (b) Fiber-coupled superconducting detector wire-bonded to the PCB, mounted on the oxygen free copper block. . . 4-4 4.3 Measurements of the critical current vs. base temperature
for different film thicknesses (8 nm, 9 nm, 10.5 nm and 13 nm) and nanowire width (50 nm, 60nm, 70 nm and 100 nm).
The red dashed line is a fit from the Ginzburg-Landau theory. 4-5 4.4 Measurements of the dark count rates vs. bias current for
different film thicknesses (8 nm, 9 nm, 10.5 nm and 13 nm) with 70 nm nanowire width. . . 4-6 4.5 (a) Normalized detection efficiency vs. bias current at 4.3 K,
5.2 K and 6.2 K for 9 nm thick film. (b) Normalized detection efficiency vs. bias current at 4.3 K, 5.2 K, 6.2 K and 7.1 K for 13 nm thick film. The measurements are performed with CW laser diodes at 785 nm, 642 nm, 515 nm and 400 nm. . . 4-8
4.6 Detection efficiency measurements at 785 nm and dark counts rate for T= 2.5 - 6.2 K vs. bias current. The inset presents the normalized detection efficiency at 670 nm vs. bias current in the same temperature range. . . 4-9 4.7 Detection efficiency measurements at 1550 nm and dark counts
rate for T= 2.5 - 5.2 K vs. bias current. . . 4-10 4.8 Timing jitter measurements (dots) and their corresponding
fits (lines) at base temperature of 4.3 K, 5.2 K and 6.2 K. The inset is the timing jitter as a function of the temperature. . . 4-11 4.9 Electrical output pulse of the SNSPD at the temperature
range of T= 4.3 - 6.2 K . . . 4-12 5.1 Schematics of the fabrication process of the SNSPDs. . . 5-3 5.2 (a) Optical microscope image of a chip with 16 SNSPDs.
(b) SEM image of the SNSPD and the series inductor. The red rectangle shows a SEM image of the U-shaped nanowire detector. . . 5-4 5.3 Normalized detection efficiency at 881 nm for two detectors
on the same chip VS bias current. . . 5-5 5.4 (a) Histograms of the critical current before (top) and after
(bottom) deposition of Si3N4. (b) Difference in critical cur- rent before and after deposition of Si3N4 for each detector, the two dashed vertical lines indicate current variation of -1 µA and 1µA, respectively. . . 5-6 5.5 (a)Timing jitter measurement after deposition of Si3N4. (b)Detector
normalized internal efficiency before and after deposition of Si3N4 as function of bias current. . . 5-7 5.6 (a)Schematic of the waveguide cross section with its dimen-
sions and the materials used. (b) 3D FDTD simulation of near field intensity distribution (normalized) of the funda- mental quasi-TE mode along 5µm of NbTiN superconducting nanowire. The light is coupled to the waveguide from the left.
(c) Simulated cross section of the electric field (normalized) in the Si3N4 waveguide before reaching the nanowire detector.
(d) Simulated cross section of the electric field (normalized) in the Si3N4 waveguide with NbTiN nanowires after 2.5µm of propagation. . . 5-7 5.7 Simulated absorption of TE and TM mode for NbTiN nanowires
located below silicon nitride waveguides as function of the length of the nanowire for 890 nm. The dashed lines show an exponential fit to the simulated absorption data. . . 5-8
5.8 Sketch of the photonic circuit with an optical picture of the selected quantum dots nanowires and a section of the ring resonator (II) and in the right a SEM image of the fabri- cated waveguide on top of the SNSPD (I). The emitted pho- tons from the QD nanowire are coupled to the silicon nitride waveguide, then filtered by a ring resonator and finally de- tected by the single photon detector. . . 5-9 5.9 (a) Spectrum of a selected quantum dot nanowire. (b) Life-
time measurement of the quantum dot nanowire performed on chip. . . 5-10 5.10 Emission spectrum of the PbS/CdS QDs at cryogenic tem-
perature. . . 5-11 5.11 (a) Map of the LRDOS results from FDTD simulation of
a dipole emitter with displacement vector d. (b) A SEM micrograph of the fabricated bowtie antenna with a small patch of PbS/CdS QDs deterministically positioned in the antenna gap. The SEM was colored to highlight the different materials. . . 5-12 5.12 The silicon nitride photonic chip consists of areas with col-
loidal QD emitters and plasmonic antennas (I), filters for pump rejection (II), a Planar Concave Grating (PCG) spec- trometer (III) of which four channels are connected to waveguide- coupled SNSPD detectors (III). All components are connected by waveguides which are surrounded by metal strips to sup- press stray light coupling (V). The false-color SEM picture (I) shows a SiN waveguide with a plasmonic bowtie antenna and a patch of colloidal QDs patterned on top of it. To suppress the 700 nm pump laser, sidewall corrugated waveguide grat- ings (II) were implemented. The QD emission was detected by U-shaped superconducting detectors placed underneath the SiN waveguides (IV). . . 5-13 5.13 (a) Schematic of the waveguide cross section with its dimen-
sions and the materials used. (b) Simulation of the absorp- tion of NbTiN nanowires below silicon nitride waveguides as function of the length of the nanowire. The dashed lines show an exponential fit to the simulated absorption data. . . 5-14 5.14 Cross-sections of the silicon nitride waveguide overlayed with
the normalized electromagnetic field power of the mode. Op- tical mode profiles in the waveguide for TE (a) and TM (d) mode. (b) and (c)( (e) and (f) ) show the TE(TM) mode when a 1µm long NbTiN nanowire is introduced below the waveguide. . . 5-15 5.15 Normalized photon counts rate and dark count rates as func-
tion of the bias current for two different SNSPDs for 1050 nm and 1300 nm. . . 5-16
5.16 (a) Typical temporal resolution of a superconducting nanowire.
The Gaussian fit displayed in red gives a timing jitter of 43
±3 ps. (b) Trace of the detection pulse used for the timing jitter measurement. The fitted data is an exponential with a decay constant of∼9 ns. A zoom of the front-edge of an electrical pulse. . . 5-17 5.17 (a) The integrated chip is wire bonded on the PCB and ready
to be measured. (b) Bottom of the home made dipstick used to perform the on-chip measurements. (c) Optical picture of the SiN waveguides containing the QDs. . . 5-18 5.18 Schematic of the cryogenic experimental set-up. The opti-
cal path of the excitation (red), imaging(white) and collec- tion(green) are highlighted in colors. The picture surround- ing by green is a picture of a SiN waveguide with deposition of PbS/CdS in the middle 4.2 K. . . 5-19 5.19 Instrument response measured with the 700 nm pulsed laser
focused on an empty antenna and the signal collected from the waveguide-connected SNSPD . . . 5-20 5.20 Typical PL decay traces for freestanding pillars of PbS/CdS
QDs and PbS/CdS QDs placed in the gap of plasmonic bowtie antennas. . . 5-20 5.21 (a) Typical PL decay traces for freestanding pillars of Pb-
S/CdS QDs and PbS/CdS QDs placed in the gap of plas- monic bowtie antennas. The experimental data is overlaid with fitted stretched exponential functions. PL lifetime from different (b) freestanding pillars of PbS/CdS QDs and (c) Pb- S/CdS QDs placed in the gap of plasmonic bowtie antennas extracted from measurements with different SNSPDs. The circled data points corresponds to the decay traces plotted in (a). . . 5-21 5.22 The PL decay traces in (a) and (b) were normalized after
subtraction of the background, and the experimental data are overlaid with fitted stretched exponential functions. . . . 5-22 5.23 The SEM micrograph in (a) shows the PCG spectrometer
with a colored overlay visualizing the propagation of light from the input to the output arms. The SEM detail in (b) shows a part of the mirror consisting of periodically arranged distributed Bragg gratings and the output couples in (c) have fine taper tips to reduce the insertion loss. . . 5-23 5.24 The emission spectrum of an embedded layer of PbS/CdS
QD at 4.2 K (a) matches the measured spectrometer channel transmissions in (b). . . 5-24
5.25 (a) PL decay traces measured with SNSPDs connected to the respective spectrometer channels and were normalized after subtraction of the background. The experimental data are overlaid with fitted stretched exponential functions. (b) Lifetimes values of the respective spectrometer channels. . . . 5-25
1.1 State-of-the-art performance of different SNSPD materials. . 1-5 4.1 Summary of the detection efficiency of the detector at 785
nm in the temperature range of 4.3 K to 6.2 K. . . 4-10 4.2 Summary of the detection efficiency of the detector at 1550
nm in the temperature range of 4.3 K to 5.2 K. . . 4-11
AFM Atomic force microscopy
a.u. Arbitrary units
AWG Arbitrary waveform generator
BCS Bardeen-Cooper-Schrieffer
CMOS Complementary metal-oxide-semiconductor
CP Cooper pair
CW Continuous wave
DBR Distributed Bragg reflector
DC Direct current
DCR Direct count rates
Eg Energy band gap
FC Ferrule connector
FDTD Finite difference time domain
FWHM Full width at half maximum
GM Gifford-McMahon
GL Ginzburg-Landau
HSQ Hydrogen silsequioxane
Ic Critical current
IR Infrared radiation
MgB2 Magnesium diboride
MoSi Molybdenum Silicide
NbN Nobium nitride
NbO Nobium oxyde
NbTiN Nobium titanium nitride
NIST National Institute of Standards and Technology
PCB Printed circuit board
PCG Planar concave grating
PCR Photon count rate
PECVD Plasma-enhanced chemical vapor deposition
PIC Photonic integrated circuit
PL Photoluminescence
PMT Photomultiplier tube
QD Quantum dot
QKD Quantum key distribution
RIE Reactive ion etching
RF Radio frequency
SEM Scanning electron microscope
Si3N4 Silicon nitride
SiO2 Silicon dioxide
SPAD Single photon avalanche diode
SPD Single photon detector
SNSPD Superconducting Nanowire Single Photon Detec- tor
SMF Single mode fiber
Tc Critical temperature
TE Transverse electric
TiO2 Titanium dioxide
TM Transverse magnetic
UV Ultra violet
WDM Wavelength division multiplexing
WSi Tungsten silicide
Fotonen zijn elementaire deeltjes die de energiekwantums van elektromagne- tische straling vormen. Ze hebben een belangrijke rol gespeeld in het ontdek- ken en bij het formaliseren van de fundamentele wetten van de kwantum- mechanica. Zoals bijvoorbeeld superpositie en verstrengeling. Bovendien zijn er, dankzij de zwakke interactie van fotonen met de omgeving, imple- mentaties met fotonen voor kwantum optische toepassingen. De controle en manipulatie van kwantumtoestanden met fotonen kan worden bereikt door experimentele opstelling met discrete elementen, of door nanometer schaal componenten ge¨ıntegreerd op een substraat. De laatste heeft als voordeel hoge stabiliteit, robuustheid, en compactheid, waardoor schaalbare archi- tecturen mogelijk zijn. Dit veld is in hoge mate ontwikkeld gedreven door wijdverbreide toepassing ervan in fiber-optische communicatie en de com- patibiliteit met CMOS industrie.
Generatie, manipulatie, en detectie van enkele fotonen zijn belangrijke bouw- stenen voor quantum information processing. Voor de uitlezing van qubit informatie worden er hoge eisen gesteld aan de detector. Voor de single- foton detectoren zijn er verschillende opties. Gevestigde technologie¨en, zo- als avalanche fotodiodes, transition-edge detectoren, en opkomende techno- logie¨en zoals superconducting nanowire single photon detectors (SNSPDs).
Vooral gedreven door de explosieve groei van quantum information proces- sing zijn SNSPDs tegenwoordig de state-of-the-art technologie voor single- foton detectie. Ze bieden bijna 100% effici¨entie van UV tot het IR golfleng- ten, lage dark-count rates, en uitzonderlijk goede tijdresoluties. Bovendien kunnen SNSPDs verkleind worden zonder compromissen te maken met de prestaties. SNSPDs bestaan uit lange nanodraden∼100 nm breed, gemaakt van een dunne (5-10 nm) supergeleidende film, waarvan de temperatuur ver beneden de supergeleidende temperatuur gehouden wordt. Een constante stroom, net onder de kritische stroom, loopt door de detector. Het detectie- mechanisme is gebaseerd op onderdrukking van de supergeleidende toestand op het moment dat een foton geabsorbeerd wordt. Gekarakteriseerd door een detecteerbare elektrische puls. De geometrie van de detector kan een- voudig aangepast worden al naar gelang van de toepassing. Zo wordt voor fiber koppeling typisch een meanderende vorm gebruikt, en voor integratie met fotonische schakelingen worden U vormige detectoren gebruikt waar-
mee de SNSPD en nanowire samen een travelling wave detector vormen.
Essenti¨ele bouwstenen voor quantuminformatieverwerkingstoepassingen zijn het genereren, manipuleren en detecteren van enkelvoudige fotonen. Om de qubit-informatie effici¨ent te kunnen uitlezen, zijn strenge prestaties van de detector vereist. Er zijn verschillende soorten single photon-detectoren beschikbaar, die goed in staat zijn om technologie zoals lawine fotodiode, overgangsrand sensor of opkomende technologie zoals de supergeleidende na- nodraads single photon-detector (SNSPD) op te stellen. Gedreven door de explosieve groei van het kwantum informatieverwerkingsveld zijn SNSPD’s tegenwoordig de state-of-the-art technologie in enkelvoudig foton tellen door het aanbieden van bijna eenheidsdetectie-effici¨entie van het UV- tot het IR-bereik, lage duistertellingssnelheid en een uitstekende tijdresolutie. Bo- vendien bieden SNSPD’s de mogelijkheid om hun voetafdruk te verkleinen met behoud van hun onge¨evenaarde prestaties. SNSPD’s bestaan uit een lange nanodraad van ∼ 100 nm breed, gemaakt van een dunne (5 − 10 nm) supergeleidende film die ver onder hun kritische temperatuur wordt gehouden. Er wordt een constante biasstroom op de detector toegepast en het detectieprincipe is gebaseerd op lokale onderdrukking van de superge- leidende toestand na absorptie van een foton en die wordt gekenmerkt door een detecteerbaar elektrisch signaal. De lay-out van de detector is eenvoudig aan te passen: een meandervorm is wenselijk voor vezelkoppeling en toe- passing met optische componenten in bulk, terwijl voor een integratie met een fotonische schakeling een meanderende-golfgeometrie zoals een eenvou- dige ”U-vorm geschikter is. In dit proefschrift worden beide architecturen gebruikt en hun prestaties onderzocht.
SNSPDs hebben hoge effici¨enties, hoge tijd resoluties, lage dark-count rates, en hoge detectie snelheden, welke cruciaal zijn voor een breed scala aan toe- passingen. Echter, het combineren van al deze eigenschappen in een device blijft tot nu toe een uitdaging. In Hoofdstuk 3, laten we een breedband NbTiN detector zien met een effici¨entie van meer dan 92%, een detectie rate van meer dan 150MHz, en een dark count rate lager dan 130Hz, in een Gifford-McMahon cryostaat. Bovendien bereiken we, na optimalisatie van het detector ontwerp en de uitlezing elektronica, een ultra lage systeem timing jitter van 14.80 ps (13.95 ps ontkoppeld) met behoud van hoge ef- fici¨entie van>75%.
Complexe fotonische geeffici¨ıntientegreerde schakelingen en complexe kwan- tum optische experimenten kunnen gerealiseerd worden door implementaties van SNSPDs in combinatie met warmtebronnen zoals cryogene elektronica, microgolf bronnen, en actieve optische componenten. Het significante aan- tal warmtebronnen heeft een negatief effect op de temperatuur die bereikt kan worden wat een nadelig effect heeft op de prestaties van de detecto- ren. Hoofdstuk 4presenteert een experimentele studie naar de prestaties van NbTiN SNSPD bij temperaturen hoger dan de typische bedrijfstempe- ratuur van Gifford-McMahon koelers. Door het detector ontwerp en film
dikte (8-13 nm) te optimaliseren is het mogelijk geweest om SNSPDs te maken die hoge prestaties, en hoge effici¨enties van het zichtbare tot tele- com gedeelte van het spectrum, laten zien bij hoge bedrijfstemperaturen.
Bij een temperatuur van 4.3 K is een effici¨entie van 82% bij 785nm, en een timing jitter van 30±0.3ps gerealiseerd. Verder hebben we bij een verge- lijkbare temperatuur voor 1550 nm een effici¨entie van 64% gemeten. Als laatste laten we zien dat tot bij temperaturen van 7 K, bijna 100% interne effici¨entie mogelijk is voor het zichtbare gedeelte van het spectrum. De hiervoor besproken eigenschappen maken SNSPDs geschikt voor gebruik in recente gedemonstreerde geminiaturisseerde closed-cycle koelplatforms met temperaturen van 4.2 K. Deze compacter koelers maken inzet van SNSPDs buiten onderzoekslaboratoria mogelijk, waarmee vele nieuwe toepassing bin- nen het bereik van SNSPDs komen.
Hoofdstuk 5focust op golfgeleider ge¨ıntegreerde SNSPDs die heel erg at- tractief zijn om te gebruiken voor het on-chip detecteren van de kwantum toestanden van licht. Een belangrijke stap naar de realisatie van geavan- ceerde ge¨ıntegreerde kwantum fotonica is de integratie van de single-foton bronnen en detectoren, met de golfgeleiders. Echter, de schaalbaarheid wordt beperkt door de prestatie-eisen die gesteld worden aan de detecto- ren. In het eerste gedeelte van dit hoofdstuk presenteren we een methode, gebaseerd op het meten van de kritische stroom, tijdresolutie en effici¨entie, voor gecontroleerde koppeling van de fotonische kanalen met voor geselec- teerde detectoren. Ook laten we zien dat depositie van een siliciumnitride laag bovenop de detectoren geen nadelig effect heeft op de prestatie van deze. Een hybride on-chip full transceiver demonstreerd de werking van de ontwikkelde methode. De transceiver bestaat uit een deterministisch ge¨ıntegreerde detector gekoppeld met een nanowre kwantum dot, door mid- del van een siliciumnitride golfgeleider en een ring resonator filter. In het tweede gedeelte van het hoofdstuk rapporteren we bovendien de werking van een tweede hybrid on-chip full-transceiver opgebouwd uit verschillende fotonische componenten en bronnen. Dit zijn plasmonische antennes (die de emissie van de IR PbS/CdS QDs versterken), en een golfgeleider, filter, en planaire concave grating gemaakt van hoge kwaliteit silicium nitride. Met deze transceiver hebben we on-chip lifetime spectroscopy van PbS/CdS col- lodiale kwantum dots by crogene temperaturen gedaan en bovendien een concurrence toename van de counts en Purcell versterking van 200 ± 50 voor QDs die deterministisch in de antenne waren geplaatst.
The photon is a fundamental particle that constitutes the energy quanta of electromagnetic radiation. Photons played a major contribution to dis- cover and to formalize fundamental laws of quantum mechanism such as superposition and entanglement. Moreover, photons have been success- fully implemented to perform quantum optics applications thanks to their weak interaction with the environment, beneficial to preserve quantum in- formation, fast speed and easily manipulated with linear optics. Control and manipulation of the photons quantum states can be accomplished by either assembling optical bulk components or by scaling down the compo- nents to the micrometer range, and integrating them on the same substrate.
The latter implementation presents the advantages of long term stability, robustness and compactness which is profitable for enabling large scale ar- chitecture. This field reached a certain maturity due to its widespread employment in fiber-optic communications and its compatibility with com- plementary metal-oxide-semiconductor (CMOS) industry.
Essential building blocks for quantum information processing applications are generation, manipulation, and detection of single photons. In order to efficiently read out the qubit information, stringent requirements are imposed on the detector. Several types of single photon detectors are avail- able, well established technology-such as avalanche photodiode, transition edge sensor or emerging technology such as the superconducting nanowire single photon-detector (SNSPD). Driven mostly by the explosive growth of the quantum information processing field, SNSPDs are nowadays the state-of-the-art technology in single photon counting because they offer near unity detection efficiency from the UV to the IR range, low dark count rate and outstanding time resolution. Moreover, SNSPDs present the capabil- ity to reduce their footprint while preserving their unmatched performance.
SNSPDs consist of a long nanowire of∼100 nm wide, made of a thin (5− 10 nm) superconducting film kept well below their critical temperature. A constant bias current is applied to the detector, and the detection principle is based on local suppression of the superconducting state after absorption of a photon and which is characterized by a detectable electrical signal.
The detector layout is easily adjustable: a meander shape is desirable for fiber coupling and application with bulk optical components, whereas for
an integration with a photonic circuit, a travelling-wave geometry such as a simple ”U”- shape is more suitable. In this thesis both architectures are employed and their performance are investigated.
Independently, fiber-coupled SNSPDs have demonstrated high efficiency, high time resolution, low dark counts, and high photon detection rates which are crucial for a wide range of applications. However, combining all per- formance parameters in a single device remains a challenge. In Chapter 3, we show a broadband NbTiN superconducting nanowire detector with an efficiency exceeding 92 %, over 150 MHz photon detection rate, and a dark count rate below 130 Hz operated in a Gifford-McMahon cryostat.
Furthermore, with careful optimization of the detector design and readout electronics, we reach an ultra-low system timing jitter of 14.80 ps (13.95 ps decoupled) while maintaining high detection efficiencies (>75%).
Realisation of large scale photonic integrated circuits or complex quantum optical experiments involve the implementation of SNSPDs in combination with heat sources such as cryogenic electronics, microwave sources and ac- tive optical components. The significant number of elements will surely increase the base temperature at the detector location and impinge on their performance. Chapter 4 presents the experimental investigation of the performance of NbTiN SNSPDs above the base temperature of a conven- tional Gifford-McMahon cryocooler. By tailoring the design and thickness (8 - 13 nm) of the detectors, high performance, high operating temperature, and high single-photon detection from the visible to telecom wavelengths are demonstrated. At 4.3 K, a detection efficiency of 82 % at 785 nm wavelength and a timing jitter of 30±0.3 ps are achieved. In addition, for 1550 nm and similar operating temperature we measured a detection efficiency of 64 %.
Finally, we show that at temperatures up to 7 K, unity internal efficiency is maintained in the visible spectrum. SNSPDs based on NbTiN material are suitable to be installed in a miniaturized closed-cycle cooling platform with 4.2 K of base temperature which has been demonstrated recently. It can allow the deployment of SNSPDs outside laboratories environments and enable a wide range of applications that require detection of weak light.
Chapter 5focus on waveguide-integrated SNSPDs which are very attrac- tive for detecting quantum states of light on-chip. An important step to realize advanced integrated quantum photonics is the integration of single photon sources and detectors with photonic waveguides. However, scalabil- ity is hindered by stringent requirements on high-performance detectors. In the first part of the chapter, we overcome the yield limitation by controlled coupling of photonic channels to pre-selected detectors based on measuring critical current, timing resolution, and detection efficiency. Moreover, we show that the high performance of the superconducting nanowire detectors, including timing jitter down to 23±3 ps is maintained after deposition of the silicon nitride layer. As a proof of concept of our approach, we demon- strate a hybrid on-chip full-transceiver consisting of a deterministically in-
tegrated detector coupled to a selected nanowire quantum dot through a silicon nitride waveguide and a ring resonator filter. In the second part of the chapter, we report also the working operation of a hybrid on-chip full- transceiver but constituted of different photonic components. These are plasmonic antennas to enhance the emission of IR-emitting colloidal Pb- S/CdS QDs, and high quality silicon nitride composed of photonic waveg- uide, filter and planar concave grating. Using these components, we per- formed on-chip lifetime spectroscopy of PbS/CdS colloidal Quantum Dots (QDs) at cryogenic temperatures and further demonstrated a concurrence increase of the counts with a maximum Purcell radiative enhancement of 200±50 for QDs deterministically placed in the antenna gap.
Introduction
The emergence of quantum mechanisms in the early 20th century through experimental observations of phenomena mainly associated with light, al- lowed scientists to mathematically describe the behaviour of particles such as atoms, photons and electrons which was not feasible with the laws of classical physics. Nowadays, several devices-such as lasers and transistors- hold a central position in our contemporary society and are established on this major science breakthrough.
A quantum state represented mathematically by the notation |Ψi, corre- sponds to particular values of attributes such as polarisation and spin of a physical system, such as photons and electrons. The control and manipula- tion of quantum states is considered to be a next step in the quantum ”rev- olution”. It is expected to boost the abilities of current technologies and/or the creation of new ones. For example, a computer based on quantum states is predicted to have enormous computing power to solve currently unsolvable problems such as electron-electron interaction between two or more molecules in chemistry. Moreover, it would help in the development of new types of materials such as high temperature superconductors. How- ever, the engineering and practical operation of these quantum devices are challenging. Thermal noise can affect the physical system causing a ran- dom modification of the quantum states and the quantum information is irretrievably lost. Therefore, quantum devices require appropriate and effi- cient isolation in order to be able to apply a number of operations such as read out and control; and, at the same time, to avoid being disrupted by the interferences induced by the environment.
Quantum bits or qubits are the basic unit to encode information, and cor- responds to the quantum version of the classical binary bit. A two level quantum mechanical system has the possibility of being either in those two
states (|0ior |1i) as a classical one but also in a superposition of these two states at the same time. The wavefunction of the system is represented by:
|Ψi=α|0i+β|1iwhereαandβare complex numbers such thatα2+β2= 1.
For a single photon, the two basis state could be the horizontal (|Hi) and the vertical (|Vi) polarisation direction. The concept of superposition can also be extended to two (or more) systems and is then called entanglement.
Two physical systems are entangled if they share a common measurable at- tribute (spin or polarization) and under certain measurements they behave like a single entity. For example, two photons entangled in polarisation have a wavefunction as the following : |Ψi= √12(|H1, H2i+|V1, V2i). The manipulation and control of these properties offer incomparable features to quantum technologies.
Qubit information is very generic and can be carried by many quantum sys- tems such as electrons, atoms and photons. To build useful devices, mainly three approaches have crystallized: qubits based on superconducting cir- cuits [1], atomic ions [2], and single photons [3]. Two categories can be distinguished: stationary qubits, that are locally limited in space and fly- ing qubits that can travel for long distances without loosing their quantum information. In this thesis we are focusing on single photons.
Single photons are the candidate of choice for quantum information pro- cessing due their low decoherence rate mostly in free space, fast propaga- tion and easy manipulated thanks to the wide range of optical components and instruments available. Moreover, photons possess several quantities for encoding the qubit information such as polarization [4], time-bin between consecutive photons [3] or along propagation of direction [5]. These features are particularly useful for long-distance secured communications by employ- ing quantum cryptography. The concept consists in transferring data that is protected by the laws of quantum mechanics. In quantum key distri- bution (QKD), the secret random key is based on the quantum state of single-photon and only know by two parties which is utilized to encrypt and decrypt messages. Performance of many protocols in QKD depends strongly on detector properties. A single photon detector with low dark count rate allows to reduce the error key rate, a high time resolution permits photon arrivals to be time-stamped very precisely and with a high count rate to increase the capacity of transmitted data. In this thesis we demonstrate a single photon detector based on NbTiN superconducting films that combines high detection efficiency, count rate, and ultra low timing jitter. Moreover the detector does not require expensive and complex cryogenic systems.
Integrated photonics provide the functionalities to control stably and pre- cisely a massive amount of photons and scaling that would enable the real- ization of large scale quantum information processing technologies. Most of
integrated photonics prototypes have been demonstrated with only a small number of components, and furthermore, the detection of photons is per- formed externally of the chip. Progresses in this direction are slow due to stringent requirements on the detectors performance. In this thesis we present an approach to overcome the yield limitation of superconducting single photon detectors by controlled coupling of photonic channels to pre- selected detectors based on measuring critical current, timing resolution, and detection efficiency. Moreover, we demonstrate fundamental photonic components suitable for cryogenic environments such as CMOS compatible dielectric low loss waveguide and wavelength division multiplexing required to accomplish efficient quantum information processing. Generation and manipulation of single photons on chip necessitate cryogenic electronic and active optical components and also include interconnection with room tem- perature modules that will inevitably alter the base temperature at the SNSPDs location and their performance. In this thesis, we demonstrate high performance SNSPDs based on NbTiN superconductor that can oper- ate up to 7 K.
The following of this chapter is to introduce the three main elements that have been used throughout this thesis.
1.1 Superconducting Nanowire Single Photon Detector
Figure 1.1: (a) SEM picture of a meander shape SNSPD. (b) SEM picture of a traveling-wave detector.
Almost 20 years ago, Gregory Goltsman et al. introduced a new type of photon detector based on ultrathin superconducting material (NbN) [6]. For a superconducting nanowire carrying a current, the absorption of a photon with energy much higher than the gap of the superconductor will suppress locally the superconductivity. Eventually this event will lead to an electrical
signal which can be measured after amplification with a pulse counter unit.
Moreover, for certain values of bias current, the detector demonstrates a single photon detection regime which corresponds to a photon count rate linearly proportional to the input optical power. This device, known as the superconducting nanowire single photon detectors (SNSPDs), demonstrated high sensitivity from visible to IR with few dark counts. In complementar- ity to its single photon sensitivity, SNSPDs presented exceptional electronic properties with a recovery time and a temporal resolution orders of magni- tude better than existing single-photon detectors. From its initial discovery, the SNSPD has been widely studied and developed by research laboratories and private companies to become the leading technology in single photon detection [7]. The main reason of the interest to improve the SNSPDs per- formance has been the rapid expansion of quantum information applications [8]. Nowadays, SNSPDs offer outstanding performance, with system detec- tion efficiencies of 80 - 93 %, from 400 nm to 1550 nm [16, 20, 23, 24]. At lower efficiencies, the detectable wavelength range is much larger and spans impressively from 5 µm [18] until the X-ray range [25]. Dark count rates can be reduced to the mHz range [14], detection count rates up to 1.5 GHz [12], subnanosecond recovery times [11] and timing jitter as low as<10 ps [10, 22]. Table 1.1 summarizes the-state-of-the-art SNSPDs in terms of sys- tem detection efficiency, dark count rate, count rate, temporal resolution, dead time and operating temperature for the four most frequently used ma- terial for SNSPDs.
Moreover, this progress has been beneficial not only for quantum science but more generally where weak light needs to be detected, such as in biomedical imaging [26], laser ranging [27] and CMOS testing [28].
The typical layout for a SNSPD consists of a long nanowire folded into a circular shape. The diameter is slightly bigger than the core of a single mode fiber allowing a proper overlap with the optical mode. Figure 1.1(a) shows the widely used meander architecture which is mainly employed for experiments with bulk optics. For on-chip integration, the light is coupled via waveguide structures and absorbed evanescently by the SNSPD which results in simple and compact architecture as shown in Figure 1.1(b). In this thesis both designs are employed.
MaterialSystemdetection efficiency(SDE),WavelengthDarkcoumtrateCountrateDeadtimeTimingjitterOperating temperatureReferenceOtherfeatures NbN90% 1550nm100cps48MHz28ns71ps2.2K[9]
-4pstimingjitterfora5µmlongnanowire[10] -Deadtimeof200psforawaveguide-integratedSNSPD(<1µmlong)[11] -1.5GHzcountratewitha16-pixelSNSPDarray[12] -SDEof82%at850nmfora50µmdiameterSNSPD[13] NbTiN92% 1310nm130cps150MHz20ns14ps2.5Kchapter3-<1cpsforawaveguide-integratedSNSPD[14] -SDEof70%at516nmand<20psfora50µmdiameterSNSPD[15] WSi93% 1550nm1000cps10MHz40ns150ps0.12K[16]-AkilopixelarrayofSNSPDs[17] -SDEof2%at5µmwavelength[18] MoSi87% 1550nm100cps-35ns76ps0.7K[19]
-SDEof84%at373nmat4Kfora56µmdiameterSNSPD[20] -SDEof98%at1550nmfora50µmdetectordiameter[21] -10pstimingjitterfora5µmlongnanowire[22] Table1.1:State-of-the-artperformanceofdifferentSNSPDmaterials.
1.2 Photonic integrated circuit
Figure 1.2: (a) Optical picture of a 300 mm wafer containing multiple PICs.
(b) Optical microscope image of a photonic integrated chip including an emitter, several micro-optical guiding components and single photon detectors. (c) Scan- ning electron microscope picture of an optical waveguide fabricated on top of a superconducting single photon detector.
Photonics is the science and technology of light (photon) in a similar way as electronics is with electric signals (electron). Photonics refers to generating, guiding, manipulating, amplifying, and detecting light. In the past few decades, light has played a major role in science and technology.
Its impact goes from the optical communications that we use daily, to the test of fundamental concepts in quantum physics. An example of the latter is the experimental demonstration of quantum entanglement by violating of Bells inequality [29] which have been accomplished with entangled pairs of photons generated by atomic radiative cascades in calcium [30]. The conventional experimental implementations consist in carefully assembling multiple blocks of bulk optical components-such as lenses, mirrors, spec- trometers, beam splitter cubes and-connecting them together via free space or optical fibers. A complete system can be composed of hundreds of opti- cal components and assembled on an optical table of several square meters.
However, this approach suffers from a lack of long-term stability, requires to be frequently realigned, bulkiness and, therefore, can be only utilized in a laboratory environment. The other way which has been proposed in the late 60’s [31] is to miniaturize the optical components down to micrometer scale and to integrate them on a common substrate. A photonic integrated circuit (PIC) is an optical device of few square millimeters as shown in Fig- ure 1.2(b), constituted of a thin dielectric layer (few hundred nanometer thick) on top of a substrate. Several PICs can be fabricated on a single wafer as shown in Figure 1.2(a). The light is guided through optical waveg- uide structures as shown in Figure 1.2(c), in a similar way as thin metal strips are guiding electrical current in an electronic integrated circuit (IC).
The optical field is located in the area with the highest refractive index, confined in two dimensions and propagates along the third dimension of the waveguide. Through technological improvements in nano-fabrication, PICs are now composed of multiple state-of-the-art single micro-optical compo- nents such as filters, light emitters, and detectors located on the same single chip. Such integration permits an improvement in terms of performances, compactness, stability, and robustness with a reduction of the cost due to the complementary metal-oxide-semiconductor (CMOS) compatibility. An- other advantage is the possibility to fabricate a hybrid chip which combines on the same substrate photonics and electronics functionalities. Therefore, PICs have a central position in classical fiber-optic communications [32] and present a great potential to be implemented in optical quantum technolo- gies.
Integrated quantum photonic (IQP) is still at its early stages. The main reason is that for such technology, each photon is important because it car- ries the qubit information and the photonic circuits need to minimize optical losses such as absorption or scattering. Whereas, in classical communica- tion, it is not so critical since the optical signal can be easily amplified along the fiber network. Nevertheless, already several material platforms have been investigated [33] with its advantages and drawbacks. Monolithic integration approaches through III-V materials such as GaAs have demon- strated on chip efficient elementary components for generations [34], active or passive manipulation [35] and detection [36] of quantum states of light.
However, waveguide losses are too high to consider building a large scale integrated circuit [37]. On the other hand, a silicon based platform is more prone to be used for IQP applications as it benefits from the highly op- timized nanofabrication recipes adapted directly from the microelectronics industry and a wide range of essential component has been demonstrated.
However, generation of single photon on silicon chips is mostly based on spontaneous four-wave mixing in ring-resonator which is a highly probabilis- tic process and requires high efficient pump extinction. As an alternative, there is of possibility to combine selected quantum dots with silicon to offer the best of both material technologies.
An optical integrated transceiver is a central device in today’s optical inter- connect technologies which enables large volumes of data to be transmitted.
It is composed of a transmitter unit (lasers and modulators), a receiver unit (photodiodes), and photonic routing circuits that are all integrated on the same chip. In a similar way, these devices could be employed for QKD applications. The generation of the secret random keys is carried out by quantum emitters, while the detection is accomplished by SNSPDs.
1.3 Quantum dots
Figure 1.3: (a) Energy band structure as function of density of states for a bulk and QDs with different size. The energy band gap (Eg) of the QD increases while its size decreases, Ref adapted from [38]. (b) General schematic of energy diagram for a quantum dot. An exciton is generated after absorption of a photon.
A quantum dot (QD) consists of a semiconductor nanocrystal that can be embedded in another semiconductor matrix which possess a generally differ- ent energy bandgap. These nanostructures have identical lattice structure than the bulk crystal. The nanoscale dimension of the quantum dot imposes the carriers to be spatially confined by physical boundaries of the material;
this results in different opto-electronic properties. Thereby, the energy spec- tra of the QD is modified by forming discrete energy levels for which the level scheme resembles a single atom, as shown in Figure 1.3(a). The di- mension of the nanoparticle will change the confinement of the charges; and if it is in all three directions, a quantum dot is obtained. For this reason semiconductor QDs are often referred as ”artificial atoms” [39]. As the QD size decreases, the energy band gap (Eg) increases, as depicted in Figure 1.3(a) and results in a blue shift of the wavelength emission. Through opti- cal or electrical excitation, carriers rapidly diffuse from the semiconductor matrix to the QD over carrier collisions and phonon relaxations, as shown with the grey arrows in Figure 1.3(b). Discrete energy levels of the QD are filled with holes and electrons, and a radiative process can take place. An electron in the lowest conduction band can interact with a hole in the high- est valence band due to their mutual Coulomb interaction to form a bound state called an exciton as depicted in red in Figure 1.3(b). The recombina- tion of the electron-hole produces the emission of a single photon with an energy of the exciton (X). There are other types of bound states that can generate a photon, for example a biexciton (XX) is formed from two exci-
tons and a negative trion (X−) which is similar to an exciton with an excess electron. Energy levels in the quantum dot are generally probed with pho- toluminescence (PL) spectroscopy by collecting the emitted single photons upon excitation. The emission wavelength of quantum dots can be tuned from visible to the infra-red by changing the composition and by adjusting the size of the quantum dot (Figure 1.4(a)). The quantum dots employed in Chapter 5 are: III-V semiconductors material, InAsP embedded in InP nanowire, and IV-VI semiconductors material, PbS/CdS core/shell for the colloidal quantum dots.
Figure 1.4: (a) Photoluminescence spectra of CdSe/ZnS and PbS/CdS core/shell colloidal QDs with different size as function of the energy. The QDs present a size-dependence of the emitted photons. The inset shows a schematic of a typical core/shell colloidal QD surrounded by ligands. Adapted from [40] (b) Photoluminescence intensity of single exciton (X), biexciton (XX) and trion (X−) of a single nanowire QD as function of the wavelength. The measurements were performed at a temperature of 4.2 K. The inset shows a SEM modified picture of a InAsP quantum dot (red) embedded in a InP tapered nanowire. Scale bar is 0.5 µm. Adapted from [41].
Colloidal QDs are composed of a small inorganic semiconductor core (1 - 10 nm in diameter) surrounded by a semiconductor shell, and are coated with organic passivating ligands, as depicted in the inset of Figure 1.4(a). Col- loidal QDs are usually synthesized by ’wet’ chemical methods. By tuning the reaction conditions and quantities of the components, different geometries can be achieved such as nanorods [42], nanoplatelets [43] or more complex shapes [44, 45]. This approach allows to synthesize low cost and mass pro- duce nanoparticles with nearly atomic precision and low dispersivity in size.
The ability to precisely control the composition, size, and shape of colloidal QDs provides great flexibility in the engineering of their electronic and op- tical properties (Figure 1.4(a)) which is not available with other types of semiconductor QDs. Therefore, colloidal QDs have been successfully used as bio-markers [46], opto-electronic devices such as solar cells [47], liquid-
crystal display TVS [48], LED [49], and integrated with photonic circuits [50–52]. Colloidal quantum dots present a great advantage to demonstrate bright single photon emission at room temperature [53, 54]. However, the optical properties of colloidal QDs still suffer from blinking effects. Col- loidal QDs can be unstable when they are out of solution and sensitive to temperature which make it challenging to integrate them with silicon based substrate.
The epitaxial QD is another type of semiconductor QD that can be fab- ricated by molecular beam epitaxy. This fabrication method exploits the self-assembly process of the QD induced by strain due to the mismatched lattices. In order to improve the emission collection, the QDs are embedded in a bottom-up grown nanowire with a smooth tapering at the end [55, 56], as shown in the inset of Figure 1.4(b). The QD (3 - 4 nm of section) is lo- cated at 200 nm from the base of the nanowire which is 1.5 - 3µm long for 250 - 300 nm in diameter. A QD nanowire provides directional bright and pure single photons emission [55] but also a natural source of polarization- entangled photons via the biexciton-exciton cascade [57, 58]. Figure 1.4(b) shows the PL spectra of a single InAsP QDs embedded in an InP nanowire from which we can distinguish the narrow line emission of an exciton (X), a biexcition (XX) and a negative trion (X−). The growth technique allows to pre-select individual dots and to easily integrate into a photonic circuit while preserving its optical properties [59]. However, homogeneity in emis- sion wavelength for this type of QDs is challenging due to random processes such as strain or defects in the surrounding semiconductor related to the growth method. In addition, the narrow lines of the PL spectrum are only obtained at cryogenic temperatures and get broader as the temperature increases mainly because of phonon interactions [60].
1.4 Scope of the thesis
The scope of this thesis is the following:
Chapter 2 introduces the working principle and performances of the well established single photon detection technologies. The main part of this chapter is dedicated to the conventional SNSPD. An introduction on the basics of superconductivity is given, followed by a description of the general detection principle and key parameters of SNSPDs. Finally a summary of the performance of the different single photon detectors is provided.
Chapter 3presents the demonstration of high performance NbTiN su-
