In the frame of the TriQuI project, we provided multiple detectors the Néel institute, CNRS, in Grenoble. The project aims at building the knowledge and technology required to create efficient triple photon generation. The group started this work by working on the design and elaboration of uncoated or coated KTP and TiO2 ridge waveguides using an original process based on precision dicing. The KTP ridged waveguides have been tested for photon pair generation. The two photon generated can be detected using two single-photon detector (fig. 5.12). Identifying the photon pair can be done with a coincidence measurement, were a fixed timing difference between the two detector outputs is a signature of a generated pair.
Figure 5.12: Experimental setup of the coincidence measurement and histogram obtained.
Image Courtesy : Augustin Vernay, Néel Institute, CNRS
As a weak power (1.4 mW) is used for the generation and the KTP crystal generation efficiency is estimated around 10−6. Thus the photons are generated at a very low rate, hence high detection efficiency and low dark counts are needed. We provided the group with several SNPSDs with efficiencies∼70% and low DCR. I had the opportunity to help the group install the detectors and ensure the proper functioning of the measurement.
Figure 5.12 shows the result of a 1 hour integration time measurement done by the Néel Institute. The very good signal over noise ratio leading to the sharp peak and near zero noise is obtained thanks to the low DCR and high SDE of the detectors. The jitter
<60ps ensures the high timing precision of the measurement allowing for the coincidences measurements.
6 Conclusion and outlook
“
A conclusion is simply the place were you got tired of thinking.Dan Chaon
”
The results presented in this works focused on improving molybdenum silicide supercon-ducting single-photon detectors characteristics.
An important amount of time has been dedicated to contribute to the work of the group on single-meander SNSPDs. At the time, system detection efficiencies of 65% had been achieved [36], and ongoing efforts focused on improving the cleanroom processes to improve the fabrication yield and optimize the quantum efficiency of the detectors.
This work led to the fabrication of detectors with 80% SDE and 26 ps timing jitter [10].
Later on, detection efficiencies up to 90% were achieved. Experimental setups for the characterization of the detectors have been developed. In particular, our work on the direct measurement of the recovery time of the detectors allowed us to gain important insights on the time dynamics of the efficiency after a detection event [42].
A specific aspect of my work has been the development of solutions to improve the maximum detection rate of the detectors and reduce their recovery time. These efforts led to detectors with maximum detection rates more than one order of magnitude higher than conventional single-meander devices of the same material. This work was published in a peer-reviewed article in 2021 [100] and a patent application was filed [101].
Different solutions to reach high detection rates and optimize other characteristics of the detectors were proposed and studied as well. In particular, working on the readout elec-tronics of the SNSPDs could help improve the detection rates by conditioning the signals of multiple detectors while minimizing the electronic crosstalk between them. Advanced superconducting electronic solution have been studies and are currently investigated in our group.
The detectors fabricated during this thesis have been of valuable use for many quantum optics applications requiring high efficiencies, low DCR and low jitter. Moreover, the improvements on maximum detection rate and recovery time of our detectors opened the way to measurements in fast QKD experiments and Heralded Single-photon Path Entanglement. Overall, the work presented here contributed to multiple publications by our group in peer-reviewed journals [43, 94, 98, 102, 88, 99]. We could as well collaborate in the international TriQui project by installing several high efficiency detectors at the Néel Institute in Grenoble, France [103].
Moreover, the work presented in this thesis has been transferred to the industry. In particular, high detection rate and PNR detectors based on the parallel antilatch solution presented here are now commercially available [104].
Outlook into the future of SNSPDs
Of course, superconducting nanowire single-photon detectors have not reached their full potential yet, and many of their current limitations will certainly be pushed back in the years to come.
Combining the best performances obtained to this day and get high SDE, low jitter and high speed is certainly feasible. It is not unimaginable that detectors exhibiting simultaneously 99% SDE, jitter under 20 ps, and detection rates above 100 MHz will be achieved by research groups working on the subject in the future. Different direction can be taken to tend to this goal, and the ones presented in this work are only a few amongst them. Combining advanced approaches, such as waveguide coupled detectors [47]
or active quenching [105], with large arrays of detectors will definitively lead to results overcoming what has been achieved to this date.
Other very interesting applications of SNSPDs have also been demonstrated. In particular, clever designs taking advantage of the thermal crosstalk between nanowires allow for position resolution of the photon absorption location on large row-column arrays [106], which open the way to many new possible applications.
More challenging routes can also be taken. Reading out large arrays of SNSPDs di-rectly at cryogenic temperature would solve the issues related to the heat load on the cryostat. Approaches based on Single-Flux-Quantum (SFQ) logic have already been demonstrated [107]. Of course, completely avoiding the use of complex cryogenic systems would lead to even more new opportunities. High Tc superconducting materials have been investigated by the scientific community for years now. Copper oxides such as YBCO1 compounds exhibitTc above the boiling point of liquid Nitrogen (77 K), which is a huge advantage for many practical applications [108]. Tc above 130 K were even obtained at atmospheric pressure [109, 110]. Detectors taking advantage of such properties would greatly contribute to the spread of SNSPDs as the standard detection solution for quantum communication applications.
While it is impossible to know exactly what the future will be made of, it is fairly reasonable to imagine that many quantum technologies will become part of our daily life in the future. Quantum Random Number Generators implemented on smartphones and laptops or Quantum Key Distribution protocols for securing communications are likely to become standards, and wide spread use of quantum computers seems to be plausible as well. The ability to detect single-photon efficiently is a necessity for many implementations of these technologies. It is probable that full camera systems sensitive to the single-photon level will be achieved at some point, and might be based on SNSPD technology, pushing further the limits of both applied and fundamental scientific research.
A Superconducting switches
Work on superconducting switches was initiated during this thesis, and I had the oppor-tunity to help fabricate early versions of the devices presented here. The results from the group presented in this section have been achieved by Patrick Remy, who dedicated important efforts to create efficient switches. This work could open the path to new readout opportunities for our detectors, and drastically improve their performances.
A.1 Working principle
Interfacing superconductor circuits with conventional electronics is challenging. Indeed, typical superconducting elements work with very low impedances and cannot drive typical semiconductor elements. A thermoelectrical device called the nanocryotron (n-tron) has been proposed to fill this purpose [111]. This three terminal device uses the μA current created by superconducting devices to create a hotspot in a large superconducting channel biased with a much larger current (figure A.1). This will force the large channel to switch to its resistive state and redirect the higher current to a semiconductor circuit.
The working principle is in fact similar to SNSPDs, with the hotspot growth being initiated by the gate current rather than the absorption of a photon. This allows for much larger width than typical SNSPDs which can create a net gain in the current provided to the semiconductor readout. Hence such a device could be used as a preamplifier between and SNSPD and its semiconductor readout. However the hotspot-growth is a relatively slow process along the length of the nanowire, which was estimated around 0.25 nm·ps−1 [112]. The device is hysteretic and not able to self-reset without external circuitry [113]. Moreover, input and output terminal are galvanically connected, hence connecting multiple SNSPDs to the same readout cannot be achieved without electronic crosstalk between them, which does not solve the issues presented in the above work.
A variation of the nanocryotron based on the thermally induced switch of the channel via a heating gate, the h-tron, was presented (figure A.2). In this case, the gate and the channel
Figure A.1: Illustration of the working principle of the n-tron. A geometrical constriction in the gate will create a hotspot that propagate trough a larger channel.
are galvanically isolated from each other. Heat generated by the gate will propagate to the channel, lower its critical current density, and make it switch to its resistive state.
The gate can either be made of a superconducting material with a constriction, or from a resistive material. It is interesting to note that this effect is actually equivalent to the unwanted thermal crosstalk observed in our multi-element detectors presented in chapter 3.
Improvement of this concept device using a multi-layer process with a resistive gate and a superconducting channel shows very promising results [85, 114]. This device provides fast switching thanks to the low gap between the channel and gate which provides high coupling and focused heating along the whole channel. Using appropriate channel current even allows for the self-reset of the device without external circuitry which allows it to drive high impedance loads. The galvanic decoupling of the input and output of the device is of great interest, as this can solve the electronic crosstalk issues encountered when trying to readout multiple SNSPDs. Figure A.3b shows SEM images of basic n-tron and h-tron structures fabricated in our group.
A.1. Working principle
Figure A.2: Illustration of the working principle of the h-tron. The gate is represented as a resistance. Joule heating will trigger the channel and make it switch from a superconductive to a normal state.
(a) SEM image of a n-tron with a gate constriction of 30 μm and a channel width of 5 μm. Due to the shape and size of the constriction, smaller gate sizes were hard to fabricate with a decent yield.
(b) SEM image of a h-tron with a gate of 50 nm and a channel of 1 μm. The gate and chan-nel are separated by a horizontal distance of 50 μm. Left inset shows the kLayout design drawing. Right inset shows a SEM zoom on the gate. Due to overetching during the fab-rication process, the gate is slightly narrower than expected.
Figure A.3: n-tron and h-tron designs fabricated during this thesis. Darkest gray is the superconducting material, lighter areas are insulating etched lines. Colors were added for clarity purpose, the gate is highlighted in red and the channel in blue.