150 100 50 0 50 100 150 200
Time [ps]
0 10000 20000 30000 40000 50000 60000 70000
N um be r of ev en ts [a . u]
Figure 3.17: Timing distribution of the photon detection event.
3.5 Photon-Number Resolution
Resolving the number of photons simultaneously detected in a single laser pulse has many applications, such as linear optics quantum computing [72] as well as quantum communication [73]. Several interesting approach exist to perform such a task. Associating a single-meander SNSPD with a time-multiplexing setup takes advantage of the detector’s low recovery time [74, 75]. Operating the detector with a lower bias current so that multi-photon event only are recorded is possible, at the cost the ability to detect single-multi-photon events [76]. Detection up to 4 photons by analysis of the output signal of a single-meander SNSPDs has been demonstrated as well [77]. A more intuitive method is of course to spread the wavepacket across a multi-element detector [78] which naturally is the case for the designs presented in this chapter.
Multipixel detectors (section 3.2) obviously allow for detecting multiple photon simultane-ously and resolution of 16 photons has been achieved using this technique [52]. Series and parallel SNSPDs (sections 3.3 and 3.4) demonstrate photon-number resolving capabilities with similar number of photons [63, 66]. Indeed the output voltage pulse amplitude is highly correlated with the number of photons absorbed in the different nanowires of those designs. In particular, antilatch parallel SNSPDs show almost perfect discrimination of the number of photons. It is nevertheless important to remember that the number of detected photons will depend on the detector efficiency as well as on the number of pixels [78]. The probability PηN(m|λ) of measuring m counts from an optical pulse
calculated as : PηN(m|λ) =
∞
X
n=m
N! m!(N −m)!
(nλ)nexp−ηλ
n! ×
m
X
j=0
(−1)j m!
j!(m−j)
1−η+(m−j)η N
n
(3.1)
Using a much higher number of elements N than the number of photonsλis therefore crucial to reduce the probability of obtaining a number of count m < λ. Equation 3.1 does not take into account different efficiencies due to pixels variability, geometries and possible misalignment of the fiber.
In order to characterize the PNR capabilities of our devices, I acquired the output voltage oscilloscope traces, while illuminating the detector with an attenuated pulsed laser. The voltage traces obtained can be seen on figure 3.18.
Figure 3.18: Persistence measurement of the output voltage pulses on a 6 nanowires antilatch parallel detector.
The histogram of the maximum amplitude shows that the pulses can be perfectly distin-guished. Nevertheless, a practical application will require the use of different discriminator thresholds at level corresponding to the different number of photon absorbed. Displaying the histogram of the number of detection vs. threshold value lets us make sure that different discriminator can be used to see the different number of photons.
Figure 3.19b shows the histogram of the counts for different threshold values (obtained from the data recorded and shown on fig. 3.18. Placing different discriminators with values of threshold within the plateau regions (e.g. 30 mV, 90 mV, 150 mV, 210 mV, 270 mV) will each correspond to a given number of multi-photon detection.
3.6. Large area SNSPDs and multimode fiber coupling
(a) Histogram of the maximum voltage of the different pulses shown on fig. 3.18. The differ-ent number of photons corresponding to the different voltage amplitudes are almost per-fectly distinguishable.
(b) Number of counts as a function of the volt-age threshold used for detection on the traces of fig. 3.18. The 6 distinguishable steps ob-tained shows that perfect discrimination of the number of photon absorbed is possible, with
∼60 mV between each value.
Figure 3.19: Characterization of the PNR capability of the antilatch design based on a set of 12’171 recorded traces.
The same antilatch parallel design was later used at ID Quantique SA in the frame of a technology transfer agreement. Detectors resolving up to 12 photons were achieved with this solution.
3.6 Large area SNSPDs and multimode fiber coupling
Large area SNSPDs are required for many applications using free-space light coupling, such as satellite laser ranging [79] and ground-satellite Quantum Key Distribution [80].
Moreover many applications require the use of multimode optical fibers (MMF), which can offer better coupling efficiencies thanks to their larger core and wider numerical aperture. However, typical SNSPDs such as those presented above cover a detection area of only 16 μm x 16 μm. This is much smaller than the core diameter of MM fibers used in communications, which is typically 50 μm or 62.5μm. The nanowire length of a multimode SNSPD covering this detection area is therefore 10 to 15 times longer than for a single mode detector. Not only is the fabrication of a large surface nanowire harder due to the high probability of local defect during the fabrication, but such a detector also suffers from a very long recovery time and loss of efficiency at rates under the MHz range.
NbN SNSPDs with a sensitive area diameter of 50μm fabricated on a photonic crystal for 850 nm detection have been reported [81]. The MMF coupled SNSPDs exhibited a SDE of 82% which is one of the highest reported for a large-area SNSPD at this wavelength.
The same group also demonstrated 100μm active-area SNSPD which achieved 65% SDE
104 105 106 107 108 109
(a) Efficiencyvs. detection rate
100 0 100 200 300 400 500 600
50µm single−meander 50µm parallel design
(b) Recovery time.
Figure 3.20: Speed improvement of 50 μm2 detection area SNSPD using an antilatch parallel design with 6 sensitive nanowires.
Data from [83]
approach is of course to use several smaller area detectors in order to increase the detection area. The largest array reported, made out of 64 independent pixels, has a diameter of 320μm with an SDE of 40% at 1550 nm [57] and was developed for deep-space optical communication.
Using the improved detection rate solution proposed in this thesis can open the way to fast multimode detectors with large detection area. Of course the fabrication of larger detectors comes with new challenges. The nanofabrication and tests of the large area detectors have been made by Emna Amri and are presented in detail in her PhD thesis [83]. Using an antilatch parallel design with 6 exposed nanowires, a recovery time of ∼260 ns was obtained and the maximum detection rate of the device was measured at
∼10 MHz [83] (see figure 3.20). For comparison, 50 μm diameter single-meander SNSPDs fabricated in the group exhibited recovery times above 400 ns (figure 3.20b).
The devices fabricated exhibited saturated efficiency plateau (although the absolute efficiency values were not measured). The jitter FWHM was measured at 104 ps, which is similar to the value obtained with a single-meander SNSPD of the same size (115 ps).
Thanks to the use of the parallel design the maximum detection rate could be greatly improved. As seen on figure 3.20a a rate of 10 MHz at 50% efficiency drop was obtained, which improves drastically the results obtained with the 50 μm single-meander SNSPD and even matches the maximum rates of our smaller 16 μm MoSi detectors.
4 Readout improvements
Using advanced readout solutions can improve the performances of SNPSDs. Reducing the fabrication complexity of our parallel designs while mitigating the electronic crosstalk in a more efficient manner can be achieved by dedicated readout electronics. Different principle are presented in the first part of this chapter, sections 4.1 and 4.2. Measurement results are discussed and compared with those obtained with the fully integrated detectors presented in chapter 3. A brief outlook of what could be the future of SNSPDs’ readout electronics is finally proposed in section 4.3.