Jitter

The timing jitter refers to the timing fluctuation between the photon absorption time and the detection time at which the event is recorded. A delay is of course expected, as the detection mechanism, propagation of the signal, amplification, and discrimination processes are not instantaneous. This delay however has to be highly consistent from one measurement to the other. It is important to characterize the jitter of our detectors, since many of the quantum experiments in which they will be used require time resolution below 100 ps [43].

Figure 2.10: Jitter characterization setup

A typical jitter measurement consists in acquiring data of a large number of detections and plot the histogram of the time recorded between the photo emission and the detection event. This is achieved by using a laser at 1550 nm from NuPhoton Technologies Inc.

with extremely short pulse duration (6 ps), which also provides an electronic trigger signal when a pulse is emitted. The pulses are attenuated to obtain an average number of photon per pulse µ∼0.01, which guarantees that detections occur at the single-photon level. A dedicated Time-Correlated Single Photon Counting module (TCSPC) is connected to the laser trigger signal and the output signal of the detector, as shown on figure 2.10. The time bins and temporal resolution of the TCSPC module used for the measurement are in the order of few ps, which guarantee high resolution.

−150 −100 −50 0 50 100 150

Time [ps]

0x10³ 2x10³ 5x10³ 8x10³ 10x10³ 12x10³ 15x10³ 18x10³ 20x10³

N um be r of oc cu re nc es

Figure 2.11: Timing distribution of the detection time with respect to the laser trigger signal. The jitter characterizes the spread of this distribution. The full width at half maximum (FWHM) jitter of the device presented here is 39.1 ps.

The timing histogram of a single-meandered SNSPDs is shown on fig. 2.11. The full width at half maximum (FWHM) of the distribution is usually defined as the jitter of the detector. Jitter values around 40 ps are typically obtained with our MoSi detectors, which is much lower than what has been reported in the past for the same material [44]. Some of our detectors even achieved values as low as 26 ps [10]. This is nevertheless higher than what can be obtained with materials as NbN and NbTiN [45] in which single-meandered SNSPDs have demonstrated jitters of ∼15 ps.

3 Detection rate improvement based on multi-elements designs

An important part of this thesis has been dedicated to overcoming limitations of conven-tional free-running SNSPDs in terms of maximum detection rate and recovery time. This chapter starts with an introduction presenting the limits of conventional single-meander SNSPDs and reviews different existing solutions (section 3.1). The solutions investigated during this thesis are presented in sections 3.2 and 3.3, and a novel design aiming at overcoming the limitations found is proposed in section 3.4. Finally, the photon-number resolving capability of those designs is discussed (section 3.5), as well as the potential of the solution to help build fast and efficient large-area detectors for multi-mode fiber coupling (section 3.6).

3.1 Reduced recovery-time and higher detection rates

The most common SNSPDs are based on a single meander and yield detection rates of tens of MHz, but the system detection efficiency decreases with detection rates in the MHz range. During this thesis, a great amount of effort has been put to overcome this limitation by testing different designs and proposing novel solutions. Indeed, as explained in chapter 1, the detection rate of SNSPDs is ultimately limited by the time dynamics of the bias current after detection, which is itself limited by electro-thermal considerations [32]. The impact on the detector detection rate and recovery time of conventional SNSPDs has been discussed in sections 2.2.2 and 2.2.3.

Shorter recovery times, hence higher detection rates, can be obtained by reducing the kinetic inductance of the detector. One approach to reduce the overall recovery time is cascade switching SNSPDs [46], also known as SNAPs, however their efficiency after detection still drop to zero as with conventional SNSPDs. Similarly, sub-nanosecond recovery times (RT) can be obtained with extremely short nanowires coupled to integrated waveguides [47], but such detectors have so far not shown high system detection efficiency when coupled to an optical fiber. Gated SNSPDs can solve the latching issue and effectively yield to detection rates of hundreds of MHz [48, 49, 50] but they are limited to applications that can be synchronized with the detector bias current. Arrays of detectors (referred in this text as “multipixels SNSPDs” and discussed in detail in section 3.2) can be used to keep the efficiency up at high detection rates [51, 52, 53]. Multi-element designs such as these have two main advantages : each element has a smaller kinetic inductance, and only one part of the whole detector is disabled after a detection event, leading to an effective non-zero efficiency after detection. In the case of multi-pixel SNSPDs, this comes at the cost of using multiple coaxial readout lines, which increases the cooling power needed to operate the system, and also demands a proportionally scaled and complex electronics readout circuitry with multiple discriminators and time-tagging units. Results obtained with this solution are presented on section 3.2. Row-columns array designs can reduce the number of coaxial lines needed, but still require at least one readout line per row and per column [54, 55], as many amplification channels, and additional processing to manage the timing-accurate readout of those different channel. Moreover, large inductors are needed to prevent crosstalk between pixels, which slows down their individual detection rate. RF-biased SNSPDs [56] allow for efficient multiplexing of detectors onto a single coaxial line, however the detection efficiency obtained with an RF bias seems to suffer from the oscillating bias current, and the complexity of RF signal generation is significantly higher than for conventional DC-biased SNSPDs.

Increasing the detection rate of SNSPDs without increasing i) the number of coaxial lines, ii) the constraints on the size and cooling power of the cryostat, and iii) the complexity and costs of the operating and readout electronics, is therefore of great practical advantage. This would contribute to spreading the use of SNSPDs further.

During this thesis, I investigated possible solutions to improve the detection rate of

3.2. Multi-Pixel SNSPDs