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IEEE Transactions on Applied Superconductivity

Single-Flux Quantum Readout Circuits for Photon-Number-Resolving Superconducting Nanowire Single-Photon Detectors

Hiroaki Myoren, Shunsuke Takeda, Masato Naruse, Tohru Taino, Jian Chen, Peiheng Wu

Published Oct 28, 2014• 10 citations
Publisher

Abstract

Superconducting nanowire single-photon detectors (SNSPDs) with series-parallel meander-type configurations are demonstrated to have photon-number-resolving capabilities and are expected to be used for an evaluation system of single-photon generators and/or an energy-resolved fast detector for a new-generation time-of-flight mass spectrometry. In this paper, we present the design, fabrication, and confirmation of correct operations of two kinds of single-flux-quantum (SFQ) digital readout circuits for an SNSPD with a series-parallel meander-type configuration. Assuming a six-parallel meander-type configuration, we designed: a six-level current discriminator using six different magnetically coupled DC/SFQ converter circuits; and a Δ-type A/D converter using a quasi-one-junction SQUID (QOS) comparator with an SFQ feedback loop. For the design of the magnetically coupled inputs, we used a 3-D inductance extraction program FastHenry and the designed circuits were fabricated using a HYPRES 4.5-kA/cm2Nb process and a National Institute of Advanced Industrial Science and Technology (AIST) standard 2.5-kA/cm2Nb process.

DOI
10.1109/tasc.2014.2365356

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Superconducting nanowire single-photon detectors (SNSPDs) with series-parallel meander-type configurations are demonstrated to have photon-number-resolving capabilities and are expected to be used for an evaluation system of single-photon generators and/or an energy-resolved fast detector for a new-generation time-of-flight mass spectrometry. In this paper, we present the design, fabrication, and confirmation of correct operations of two kinds of single-flux-quantum (SFQ) digital readout circuits for an SNSPD with a series-parallel meander-type configuration. Assuming a six-parallel meander-type configuration, we designed: a six-level current discriminator using six different magnetically coupled DC/SFQ converter circuits; and a Δ-type A/D converter using a quasi-one-junction SQUID (QOS) comparator with an SFQ feedback loop. For the design of the magnetically coupled inputs, we used a 3-D inductance extraction program FastHenry and the designed circuits were fabricated using a HYPRES 4.5-kA/cm2Nb process and a National Institute of Advanced Industrial Science and Technology (AIST) standard 2.5-kA/cm2Nb process.
Superconducting nanowire single-photon detectors (SNSPDs) with series-parallel meander-type configuration are demonstrated to have photon-number- resolving (PNR) capabilities. Based on this unique feature, SNSPDs are expected to be used in the evaluation of single photon sources. However, increasing the number of SNSPDs causes an increase in the heat flow and further degradation in the timing jitter. To resolve this problem, the use of single flux quantum (SFQ) logic circuits is proposed. In this study, we present the read-out circuit for the SNSPD array, based on the SFQ logic. Our design of the read- out circuit makes it feasible to simultaneously process six SNSPD signals at 500 MHz and amplify the output voltage as an input to other circuits without the need for any amplifiers. We confirm using simulations that our designed read-out circuit has these capabilities. Furthermore, it is shown that the output voltage increases by 4.8 mV, proportional to the number of SNSPD signals and the output voltage is between 4.8 and 27.8 mV.
A read-out circuit for photon-number-resolving superconducting nanowire single-photon detector (SNSPD) based on the single-flux-quantum (SFQ) circuit was designed and the basic performance of the fabricated circuit was confirmed. The current pulse output from an SNSPD array was converted to a SFQ pulse. The SFQ pulse triggered two stacks of double-flux-quantum (DFQ) amplifiers. A single block of a 40-series DFQ amplifier (DFQA) generated 2.3-mV voltage with a rising time of 400 ps for an operation frequency of 1.25 GHz, and the two stacks of DFQA generated 4.6-mV voltage when the SFQ pulse trains supplied both the DFQAs. The total voltage generated by twelve-series of 40-series DFQAs was expected to be 27.6 mV, with the same rising time of 400 ps. Using the fabricated circuit, the 40-series DFQA and the two stacks of the 40-series DFQA generated 2.3- and 4.6-mV voltage, respectively. The output voltage of the SFQ readout circuit was compatible to a room-temperature photon counter with a wide-band low-noise amplifier operating at low temperature.
Superconducting nanowire single-photon detectors (SNSPDs) are typical switching devices capable of detecting single photons with almost 100% detection efficiency. However, they cannot determine the exact number of incident photons during a detection event. Multi-pixel SNSPDs employing multiple read-out channels can provide photon number resolvability (PNR), but they require increased cooling power and costly multi-channel electronic systems. In this work, a single-flux quantum (SFQ) circuit is employed, and PNR based on multi-pixel SNSPDs is successfully demonstrated. A multi-input magnetically coupled DC/SFQ converter (MMD2Q) circuit with a mutual inductance M is used to combine and record signals from a multi-pixel SNSPD device. The designed circuit is capable of discriminating the amplitude of the combined signals in accuracy of Φ 0/M with Φ 0 being a single magnetic flux quantum. By employing the MMD2Q circuit, the discrimination of up to 40 photons can be simulated. A 4-parallel-input MMD2Q circuit is fabricated, and a PNR of 3 is successfully demonstrated for an SNSPD array with one channel reserved for the functional verification. The results confirm that an MMD2Q circuit is an effective tool for implementing PNR with multi-pixel SNSPDs.
We demonstrate the transfer of single photon triggered electrical pulses from a superconducting nanowire single photon detector (SNSPD) to a single flux quantum (SFQ) pulse. We describe design and test of a digital SFQ based SNSPD readout circuit and demonstrate its correct operation. Both circuits (SNSPD and SFQ) operate under the same cryogenic conditions and are directly connected by wire bonds. A future integration of the present multi-chip configuration seems feasible because both fabrication process and materials are very similar. In contrast to commonly used semiconductor amplifiers, SFQ circuits combine very low power dissipation (a few microwatts) with very high operation speed, thus enabling count-rates of several gigahertz. The SFQ interface circuit simplifies the SNSPD readout and enables large numbers of detectors for future compact multi-pixel systems with single photon counting resolution. The demonstrated circuit has great potential for scaling the present interface solution to 1,000 detectors by using a single SFQ chip.
We designed and tested the digital readout circuitry for superconducting nanowire single-photon detectors (SNSPDs). The designed time-to-digital converter (TDC) comprises a decision-making block, a clock controller, and a counter with parallel-to-serial (P2S) interface. We also designed an on-chip pattern generator to imitate a digitized SNSPD response that allowed us to screen the circuitry without bonding actual SNSPDs. Both rapid single-flux quantum (RSFQ) and its energy-efficient rapid single-flux quantum (ERSFQ) versions were designed for comparison and debugging purposes. To optimize the design of the feeding Josephson transmission line (FJTL) required by the ERSFQ variant, we compared different power grid structures and types of FJTLs. Using an 8-bit counter with P2S interface as a device under test, we also investigated the effect of FJTL's size on the bias margins. FJTL with a 75% junction overhead is sufficient for the ERSFQ circuit to match the margins of its RSFQ counterpart. All chips have been fabricated at MIT-LL using the SFQ5ee process node. Experimental results and future research directions are presented.
Superconducting single-photon detectors (SSPDs) based on NbN nanowires possess properties such as high counting rate, low dark count rate and small timing jitter, which are promising characteristics for a quantum key distribution system. An SSPD array has recently attracted attention, because it can provide a higher counting rate, and spatial and photon-number resolutions. We propose readout electronics using single-flux-quantum (SFQ) circuits to process a large number of output signals from an SSPD array. We designed and tested an interface circuit between an SSPD and SFQ circuit, and successfully observed SFQ pulse generation for an input pulse with a current amplitude of 17.4 ~ 36.4 muA , which is comparable with an output current amplitude of the SSPD. The interface circuit was also operational for an input pulse duration of 1.6 ns, which is shorter than the typical recovery time of the SSPD.