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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.
©2011 Optical Society of America
1. Introduction
The ability to detect single photons with high detection efficiency is one of the major breakthroughs in modern physics. SNSPDs are a key enabling technology with great importance for the instruments used in optical quantum information processing, quantum cryptography, quantum key distribution, and deep space communication, where fast detectors with high sensitivity and high detection efficiency are required [1]. When high sensitivity is needed, SNSPDs provide high speed, high efficiency, and single photon resolution at near-infrared wavelengths [2,3]. The photon resolving functionality is their key feature, which will enable many future applications in the field of quantum communication and information processing [4–6].
SNSPDs with a small detection area produce voltage pulses with about 1 ns pulse width as a response to each absorbed photon. The response time is mainly defined by the inductance of the nanowire meander [7]. The typical readout scheme consists of a bias tee, a microwave amplifier and a pulse counter. The weak voltage pulses require cryogenic low noise amplifiers with wide bandwidth. The high complexity of such a readout scheme is the most serious challenge for both multi-pixel detector arrays and imaging systems [8]. Today, only a very few detectors can be installed in a system due to rapidly growing complexity and total power consumption [9]. The connection between SNSPD and single flux quantum electronics is very natural because voltage level, switching speed and operation temperature are matching perfectly. This concept was first proposed by Terai et al. [10]. The same group demonstrated recently the measurement of electrical signals from an SNSPD using an SFQ circuit [11]. In their experiment, the detector and the SFQ chip are installed in different Dewars. A long electrical connection of about 3 m as well as a capacitive decoupling was used. SNSPDs are typically very sensitive to electrical signals and especially to reflections and noise from the readout circuit. An wide-band matching is not possible because the impedance of the SNSPD varies rapidly between purely inductive in the superconducting state and high ohmic after a photon absorption. The SNSPD is typically optimized for high detection efficiency and generates therefore very weak electrical pulses. The transfer of these pulses over a certain distance as well as a connection to a semiconductor amplifier sets certain constrains on the minimum pulse amplitude. To obtain a high count rate (above 109 photons per second) and a high number of detectors at the same time, we require a readout electronics which is able to operate in very close vicinity to the detector itself. We demonstrate for the first time a compact multi-chip-module (MCM) involving one chip containing the SNSPD and a second chip containing the digital SFQ readout circuit. This enables a very close packaging of the detector and the readout electronics which avoids any long connections between detector and readout electronics.
2. Operation Principle
The SNSPD is a narrow superconducting wire, whose thickness is less than the electron thermalization length and is maintained at a temperature much lower than the critical temperature of the superconductor. The wire is biased below its critical current. After a photon absorption, a localized region is formed within the superconducting wire, a so-called hot spot. As a result, the entire cross-section of the SNSPD stripe becomes resistive, leading to a voltage signal across the contacts of the structure. After the growth phase, the hot spot size is reduced in time due to electron–phonon relaxation and, subsequently, the escape of phonons into the substrate, as well as quasiparticle out-diffusion. This operation principle of an SNSPD leads to a number of design and fabrication requirements that have to be met in order to produce a functional device [12].
The typical amplitude of the generated output voltage of the detector is in the range of 1 mV, which is well suitable for a direct electrical connection to SFQ electronics [13]. This digital superconductor electronics family represents the binary data by the presence or absence of single magnetic flux quanta Φ0 = h/2e ≈ 2.07 · 10−15 Vs inside a small superconducting loop. Here, e denotes the elementary charge and h the Planck’s constant. These loops are interrupted by Josephson junctions, which act as active switchings elements for a controlled transfer of single flux quanta between different loops of the circuit. Each photon, which gets absorbed by the SNSPD generates a voltage pulse which propagates to the input transformer of the SFQ circuit.
The schematic diagram of the SFQ circuit and the detector is shown in Fig. 1(a) and a microphotograph of the SFQ circuit is shown in Fig. 1(b). The voltage supply of the SFQ circuit provides a current bias of junction J3 (about 80% of its critical current). A smaller amount of current is flowing through the junctions J1 and J2. The dynamics of the Josephson junctions is much faster than the input signal, which allows the SFQ circuit to keep track of the input signal at all times. The rising edge of an input pulse creates an extra current in the secondary coil of the transformer. This extra current is subtracted from the bias current of J1 and J2, but added to the current through J3. If the resulting current in J3 exceeds its critical current, it switches by creating a single magnetic flux quantum. Due to the related magnetic induction, the current distribution in the loop changes resulting in a large current flowing through J1 and J2. At the same time, the current also triggers the switching of J4 which releases a single flux quantum towards the output. As soon as the input current reduces again, this current gets even larger and the pair of junctions J1 and J2 will switch (the weaker junction J1 first and the second junction will immediately follow). The readout circuit returns to its initial condition and is sensitive to detect the next voltage pulse. The circuit parameters are designed to allow only one internal state under the condition of no input current. As soon as the SNSPD returned to the superconducting state there is no current flowing into the SFQ circuit which ensures a proper reset after each detected photon.
Fig. 1 Schematic diagram of the SFQ circuit with connected SNSPD (a) and microphotograph of the SFQ circuit (b). The blue crosses denote Josephson junctions.
The flux quantization in superconducting loops provides a natural quantization of each voltage pulse. By Faraday’s law, each transfer of flux is connected to a transient voltage pulse ∫v(t)dt = Φ0. The detector has a voltage supply, which creates a current in the superconducting nanowire. If a photon is absorbed, a part of the nanowire becomes normal conducting and its bias current is redistributed into the input transformer of the SFQ circuit. The main challenge in this development was to match the detector and its bias scheme to the superconducting circuit. The inherent hysteresis of the detector causes latching which prevents a fast reset after a single photon absorption. To overcome this problem, we implemented a voltage bias scheme consisting of an LR-network with a time constant of about 5 ns [14, 15]. During switching, the SNSPD performs a large impedance jump from zero to a large impedance in the kilo ohm range. This renders it difficultly to match the impedance of the transmission line, and therefore handicaps the pulse transfer over long distances. We avoided this problem by using only a very short connection between the detector and the readout circuit. All implementations reported in the literature use a coupling capacitor, mainly to avoid parasitic current redistributions caused by the readout amplifier. From the detector point of view, the SFQ circuit is an LR-network with mutually coupling to further circuity. Any dc-current in the secondary coil of the transformer does not allow a dc-current to flow towards the detector. Therefore our readout circuit can not create parasitic bias currents for the detector and we can use for the first time a dc coupling between the SNSPD and the readout circuit. This enables covering the whole detection bandwidth from zero to the maximum count rate of the detector.
3. Circuit sensitivity
The sensitivity of the SFQ interface circuit is a crucial parameter to ensure the reliable detection of all pulses from the detector. The design of the input transformer enables us to define a certain sensitivity by setting a certain current gain. On the other hand, an improved sensitivity is payed by a reduced bandwidth which limits the maximum possible count rate. Our present design uses an input bandwidth from dc to 8 GHz, which is sufficiently higher than the recovery time of the detector itself. Therefore we can ensure the detection of individual photons with a spacing of at least 125 ps between two pulses. On the other hand this bandwidth is low enough to prevent a back action from the high frequency transients of the SFQ circuit (above 40 GHz) to the detector. A bunch of simultaneous photons is detected as a single photon event. Superconductor electronics offer the potential for on-chip counting and processing at such a high speed.
Before connecting to the superconducting detector, we measured the bit error rate of the SFQ interface circuit by using imitated pulses from a room temperature signal generator. Fig. 2 shows the detection error rate versus the SFQ supply current. In this experiment, signals with 25 μA amplitude are barely detectable by the SFQ circuit. We believe this experiment gives a pessimistic view of the sensitivity that will be obtainable when the input is from the detector because of the use of a room temperature source.
Fig. 2 Measured sensitivity of the SFQ readout circuit versus bias supply current. The traces correspond to different amplitudes of the applied input current.
To ensure an SNSPD pulse with an amplitude large enough for error-free detection, we used a detector with a relatively large critical current of 136 μA. A further increase in this sensitivity is possible at the price of reduced bandwidth. A 2 GHz bandwidth would allow a larger current gain of the transformer, enabling the detection of pulses with an amplitude as low as 6 μA. This will be sufficient to apply the proposed readout circuit to all existing types of superconducting single photon detectors.
4. Experimental set-up
The detector and the SFQ chip are mounted on a brass carrier. Aluminum wire bonds connect both chips. The digital SFQ output is matched on-chip to 50 Ω impedance and the chip is wire-bonded to a microstrip line, which is soldered to an SMA connector. A coaxial cable guides the digital output data from cryogenic temperature to room temperature. An optical fiber is mounted to a second adjustable brass plate which is aligned manually by viewing the transmission of fiber-coupled 650 nm light onto the detector. The positioning between the fiber and the chip holder was not done very precisely because the intention of this work was a feasibility proof of the electrical interface between the detector and the SFQ circuit. Therefore, the present setup suffers from a low system detection efficiency, owing to the difficulty of coupling light to such a small detector area.
A low noise voltage source was used to bias the detector. A small discrete inductor was directly connected to the detector. It filters high-frequency noise and stabilizes the supply current defined by the resistor value and the supply voltage. The power supply of the SFQ circuit is realized by four individual dc voltages with a design value of 2.5 mV. For observing the output signal on the oscilloscope, we used a room temperature amplifier with 80 dB gain. The bandwidth of this particular amplifier (dc–50 kHz) limited the maximum count rate during this experiment, but we can easily remove this bottleneck and enable count rates up to 2 GHz [16].
The SFQ input uses a resistor in series with the transformer, because the detector is usually connected to an output line with 50 Ω characteristic impedance. The back action of fast transients from the SFQ circuit to the detector is minimized by decoupling both components with a superconducting transformer. The present experiments where performed at low bias current values and we could not observe any evidence for transient back actions from the SFQ circuit. Circuit simulations show that the estimated back actions is below 1% of the critical current of the detector which should allow its operation at much higher bias currents. The transformer also provides current amplification by a factor of five. The edge-triggered nature of the SFQ circuit ensures reliable pulse detection independent on the pulse width.
All experiments were performed in vacuum at 4.2 K. Fig. 3 shows the experimental configuration including SFQ and SNSPD chips. To minimize the electromagnetic interference with the power grid, we use individual battery powered voltage supplies for the detector and the SFQ circuit. To distinguish clearly between photon-triggered optical and parasitic electrical events (dark counts and noise-induced switching error), we modulated the optical signal of the laser diode and analyzed the signal correlation. In the presented experiments, the bias current of the detector is rather low and on average we observed 1 to 100 counts during each “on” phase of the laser. We very rarely observed counts during the “off” phase of the laser. The correlation between the optical modulation and the reconstructed photon counts was always larger than 95%.
Fig. 4 shows the assembly of the MCM. The SNSPD consists of a 10 nm thick NbN meander with 100 nm line width and 200 nm pitch on a sapphire substrate. The covered area of the detector is 3.5 μm×4.0 μm. The fabrication and device characteristics are described in detail elsewhere [12].
Fig. 4 Illustration of both chips connected by wire bonds (a) and MCM mounted on a brass table (b). Detailed pictures show the central section of the SNSPD (c) and of the SFQ circuit (d), respectively. The SFQ chip supports four individual input channels, of which only one is used in this experiment.
The SFQ chip was fabricated by using the standard foundry process for superconducting electronics of IPHT Jena, Germany. The characteristics of this ISO 9001 certified process are a critical Josephson current density of 1 kA cm−2 and a minimum junction size of 12.5 μm2 [17]. The circuit contains a Nb groundplane and two Nb wiring layers The evaluated circuit utilizes 22 Josephson junctions (for one channel only). A printed circuit board is used for the dc-power connections of the chip. The distance between the detector and the SFQ circuit is about 4 mm. SNSPD and SFQ are directly connected by bond wires in coplanar configuration.
5. Experimental transformation of single photons into single flux quanta
Each absorbed photon triggers the generation of an SFQ pulse which is latched in an on-chip flip-flop circuit. The SFQ circuit provides a non-return to zero voltage output signal. As a result, we can observe a static change of the voltage level, which toggles between two levels. These switching points are indicated by arrows in Fig. 5(a). All data sets were recorded with a digital oscilloscope. The lower traces in Fig. 5(a) and (b) show recoded data sets for two different bias currents of the detector. When we increased the bias current of the detector, we confirmed the exponential increase of the count rate (see Fig. 5(c)) as expected [18]. Left in complete darkness, we did not observe any dark counts. That can be seen by the toggling free areas during darkness in Fig. 5(b). The dark count rate for detectors with this width is typically larger than zero only for bias currents above 85% of the critical current [19].
Fig. 5 The measured output data of the SFQ circuit is shown together with the modulation signal for the laser. Each jump in the voltage trace corresponds to an absorbed photon. Traces (a) and (b) show data records for detector normalized bias currents of 69.5% and 72.8%, respectively. Each circle in (c) denotes an individual measurement and the line shows a fit with theory.
A second type of experiments is based on a sinusoidal modulation of the light intensity. The current of the laser diode was modulated with a sine-wave. Fig. 6 shows a good agreement of the reconstructed waveform with the applied intensity of light. The visible deviations can be explained by the non-linear dependence of the laser diode on the supply current as well as by a large random influence due to the very low count rate.
Fig. 6 Measured digital output data for a sine-wave modulated optical input signal. The average pulse density is used to reconstruct the photon density of the input signal.
In our experiment, both circuits are galvanically coupled and operate under the same cryogenic condition next to each other. The electrical pulse of the SNSPD triggers the generation of a single magnetic flux quantum about 50 ps after the single photon absorption by the detector. From this moment, the information about the quantum event “a single photon was absorbed in the detector” is available in the digital domain and SFQ electronics provides many different solutions for counting, multiplexing, and digitally processing this information with very low power consumption. The jitter of SFQ circuits is below 0.2 ps [20,21], which by far outperforms the typical timing jitter of semiconductor SNSPD readout circuits, which is about 30 ps [22]. Since SFQ signal processing adds only a very small fraction to the overall detection jitter, it is also possible to apply time multiplexing schemes to combine the signals of 10–100 detectors into a single readout channel.
6. Conclusion
The unique feature of the digital SNSPD readout circuit is the transformation of one quantized portion of energy (single photon) into an other quantized unit (single magnetic flux quantum) by using a similar superconducting thin-film technology. Our experiments demonstrates the reliable detection of single photons by direct electrical conversion into single flux quanta. The proposed concept is suitable for direct scaling to a multi-channel system with single photon counting resolution on two chips: one for the detector array and the other one for the multi-input SFQ circuit.
The major drawback of all semiconductor based readout approaches is their high power consumption. The SFQ implementation has even the potential for further reduction down to about 1 μW per channel by lowering the supply voltage from 2.5 mV down to 250 μV [23]. This is about one-thousand times less than the power consumption of a cryogenic semiconductor amplifier. The number of readout channels is only limited by the available integration density for the superconducting chip and the thermal budget. We can estimate that one readout channel requires a chip area less than 0.1 mm2 and consumes about 1 μW of power. Therefore, scaling of this concept is straightforward and provides a promising solution for readout, multiplexing and processing by a single SFQ chip of the signals from 103 detectors. The interface offers for the first time a scalable approach for building multi-pixel arrays and cameras with a very high number of detectors. Due to the similarity of the fabrication technologies for the SNSPD and the SFQ circuit, the integration of our hybrid approach on a single chip seems feasible in the near future.
Acknowledgments
The authors thank J. Kunert from the Institute of Photonic Technology, Jena, Germany for helpful discussions during this research. This work was supported in parts by the Promotion of Excellence of Ilmenau University of Technology and in parts by the Germany Research Foundation DFG CFN A4.3.
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