We present experimental results from a Quantum Key Distribution (QKD) setup based on entanglement, where polarization entangled photon pairs are distributed after narrowband filtering and followed by detection employing polarization-based receivers. A comparative analysis of the performance of InGaAs single photon avalanche detectors (SPADs) and superconducting nanowire single photon detectors (SNSPDs), reveals a notable advantage of SNSPDs for QKD applications. The entangled photon pairs are generated using a Sagnac-based polarization entanglement source, and subsequently split by Dense Wavelength Division Multiplexers (DWDMs). These entangled photon pairs are sent to two polarization-based receivers facilitating the comparison between the detection using InGaAs SPADs and SNSPDs. The results emphasize the superior performance of SNSPDs reflected in an around 80 times higher coincidence rate, therefore tolerating around 8 dB higher attenuation values. For both configurations and all considered attenuation levels, the fidelity and quantum bit error rate (QBER) are well inside the respective bounds for secure key generation. This work provides valuable insights for the optimization of QKD systems.
We present experimental results from a Quantum Key Distribution (QKD) setup based on entanglement, where polarization entangled photon pairs are distributed after narrowband filtering and followed by detection employing polarization-based receivers. A comparative analysis of the performance of InGaAs single photon avalanche detectors (SPADs) and superconducting nanowire single photon detectors (SNSPDs), reveals a notable advantage of SNSPDs for QKD applications. The entangled photon pairs are generated using a Sagnac-based polarization entanglement source, and subsequently split by Dense Wavelength Division Multiplexers (DWDMs). These entangled photon pairs are sent to two polarization-based receivers facilitating the comparison between the detection using InGaAs SPADs and SNSPDs. The results emphasize the superior performance of SNSPDs reflected in an around 80 times higher coincidence rate, therefore tolerating around 8 dB higher attenuation values. For both configurations and all considered attenuation levels, the fidelity and quantum bit error rate (QBER) are well inside the respective bounds for secure key generation. This work provides valuable insights for the optimization of QKD systems.
We present the latest results on two kinds of photon detectors: single photon detectors (SPDs) and photon number resolving detector (PNRD). We developed high speed and low noise SPDs using superconducting nano-wire (abbreviated by SNSPD) and semiconductor (InGaAs) avalanche photodiode (APD). The SNSPD system has totally four channels all of which have the detection eciency higher than 16% at 100Hz dark count rate. The InGaAs APD system also has four channels and the best performance is represented by the after-pulse probability of 0.61%, the dark count probability of 0.71×10-6 (~1kHz), and the detection eciency of 10.9%. Both systems were applied to wavelength division multiplexing quantum key distribution (WDM-QKD) operated at 1.2GHz repetition rate in a eld environment. The PNRD is made of superconducting transition edge sensor. It was applied to the implementation of quantum receiver which could beat the homodyne limit of the bit error rate of binary coherent states. We discuss future perspective of quantum communications with those photon detection technologies, including multi-user QKD networks and low-power high capacity communications.
In a commercial entanglement-distribution system over telecom fibre, the single-photon detectors are the largest part of the hardware cost. In entanglement-based quantum key distribution (QKD), both photons of each pair must be detected. With two telecom photons, each arm needs either an InGaAs single-photon avalanche diode (SPAD) of low efficiency or a superconducting nanowire single-photon detector (SNSPD) with its own cryostat. We study a highly nondegenerate photon-pair source that needs a telecom detector on one arm only. Two periodically poled KTiOPO4 (ppKTP) crystals pumped at 405~nm emit pairs at 548 and 1550~nm. A Si-SPAD with 82\% efficiency detects the 548~nm photon at the source node, and only the 1550~nm photon travels through fibre. A first, unoptimised module in a polarisation-correlated configuration generates about 1.5×106 pairs/s per mW of pump power. It collects 13\% of the pairs on the 548~nm arm and 23\% on the 1550~nm arm. In an asymptotic BBM92 model, we compare its links with generic 2×1550~nm links that use a bright published source at the same pump power. At 50~km, our link with one SNSPD costs about 25\% more per secret bit than the generic link with one SNSPD. With optimistic target values of 52\% collection and twice the brightness, it costs about 40\% as much as the generic link with two SNSPDs. Our link without a cryostat then costs about 70\% less than the generic link with one SNSPD. If every link runs at its own best pump power, the two-SNSPD link is again the cheapest. The source can also serve clock synchronisation and detector calibration, which need only time-correlated pairs. All key rates assume the entangled configuration of the source, which is under development.
Experimental results obtained with silicon single photon avalanche diodes (SPADs) in quantum key distribution (QKD) at short wavelengths reveal remarkable potential for application in local area networks (LAN) and for free-space transmission at high rate. Actual application prospects, however, depend on the performance level and on the suitability of practical systems using the available silicon SPAD devices. They can be essentially divided in two groups: planar p-n junction structures with a thin depletion layer (typically 1 μm); and reach-through structures with a thick depletion layer (from 20 μm to 150μm). The physical mechanisms that control the device behaviour were investigated and the effect on the key parameters of the detector (quantum detection efficiency, dark counting rate, afterpulsing probability and photon-timing jitter) were thoroughly assessed. A quantitative analysis was made of the influence of such parameters on the quantum bit error rate (QBER). Actual parameters were measured and the attainable performance and system suitability of the two device types evaluated. Comparable performance is obtained, but from a system viewpoint thin SPADs appear inherently better suited to high-rate QKD applications, because of their faster response time, ruggedness, low voltage, low power dissipation and fabrication technology, which is simple, efficient, economical and compatible with monolithic integration of detector and associated circuits.
We provide a direct comparison between the InGaAs avalanche photodiode (APD) and the NbN superconducting single photon detector (SSPD) for applications in fiber-based quantum cryptography. The quantum efficiency and dark count rate were measured for each detector, and used to calculate the quantum bit error rate (QBER) and shared key rate for a QKD link. The results indicate that, despite low quantum efficiency, the speed of the SSPD makes it a superior detector for quantum information applications. Finally, we present results of an initial integration of an SSPD into a receiver node of the DARPA quantum network to perform quantum key distribution. Encoding quantum information on single photons has enabled the realization of a number of unique communication schemes such as the implementation of the BB84 protocol for quantum key distribution 1 and quantum dense coding. 2 Furthermore, the implementation of these schemes using photons at telecommunication wavelengths has permitted their extension to fiber optic networks, 3,4 drastically increasing the practicality of quantum information processing. To date, the state of the art in commercially available single photon detection at telecommunication wavelengths (1550 nm) has been the thermo-electrically cooled InGaAs avalanche photodiode (APD). The cooled InGaAs APD is generally operated at T = -60°C in the reverse-biased Geiger mode. Though these detectors have been successfully implemented in operating QKD links in optical fiber, the InGaAs APD suffers from afterpulsing effects that limit the key exchange rates. New SPD (single photon detector) technologies based on superconducting materials are emerging that hold tremendous promise for drastically increasing the photon counting rate and eliminating dark noise.57 The SSPD consists of a NbN thin film (t ∼ 5 nm) patterned into a narrow (w ∼ 100 nm) microstrip meander that is ∼ 500μm in length. The detector is cooled to T < 4K in a closed cycle refrigeration unit and a bias current I b is supplied to the device just below the superconducting critical current (I c ). When a photon is incident upon the microstrip superconductivity is broken in the region local to photon absorption and a normal state region is formed in the device. The current passing through the device causes a measurable voltage pulse in the detector that can be amplified and shaped to act as an input for traditional logic circuitry. Due to the unique electronic bandstructure in a superconducting material the intrinsic response time of the device is on the order of picoseconds, so that the repetition rate may be increased above 1 GHz 8 without being susceptible to afterpulsing effects that plague the InGaAs APD. 9 To make a direct comparison between the InGaAs APD and the SSPD both were placed in a SPD characterization testbed to measure quantum efficiency and dark count rate at a variety of bias conditions. The QBER for a QKD link is calculated from the measured parameters and then compared to requirements for a provably secure QKD link. Following detector characterization, one of the nodes of the DARPA Quantum Network operated by BBN Technologies 3 utilized an SSPD in the receiver to transmit key material. The demonstration provides an in situ comparison between the InGaAs APD, normally operated in the BBN link, and the SSPD.
Entangled photon sources (EPSs) are essential building blocks for scalable quantum communication and quantum key distribution (QKD). We present a stable, highly nondegenerate EPS based on type-0 spontaneous parametric down-conversion (SPDC) in a crossed-crystal configuration, generating photon pairs at 680~nm and 1550~nm when pumped by a 473~nm laser. This wavelength combination, reported here for the first time, simultaneously benefits from the peak detection efficiency of the most of the Si-SPADs in the visible/near-infrared spectral range and the low-loss fiber transmission of the telecom C-band. This configuration provides the most favorable balance between performance and cost for detection using Si-SPADs and InGaAs detectors. The source exhibits a measured spectral bandwidth of 300~GHz, corresponding to a spectral brightness of up to 1.9×103~pairs~s−1~mW−1~GHz−1. Heralding efficiencies reach 18~\% (signal) and 34~\% (idler) with Si-SPAD and superconducting nanowire single-photon detectors (SNSPD) detection. The entangled state achieves visibilities of (97.3±1.0)% in the H/V basis and (94.9±1.6)% in the D/A basis, yielding a fidelity of ≥(96.1±1.3)%. These results establish the presented EPS as a practical wavelength-hybrid platform for fiber-based QKD and emerging long-haul quantum network architectures.
Two approaches to single-photon detection at telecom wavelengths have been investigated, using semiconductor and superconductor technologies. The low temperature operation of free-running InGaAs/InP negative feedback avalanche diodes (NFADs) was studied, demonstrating that they can operate with an extremely low dark count rate, which opens up a host of new applications. Secondly, superconducting nanowire single-photon detectors (SNSPDs) based on amorphous materials were developed, fabricated and characterized. These detectors are highly attractive because they can achieve excellent attributes simultaneously, such as high efficiency, low noise, good temporal resolution, no afterpulsing and high count rates. Finally, it was demonstrated that the use of NFADs can enable record distance quantum key distribution (QKD) over 307 km of optical fiber. A next generation QKD system has been proposed and numerically analyzed, aiming at extending the record distance by an additional 100 km and increasing the state-of-the-art secret key rate by over an order of magnitude.