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Quantum cryptography immune from detector attacks is realized by the development of a source of indistinguishable laser pulses based on optically seeded gain-switched lasers. Key rates exceeding 1 Mb s−1 are demonstrated in the finite-size regime.

© 2018 American Physical Society. Fast-gated avalanche photodiodes (APDs) are the most commonly used single photon detectors for high-bit-rate quantum key distribution (QKD). Their robustness against external attacks is crucial to the overall security of a QKD system, or even an entire QKD network. We investigate the behavior of a gigahertz-gated, self-differencing (In,Ga)As APD under strong illumination, a tactic Eve often uses to bring detectors under her control. Our experiment and modeling reveal that the negative feedback by the photocurrent safeguards the detector from being blinded through reducing its avalanche probability and/or strengthening the capacitive response. Based on this finding, we propose a set of best-practice criteria for designing and operating fast-gated APD detectors to ensure their practical security in QKD.

Quantum key distribution (QKD) has matured rapidly towards practical use for protecting fiber communication infrastructures due to its unique ability of transmitting information-theoretically secure digital keys. Here, we report key advances in QKD that allow modulator-free transmitter [1], application into existing fiber infrastructures [2,3] and cryogen-free long-distance operation [4].

Quantum key distribution (QKD) provides an attractive means for securing communications in optical fibre networks. However, deployment of the technology has been hampered by the frequent need for dedicated dark fibres to segregate the very weak quantum signals from conventional traffic. Up until now the coexistence of QKD with data has been limited to bandwidths that are orders of magnitude below those commonly employed in fibre optic communication networks. Using an optimised wavelength divisional multiplexing scheme, we transport QKD and the prevalent 100 Gb/s data format in the forward direction over the same fibre for the first time. We show a full quantum encryption system operating with a bandwidth of 200 Gb/s over a 100 km fibre. Exploring the ultimate limits of the technology by experimental measurements of the Raman noise, we demonstrate it is feasible to combine QKD with 10 Tb/s of data over a 50 km link. These results suggest it will be possible to integrate QKD and other quantum photonic technologies into high bandwidth data communication infrastructures, thereby allowing their widespread deployment.

We report on a gated single-photon detector based on InGaAs/InP avalanche photodiodes (APDs) with a single-photon detection efficiency exceeding 55% at 1550 nm. Our detector is gated at 1 GHz and employs the self-differencing technique for gate transient suppression. It can operate nearly dead time free, except for the one clock cycle dead time intrinsic to self-differencing, and we demonstrate a count rate of 500 Mcps. We present a careful analysis of the optimal driving conditions of the APD measured with a dead time free detector characterization setup. It is found that a shortened gate width of 360 ps together with an increased driving signal amplitude and operation at higher temperatures leads to improved performance of the detector. We achieve an afterpulse probability of 7% at 50% detection efficiency with dead time free measurement and a record efficiency for InGaAs/InP APDs of 55% at an afterpulse probability of only 10.2% with a moderate dead time of 10 ns., Comment: 10 pages, 4 figures

Quantum key distribution (QKD) uniquely allows distribution of cryptographic keys with security verified by quantum mechanical limits. Both protocol execution and subsequent applications require the assistance of classical data communication channels. While using separate fibers is one option, it is economically more viable if data and quantum signals are simultaneously transmitted through a single fiber. However, noise-photon contamination arising from the intense data signal has severely restricted both the QKD distances and secure key rates. Here, we exploit a novel temporal-filtering effect for noise-photon rejection. This allows high-bit-rate QKD over fibers up to 90 km in length and populated with error-free bidirectional Gb/s data communications. With high-bit rate and range sufficient for important information infrastructures, such as smart cities and 10 Gbit Ethernet, QKD is a significant step closer towards wide-scale deployment in fiber networks., 7 pages, 5 figures

We demonstrate high count rate single photon detection at telecom wavelengths using a thermoelectrically-cooled semiconductor diode. Our device consists of a single InGaAs avalanche photodiode driven by a 2 GHz gating frequency signal and coupled to a tuneable self-differencing circuit for enhanced detection sensitivity. We find the count rate is linear with the photon flux in the single photon detection regime over approximately four orders of magnitude, and saturates at 1 gigacount/s at high photon fluxes. This result highlights promising potential for APDs in high bit rate quantum information applications., Electronics Letters (2012)

We demonstrate the use of two high speed avalanche photodiodes in exploring higher order photon correlations. By employing the photon number resolving capability of the photodiodes the response to higher order photon coincidences can be measured. As an example we show experimentally the sensitivity to higher order correlations for three types of photon sources with distinct photon statistics. This higher order correlation technique could be used as a low cost and compact tool for quantifying the degree of correlation of photon sources employed in quantum information science.

We demonstrate multi-gigahertz photon-counting at 1550nm using self-differencing In-GaAs APDs. The quantum efficiency is characterized as 23.5% at an afterpulse probability of 4.84%. The device will further increase the bit-rate for fiber quantum key distribution.

We demonstrate an efficient photon number detector for visible wavelengths using a silicon avalanche photodiode. Under subnanosecond gating, the device is able to resolve up to four photons in an incident optical pulse. The detection efficiency at 600 nm is measured to be 73.8%, corresponding to an avalanche probability of 91.1% of the absorbed photons, with a dark count probability below 1.1x10^{-6} per gate. With this performance and operation close to room temperature, fast-gated silicon avalanche photodiodes are ideal for optical quantum information processing that requires single-shot photon number detection.

We show that semiconductor avalanche photodiodes can exhibit diminutive amplification noise during the early evolution of avalanches. The noise is so low that the number of locally excited charges that seed each avalanche can be resolved. These findings constitute an important step towards realization of a solid-state noiseless amplifier for quantum information processing. Moreover, we believe that the experimental setup used, i.e., time-resolving locally excited avalanches, will become a useful tool for optimizing the number resolution.

We report a 1.036 GHz gated Geiger mode InGaAs avalanche photodiode with a detection dead time of just 1.93 ns. This is demonstrated by full recovery of the detection efficiency two gate cycles after a detection event, as well as a measured maximum detection rate of 497 MHz. As an application, we measure the second order correlation function $g^{(2)}$ of the emission from a diode laser with a single detector which works reliably at high speed owing to the extremely short dead time of the detector. The device is ideal for high bit rate fiber wavelength quantum key distribution and photonic quantum computing., 10 pages, 4 figures. Updated to published version

We report the first gigahertz clocked decoy-protocol quantum key distribution (QKD) system, with a record secure key rate of 1.02 Mbit/s over a fiber distance of 20 km and 10.1 kbit/s over 100 km.

A quantum random number generator (QRNG) based on gated single photon detection of an InGaAs photodiode at GHz frequency is demonstrated. Owing to the extremely long coherence time of each photon, each photons' wavefuntion extends over many gating cycles of the photodiode. The collapse of the photon wavefunction on random gating cycles as well as photon random arrival time detection events are used to generate sequences of random bits at a rate of 4.01 megabits/s. Importantly, the random outputs are intrinsically bias-free and require no post-processing procedure to pass random number statistical tests, making this QRNG an extremely simple device.

We report a demonstration of quantum key distribution (QKD) at GHz clock rates with InGaAs avalanche photodiodes (APDs) operating in a self-differencing mode. Such a mode of operation allows detection of extremely weak avalanches so that the detector afterpulse noise is sufficiently suppressed. The system is characterized by a secure bit rate of 2.37 Mbps at 5.6 km and 27.9 kbps at 65.5 km when the fiber dispersion is not compensated. After compensating the fiber dispersion, the QKD distance is extended to 101 km, resulting in a secure key rate of 2.88 kbps. Our results suggest that InGaAs APDs are very well suited to GHz QKD applications., Comment: 4 pages, 4 figures

Experimental one-way decoy pulse quantum key distribution running continuously for 60 hours is demonstrated over a fiber distance of 20km. We employ a decoy protocol which involves one weak decoy pulse and a vacuum pulse. The obtained secret key rate is on average over 10kbps. This is the highest rate reported using this decoy protocol over this fiber distance and duration., Accepted for publication in Optics Express

InGaAs avalanche photodiodes (APDs) are convenient for single photon detection in the near infrared (NIR) including the fiber communication bands (1.31∕1.55μm). However, to suppress afterpulse noise due to trapped avalanche charge, they must be gated with megahertz repetition frequencies, thereby severely limiting the count rate in NIR applications. Here, the authors show gating frequencies for InGaAs APDs well beyond 1GHz. Using a self-differencing technique to sense much weaker avalanches, the authors reduce drastically afterpulse noise. At 1.25GHz, they obtain a detection efficiency of 10.8% with an afterpulse probability of 6.16%. In addition, the detector features low jitter (55ps) and a count rate of 100MHz.

We demonstrate quantum key distribution (QKD) with bidirectional 10 Gb/s classical data channels in a single fiber using dense wavelength division multiplexing. Record secure key rates of 2.38 Mbps and fiber distances up to 70 km are achieved. Data channels are simultaneously monitored for error-free operation. The robustness of QKD is further demonstrated with a secure key rate of 445 kbps over 25 km, obtained in the presence of data lasers launching conventional 0 dBm power. We discuss the fundamental limit for the QKD performance in the multiplexing environment., 10 pages, 4 figures

Here we report the first demonstration of entanglement distribution over a record distance of 200 km which is of sufficient fidelity to realize secure communication. In contrast to previous entanglement distribution schemes, we use detection elements based on practical avalanche photodiodes (APDs) operating in a self-differencing mode. These APDs are low-cost, compact and easy to operate requiring only electrical cooling to achieve high single photon detection efficiency. The self-differencing APDs in combination with a reliable parametric down-conversion source demonstrate that entanglement distribution over ultra-long distances has become both possible and practical. Consequently the outlook is extremely promising for real world entanglement-based communication between distantly separated parties.

We report in this paper a gigahertz clocked quantum key distribution (QKD) system using only practical components. Compact semiconductor avalanche photodiodes are used for high-speed single photon detection, whereas an attenuated laser is used as a light source in combination with the decoy protocol for guarding against photon-number-splitting attacks. The system is characterized by secure key rates of 1.02 Mbit s−1 for a fibre distance of 20 km and 10.1 kbit s−1 for 100 km. The suitability of standard single mode fibres as the quantum channel is also demonstrated.

Entanglement distribution over 200 kilometers of optical fiber is demonstrated with practical, low-cost, InGaAs avalanche photodiodes. High coincidence count rates are also observed indicating entangled based quantum key distribution is feasible over ultra-long distances.

Practical and low-cost self-differencing InGaAs avalanche photodiodes have been successfully applied to ultra-long distance and efficient entanglement distribution over 200kilometers of optical fiber.