arXiv subjects

Andrew J. Shields

Publications and source records attributed to Andrew J. Shields.

At least 37 records · Page 2Linked to original sources

arXiv2020

Active reset of a radiative cascade for superequilibrium entangled photon generation

The generation rate of entangled photons emitted from cascaded few-level systems is intrinsically limited by the lifetime of the radiative transitions. Here, we overcome this limit for entangled photon pairs from quantum dots via a novel driving regime based on an active reset of the radiative cascade. We show theoretically and experimentally the driving regime to enable the generation of entangled photon pairs with higher fidelity and intensity compared to the optimum continuously driven equilibrium state. Finally, we electrically generate entangled photon pairs with a total fidelity of $(79.5 \pm 1.1)\%$ at a record clock rate of 1.15GHz.

quant-ph↗

arXiv2019

Intrinsic mitigation of the after-gate attack in quantum key distribution through fast-gated delayed detection

The information theoretic security promised by quantum key distribution (QKD) holds as long as the assumptions in the theoretical model match the parameters in the physical implementation. The superlinear behaviour of sensitive single-photon detectors represents one such mismatch and can pave the way to powerful attacks hindering the security of QKD systems, a prominent example being the after-gate attack. A longstanding tenet is that trapped carriers causing delayed detection can help mitigate this attack, but despite intensive scrutiny, it remains largely unproven. Here we approach this problem from a physical perspective and find new evidence to support a detector's secure response. We experimentally investigate two different carrier trapping mechanisms causing delayed detection in fast-gated semiconductor avalanche photodiodes, one arising from the multiplication layer, the other from the heterojunction interface between absorption and charge layers. The release of trapped carriers increases the quantum bit error rate measured under the after-gate attack above the typical QKD security threshold, thus favouring the detector's inherent security. This represents a significant step to avert quantum hacking of QKD systems.

quant-ph↗

arXiv2019

Real-time inteferometric quantum random number generation on chip

We demonstrate on-chip quantum random number generation at high data rates using the random phases of gain-switched laser pulses. Interference of the gain-switched pulses produced by two independent semiconductor lasers is performed on a photonic integrated circuit (PIC) and the resulting pulse train is received and processed in real-time using homebuilt capture electronics consisting a field programmable gate array (FPGA) and a 10-bit digitizer. Random numbers with low correlation coefficient are shown for pulse clock rates of 1 GHz and data rates of 8 Gbps. The random numbers are also shown to successfully pass all tests within the National Institute for Standards and Technology (NIST) test suite. The system provides genuine random numbers in a compact platform that can be readily integrated into existing quantum cryptographic technology.

physics.optics↗

arXiv2019

A Modulator-Free Quantum Key Distribution Transmitter Chip

Quantum key distribution (QKD) has convincingly been proven compatible with real life applications. Its wide-scale deployment in optical networks will benefit from an optical platform that allows miniature devices capable of encoding the necessarily complex signals at high rates and with low power consumption. While photonic integration is the ideal route toward miniaturisation, an efficient route to high-speed encoding of the quantum phase states on chip is still missing. Consequently, current devices rely on bulky and high power demanding phase modulation elements which hinder the sought-after scalability and energy efficiency. Here we exploit a novel approach to high-speed phase encoding and demonstrate a compact, scalable and power efficient integrated quantum transmitter. We encode cryptographic keys on-demand in high repetition rate pulse streams using injection-locking with deterministic phase control at the seed laser. We demonstrate record secure-key-rates under multi-protocol operation. Our modulator-free transmitters enable the development of high-bit rate quantum communications devices, which will be essential for the practical integration of quantum key distribution in high connectivity networks.

quant-ph↗

arXiv2018

Overcoming the rate-distance barrier of quantum key distribution without using quantum repeaters

Quantum key distribution (QKD) allows two distant parties to share encryption keys with security based on physical laws. Experimentally, it has been implemented with optical means, achieving key rates of 1.26 Megabit/s over 50 kilometres (km) of standard optical fibre and 1.16 bit/hour over 404 km of ultralow-loss fibre in a measurement-device-independent configuration. Increasing the bit rate and range of QKD is a formidable, but important, challenge. A related target, currently considered unfeasible without quantum repeaters, is overcoming the fundamental rate-distance limit of point-to-point QKD. Here we introduce a conceptually new scheme where pairs of phase-randomised optical fields are first generated at two distant locations and then combined at a central measuring station. The fields imparted with the same random phase are "twins" and can be employed to distil a quantum key, as we prove under an explicit security assumption. The key rate of this Twin-Field QKD (TF-QKD) shows the same dependence on distance as a quantum repeater, scaling with the square-root of the channel transmittance, irrespective of whom is in control of the measuring station. Differently from a quantum repeater, however, the new scheme is feasible with current technology and presents manageable levels of noise even on 550 km of standard optical fibre. This is promising to overcome the QKD rate-distance barrier and to greatly extend the range of secure quantum communications.

quant-ph↗

arXiv2018

Intensity modulation as a preemptive measure against blinding of single-photon detectors based on self-differencing cancellation

Quantum key distribution is rising as an important cryptographic primitive for protecting the communication infrastructure in the digital era. However, its implementation security is often weakened by components whose behavior deviates from what is expected. Here, we analyse the response of a self-differencing avalanche photodiode, a key enabler for high speed quantum key distribution, to intense light shone from a continuous-wave laser. Under incorrect settings, the cancellation entailed by the self-differencing circuitry can make the detector insensitive to single photons. However, we experimentally demonstrate that even in such cases intensity modulation can be used as an effective measure to restore the detector's expected response to the input light.

quant-ph↗

arXiv2018

Long-term transmission of entangled photons from single quantum dot over deployed fiber

Non-classical light sources based on a single quantum emitter are considered as core technology for multiple quantum network architectures. A large variety of sources has been developed, but the generated photons remained far from being utilized in established standard fiber networks. Here, we report a week-long transmission of polarization-entangled photons from a single InAs/GaAs quantum dot over a metropolitan network fiber. The emitted photons are in the telecommunication O-band, favored for fiber optical communication. We employ a polarization stabilization system overcoming changes of birefringence introduced by 18.23km of installed fiber. Stable transmission of polarization-encoded entanglement with a high fidelity of 91% is achieved, facilitating the operation of sub-Poissonian quantum light sources over existing fiber networks.

quant-ph↗

arXiv2018

Long term test of a fast and compact Quantum Random Number Generator

Random numbers are an essential resource to many applications, including cryptography and Monte Carlo simulations. Quantum random number generators (QRNGs) represent the ultimate source of randomness, as the numbers are obtained by sampling a physical quantum process that is intrinsically probabilistic. However, they are yet to be widely employed to replace deterministic pseudo random number generators (PRNG) for practical applications. QRNGs are regarded as interesting devices. However they are slower than PRNGs for simulations and are typically seen as clumsy laboratory prototypes, prone to failures and unreliable for cryptographic applications. Here we overcome these limitations and demonstrate a compact and self-contained QRNG capable of generating random numbers at a pace of 8 Gbit/s uninterruptedly for 71 days. During this period, the physical parameters of the quantum process were monitored in real time by self-checking functions implemented in the generator itself. At the same time, the output random numbers were analyzed with the most stringent suites of statistical tests. The analysis shows that the QRNG under test sustained the continuous operation without physical instabilities or hardware failures. At the same time, the output random numbers were analyzed with the most stringent suites of statistical tests, which were passed during the whole operation time. This extensive trial demonstrates the reliability of a robustly designed QRNG and paves the way to its use in practical applications based on randomness.

quant-ph↗

arXiv2017

Manipulating photon coherence to enhance the security of practical quantum key distribution

Quantum key distribution (QKD) allows two users to communicate with theoretically provable secrecy by encoding information on photonic qubits. Current encoders are complex, however, which reduces their appeal for practical use and introduces potential vulnerabilities to quantum attacks. Distributed-phase-reference (DPR) systems were introduced as a simpler alternative, but have not yet been proven practically secure against all classes of attack. Here we demonstrate the first DPR QKD system with information-theoretic security. Using a novel light source, where the coherence between pulses can be controlled on a pulse-by-pulse basis, we implement a secure DPR system based on the differential quadrature phase shift protocol. The system is modulator-free, does not require active stabilization or a complex receiver, and also offers megabit per second key rates, almost three times higher than the standard Bennett-Brassard 1984 (BB84) protocol. This enhanced performance and security highlights the potential for DPR protocols to be adopted for real-world applications.

quant-ph↗

arXiv2017

Modulator-free coherent-one-way quantum key distribution

Time-bin encoding is an attractive method for transmitting photonic qubits over long distances with minimal decoherence. It allows a simple receiver for quantum key distribution (QKD) that extracts a key by measuring time of arrival of photons and detects eavesdropping by measuring interference of pulses in different time bins. In the past, coherent pulses have been generated using a CW laser and an intensity modulator. A greatly simplified transmitter is proposed and demonstrated here that works by directly modulating the laser diode. Coherence between pulses is maintained by a weak seed laser. The modulator-free source creates time-bin encoded pulses with a high extinction ratio (29.4 dB) and an interference visibility above 97 %. The resulting QKD transmitter gives estimated secure key rates up to 4.57 Mbit/s, the highest yet reported for coherent-one-way QKD, and can be programmed for all protocols using weak coherent pulses.

quant-ph↗

arXiv2017

Long-distance quantum key distribution secure against coherent attacks

Quantum key distribution (QKD) permits information-theoretically secure transmission of digital encryption keys, assuming that the behaviour of the devices employed for the key exchange can be reliably modelled and predicted. Remarkably, no assumptions have to be made on the capabilities of an eavesdropper other than that she is bounded by the laws of Nature, thus making the security of QKD "unconditional". However, unconditional security is hard to achieve in practice. For example, any experimental realisation can only collect finite data samples, leading to vulnerabilities against coherent attacks, the most general class of attacks, and for some protocols the theoretical proof of robustness against these attacks is still missing. For these reasons, in the past many QKD experiments have fallen short of implementing an unconditionally secure protocol and have instead considered limited attacking capabilities by the eavesdropper. Here, we explore the security of QKD against coherent attacks in the most challenging environment: the long-distance transmission of keys. We demonstrate that the BB84 protocol can provide positive key rates for distances up to 240 km without multiplexing of conventional signals, and up to 200 km with multiplexing. Useful key rates can be achieved even for the longest distances, using practical thermo-electrically cooled single-photon detectors.

quant-ph↗

arXiv2016

Ultra-high bandwidth quantum secured data transmission

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.

quant-ph↗

arXiv2016

Universal growth scheme for entanglement-ready quantum dots

Efficient sources of individual pairs of entangled photons are required for quantum networks to operate using fibre optic infrastructure. Entangled light can be generated by quantum dots (QDs) with naturally small fine-structure-splitting (FSS) between exciton eigenstates. Moreover, QDs can be engineered to emit at standard telecom wavelengths. To achieve sufficient signal intensity for applications, QDs have been incorporated into 1D optical microcavities. However, combining these properties in a single device has so far proved elusive. Here, we introduce a growth strategy to realise QDs with small FSS in the conventional telecom band, and within an optical cavity. Our approach employs droplet-epitaxy of InAs quantum dots on (001) substrates. We show the scheme improves the symmetry of the dots by 72%. Furthermore, our technique is universal, and produces low FSS QDs by molecular beam epitaxy on GaAs emitting at ~900nm, and metal-organic vapour phase epitaxy on InP emitting at 1550 nm, with mean FSS 4x smaller than for Stranski-Krastanow QDs.

quant-ph↗

arXiv2015

Quantum secured gigabit optical access networks

Optical access networks connect multiple endpoints to a common network node via shared fibre infrastructure. They will play a vital role to scale up the number of users in quantum key distribution (QKD) networks. However, the presence of power splitters in the commonly used passive network architecture makes successful transmission of weak quantum signals challenging. This is especially true if QKD and data signals are multiplexed in the passive network. The splitter introduces an imbalance between quantum signal and Raman noise, which can prevent the recovery of the quantum signal completely. Here we introduce a method to overcome this limitation and demonstrate coexistence of multi-user QKD and full power data traffic from a gigabit passive optical network (GPON). The dual feeder implementation is compatible with standard GPON architectures and can support up to 128 users, highlighting that quantum protected GPON networks could be commonplace in the future.

quant-ph↗

arXiv2015

Practical security bounds against the Trojan-horse attack in quantum key distribution

In the quantum version of a Trojan-horse attack, photons are injected into the optical modules of a quantum key distribution system in an attempt to read information direct from the encoding devices. To stop the Trojan photons, the use of passive optical components has been suggested. However, to date, there is no quantitative bound that specifies such components in relation to the security of the system. Here, we turn the Trojan-horse attack into an information leakage problem. This allows us quantify the system security and relate it to the specification of the optical elements. The analysis is supported by the experimental characterization, within the operation regime, of reflectivity and transmission of the optical components most relevant to security.

quant-ph↗

arXiv2015

Quantum photonics hybrid integration platform

Fundamental to integrated photonic quantum computing is an on-chip method for routing and modulating quantum light emission. We demonstrate a hybrid integration platform consisting of arbitrarily designed waveguide circuits and single photon sources. InAs quantum dots (QD) embedded in GaAs are bonded to an SiON waveguide chip such that the QD emission is coupled to the waveguide mode. The waveguides are SiON core embedded in a SiO2 cladding. A tuneable Mach Zehnder modulates the emission between two output ports and can act as a path-encoded qubit preparation device. The single photon nature of the emission was verified by an on-chip Hanbury Brown and Twiss measurement.

physics.optics↗

arXiv2015

Security bounds for efficient decoy-state quantum key distribution

Information-theoretical security of quantum key distribution (QKD) has been convincingly proven in recent years and remarkable experiments have shown the potential of QKD for real world applications. Due to its unique capability of combining high key rate and security in a realistic finite-size scenario, the efficient version of the BB84 QKD protocol endowed with decoy states has been subject of intensive research. Its recent experimental implementation finally demonstrated a secure key rate beyond 1 Mbps over a 50 km optical fiber. However the achieved rate holds under the restrictive assumption that the eavesdropper performs collective attacks. Here, we review the protocol and generalize its security. We exploit a map by Ahrens to rigorously upper bound the Hypergeometric distribution resulting from a general eavesdropping. Despite the extended applicability of the new protocol, its key rate is only marginally smaller than its predecessor in all cases of practical interest.

quant-ph↗

arXiv2015

GHz-gated InGaAs/InP single-photon detector with detection efficiency exceeding 55% at 1550 nm

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.

quant-ph↗