Quantum key distribution (QKD) allows users to generate shared encryption keys that are guaranteed to be theoretically secure by the laws of quantum mechanics. Most implementations use light pulses that are attenuated by the propagation medium. This leads to a fundamental rate-distance limit in QKD that was thought to be impossible to overcome with current technology. The recent proposal of "Twin-Field QKD", however, changed this belief and showed how to overcome this limit and perform long-distance QKD with present-day technology.
In my talk I will review the most significant theoretical and experimental results in this emerging and rapidly growing research area.
The practical combination of quantum cryptography and classical communications will require convergence of their technologies. In this pivotal time where both fields are transitioning towards photonic integrated architectures, it is essential to develop devices that fully leverage their hardware compatibilities, while still addressing the key issues of cost reduction, miniaturization and infrastructure energetic footprint, essential for future high- bandwidth, low-latency networks. Here, we address these issues by developing an on-chip transmitter consisting of just 3 building blocks but capable of transmitting both quantum encrypted photons and classical multi-level modulation signals. By combining optical injection locking and direct phase modulation we are able to encode pulse trains with multiple levels of differential phase, without the need of high-speed electro-optic modulators and their associated power footprint. We generate return-to-zero differential phase shift keying signals with up to 16 distinct levels. Moreover, we demonstrate multi-protocol quantum key distribution delivering state-of-the-art secure key rates. Our on-chip transmitter will facilitate the flexible combination of quantum and classical communications within a single, power-efficient device that can readily be integrated in existing high connectivity networks.
Quantum key distribution (QKD) allows two users to communicate with theoretically provable
secrecy . This is vitally important to secure the confidential data of governments, businesses
and individuals. As the technology is adopted by a wider audience, a quantum network will
become necessary for multi-party communication, as in the classical communication networks in
use today. Unfortunately, a number of phase-encoded QKD protocols based on weak coherent
pulses have been developed. Whilst the first protocol, proposed by Bennett and Brassard
in 1984 (BB84), is still commonly used, other protocols such as differential phase shift  or
coherent one way QKD  are also adopted. Each protocol has its benefits; however all would
require a different transmitter and receiver, increasing the complexity and cost of quantum
In this work we demonstrate a multi-protocol transmitter [4-6] that also has the benefits of
small footprint, low power consumption and low complexity. We use this transmitter to give the
first experimental demonstration of an improved version of the BB84 protocol, known as the
differential quadrature phase shift protocol. We have achieved megabit per second secure key
rates at short distances, and have shown secure key rates that are, on average, 2.71 times higher
than the standard BB84 protocol. This enhanced performance over such a commonly adopted
protocol, at no expense to experimental complexity, could lead to a widespread migration to
the new protocol.
The security of the BB84 protocol relies on each signal and reference pulse pair being globally
phase randomised with respect to all other pulse pairs. In the DQPS protocol, blocks with a
length L ≥ 2 are used and each block has a globally random phase with respect to all other blocks.
Implementing this protocol would ordinarily require a high-speed random number generator and
a phase modulator. As well as increasing device complexity, it would also require an unrealistic
continuous source of electrical modulation signals for complete security. The transmitter we
use injects light from a master laser diode into a 2 GHz gain-switched slave laser diode. The
principal of optical injection locking means that the slave laser inherits the phase of the master
laser. We apply modulations to the master laser current within a block to control the phase
of the slave laser output pulses, and then drive the master laser below threshold for a short
period of time when phase randomisation is required. This ensures the lasing comes from below
threshold, thus the phase adopted by the slave laser pulse is completely random. We perform
an autocorrelation measurement on the blocks to show their randomness.
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Technological advances in quantum computers and number theory have the potential to compromise the security of existing cryptographic protocols. Quantum key distribution (QKD) offers the possibility of information theoretic security and is theoretically unbreakable. Therefore it is the natural candidate to face the above digital threat.
However, in implementing QKD, it is important to check that the components employed do not deviate from their expected behaviour, to avoid opening the door to new security loopholes . For this reason, it is necessary to characterise the real behaviour of the components, build reliable models and include them in the security analysis.
Here we introduce a set of techniques and measurements to ease this characterisation process. We discuss explicit examples applied to the source , the boundaries  and the detection unit  of a QKD apparatus. These methods pave the way to the future certification of QKD systems.
 K. Tamaki, M. Curty, and M. Lucamarini, “Decoy-state quantum key distribution with a leaky source,” New J. Phys 18, 65008 (2016).
 J. F. Dynes et al., “Testing the photon-number statistics of a quantum key distribution light source,” arXiv:1711.00440 (2017).
 M. Lucamarini et al., “Practical Security Bounds Against the Trojan-Horse Attack in Quantum Key Distribution,” Phys. Rev. X 5, 031030 (2015).
 A. Koehler-Sidki et al., “Setting best practice criteria for self-differencing avalanche photodiodes in quantum key distribution,” SPIE Proc. 10442, Quant. Inf. Sci. Tech. III, 104420L (2017).
Quantum key distribution (QKD)1 is a quantum technology already present in the market. This technology will become an essential point to secure our communication systems and infrastructure when today’s public key cryptography will be broken by either a mathematical algorithm or by, eventually, the development of quantum computers. One of the main task of quantum metrology and standardization in the next future is ensuring that QKD apparatus works as expected, and appropriate countermeasures against quantum hacking are taken. In this paper, we discuss the security of one of the QKD most critical (and quantum-hackered) components, i.e., single photon detectors based on fiber-pigtailed InGaAs SPADs. We analyze their secondary photon emission (backflash light) that can be exploited by an eavesdropper (Eve) to gain information without introducing errors in the key. We observed a significant light leakage from the detection event of fiber-pigtailed InGaAs SPADs. This may represent a significant security threat in all QKD apparatus. We provide a method to quantify the amount of potential information leakage, and we propose a solution to fix this potential security bug in practical QKD apparatus.
In recent years, the security of avalanche photodiodes as single photon detectors for quantum key distribution has been subjected to much scrutiny. The most prominent example of this surrounds the vulnerability of such devices to blinding under strong illumination. We focus on self-differencing avalanche photodiodes, single photon detectors that have demonstrated count rates exceeding 1 GCounts/s resulting in secure key rates over 1 MBit/s. These detectors use a passive electronic circuit to cancel any periodic signals thereby enhancing detection sensitivity. However this intrinsic feature can be exploited by adversaries to gain control of the devices using illumination of a moderate intensity. Through careful experimental examinations, we define here a set of criteria for these detectors to avoid such attacks.
A quantum bug, or "qbug", is the fundamental unit of problems in quantum key distribution. It can include a
particular attack, an inaccurate use of the technology, a loophole of the theory or a hidden side channel. In this
manuscript we detail one of them, related to the choice of the protocol and its security proof in the finite-size
scenario. The treatment makes use of linear programming, a tool that well adapts to the practical constraints
imposed by an actual quantum key distribution set up.
Quantum communication, in particular, quantum key distribution (QKD) is moving ever closer to real world
implementation. However, for successful QKD system deployment, the QKD system components must be robustly
designed and feature highly reliable operation. In this paper we focus on one important aspect of any quantum
communication system: the single photon detector. In particular our interest is centered upon the InGaAs avalanche
photodiode (APD) single photon detector operating in a
self-differencing (SD) mode. Such a detector features high clock
frequencies of up to 3GHz, high photon count rates as well as detection efficiencies approaching 20% with low
afterpulsing. We show successful operation of a high bit rate QKD system using this SD-APD technology in a real world
We demonstrate an efficient photon number detector for visible wavelengths using a fast-gated silicon avalanche
photodiode. Using sub-nanosecond voltage gates with a self-differencing circuit, the device is able to resolve up to four
photons in an incident optical pulse, with a detection probability of up to 91.1 % at 1 GHz. With this performance and
close to room temperature operation, fast-gated silicon avalanche photodiodes are ideal for optical quantum information
processing that requires single-shot photon number detection.
We present the first demonstration of telecom fiber-based quantum key distribution using single photons from
a quantum dot in a pillar microcavity. The source offers both telecommunication wavelength operation at 1.3
microns and Purcell enhancement of the spontaneous emission rate. Several emission lines from the InAs/GaAs
quantum dot are identified, including the exciton-biexciton cascade and charged excitonic emission. We show an
order of magnitude increase in the collected intensity of the emission from a charged excitonic state when temperature
tuned onto resonance with the HE11 mode of the pillar microcavity, as compared to the off-resonance
intensity. Above- and below-GaAs-bandgap optical excitation was used and the effect of the excitation energy
on the photoluminescence investigated. Exciting below the GaAs-bandgap offers significant improvement in the
quality of the single photon emission and a reduction of the multi-photon probability to 0.1 times the value for
Poissonian light was measured, before subtraction of detector dark counts, the lowest value recorded to date
for a quantum dot source at a fibre wavelength. We observe also the first evidence of Purcell enhancement of
the spontaneous emission rate for a single telecommunication wavelength quantum dot in a pillar microcavity.
We have incorporated the source into a phase encoded interferometric scheme implementing the BB84 quantum
cryptography protocol and distributed a key, secure from the pulse splitting attack, over standard telecommunication
optical fibre. We show a transmission distance advantage over that possible with (length-optimized)
uniform intensity weak coherent pulses at 1310 nm in the same system.
Single photon sources are important components for future quantum communication networks. Lights emitting diodes with emission from an embedded self-organized quantum dot offer compact semiconductor sources that can be easily fabricated using standard photolithographic techniques. In this paper, progress towards an electrically driven 1300 nm quantum dot single photon emitter for fiber optic based applications are addressed. Low density longer wavelength emissions were achieved by exploiting the second critical growth threshold for large self-assembled InAs quantum dots on GaAs. The single photon collection efficiency was improved by incorporating the quantum dots between GaAs/AlxGa1-xAs distributed Bragg reflector mirror stacks and laterally confined inside etched micropillars. Resonance of the microcavity mode with the InAs quantum dot emission leads to an enhancement in the collection intensity. Emission from an active quantum dot was collected using a confocal microscope and coupled directly into a single mode fiber. Strong suppression in the multiphoton emission rate was verified by a custom Hanbury-Brown and Twiss interferometer set-up with optical fibers and InGaAs single photon avalanche photodetectors. Integration of electrical contacts with a planar resonant microcavity structure for a single photon light emitting diode is also discussed. Electroluminescence spectra recorded on such a device revealed sharp lines due to the charge recombination in a quantum dot. Correlation measurements on a single quantum dot line showed the suppression of multiphoton emission for an electrically driven source near 1300 nm for the first time.
We review progress in realizing a semiconductor source of single photons and photon pairs based on the emission of individual self-assembled quantum dots. Integration of the quantum dot into a pillar microcavity produces a strong Purcell enhancement of the radiative recombination rate, resulting in photon collection efficiencies into a lens of ~10%. The residual multi-photon emission is found to derive from the emission of other layers within the structure, such that under resonant laser excitation of the dot a greater than 50-fold reduction in the 2-photon rate can be achieved compared to a laser of the same average intensity. The polarization of the emitted photons can be controlled and selected in appropriately designed cavities. Through careful control of the dot growth conditions, we realize a single photon source at the fiber compatible wavelength of 1300nm. This is achieved by utilizing a second critical InAs coverage to produce a low density of large, long wavelength InAs quantum dots. We demonstrate also an electrically driven planar cavity structure with photon collection efficiencies into a lens of ~5%, corresponding to an order of magnitude enhancement in the photon collection compared to dots in a bulk semiconductor LED. Single photon emission is demonstrated for both the biexciton and exciton state of the quantum dot.
We report the realization of the first electrically-driven single photon source. The device is based on a GaAs p-i-n diode containing self-assembled InAs quantum dots embedded within the i-region. Time resolved electroluminescence from the device yields information on the radiative lifetimes of the single quantum dots involved. Applying a continuous current a dip is measured in the second-order correlation function at zero time delay indicating that the luminescence is antibunched. Under pulsed excitation by subnanosecond voltage pulses, strong suppression of multi-photon pulses is achieved with respect to a laser of the same average intensity. The results suggest that conventional semiconductor technology can be used to mass-produce a single photon source for applications in quantum information technology.