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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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.
Recently, it has been demonstrated that by recovering the amplitude and phase of the backscattered optical signal, a ΦOTDR using pulse coding can be treated as a fully linear system in terms of trace coding/decoding, thus allowing for the use of tens of thousands of bits with a dramatic improvement of the system performance. In this communication, as a continuation of previous work by the same authors, a preliminary study aiming at characterizing the limits of the system in terms of maximum usable code length is presented. Using a code exceeding 1million bits over a duration of 0.26ms, it is observed that fiber optical path variations exceeding ≈π occurring over a time inferior to the pulse code length can lead to localized fading in the ΦOTDR trace. The occurrence, positions and form of the fading points along the ΦOTDR trace is observed to be strongly dependent on the type, frequency and amplitude of the perturbations applied to the fiber.
We investigate the transmission performance, with and without digital backpropagation, in wavelength-division-multiplexed-
(WDM) transmission systems using polarisation-division-multiplexed (PDM) differentially-encoded
quadrature phase shift keying (QPSK) and 16-quadrature amplitude modulation (16QAM). We consider transmission at
56GBd, 28GBd, 14GBd and 7GBd per WDM-channel while varying the channel spacing from 100GHz to 12.5GHz to
maintain constant spectral efficiency per modulation format. The symmetrical split-step method based on the Manakov
equation was employed to backpropagate the central WDM-channel with the help of receiver based digital signal
processing. Performance of digital backpropagation was found to decrease with reduced symbol-rate, owing to the
smaller proportion of the overall spectrum which is backpropagated in these cases. At higher symbol-rates digital
backpropagation is up to 0.5dB more effective for 16QAM than for the QPSK format, which we attribute to the
increased influence of intra-channel nonlinearities for formats with multiple intensity values leading in turn to a higher
benefit when intra-channel effects are compensated for. Improvement in launch power @ BER=3x10<sup>-3</sup> amounts to 1.9dB
for 56GBd QPSK and 2.4dB of 56GBd 16QAM.
Optical communications is undergoing a digital revolution, as digital signal processing (DSP) emerges as a practical
solution for robust long-haul transmission. In contrast to previous systems, in which optical dispersion compensation
was considered a necessity, recently uncompensated long-haul transmission has been demonstrated using DSP at the
transmitter, receiver, or a combination of the two. We review the historic developments of electronic signal processing
as applied to optical communications before focusing the latest developments, namely digital coherent systems. We then
discuss the salient features of the electronic signal processing schemes currently under investigation before discussing
the challenges and opportunities offered by electronic signal processing.