A quantum key distribution protocol for rapid denial of service detection

We introduce a quantum key distribution protocol designed to expose fake users that connect to Alice or Bob for the purpose of monopolising the link and denying service. It inherently resists attempts to exhaust Alice and Bob's initial shared secret, and is 100% efficient, regardless of the number of qubits exchanged above the finite key limit. Additionally, secure key can be generated from two-photon pulses, without having to make any extra modifications. This is made possible by relaxing the security of BB84 to that of the quantum-safe block cipher used for day-to-day encryption, meaning the overall security remains unaffected for useful real-world cryptosystems such as AES-GCM being keyed with quantum devices.Comment: 13 pages, 3 figures. v2: Shifted focus of paper towards DoS and added protocol 4. v1: Accepted to QCrypt 201

Do the laws of physics prohibit counterfactual communication?

It has been conjectured that counterfactual communication is impossible, even for post-selected quantum particles. We strongly challenge this by proposing exactly such a counterfactual scheme where---unambiguously---none of Alice's photons that make it has been to Bob. We demonstrate counterfactuality theoretically and experimentally by means of weak measurements, as well as conceptually using consistent histories. Importantly, the accuracy of Alice learning Bob's bit can be made arbitrarily close to unity with no trace left by Bob on Alice's photon.Comment: Experiment conducted in the lab, showing no weak trace from Bob at either D0 or D1. 5 pages, 5 figure

On the effects of self- and cross-phase modulation on photon purity for four-wave mixing photon-pair sources

We consider the effect of self-phase modulation and cross-phase modulation on the joint spectral amplitude of photon pairs generated by spontaneous four-wave mixing. In particular, the purity of a heralded photon from a pair is considered, in the context of schemes that aim to maximise the purity and minimise correlation in the joint spectral amplitude using birefringent phase-matching and short pump pulses. We find that non-linear phase modulation effects will be detrimental, and will limit the quantum interference visibility that can be achieved at a given generation rate. An approximate expression for the joint spectral amplitude with phase modulation is found by considering the group velocity walk-off between each photon and the pump, but neglecting the group-velocity dispersion at each wavelength. The group-velocity dispersion can also be included with a numerical calculation, and it is shown that it only has a small effect on the purity for the realistic parameters considered.Comment: 11 pages, 10 figure

Counterfactual Ghost Imaging

We give a protocol for ghost imaging in a way that is always counterfactual - while imaging an object, no light interacts with that object. This extends the idea of counterfactuality beyond communication, showing how this interesting phenomenon can be leveraged for metrology. Given, in the infinite limit, no photons ever go to the imaged object, it presents a method of imaging even the most light-sensitive of objects without damaging them. Even when not in the infinite limit, it still provides a many-fold improvement in visibility and signal-to-noise ratio over previous protocols, with over an order of magnitude reduction in absorbed intensity.Comment: 7 pages, 6 figures, matches published versio

Comment on ``Scheme of the arrangement for attack on the protocol BB84"

In a recent paper (Scheme of the arrangement for attack on the protocol BB84, Optik 127(18):7083-7087, Sept 2016), a protocol was proposed for using weak measurement to attack BB84. This claimed the four basis states typically used could be perfectly discriminated, and so an interceptor could obtain all information carried. We show this attack fails when considered using standard quantum mechanics, as expected - such ``single-shot" quantum state discrimination is impossible, even using weak measurement.Comment: 3 pages, 1 figure, accepted for publication by Opti

In-situ measurements of fabrication induced strain in diamond photonic-structures using intrinsic colour centres

Diamond has established itself as an ideal material for photonics and optomechanics, due to its broad-band transparency and hardness. In addition, colour centres hosted within its lattice such as the nitrogen-vacancy (NV) centre, have become leading candidates for use in quantum information processing, and quantum sensors. The fabrication of nanoscale devices coupled to high quality NVs has been an outstanding challenge due to their sensitivity to magnetic, electric and strain fields within their local environment. In this work, we show how the NV centre’s ground state electron spin can be used as an embedded atomic-scale probe of the local strain caused by focused ion beam milling of nanoscale devices. This technique can thus be used to measure, and optimise material and device fabrication processes to allow diamond to reach its full potential