arXiv subjects

A. Broadbent

Publications and source records attributed to A. Broadbent.

4 recordsLinked to original sources

arXiv2013

Quantum computing on encrypted data

The ability to perform computations on encrypted data is a powerful tool for protecting privacy. Recently, protocols to achieve this on classical computing systems have been found. Here we present an efficient solution to the quantum analogue of this problem that enables arbitrary quantum computations to be carried out on encrypted quantum data. We prove that an untrusted server can implement a universal set of quantum gates on encrypted quantum bits (qubits) without learning any information about the inputs, while the client, knowing the decryption key, can easily decrypt the results of the computation. We experimentally demonstrate, using single photons and linear optics, the encryption and decryption scheme on a set of gates sufficient for arbitrary quantum computations. Because our protocol requires few extra resources compared to other schemes it can be easily incorporated into the design of future quantum servers. These results will play a key role in enabling the development of secure distributed quantum systems.

quant-ph↗

arXiv2006

Entanglement swapping, light cones and elements of reality

Recently, a number of two-participant all-versus-nothing Bell experiments have been proposed. Here, we give local realistic explanations for these experiments. More precisely, we examine the scenario where a participant swaps his entanglement with two other participants and then is removed from the experiment; we also examine the scenario where two particles are in the same light cone, i.e. belong to a single participant. Our conclusion is that, in both cases, the proposed experiments are not convincing proofs against local realism.

quant-ph↗

arXiv2005

On the power of non-local boxes

A non-local box is a virtual device that has the following property: given that Alice inputs a bit at her end of the device and that Bob does likewise, it produces two bits, one at Alice's end and one at Bob's end, such that the XOR of the outputs is equal to the AND of the inputs. This box, inspired from the CHSH inequality, was first proposed by Popescu and Rohrlich to examine the question: given that a maximally entangled pair of qubits is non-local, why is it not maximally non-local? We believe that understanding the power of this box will yield insight into the non-locality of quantum mechanics. It was shown recently by Cerf, Gisin, Massar and Popescu, that this imaginary device is able to simulate correlations from any measurement on a singlet state. Here, we show that the non-local box can in fact do much more: through the simulation of the magic square pseudo-telepathy game and the Mermin-GHZ pseudo-telepathy game, we show that the non-local box can simulate quantum correlations that no entangled pair of qubits can in a bipartite scenario and even in a multi-party scenario. Finally we show that a single non-local box cannot simulate all quantum correlations and propose a generalization for a multi-party non-local box. In particular, we show quantum correlations whose simulation requires an exponential amount of non-local boxes, in the number of maximally entangled qubit pairs.

quant-ph↗

arXiv1996

Detection of Large Scale Structure in a $B < 17^{m}$ Galaxy Redshift Survey

We report on results from the Durham/UKST Galaxy Redshift Survey where we have found large scale ``cellular'' features in the galaxy distribution. These have spatial 2-point correlation function power significantly in excess of the predictions of the standard cold dark matter cosmological model$^{1}$, supporting the previous observational results from the APM survey$^{2,3}$. At smaller scales, the 1-D pairwise galaxy velocity dispersion is measured to be $\bf 387^{+96}_{-62}$ kms$^{-1}$ which is also inconsistent with the prediction of the standard cold dark matter model$^{1}$. Finally, the survey has produced the most significant detection yet of large scale redshift space distortions due to dynamical infall of galaxies$^{4}$. An estimate of $\bf Ω^{0.6}/b = 0.55 \pm 0.12$ is obtained which is consistent either with a low density Universe or a critical density Universe where galaxies are biased tracers of the mass.

astro-ph↗