arXiv subjects

Jonatan Bohr Brask

Publications and source records attributed to Jonatan Bohr Brask.

60 records · Page 4Linked to original sources

arXiv2013

Testing nonlocality of a single photon without a shared reference frame

The question of testing the nonlocality of a single photon has raised much debate over the last years. The controversy is intimately related to the issue of providing a common reference frame for the observers to perform their local measurements. Here we address this point by presenting a simple scheme for demonstrating the nonlocality of a single photon which does not require a shared reference frame. Specifically, Bell inequality violation can be obtained with certainty with unaligned devices, even if the relative frame fluctuates between each experimental run of the Bell test. Our scheme appears feasible with current technology, and may simplify the realization of quantum communication protocols based on single-photon entanglement.

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arXiv2013

Device-independent quantum key distribution with spin-coupled cavities

Device-independent quantum key distribution (DIQKD) guarantees the security of a shared key without any assumptions on the apparatus used, provided that the observed data violate a Bell inequality. Such violation is challenging experimentally due to channel losses and photo-detection inefficiencies. Here we describe a realistic DIQKD protocol based on interaction between light and spins stored in cavities, which allows a heralded mapping of polarisation entanglement of light onto the spin. The spin state can subsequently be measured with near unit efficiency. Heralding alleviates the effect of channel loss, and as the protocol allows for local heralding, the spin decay is not affected by the communication time between the parties, making Bell inequality violation over an arbitrary distance possible. We compute the achievable key rates of the protocol, based on recent estimates of experimentally accessible parameter values and compare to the other known DIQKD protocol, which is entirely optical. We find significant improvements in terms of key bits per source use. For example we gain about five orders of magnitude over a distance of 75km, for realistic parameter values.

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arXiv2012

Robust nonlocality tests with displacement-based measurements

Lately, much interest has been directed towards designing setups that achieve decisive tests of local realism. Here we present Bell tests with measurements based on linear optical displacements and single-photon detection. The scheme displays good tolerance to loss. In particular, for entangled squeezed states, we find thresholds compatible with current efficiencies of detectors and sources. Furthermore, the scheme is easily extendible to any number of observers, allowing observation of multipartite nonlocality for a single photon shared among multiple modes. We also consider the case of atom-photon entanglement, where the loss threshold can be lowered further, as well as local filters compensating transmission and coupling inefficiencies at the source.

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arXiv2012

Bell tests for continuous variable systems using hybrid measurements and heralded amplifiers

We present Bell tests for optical continuous variable systems, combining both hybrid measurements (i.e. measuring both particle and wave aspects of light) and heralded amplifiers. We discuss two types of schemes, in which the amplifier is located either at the source, or at the parties' laboratories. The inclusion of amplifiers helps to reduce the detrimental effect of losses in the setup. In particular, we show that the requirements in terms of detection efficiency and transmission losses are significantly reduced, approaching the experimentally accessible regime.

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arXiv2011

Feasibility of loophole-free nonlocality tests with a single photon

Recently much interest has been directed towards designing setups that achieve realistic loss thresholds for decisive tests of local realism, in particular in the optical regime. We analyse the feasibility of such Bell tests based on a W-state shared between multiple parties, which can be realised for example by a single photon shared between spatial modes. We develop a general error model to obtain thresholds on the efficiencies required to violate local realism, and also consider two concrete optical measurement schemes.

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arXiv2008

Memory Imperfections in Atomic Ensemble-based Quantum Repeaters

Quantum repeaters promise to deliver long-distance entanglement overcoming noise and loss in realistic quantum channels. A promising class of repeaters, based on atomic ensemble quantum memories and linear optics, follow the proposal by Duan et al [Nature 414, 413, 2001]. Here we analyse this protocol in terms of a very general model for the quantum memories employed. We derive analytical expressions for scaling of entanglement with memory imperfections, dark counts, loss and distance, and apply our results to two specific quantum memory protocols. Our methods apply to any quantum memory with an interaction Hamiltonian at most quadratic in the mode operators and are in principle extendible to more recent modifications of the original DLCZ proposal.

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