15 research outputs found
Experimental realization of a programmable quantum-state discriminator and a phase-covariant quantum multimeter
We present an optical implementation of two programmable quantum measurement
devices. The first one serves for unambiguous discrimination of two
nonorthogonal states of a qubit. The particular pair of states to be
discriminated is specified by the quantum state of a program qubit. The second
device can perform von Neumann measurements on a single qubit in any basis
located on the equator of the Bloch sphere. Again, the basis is selected by the
state of a program qubit. In both cases the data and program qubits are
represented by polarization states of photons. The experimental apparatus
exploits the fact that two Bell states can be distinguished solely by means of
linear optics. The outcome corresponding to the remaining two Bell states
represents an inconclusive result.Comment: 7 pages, 7 figure
Imperfect 1-Out-of-2 Quantum Oblivious Transfer: Bounds, a Protocol, and its Experimental Implementation
Oblivious transfer is an important primitive in modern cryptography.
Applications include secure multiparty computation, oblivious sampling,
e-voting, and signatures.
Information-theoretically secure perfect 1-out-of 2 oblivious transfer is
impossible to achieve. Imperfect variants, where both participants' ability to
cheat is still limited, are possible using quantum means while remaining
classically impossible. Precisely what security parameters are attainable
remains unknown.
We introduce a theoretical framework for studying semi-random quantum
oblivious transfer, which is shown equivalent to regular oblivious transfer in
terms of cheating probabilities. We then use it to derive bounds on cheating.
We also present a protocol with lower cheating probabilities than previous
schemes, together with its optical realisation.Comment: 20 pages, 1 figur
Entanglement of coherent states and decoherence
A possibility to produce entangled superpositions of strong coherent states
is discussed. A recent proposal by Howell and Yazell [Phys. Rev. A 62, 012102
(2000)] of a device which entangles two strong coherent coherent states is
critically examined. A serious flaw in their design is found. New modified
scheme is proposed and it is shown that it really can generate non-classical
states that can violate Bell inequality. Moreover, a profound analysis of the
effect of losses and decoherence on the degree of entanglement is accomplished.
It reveals the high sensitivity of the device to any disturbances and the
fragility of generated states
Space QUEST mission proposal: experimentally testing decoherence due to gravity
Models of quantum systems on curved space-times lack sufficient experimental
verification. Some speculative theories suggest that quantum properties, such
as entanglement, may exhibit entirely different behavior to purely classical
systems. By measuring this effect or lack thereof, we can test the hypotheses
behind several such models. For instance, as predicted by Ralph and coworkers
[T C Ralph, G J Milburn, and T Downes, Phys. Rev. A, 79(2):22121, 2009, T C
Ralph and J Pienaar, New Journal of Physics, 16(8):85008, 2014], a bipartite
entangled system could decohere if each particle traversed through a different
gravitational field gradient. We propose to study this effect in a ground to
space uplink scenario. We extend the above theoretical predictions of Ralph and
coworkers and discuss the scientific consequences of detecting/failing to
detect the predicted gravitational decoherence. We present a detailed mission
design of the European Space Agency's (ESA) Space QUEST (Space - Quantum
Entanglement Space Test) mission, and study the feasibility of the mission
schema.Comment: 18 pages, 13 figures, included radiation damage to detectors in
appendi