177 research outputs found
Quantum cryptographic ranging
We present a system to measure the distance between two parties that allows
only trusted people to access the result. The security of the protocol is
guaranteed by the complementarity principle in quantum mechanics. The protocol
can be realized with available technology, at least as a proof of principle
experiment.Comment: 2 pages, 1 figure. Contribution to the proceedings of the IV edition
of the Garda Lake Workshop "Mysteries, Puzzles and Paradoxes in Quantum
Mechanics
The time as an emergent property of quantum mechanics, a synthetic description of a first experimental approach
The "problem of time" in present physics substantially consists in the fact
that a straightforward quantization of the general relativistic evolution
equation and constraints generates for the Universe wave function the
Wheeler-De Witt equation, which describes a static Universe. Page and Wootters
considered the fact that there exist states of a system composed by entangled
subsystems that are stationary, but one can interpret the component subsystems
as evolving: this leads them to suppose that the global state of the universe
can be envisaged as one of this static entangled state, whereas the state of
the subsystems can evolve. Here we synthetically present an experiment, based
on PDC polarization entangled photons, that allows showing with a practical
example a situation where this idea works, i.e. a subsystem of an entangled
state works as a "clock" of another subsystem
Noise, errors and information in quantum amplification
We analyze and compare the characterization of a quantum device in terms of noise, transmitted bit-error-rate (BER) and mutual information, showing how the noise description is meaningful only for Gaussian channels. After reviewing the description of a quantum communication channel, we study the insertion of an amplifier. We focus attention on the case of direct detection, where the linear amplifier has a 3 decibels noise figure, which is usually considered an unsurpassable limit, referred to as the standard quantum limit (SQL). Both noise and BER could be reduced using an ideal amplifier, which is feasible in principle. However, just a reduction of noise beyond the SQL does not generally correspond to an improvement of the BER or of the mutual information. This is the case of a laser amplifier, where saturation can greatly reduce the noise figure, although there is no corresponding improvement of the BER. Such mechanism is illustrated on the basis of Monte Carlo simulations
Orthogonality relations in Quantum Tomography
Quantum estimation of the operators of a system is investigated by analyzing
its Liouville space of operators. In this way it is possible to easily derive
some general characterization for the sets of observables (i.e. the possible
quorums) that are measured for the quantum estimation. In particular we analyze
the reconstruction of operators of spin systems.Comment: 10 pages, 2 figure
Ancilla-assisted schemes are beneficial for Gaussian state phase estimation
We study interferometry with Gaussian states and show that an ancilla-assisted scheme outperforms coherent state interferometry for all levels of loss. We also compare the ancilla-assisted scheme to other interferometric schemes involving squeezing, and show that it has the most advantage in the high-loss, high photon-number regime. In fact, in the presence of high loss, it outperforms many other strategies proposed to date. We find the optimal measurement observable for each scheme discussed. We also find that, with the appropriate measurement, the achievable precision of the proposal by Caves [Phys. Rev. D 23, 1693 (1981)] can be improved upon, and is less vulnerable to losses than previously thought
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