1,564 research outputs found
Super-Resolving Quantum Radar: Coherent-State Sources with Homodyne Detection Suffice to Beat the Diffraction Limit
There has been much recent interest in quantum metrology for applications to
sub-Raleigh ranging and remote sensing such as in quantum radar. For quantum
radar, atmospheric absorption and diffraction rapidly degrades any actively
transmitted quantum states of light, such as N00N states, so that for this
high-loss regime the optimal strategy is to transmit coherent states of light,
which suffer no worse loss than the linear Beer's law for classical radar
attenuation, and which provide sensitivity at the shot-noise limit in the
returned power. We show that coherent radar radiation sources, coupled with a
quantum homodyne detection scheme, provide both longitudinal and angular
super-resolution much below the Rayleigh diffraction limit, with sensitivity at
shot-noise in terms of the detected photon power. Our approach provides a
template for the development of a complete super-resolving quantum radar system
with currently available technology.Comment: 23 pages, content is identical to published versio
Nonlocal entanglement of coherent states, complementarity, and quantum erasure
We describe a nonlocal method for generating entangled coherent states of a two-mode field wherein the field modes never meet. The proposed method is an extension of an earlier proposal [C. C. Gerry, Phys. Rev. A 59, 4095 (1999)] for the generation of superpositions of coherent states. A single photon injected into a Mach-Zehnder interferometer with cross-Kerr media in both arms coupling with two external fields in coherent states produces entangled coherent states upon detection at one of the output ports. We point out that our proposal can be alternatively viewed as a which path experiment, and in the case of only one external field, we describe the implementation of a quantum eraser
The parity operator in quantum optical metrology
Photon number states are assigned a parity of if their photon number is even
and a parity of if odd. The parity operator, which is minus one to the power of
the photon number operator, is a Hermitian operator and thus a quantum
mechanical observable though it has no classical analog, the concept being
meaningless in the context of classical light waves. In this paper we review
work on the application of the parity operator to the problem of quantum
metrology for the detection of small phase shifts with quantum optical
interferometry using highly entangled field states such as the so-called N00N
states, and states obtained by injecting twin Fock states into a beam splitter.
With such states and with the performance of parity measurements on one of the
output beams of the interferometer, one can breach the standard quantum limit,
or shot-noise limit, of sensitivity down to the Heisenberg limit, the greatest
degree of phase sensitivity allowed by quantum mechanics for linear phase
shifts. Heisenberg limit sensitivities are expected to eventually play an
important role in attempts to detect gravitational waves in interferometric
detection systems such as LIGO and VIRGO.Comment: to be published in Contemporary Physic
Timing of pair production in time-dependent force fields
We examine the creation and annihilation dynamics for electron-positron pairs in a time-dependent but subcritical electric force using a simplified model system. Numerical and semianalytical solutions to computational quantum field theory show that despite the continuity of the quantum field operator in time, the actual number of created particles can change in a discontinuous way if the field changes abruptly. The number of permanently created particles after the pulse, however, increases continuously with the duration of the electric field pulse, suggesting a transition from an exclusive annihilation to a creation regime
Unitary and nonunitary approaches in quantum field theory
We use a simplified essential state model to compare two quantum field theoretical approaches to study the creation of electron-positron pairs from the vacuum. In the unitary approach the system is characterized by a state with different numbers of particles that is described by occupation numbers and evolves with conserved norm. The nonunitary approach can predict the evolution of wave functions and density operators with a fixed number of particles but time-dependent norms. As an example to illustrate the differences between both approaches, we examine the degree of entanglement for the Klein paradox, which describes the creation of an electron-positron pair from vacuum in the presence of an initial electron. We demonstrate how the Pauli blocking by the initial electron comes at the expense of a gain in entanglement of this electron with the created electron as well as with the created positron
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