539 research outputs found
Generating and Revealing a Quantum Superposition of Electromagnetic Field Binomial States in a Cavity
We introduce the -photon quantum superposition of two orthogonal
generalized binomial states of electromagnetic field. We then propose, using
resonant atom-cavity interactions, non-conditional schemes to generate and
reveal such a quantum superposition for the two-photon case in a single-mode
high- cavity. We finally discuss the implementation of the proposed schemes.Comment: 4 pages, 3 figures. Title changed (published version
Microwave Photon Detector in Circuit QED
Quantum optical photodetection has occupied a central role in understanding
radiation-matter interactions. It has also contributed to the development of
atomic physics and quantum optics, including applications to metrology,
spectroscopy, and quantum information processing. The quantum microwave regime,
originally explored using cavities and atoms, is seeing a novel boost with the
generation of nonclassical propagating fields in circuit quantum
electrodynamics (QED). This promising field, involving potential developments
in quantum information with microwave photons, suffers from the absence of
photodetectors. Here, we design a metamaterial composed of discrete
superconducting elements that implements a high-efficiency microwave photon
detector. Our design consists of a microwave guide coupled to an array of
metastable quantum circuits, whose internal states are irreversibly changed due
to the absorption of photons. This proposal can be widely applied to different
physical systems and can be generalized to implement a microwave photon
counter.Comment: accepted in Phys. Rev. Let
Non-Local Quantum Gates: a Cavity-Quantum-Electro-Dynamics implementation
The problems related to the management of large quantum registers could be
handled in the context of distributed quantum computation: unitary non-local
transformations among spatially separated local processors are realized
performing local unitary transformations and exchanging classical
communication. In this paper, we propose a scheme for the implementation of
universal non-local quantum gates such as a controlled-\gate{NOT} (\cnot)
and a controlled-quantum phase gate (\gate{CQPG}). The system we have chosen
for their physical implementation is a Cavity-Quantum-Electro-Dynamics (CQED)
system formed by two spatially separated microwave cavities and two trapped
Rydberg atoms. We describe the procedures to follow for the realization of each
step necessary to perform a specific non-local operation.Comment: 12 pages, 5 figures, RevTeX; extensively revised versio
Manipulating mesoscopic multipartite entanglement with atom-light interfaces
Entanglement between two macroscopic atomic ensembles induced by measurement
on an ancillary light system has proven to be a powerful method for engineering
quantum memories and quantum state transfer. Here we investigate the
feasibility of such methods for generation, manipulation and detection of
genuine multipartite entanglement between mesoscopic atomic ensembles. Our
results extend in a non trivial way the EPR entanglement between two
macroscopic gas samples reported experimentally in [B. Julsgaard, A. Kozhekin,
and E. Polzik, Nature {\bf 413}, 400 (2001)]. We find that under realistic
conditions, a second orthogonal light pulse interacting with the atomic
samples, can modify and even reverse the entangling action of the first one
leaving the samples in a separable state.Comment: 8 pages, 6 figure
Time evolution of a superposition of dressed oscillator states in a cavity
Using the formalism of {\it renormalized} coordinates and \textit{dressed}
states introduced in previous publications, we perform a nonperturbative study
of the time evolution of a superposition of two states, the ground state and
the first excited level of a harmonic oscillator, the system being confined in
a perfectly reflecting cavity of radius . For , we find
dissipation with dominance of the interference terms of the density matrix, in
both weak- and strong-coupling regimes. For small values of all elements of
the density matrix present an oscillatory behavior as times goes on and the
system is not dissipative. In both cases, we obtain improved theoretical
results with respect to those coming from perturbation theory.Comment: 15 pages, LATEX, 3 figures; version to appear in J. Phys. A - Math.
Theo
Spontaneously generated atomic entanglement in free space: reinforced by incoherent pumping
We study spontaneously generated entanglement (SGE) between two identical
multilevel atoms in free space via vacuum-induced radiative coupling. We show
that the SGE in two-atom systems may initially increase with time but
eventually vanishes in the time scale determined by the excited state lifetime
and radiative coupling strength between the two atoms. We demonstrate that a
steady-state SGE can be established by incoherently pumping the excited states
of the two-atom system. We have shown that an appropriate rate of incoherent
pump can help producing optimal SGE. The multilevel systems offer us more
chanel to establish entanglement. The system under consideration could be
realized in a tight trap or atoms/ions doped in a solid substrate.Comment: have some difference with published version (please see PRA
Environment assisted entanglement enhancement
We consider dissipative atom-cavity systems and show that their collective
dynamics leads to the maximization of entanglement for intermediate values of
the cavity leakage parameter . We discuss possible ways the reservoir
influences entanglement. We first consider the entanglement of a single
two-level atom with a microwave cavity that is coupled to another cavity. We
show that the atom-cavity entanglement can be made to increase with cavity
leakage. We next show that the entanglement between two atoms passing
successively through a cavity can be maximised for intermediate values of
. We finally consider the micromaser where the increase of two-atom
entanglement for stronger cavity-environment coupling is demonstrated for
experimentally attainable values of the micromaser parameters.Comment: 4 pages, Revtex, 1 eps figure; minor changes to match with published
versio
Quantum Zeno dynamics of a field in a cavity
We analyze the quantum Zeno dynamics that takes place when a field stored in
a cavity undergoes frequent interactions with atoms. We show that repeated
measurements or unitary operations performed on the atoms probing the field
state confine the evolution to tailored subspaces of the total Hilbert space.
This confinement leads to non-trivial field evolutions and to the generation of
interesting non-classical states, including mesoscopic field state
superpositions. We elucidate the main features of the quantum Zeno mechanism in
the context of a state-of-the-art cavity quantum electrodynamics experiment. A
plethora of effects is investigated, from state manipulations by phase space
tweezers to nearly arbitrary state synthesis. We analyze in details the
practical implementation of this dynamics and assess its robustness by
numerical simulations including realistic experimental imperfections. We
comment on the various perspectives opened by this proposal
Decoherence assisting a measurement-driven quantum evolution process
We study the problem of driving an unknown initial mixed quantum state onto a
known pure state without using unitary transformations. This can be achieved,
in an efficient manner, with the help of sequential measurements on at least
two unbiased bases. However here we found that, when the system is affected by
a decoherence mechanism, only one observable is required in order to achieve
the same goal. In this way the decoherence can assist the process. We show
that, depending on the sort of decoherence, the process can converge faster or
slower than the method implemented by means of two complementary observables.Comment: Four pages, three figures included ([email protected]
Realization of a superconducting atom chip
We have trapped rubidium atoms in the magnetic field produced by a
superconducting atom chip operated at liquid Helium temperatures. Up to
atoms are held in a Ioffe-Pritchard trap at a distance of 440
m from the chip surface, with a temperature of 40 K. The trap
lifetime reaches 115 s at low atomic densities. These results open the way to
the exploration of atom--surface interactions and coherent atomic transport in
a superconducting environment, whose properties are radically different from
normal metals at room temperature.Comment: Submitted to Phys. Rev. Let
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