231 research outputs found
Virtual-photon-induced quantum phase gates for two distant atoms trapped in separate cavities
We propose a scheme for implementing quantum gates for two atoms trapped in
distant cavities connected by an optical fiber. The effective long-distance
coupling between the two distributed qubits is achieved without excitation and
transportation of photons through the optical fiber. Since the cavity modes and
fiber mode are never populated and the atoms undergo no transitions, the gate
operation is insensitive to the decoherence effect when the thermal photons in
the environment are negligible. The scheme opens promising perspectives for
networking quantum information processors and implementing distributed and
scalable quantum computation
Reply to Cereceda's comment on "Quantum nonlocality for a three-particle nonmaximally entangled state without inequaltiy"
This is to reply to Cereceda's comment on "Quantum nonlocality for a
three-particle nonmaximally entangled state without inequaltiy"Comment: 4 pages, no figure
Generation of entangled states for many multilevel atoms in a thermal cavity and ions in thermal motion
We propose a scheme for generating entangled states for two or more
multi-level atoms in a thermal cavity. The photon-number dependent parts in the
effective Hamiltonian are canceled with the assistant of a strong classical
field. Thus the scheme is insensitive to both the cavity decay and the thermal
field. The scheme does not require individual addressing of the atoms in the
cavity. The scheme can also be used to generate entangled states for many hot
multi-level ions
Quantum information processing and multiatom entanglement engineering with a thermal cavity
We propose a scheme for realizing two-qubit quantum phase gates with atoms in
a thermal cavity. The photon-number dependent parts in the evolution operator
are canceled with the assistant of a strong classical field. Thus the scheme is
insensitive to the thermal field. In the scheme the detuning between the atoms
and the cavity is equal to the atom-cavity coupling strength and thus the gates
operate at a high speed, which is also important in view of decoherence. The
scheme can be generalized to generate multiatom entangled states with a thermal
cavity
Macroscopic superposition and entanglement for displaced thermal fields induced by a single atom
We show that a cavity field can evolve from an initial displaced mixed
thermal state to a macroscopic superpositions of displaced thermal states via
resonant interaction with a two-level atom. As a macroscopic system (meter) is
really in a mixed state before coupling with the microscopic system at some
temperature, our result is important for studying the quantum measurement
problem and decoherence under real conditions. For the two-mode case,
entanglement of displaced thermal states between the modes can be obtained
Open system geometric phase based on system-reservoir joint state evolution
The geometric phase is of fundamental interest and plays an important role in
quantum information processing. However, the definition and calculation of this
phase for open systems remains a problem due to the lack of agreement on
generalizations of the parallel transport condition to mixed state nonunity
evolutions. Here we tackle this problem by associating the open system
geometric phase with the parallel transport of the joint system-reservoir
state. Our approach not only provides a way around the nonunitary evolution
obstacle, but also sheds light on the relation between the geometric phase and
the system-reservoir entanglement, which has not been investigated. Based on
this approach, we calculate the geometric phase of different quantum systems
subject to energy decay, showing that it is robust against decoherence, which
is in distinct contrast with previous results.Comment: 6 pages + Supplementary informatio
Macroscopic displaced thermal field as the entanglement catalyst
We show that entanglement of multiple atoms can arise via resonant
interaction with a displaced thermal field with a macroscopic photon-number.
The cavity field acts as the catalyst, which is disentangled with the atomic
system after the operation. Remarkably, the entanglement speed does not
decrease as the average photon-number of the mixed thermal state increases. The
atoms may evolve to a highly entangled state even when the photon-number of the
cavity mode approaches infinity.Comment: Quantum Information & Computation Volume 7 Issue 8, November 200
Two-photon absorption and emission by Rydberg atoms in coupled cavities
We study the dynamics of a system composed of two coupled cavities, each
containing a single Rydberg atom. The interplay between Rydberg-Rydberg
interaction and photon hopping enables the transition of the atoms from the
collective ground state to the double Rydberg excitation state by individually
interacting with the hybrid cavity modes and suppressing the up conversion
process between them. The atomic transition is accompanied by the two-photon
absorption and emission of the hybrid modes. Since the energy level structure
of the atom-cavity system is photon number dependent, there is only a pair of
states being in the two-photon resonance. Therefore, the system can act as a
quantum nonlinear absorption filter through the nonclassical quantum process,
converting coherent light field into a non-classical state. Meanwhile, the
vacuum field in the cavity inspires the Rydberg atoms to simultaneously emit
two photons into the hybrid mode, resulting in obvious emission enhancement of
the mode.Comment: 7 pages, 6 figure
Quantum signature for laser-driven correlated excitation of Rydberg atoms
The excitation dynamics of a laser-driven Rydberg gas exhibits a cooperative
effect due to the interatomic Rydberg-Rydberg interaction, but the large
many-body system with inhomogeneous Rydberg coupling is hard to exactly solved
or numerically study by density matrix equations. In this paper, we find that
the laser-driven Rydberg gas with most of the atoms being in the ground state
can be described by a simplified interaction model resembling the optical Kerr
effect if the distance-dependent Rydberg-Rydberg interaction is replaced by an
infinite-range coupling. We can then quantitatively study the effect of the
quantum fluctuations on the Rydberg excitation with the interatomic correlation
involved and analytically calculate the statistical characteristics of the
excitation dynamics in the steady state, revealing the quantum signature of the
driven-dissipative Rydberg gas. The results obtained here will be of great
interest for other spin-1/2 systems with spin-spin coupling.Comment: Main text (8 pages, 8 figures
Generation of atomic and field squeezing by adiabatic passage and symmetry breaking
We propose an efficient scheme for realizing squeezing for both an atomic
ensemble and a cavity field via adiabatic evolution of the dark state of the
atom-cavity system. Controlled symmetry breaking of the Hamiltonian ensures a
unique dark state for the total system, in which the atomic system or cavity
mode is squeezed depending upon the choice of the detunings. Since the
generation of the atomic squeezed state requires neither the cavity mode nor
the atomic system to be excited, the decoherence effects are effectively
suppressed. The scheme is insensitive to the uncertainty in the atomic number
and imperfect timing, and the time needed for the generation of the desired
squeezed state decreases as the size of the system grows. The required
experimental techniques are within the scope of what can be obtained in the
present cavity QED setups.Comment: to be published in Physical Review
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