Impurity entropy of junctions of multiple quantum wires

We calculate the zero-temperature impurity entropy of a junction of multiple quantum wires of interacting spinless fermions. Starting from a given single-particle S-matrix representing a fixed point of the renormalization group (RG) flows, we carry out fermionic perturbation theory in the bulk interactions, with the perturbation series summed in the random phase approximation (RPA). The results agree completely with boundary conformal field theory (BCFT) predictions of the ground state degeneracy, and also with known RG flows through the g-theorem.Comment: 21 pages, 4 figure

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

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

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

Unconventional geometric quantum phase gates with a cavity QED system

We propose a scheme for realizing two-qubit quantum phase gates via an unconventional geometric phase shift with atoms in a cavity. In the scheme the atoms interact simultaneously with a highly detuned cavity mode and a classical field. The atoms undergo no transitions during the gate operation, while the cavity mode is displaced along a circle in the phase space, aquiring a geometric phase conditional upon the atomic state. Under certain conditions, the atoms are disentangled with the cavity mode and thus the gate is insensitive to both the atomic spontaneous emission and the cavity decay

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

A fermionic approach to tunneling through junctions of multiple quantum wires

Junctions of multiple one-dimensional quantum wires of interacting electrons have received considerable theoretical attention as a basic constituent of quantum circuits. While results have been obtained on these models using bosonization and Density Matrix Renormalization Group (DMRG) methods, another powerful technique is based on direct perturbation theory in the bulk interactions, combined with the Renormalization Group (RG) and summed in the Random Phase Approximation (RPA). This technique has so far only been applied to the case where finite length interacting wires are attached to non-interacting Fermi liquid leads. We reformulate it in terms of the single-particle S-matrix, formally unifying treatments of junctions of different numbers of leads, and extend this method to cover the case of infinite length interacting leads obtaining results on 2-lead and 3-lead junctions in good agreement with previous bosonization and DMRG results.Comment: 24 pages, 5 figures; v