Oscillations of the purity in the repeated-measurement-based generation of quantum states

Repeated observations of a quantum system interacting with another one can drive the latter toward a particular quantum state, irrespectively of its initial condition, because of an {\em effective non-unitary evolution}. If the target state is a pure one, the degree of purity of the system approaches unity, even when the initial condition of the system is a mixed state. In this paper we study the behavior of the purity from the initial value to the final one, that is unity. Depending on the parameters, after a finite number of measurements, the purity exhibits oscillations, that brings about a lower purity than that of the initial state, which is a point to be taken care of in concrete applications.Comment: 5 pages, 3 figure

Extraction of an Entanglement by Repetition of the Resonant Transmission of an Ancilla Qubit

A scheme for the extraction of entanglement in two noninteracting qubits (spins) is proposed. The idea is to make use of resonant transmission of ancilla qubit through the two fixed qubits, controlled by the entanglement in the scatterers. Repetition of the resonant transmission extracts the singlet state in the target qubits from their arbitrary given state. Neither the preparation nor the post-selection of the ancilla spin is required, in contrast to the previously proposed schemes.Comment: 14 pages, 7 figure

Injection and detection of spin in a semiconductor by tunneling via interface states

Injection and detection of spin accumulation in a semiconductor having localized states at the interface is evaluated. Spin transport from a ferromagnetic contact by sequential, two-step tunneling via interface states is treated not in itself, but in parallel with direct tunneling. The spin accumulation induced in the semiconductor channel is not suppressed, as previously argued, but genuinely enhanced by the additional spin current via interface states. Spin detection with a ferromagnetic contact yields a weighted average of the spin accumulation in the channel and in the localized states. In the regime where the spin accumulation in the localized states is much larger than that in the channel, the detected spin signal is insensitive to the spin accumulation in the localized states and the ferromagnet probes the spin accumulation in the semiconductor channel.Comment: 7 pages, 2 figures. Theory onl

Entanglement Purification through Zeno-like Measurements

We present a novel method to purify quantum states, i.e. purification through Zeno-like measurements, and show an application to entanglement purification.Comment: 5 pages, 1 figure; Contribution to the Proceedings of "Mysteries, Puzzles and Paradoxes in Quantum Mechanics", Gargnano, Italy, 2003 (to be published in J. Mod. Opt.

Thermal spin current and magnetothermopower by Seebeck spin tunneling

The recently observed Seebeck spin tunneling, the thermoelectric analog of spin-polarized tunneling, is described. The fundamental origin is the spin dependence of the Seebeck coefficient of a tunnel junction with at least one ferromagnetic electrode. Seebeck spin tunneling creates a thermal flow of spin-angular momentum across a tunnel barrier without a charge tunnel current. In ferromagnet/insulator/semiconductor tunnel junctions this can be used to induce a spin accumulation (\Delta \mu) in the semiconductor in response to a temperature difference (\Delta T) between the electrodes. A phenomenological framework is presented to describe the thermal spin transport in terms of parameters that can be obtained from experiment or theory. Key ingredients are a spin-polarized thermoelectric tunnel conductance and a tunnel spin polarization with non-zero energy derivative, resulting in different Seebeck tunnel coefficients for majority and minority spin electrons. We evaluate the thermal spin current, the induced spin accumulation and \Delta\mu/\Delta T, discuss limiting regimes, and compare thermal and electrical flow of spin across a tunnel barrier. A salient feature is that the thermally-induced spin accumulation is maximal for smaller tunnel resistance, in contrast to the electrically-induced spin accumulation that suffers from the impedance mismatch between a ferromagnetic metal and a semiconductor. The thermally-induced spin accumulation produces an additional thermovoltage proportional to \Delta\mu, which can significantly enhance the conventional charge thermopower. Owing to the Hanle effect, the thermopower can also be manipulated with a magnetic field, producing a Hanle magnetothermopower.Comment: 10 pages, 3 figures, 1 tabl

Distillation of Entanglement between Distant Systems by Repeated Measurements on Entanglement Mediator

A recently proposed purification method, in which the Zeno-like measurements of a subsystem can bring about a distillation of another subsystem in interaction with the former, is utilized to yield entangled states between distant systems. It is shown that the measurements of a two-level system locally interacting with other two spatially separated not coupled subsystems, can distill entangled states from the latter irrespectively of the initial states of the two subsystems.Comment: 11 pages, 2 figures; the version accepted for publication in Phys. Rev.

Resonant Scattering Can Enhance the Degree of Entanglement

Generation of entanglement between two qubits by scattering an entanglement mediator is discussed. The mediator bounces between the two qubits and exhibits a resonant scattering. It is clarified how the degree of the entanglement is enhanced by the constructive interference of such bouncing processes. Maximally entangled states are available via adjusting the incident momentum of the mediator or the distance between the two qubits, but their fine tunings are not necessarily required to gain highly entangled states and a robust generation of entanglement is possible.Comment: 7 pages, 13 figure

Distillation by repeated measurements: continuous spectrum case

Repeated measurements on a part of a bipartite system strongly affect the other part not measured, whose dynamics is regulated by an effective contracted evolution operator. When the spectrum of this operator is discrete, the latter system is driven into a pure state irrespective of the initial state, provided the spectrum satisfies certain conditions. We here show that even in the case of continuous spectrum an effective distillation can occur under rather general conditions. We confirm it by applying our formalism to a simple model.Comment: 4 pages, 2 figure

Entanglement Generation by Qubit Scattering in Three Dimensions

A qubit (a spin-1/2 particle) prepared in the up state is scattered by local spin-flipping potentials produced by the two target qubits (two fixed spins), both prepared in the down state, to generate an entangled state in the latter when the former is found in the down state after scattering. The scattering process is analyzed in three dimensions, both to lowest order and in full order in perturbation, with an appropriate renormalization for the latter. The entanglement is evaluated in terms of the concurrence as a function of the incident and scattering angles, the size of the incident wave packet, and the detector resolution, to clarify the key elements for obtaining an entanglement with high quality. The characteristics of the results are also discussed in the context of (in)distinguishability of alternative paths for a quantum particle.Comment: 21 pages, 19 figures, the final versio