1,502 research outputs found
Lower bound for electron spin entanglement from beamsplitter current correlations
We determine a lower bound for the entanglement of pairs of electron spins
injected into a mesoscopic conductor. The bound can be expressed in terms of
experimentally accessible quantities, the zero-frequency current correlators
(shot noise power or cross-correlators) after transmission through an
electronic beam splitter. The effect of spin relaxation (T_1 processes) and
decoherence (T_2 processes) during the ballistic coherent transmission of the
carriers in the wires is taken into account within Bloch theory. The presence
of a variable inhomogeneous magnetic field allows the determination of a useful
lower bound for the entanglement of arbitrary entangled states. The decrease in
entanglement due to thermally mixed states is studied. Both the entanglement of
the output of a source (entangler) and the relaxation (T_1) and decoherence
(T_2) times can be determined.Comment: 4 pages, 3 figure
Role of hyperfine interaction for cavity-mediated coupling between spin qubits
We consider two qubits interacting by means of an optical cavity, where each
qubit is represented by a single electron spin confined to a quantum dot. It is
known that electron spins in III-V semiconductor quantum dots are affected by
the decoherence due to the hyperfine interaction with nuclear spins. Here we
show that the interaction between two qubits is influenced by the Overhauser
field as well. Starting from an unpolarizied nuclear ensemble, we investigate
the dependance of the fidelities for two-qubit gates on the Overhauser field.
We include the hyperfine interaction perturbatively to second order in our
analytical results, and to arbitrary precision numerically.Comment: 7 pages, 7 figure
Quantum gates between capacitively coupled double quantum dot two-spin qubits
We study the two-qubit controlled-not gate operating on qubits encoded in the
spin state of a pair of electrons in a double quantum dot. We assume that the
electrons can tunnel between the two quantum dots encoding a single qubit,
while tunneling between the quantum dots that belong to different qubits is
forbidden. Therefore, the two qubits interact exclusively through the direct
Coulomb repulsion of the electrons. We find that entangling two-qubit gates can
be performed by the electrical biasing of quantum dots and/or tuning of the
tunneling matrix elements between the quantum dots within the qubits. The
entangling interaction can be controlled by tuning the bias through the
resonance between the singly-occupied and doubly-occupied singlet ground states
of a double quantum dot.Comment: 12 pages, 7 figure
Entangled photons from the polariton vacuum in a switchable optical cavity
We study theoretically the entanglement of two-photon states in the ground
state of the intersubband cavity system, the so-called polariton vacuum. The
system consists of a sequence of doped quantum wells located inside a
microcavity and the photons can interact with intersubband excitations inside
the quantum wells. Using an explicit solution for the ground state of the
system, operated in the ultrastrong coupling regime, a post-selection is
introduced, where only certain two-photon states are considered and analyzed
for mode entanglement. We find that a fast quench of the coupling creates
entangled photons and that the degree of entanglement depends on the absolute
values of the in-plane wave vectors of the photons. Maximally entangled states
can be generated by choosing the appropriate modes in the post-selection.Comment: 9+ pages, 7 figure
Weak measurement of quantum dot spin qubits
The theory of weak quantum measurements is developed for quantum dot spin
qubits. Building on recent experiments, we propose a control cycle to prepare,
manipulate, weakly measure, and perform quantum state tomography. This is
accomplished using a combination of the physics of electron spin resonance,
spin blockade, and Coulomb blockade, resulting in a charge transport process.
We investigate the influence of the surrounding nuclear spin environment, and
find a regime where this environment significantly simplifies the dynamics of
the weak measurement process, making this theoretical proposal realistic with
existing experimental technology. We further consider spin-echo refocusing to
combat dephasing, as well as discuss a realization of "quantum undemolition",
whereby the effects of quantum state disturbance are undone.Comment: 8 pages, 2 figure
Double-Occupancy Errors, Adiabaticity, and Entanglement of Spin-Qubits in Quantum Dots
Quantum gates that temporarily increase singlet-triplet splitting in order to
swap electronic spins in coupled quantum dots, lead inevitably to a finite
double-occupancy probability for both dots. By solving the time-dependent
Schr\"odinger equation for a coupled dot model, we demonstrate that this does
not necessarily lead to quantum computation errors. Instead, the coupled dot
ground state evolves quasi-adiabatically for typical system parameters so that
the double-occupancy probability at the completion of swapping is negligibly
small. We introduce a measure of entanglement which explicitly takes into
account the possibilty of double occupancies and provides a necessary and
sufficient criterion for entangled states.Comment: 9 pages, 4 figures include
Validity and Reliability of the Lesbian, Gay, Bisexual Working Alliance Self-Efficacy Scales
In this paper, the authors report on the development and initial psychometric evaluation of the Lesbian, Gay, and Bisexual Working Alliance Self-Efficacy Scales (LGB-WASES) with data collected from two studies and 534 counseling trainees. Exploratory factor analysis results yielded a 32-item scale with a three-factor model (a) Emotional Bond, (b) Establishing Tasks, and (c) Setting Goals. LGB-WASES scores were internally consistent and remained stable over a 3-week period. Construct validity evidence suggests the LGB-WASES scores were (a) positively related to general perceptions of counseling self-efficacy and multicultural counseling competency, (b) negatively related to attitudes toward lesbians and gay men, and (d) unrelated to social desirability. Recommendations for future research are also discussed
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