1,687 research outputs found
Quantum entanglement and fixed-point bifurcations
How does the classical phase space structure for a composite system relate to
the entanglement characteristics of the corresponding quantum system? We
demonstrate how the entanglement in nonlinear bipartite systems can be
associated with a fixed point bifurcation in the classical dynamics. Using the
example of coupled giant spins we show that when a fixed point undergoes a
supercritical pitchfork bifurcation, the corresponding quantum state - the
ground state - achieves its maximum amount of entanglement near the critical
point. We conjecture that this will be a generic feature of systems whose
classical limit exhibits such a bifurcation.Comment: v2: Structure of the paper changed for clarity, reduced length, now 9
pages with 6 figure
Charge Transport in a Quantum Electromechanical System
We describe a quantum electromechanical system(QEMS) comprising a single
quantum dot harmonically bound between two electrodes and facilitating a
tunneling current between them. An example of such a system is a fullerene
molecule between two metal electrodes [Park et al., Nature, 407, 57 (2000)].
The description is based on a quantum master equation for the density operator
of the electronic and vibrational degrees of freedom and thus incorporates the
dynamics of both diagonal (population) and off diagonal (coherence) terms. We
derive coupled equations of motion for the electron occupation number of the
dot and the vibrational degrees of freedom, including damping of the vibration
and thermo-mechanical noise. This dynamical description is related to
observable features of the system including the stationary current as a
function of bias voltage.Comment: To appear in Phys. Rev. B., 13 pages, single colum
Mesoscopic one-way channels for quantum state transfer via the Quantum Hall Effect
We show that the one-way channel formalism of quantum optics has a physical
realisation in electronic systems. In particular, we show that magnetic edge
states form unidirectional quantum channels capable of coherently transporting
electronic quantum information. Using the equivalence between one-way photonic
channels and magnetic edge states, we adapt a proposal for quantum state
transfer to mesoscopic systems using edge states as a quantum channel, and show
that it is feasible with reasonable experimental parameters. We discuss how
this protocol may be used to transfer information encoded in number, charge or
spin states of quantum dots, so it may prove useful for transferring quantum
information between parts of a solid-state quantum computer.Comment: 4 pages, 3 figure
Quantum state transfer between a Bose-Einstein condensate and an optomechanical mirror
In this paper we describe a scheme for state transfer between a trapped
atomic Bose condensate and an optomechanical end-mirror mediated by a cavity
field. Coupling between the mirror and the cold gas arises from the fact that
the cavity field can produce density oscillations in the gas which in turn acts
as an internal Bragg mirror for the field. After adiabatic elimination of the
cavity field we find that the hybrid system of the gas and mirror is described
by a beam splitter Hamiltonian that allows for state transfer, but only if the
quantum nature of the cavity field is retained
Quantum open-systems approach to current noise in resonant tunneling junctions
A quantum Markovian master equation is derived to describe the current noise in resonant tunneling devices. This equation includes both incoherent and coherent quantum tunneling processes. We show how to obtain the population master equation by adiabatic elimination of quantum coherences in the presence of elastic scattering. We calculate the noise spectrum for a double well device and predict subshot noise statistics for strong tunneling between the wells. The method is an alternative to Green's function methods and population master equations for very small coherently coupled quantum dots
Reduction in laser-intensity fluctuations by a feedback-controlled output mirror
We present the theory of a laser in which the transmittivity of one output mirror is controlled by a current derived from a photodetector illuminated by the output light from that end of the cavity. That is, one output port of the laser is controlled by a feedback loop. We calculate the photon statistics inside the cavity. We also calculate the spectrum of intensity fluctuations for the light leaving the cavity through the output mirror not controlled by feedback. We show that intensity fluctuations inside the cavity may be reduced to 50% below the Poissonian limit while outside the cavity the reduction is at best 25% of the shot-noise limit. These optimum results, however, are not achieved under the same operating conditions
Continuous quantum non-demolition measurement of Fock states of a nanoresonator using feedback-controlled circuit QED
We propose a scheme for the quantum non-demolition (QND) measurement of Fock
states of a nanomechanical resonator via feedback control of a coupled circuit
QED system. A Cooper pair box (CPB) is coupled to both the nanoresonator and
microwave cavity. The CPB is read-out via homodyne detection on the cavity and
feedback control is used to effect a non-dissipative measurement of the CPB.
This realizes an indirect QND measurement of the nanoresonator via a
second-order coupling of the CPB to the nanoresonator number operator. The
phonon number of the Fock state may be determined by integrating the stochastic
master equation derived, or by processing of the measurement signal.Comment: 5 pages, 3 figure
Dynamical creation of entanglement by homodyne-mediated feedback
For two two-level atoms coupled to a single-mode cavity field that is driven
and heavily damped, the steady-state can be entangled by shining an
un-modulated driving laser on the system [S.Schneider, G. J. Milburn Phys. Rev
A 65, 042107, 2002]. We present a scheme to significantly increase the
steady-state entanglement by using homodyne-mediated feedback, in which the
driving laser is modulated by the homodyne photocurrent derived from the cavity
output. Such feedback can increase the nonlinear response to both the
decoherence process of the two-qubit system and the coherent evolution of
individual qubits. We present the properties of the entangled states using the
SO(3) Q function.Comment: 8 page
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