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