D. L. Zhou

Necessary and sufficient conditions for local creation of quantum correlation

Quantum correlation can be created by a local operation from some initially classical states. We prove that the necessary and sufficient condition for a local trace-preserving channel to create quantum correlation is that it is not a commutativity-preserving channel. This condition is valid for arbitrary finite dimension systems. We also derive the explicit form of commutativity-preserving channels. For a qubit, a commutativity-preserving channel is either a completely decohering channel or a mixing channel. For a three-dimensional system (qutrit), a commutativity-preserving channel is either a completely decohering channel or an isotropic channel.

Quantum correlating power of local quantum channels

We define quantum-correlating power (QCP) of a local quantum channel acting on the left part of a bipartite quantum system as the maximum amount of left quantum correlation that can be created by this channel. We prove that for any local channel, the optimal input state, which corresponds to the maximum quantum correlation in the output state, must be a classical-classical state. Further, the single-qubit channels with maximum QCP can be found in the class of channels which take their optimal input states to rank-two quantum-classical states. A superactivation property of QCP, that is, two zero-QCP channels can constitute a positive-QCP channel, is observed and discussed for single-qubit phase damping channels. The analytic expression for QCP of single-qubit amplitude damping channel is obtained. DOI: 10.1103/PhysRevA.87.032340

Mean-field dynamics of a Bose Josephson junction in an optical cavity

We study the mean-field dynamics of a Bose Josephson junction which is dispersively coupled to a single mode of a high-finesse optical cavity. An effective classical Hamiltonian for the Bose Josephson junction is derived, and its dynamics is studied from the perspective of a phase portrait. It is shown that the strong condensate-field interplay does alter the dynamics of the Bose Josephson junction drastically. The possibility of coherent manipulating and in situ observation of the dynamics of the Bose Josephson junction is discussed. Cavity quantum electrodynamics cavity QED has now grown into a paradigm in the study of the matter-field interaction. To tailor the atom-field coupling effectively, a high degree of control over the center-of-mass motion of the atoms is essential. Although previous works have focused on the few-atom level 1, recently, a great step was made as two groups succeeded independently in coupling a BoseEinstein condensate to a single-cavity mode 2,3. That is, a single-cavity mode dressed condensate has been achieved. This opens up a new regime in both the fields of cavity QED and ultracold atom physics. In the condensate, all the atoms occupy the same motional mode and couple identically to the cavity mode, thus realizing the Dicke model 4 in a broad sense. As shown by these experiments, the condensate is quite robust; it would not be destroyed by its interaction with the cavity mode in the time scale of the experiments. In this paper, we investigate the mean-field dynamics of a Bose Josephson junction BJJ5, which is coupled to a driven cavity mode. This extends our previous work to the many-atom case 6. The system may be constructed by splitting a Bose-Einstein condensate, which is already coupled to a single-cavity mode, into two weakly linked condensates, as can be done in a variety of ways 7‐11. We restrict ourselves to the large-detuning and low-excitation limit, so that atomic spontaneous emission can be neglected. In this limit, the effect of the strong coupling between the atoms and the field, seen by the field, is to shift the cavity resonance frequency and hence modify the field intensity. Unlike the single-condensate case, we now have two, which may couple with different strengths to the cavity mode because of the position dependence of the atom-field coupling. Consequently, the field dynamics couples to the tunneling dynamics of the BJJ and vice versa. It is the very interplay between the two sides that we are interested in. The interplay is made possible by the greatly enhanced atom-field coupling in a microcavity, which is unique in the context of cavity QED. In the usual optical traps and optical lattices, the interaction between the atoms and the light field is one way in the sense that the light field affects the motion of the atoms effectively, while the atoms have little back-action on the light field—the laser intensity is almost the same with or without the presence of the atoms. We would like to stress that, although there had already been some experimental investigations on this subject and phenomena such as dispersive optical bistability were observed 12,13, all of them dealt with thermally cold atoms. However, here, the long-range coherence of the condensates will surely make a difference. The Hamiltonian of the system consists of three parts: H = Ha + Hf + Hint. 1 Ha is the canonical Bose-Josephson-junction Hamiltonian in the two-mode approximation =1 throughout,

Cavity QED with cold atoms trapped in a double-well potential

We investigate the interplay dynamics of a cavity QED system, where the two-level atoms are trapped in a double-well potential, and the cavity mode, with a frequency largely detuned from the atomic level splitting, is driven by a probe laser. The interaction between the center-of-mass motion of the atoms and the cavity mode is induced by the position-dependent atom-field coupling. The dynamics of the system is characterized by two distinct time scales, the inverse of the atomic interwell tunneling rate and the inverse of the cavity loss rate. The system shows drastically different (quasi)steady behaviors in the short- and long-time intervals, and the detection of the statistics of atom number distribution from the transmission spectra is available only in the short-time interval.

Nonlinear dynamics of a cigar-shaped Bose-Einstein condensate in an optical cavity

The structural and physical properties of the recently discovered electronic ferroelectric materials LuFe2O4 and Lu2Fe3O7 have been investigated for Mg substitution of Fe. X-ray diffraction data demonstrate that the lattice parameters in both systems change progressively with increasing Mg content, with a smaller unit cell volume on replacing Fe2+ by Mg2+. X-ray absorption near-edge spectroscopy experiments at the Fe K-edge show that the average Fe oxidation state is slightly increased along with Mg doping in Lu2Fe3O7 materials, consistent with isomorphous replacement of Fe2+ by Mg2+. Measurements of dielectric properties demonstrate that Mg doping could have an effect on the electron hopping energy between Fe2+ and Fe3+ ions. Transmission electron microscopy and magnetization analysis reveal that Mg doping in LuFe2O4 has a much greater influence than in Lu2Fe3O7 on both the charge ordering and the low-temperature magnetic properties.

Discontinuity of maximum entropy inference and quantum phase transitions

In this paper, we discuss the connection between two genuinely quantum phenomena --- the discontinuity of quantum maximum entropy inference and quantum phase transitions at zero temperature. It is shown that the discontinuity of the maximum entropy inference of local observable measurements signals the non-local type of transitions, where local density matrices of the ground state change smoothly at the transition point. We then propose to use the quantum conditional mutual information of the ground state as an indicator to detect the discontinuity and the non-local type of quantum phase transitions in the thermodynamic limit.

Single-photon scattering on a strongly dressed atom

We use the generalized rotating-wave approximation approach [Irish, Phys. Rev. Lett. 99, 173601 (2007)] to study single-photon scattering on a two-level system (TLS) with arbitrarily strong coupling to a local mode in a one-dimensional (1D) coupled-resonator array. We obtain the scattering amplitudes by an analytical method, which works well in a broad parameter region, confirmed by independent numerical results. In particular, when the resonator mode is far off resonance with the TLS, our results appear more reasonable than the ones from the standard adiabatic approximation. The approach is further extended to cases with a 1D resonator array strongly coupled to more than one TLS.

Mosaic spin models with topological order

We study a class of two-dimensional spin models with the Kitaev-type couplings in mosaic structure lattices to implement topological orders. We show that they are exactly solvable by reducing them to some free Majorana fermion models with gauge symmetries. The typical case with a 4-8-8 close packing is investigated in detail to display the quantum phases with Abelian and non-Abelian anyons. Its topological properties characterized by Chern numbers are revealed through the edge modes of its spectrum.

Fast entanglement of two charge-phase qubits through nonadiabatic coupling to a large Josephson junction

We propose a theoretical protocol for quantum logic gates between two Josephson junction charge-phase qubits through the control of their coupling to a large junction whose Josephson coupling energy is much larger than its Coulomb charge energy. In the low excitation limit of the large junction, it behaves effectively as a quantum data-bus mode of a harmonic oscillator. Our protocol can be fast since it does not require the data-bus to stay adiabatically in its ground state, as such it can be implemented over a wide parameter regime independent of the data-bus quantum state.

Comment on some results of Erdahl and the convex structure of reduced density matrices

In [J. Math. Phys. 13, 1608–1621 (1972)], Erdahl10.1063/1.1665885 considered the convex structure of the set of N-representable 2-body reduced density matrices in the case of fermions. Some of these results have a straightforward extension to the m-body setting and to the more general quantum marginal problem. We describe these extensions, but cannot resolve a problem in the proof of Erdahls claim that every extreme point is exposed in finite dimensions. Nevertheless, we can show that when 2m ⩾ N every extreme point of the set of N-representable m-body reduced density matrices has a unique pre-image in both the symmetric and anti-symmetric setting. Moreover, this extends to the quantum marginal setting for a pair of complementary m-body and (N − m)-body reduced density matrices.In [J. Math. Phys. 13, 1608–1621 (1972)], Erdahl10.1063/1.1665885 considered the convex structure of the set of N-representable 2-body reduced density matrices in the case of fermions. Some of these results have a straightforward extension to the m-body setting and to the more general quantum marginal problem. We describe these extensions, but cannot resolve a problem in the proof of Erdahls claim that every extreme point is exposed in finite dimensions. Nevertheless, we can show that when 2m ⩾ N every extreme point of the set of N-representable m-body reduced density matrices has a unique pre-image in both the symmetric and anti-symmetric setting. Moreover, this extends to the quantum marginal setting for a pair of complementary m-body and (N − m)-body reduced density matrices.