Showing papers by "Yu-xi Liu published in 2009"

Effective Hamiltonian approach to the Kerr nonlinearity in an optomechanical system

Abstract: Using the Born-Oppenheimer approximation, we derive an effective Hamiltonian for an optomechanical system that leads to a nonlinear Kerr effect in the system's vacuum. The oscillating mirror at one edge of the optomechanical system induces a squeezing effect in the intensity spectrum of the cavity field. A near-resonant laser field is applied at the other edge to drive the cavity field in order to enhance the Kerr effect. We also propose a quantum-nondemolition-measurement setup to monitor a system with two cavities separated by a common oscillating mirror based on our effective Hamiltonian approach.

Cooling and squeezing the fluctuations of a nanomechanical beam by indirect quantum feedback control

Abstract: We study cooling and squeezing the fluctuations of a nanomechanical beam using quantum feedback control. In our model, the nanomechanical beam is coupled to a transmission line resonator via a superconducting quantum interference device. The leakage of the electromagnetic field from the transmission line resonator is measured using homodyne detection. This measured signal is then used to design a quantum feedback control signal to drive the electromagnetic field in the transmission line resonator. Although the control is imposed on the transmission line resonator, this quantum feedback control signal indirectly affects the thermal motion of the nanomechanical beam via the inductive beam-resonator coupling, making it possible to cool and squeeze the fluctuations of the beam, allowing it to approach the standard quantum limit.

Quantum switch for single-photon transport in a coupled superconducting transmission-line-resonator array

Abstract: We propose and study an approach to realize quantum switch for single-photon transport in a coupled superconducting transmission-line-resonator (TLR) array with one controllable hopping interaction. We find that the single photon with arbitrary wave vector can transport in a controllable way in this system. We also study how to realize controllable hopping interaction between two TLRs via a Cooper-pair box (CPB). When the frequency of the CPB is largely detuned from those of the two TLRs, the variables of the CPB can be adiabatically eliminated and thus a controllable interaction between two TLRs can be obtained.

Efficient quantum circuits for one-way quantum computing.

Abstract: While Ising-type interactions are ideal for implementing controlled phase flip gates in one-way quantum computing, natural interactions between solid-state qubits are most often described by either the XY or the Heisenberg models. We show an efficient way of generating cluster states directly using either the imaginary SWAP (iSWAP) gate for the XY model, or the sqrt[SWAP] gate for the Heisenberg model. Our approach thus makes one-way quantum computing more feasible for solid-state devices.

Superconducting qubits can be coupled and addressed as trapped ions

Abstract: Exploiting the intrinsic nonlinearity of superconducting Josephson junctions, we propose a scalable circuit with superconducting qubits (SCQs) which is very similar to the successful one now being used for trapped ions. The SCQs are coupled to the ``vibrational'' mode provided by a superconducting $LC$ circuit or its equivalent (e.g., a superconducting quantum interference device). Both single-qubit rotations and qubit-$LC$-circuit couplings and/or decouplings can be controlled by the frequencies of the time-dependent magnetic fluxes. The circuit is scalable since the qubit-qubit interactions, mediated by the $LC$ circuit, can be selectively performed, and the information transfer can be realized in a controllable way.

Generating stationary entangled states in superconducting qubits

Abstract: When a two-qubit system is initially maximally entangled, two independent decoherence channels, one per qubit, would greatly reduce the entanglement of the two-qubit system when it reaches its stationary state. We propose a method on how to minimize such a loss of entanglement in open quantum systems. We find that the quantum entanglement of general two-qubit systems with controllable parameters can be controlled by tuning both the single-qubit parameters and the two-qubit coupling strengths. Indeed, the maximum fidelity ${F}_{\text{max}}$ between the stationary entangled state, ${\ensuremath{\rho}}_{\ensuremath{\infty}}$, and the maximally entangled state, ${\ensuremath{\rho}}_{m}$, can be about $2/3\ensuremath{\approx}\text{max}{\text{tr}({\ensuremath{\rho}}_{\ensuremath{\infty}}{\ensuremath{\rho}}_{m})}={F}_{\text{max}}$, corresponding to a maximum stationary concurrence, ${C}_{\text{max}}$, of about $1/3\ensuremath{\approx}C({\ensuremath{\rho}}_{\ensuremath{\infty}})={C}_{\text{max}}$. This is significant because the quantum entanglement of the two-qubit system can be produced and kept, even for a long time. We apply our proposal to several types of two-qubit superconducting circuits and show how the entanglement of these two-qubit circuits can be optimized by varying experimentally controllable parameters.

Perfect function transfer and interference effects in interacting boson lattices

Abstract: We show how to perfectly transfer, without state initialization and remote collaboration, arbitrary functions in interacting boson lattices. We describe a possible implementation of state transfer through bosonic atoms trapped in optical lattices or polaritons in on-chip coupled cavities. Significantly, a family of Hamiltonians, both linear and nonlinear, is found which are related to the Bose-Hubbard model and that enable the perfect transfer of arbitrary functions. It is shown that the state transfer between two sites in two-dimensional lattices can result in quantum interference due to the different numbers of intermediate sites in different paths. The signature factor in nuclear physics can be useful to characterize this quantum interference.

Universal existence of exact quantum state transmissions in interacting media

Abstract: We consider an exact state transmission, where a density matrix in one information processor A at time t=0 is exactly equal to that in another processor B at a later time. We demonstrate that there always exists a complete set of orthogonal states, which can be employed to perform the exact state transmission. Our result is very general in the sense that it holds for arbitrary media between the two processors and for any time interval. We illustrate our results in terms of models of spin, fermionic, and bosonic chains. This complete set can be used as a basis to study the perfect state transfer which is associated with degenerate subspaces of this set of states. Interestingly, this formalism leads to a proposal of perfect state transfer via adiabatic passage, which does not depend on the specific form of the driving Hamiltonian.

Observation of B0→ΛΛ̄K0 and B0→Λ Λ̄K*0 at Belle

Abstract: We study the charmless decays B -> Lambda(Lambda) over barh , where h stands for pi(+), K+, K-0, K*(+), or K*(0), using a 605 fb(-1) data sample collected at the Gamma(4S) resonance with the Belle detector at the KEKB asymmetric energy e(+)e(-) collider. We observe B-0 -> Lambda(Lambda) over barK(0) and B-0 -> Lambda(Lambda) over barK*(0) with branching fractions of (4.76(-0.68)(+0.84) (stat) +/- 0. 61 (syst)) x 10(-6) and (2.46(-0.72)(+0.87) +/- 0.34) x 10(-6), respectively. The significances of these signals in the threshold-mass enhanced mass region, M Lambda(Lambda) over bar Lambda K+) = (3.38(-0.36)(+0.41) +/- 0.41) x 10(-6) with better accuracy, and report the following measurement or 90% confidence level upper limit in the threshold-mass-enhanced region: B(B+ -> Lambda(Lambda) over barK*(+)) = (2.19(-0.88)(+1.13) +/- 0.33) x 10(-6) with 3.7 sigma significance; B(B+ -> Lambda(Lambda) over bar pi(+)) Lambda (D) over bar0 yields a branching fraction B(B-0 -> Lambda(Lambda) over bar(D) over bar (0)) = (1.05(-0.44)(+0.57) +/- 0.14) x 10(-5). This may be compared with the large, similar to 10(-4), branching fraction observed for B-0 -> p (p) over bar(D) over bar (0). . The M Lambda(Lambda) over bar enhancements near threshold and related angular distributions for the observed modes are also reported.

Phase gate of one qubit simultaneously controlling n qubits in a cavity or coupled to a resonator

Abstract: We propose how to realize a three-step controlled-phase gate of one qubit simultaneously controlling $n$ qubits in a cavity or coupled to a resonator. The $n$ two-qubit controlled-phase gates, forming this multiqubit phase gate, can be performed simultaneously. The operation time of this phase gate is independent of the number $n$ of qubits. This phase gate controlling at once $n$ qubits is insensitive to the initial state of the cavity mode and can be used to produce an analogous CNOT gate simultaneously acting on $n$ qubits. We present two alternative approaches to implement this gate. One approach is based on tuning the qubit frequency while the other method tunes the resonator frequency. Using superconducting qubits coupled to a resonator as an example, we show how to implement the proposed gate with one superconducting qubit simultaneously controlling $n$ qubits selected from $N$ qubits coupled to a resonator ($1

Phase gate of one qubit simultaneously controlling $n$ qubits in a cavity

Abstract: We propose how to realize a three-step controlled-phase gate of one qubit simultaneously controlling $n$ qubits in a cavity or coupled to a resonator. The $n$ two-qubit controlled-phase gates, forming this multiqubit phase gate, can be performed simultaneously. The operation time of this phase gate is independent of the number $n$ of qubits. This phase gate controlling at once $n$ qubits is insensitive to the initial state of the cavity mode and can be used to produce an analogous CNOT gate simultaneously acting on $n$ qubits. We present two alternative approaches to implement this gate. One approach is based on tuning the qubit frequency while the other method tunes the resonator frequency. Using superconducting qubits coupled to a resonator as an example, we show how to implement the proposed gate with one superconducting qubit simultaneously controlling $n$ qubits selected from $N$ qubits coupled to a resonator ($1

Evidence of time-dependent CP violation in the decay B0→D*+D *-

Abstract: We report a measurement of the CP-odd fraction and the time-dependent CP violation in B-0 -> D*+D*- decays, using 657x10(6) BB events collected at the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider. We measure a CP-odd fraction of R-perpendicular to=0.125 +/- 0.043(stat)+/- 0.023(syst). From the distributions of the proper-time intervals between a B-0 -> D*+D*- decay and the other B meson in the event, we obtain evidence of CP violation with measured parameters A(D)(*+)D(*-)=0.15 +/- 0.13(stat)+/- 0.04(syst) and SD*+D*-=-0.96 +/- 0.25(stat)(-0.16)(+0.13)(syst).

Probing Nano-Mechanical QED Effects

Abstract: Center for Theoretical Physics, Physics Department, Center for the Study of Complex Systems,The University of Michigan, Ann Arbor, Michigan 48109-1040, USA(Dated: February 15, 2009)We propose and study an “intrinsic probing” approach, without introducing any external detector,to mimic cavity QED effects in a qubit-nanomechanical resonator system This metallic nanome-chanical resonator can act as an intrinsic detector when a weak driving current passes through itThe nanomechanical resonator acts as both the cavity and the detector A cavity QED-like effectis demonstrated by the correlation spectrum of the electromotive force between the two ends ofthe nanomechanical resonator Using the quantum regression theorem and perturbation theory, weanalytically calculate the correlation spectrum In the weak driving limit, we study the effect on thevacuum Rabi splitting of both the strength of the driving as well as the frequency-detuning betweenthe charge qubit and the nanomechanical resonator Numerical calculations confirm the validity ofour intrinsic probing approach

Quantum-feedback-control induced bifurcation and its application to qubit readout

Abstract: Sensitive measurement of the state of a qubit is crucial to the implementation of quantum information processing. This paper proposes a strategy to utilize a static bifurcation induced by nonlinear quantum feedback control to read out the state of a charge qubit that is dispersively coupled to a transmission line resonator. Compared with the traditional dispersive qubit readout strategy, the main advantage of our proposal is that even a weak qubit-resonator coupling is hopeful to lead to a strong measurement by varying the nonlinear feedback control parameter. This work opens up new perspectives for the applications of nonlinear quantum feedback control.

Spin-squeezing suddenly vanishes under decoherence

Abstract: Xiaoguang Wang, 2 Adam Miranowicz, 3 Yu-xi Liu, 4 C. P. Sun, and Franco Nori 6 Advanced Science Institute, The Institute of Physical and Chemical Research (RIKEN), Wako-shi, Saitama 351-0198, Japan Zhejiang Institute of Modern Physics, Department of Physics, Zhejiang University, Hangzhou 310027, China Faculty of Physics, Adam Mickiewicz University, 61-614 Poznan, Poland Institute of Microelectronics and Tsinghua National Laboratory for Information Science and Technology, Tsinghua University, Beijing 100084, China Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing 100190, China Department of Physics, Center for Theoretical Physics, The University of Michigan, Ann Arbor, Michigan 48109-1120, USA