arXiv subjects

Yu-xi Liu

Publications and source records attributed to Yu-xi Liu.

At least 145 records · Page 8Linked to original sources

arXiv2009

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

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 wavevector can transport in a controllable way in this system. We also study how to realize controllable hopping interaction between two TLRs via a superconducting quantum interference device (SQUID). When the frequency of the SQUID is largely detuned from those of the two TLRs, the variables of the SQUID can be adiabatically eliminated and thus a controllable interaction between two TLRs can be obtained.

quant-ph↗

arXiv2009

Multi-Stability of Electromagnetically Induced Transparency in Atom-Assisted Optomechanical Cavities

We study how an oscillating mirror affects the electromagnetically induced transparency (EIT) of an atomic ensemble, which is confined in a gas cell placed inside a micro-cavity with an oscillating mirror in one end. The oscillating mirror is modeled as a quantum mechanical harmonic oscillator. The cavity field acts as a probe light of the EIT system and also produces a light pressure on the oscillating mirror. The back-action from the mirror to the cavity field results in several (from one to five) steady-states for this atom-assisted optomechanical cavity, producing a complex structure in its EIT. We calculate the susceptibility with respect to the few (from one to three) stable solutions found here for the equilibrium positions of the oscillating mirror. We find that the EIT of the atomic ensemble can be significantly changed by the oscillating mirror, and also that the various steady states of the mirror have different effects on the EIT.

quant-ph↗

arXiv2009

Universal Existence of Exact Quantum State Transmissions in Interacting Media

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 bases to study the perfect state transfer, which is associated with degenerated 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.

quant-ph↗

arXiv2009

Probing Nano-Mechanical QED Effects

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 nanomechanical resonator can act as an intrinsic detector when a weak driving current passes through it. The nanomechanical resonator acts as both the cavity and the detector. A cavity QED-like effect is demonstrated by the correlation spectrum of the electromotive force between the two ends of the nanomechanical resonator. Using the quantum regression theorem and perturbation theory, we analytically calculate the correlation spectrum. In the weak driving limit, we study the effect on the vacuum Rabi splitting of both the strength of the driving as well as the frequency-detuning between the charge qubit and the nanomechanical resonator. Numerical calculations confirm the validity of our intrinsic probing approach.

quant-ph↗

arXiv2009

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

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 (SQUID). 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.

quant-ph↗

arXiv2008

Quantum super-cavity with atomic mirrors

We study single-photon transport in an array of coupled microcavities where two two-level atomic systems are embedded in two separate cavities of the array. We find that a single-photon can be totally reflected by a single two-level system. However, two separate two-level systems can also create, between them, single-photon quasi-bound states. Therefore, a single two-level system in the cavity array can act as a mirror while a different type of cavity can be formed by using two two-level systems, acting as tunable "mirrors", inside two separate cavities in the array. In analogy with superlattices in solid state, we call this new "cavity inside a coupled-cavity array" a super-cavity. This supercavity is the quantum analog of Fabry-Perot interferometers. Moreover, we show that the physical properties of this quantum super-cavity can be adjusted by changing the frequencies of these two-level systems.

quant-ph↗

arXiv2008

Controllable scattering of photons inside a one-dimensional resonator waveguide

We analyze coherent transport of photons, which propagate in a one-dimensional coupled-resonator waveguide (CRW) and are scattered by a controllable two-level system located inside the CRW. Our approach, which uses discrete coordinates, unifies "low" and "high" energy effective theories for single photon scattering. We show that the controllable two-level system can behave as a quantum switch for the coherent transport of photons. This study may inspire new electro-optical single-photon quantum devices. We also suggest an experimental setup based on superconducting transmission line resonators and qubits

cond-mat.supr-con↗

arXiv2008

Protecting entanglement in superconducting qubits

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 protected by tuning both the single-qubit parameters and the two-qubit coupling strengths. Indeed, the maximum fidelity $F_{\rm max}$ between the stationary entangled state, $ρ_{\infty}$, and the maximally-entangled state, $ρ_m$, can be about $2/3\approx\max\{{\rm tr}(ρ_{\infty}ρ_m)\}=F_{\rm max}$, corresponding to a maximum stationary concurrence, $C_{\rm max}$, of about $1/3\approx C(ρ_{\infty})=C_{\rm max}$. This is significant because the quantum entanglement of the two-qubit system can be protected, 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.

quant-ph↗

arXiv2008

Efficient purification protocols using iSWAP gates in solid-state qubits

We show an efficient purification protocol in solid-state qubits by replacing the usual bilateral CNOT gate by the bilateral iSWAP gate. We also show that this replacement can be applied to breeding and hashing protocols, which are useful for quantum state purification. These replacements reduce the number of fragile and cumbersome two-qubit operations, making more feasible quantum-information-processing with solid-state qubits. As examples, we also present quantitative analyses for the required time to perform state purification using either superconducting or semiconducting qubits.

quant-ph↗

arXiv2008

Efficient quantum circuits for one-way quantum computing

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 iSWAP gate for the XY model, or the $\sqrt{\rm SWAP}$ gate for the Heisenberg model. Our approach thus makes one-way quantum computing more feasible for solid-state devices.

cond-mat.mes-hall↗

arXiv2008

Cavity optomechanical coupling assisted by an atomic gas

We theoretically study a cavity filled with atoms, which provides the optical-mechanical interaction between the modified cavity photonic field and a movable mirror at one end. We show that the cavity field ``dresses'' these atoms, producing two types of polaritons, effectively enhancing the radiation pressure of the cavity field upon the end mirror, as well as establishing an additional squeezing mode of the end mirror. This squeezing produces an adiabatic entanglement, which is absent in usual vacuum cavities, between the oscillating mirror and the rest of the system. We analyze the entanglement and quantify it using the Loschmidt echo and fidelity.

quant-ph↗

arXiv2008

Control of photon propagation via electromagnetically induced transparency in lossless media

We study the influence of a lossless material medium on the coherent storage and quantum state transfer of a quantized probe light in an ensemble of $Λ$-type atoms. The medium is modeled as uniformly distributed two-level atoms with same energy level spacing, coupling to a probe light. This coupled system can be simplified to a collection of two-mode polaritons which couple to one transition of the $Λ$-type atoms. We show that, when the other transition of $Λ$-type atoms is controlled by a classical light, the electromagnetically induced transparency can also occur for the polaritons. In this case the coherent storage and quantum transfer for photon states are achievable through the novel dark states with respect to the polaritons. By calculating the corresponding dispersion relation, we find the ensemble of the three-level atoms with $Λ$-type transitions may serve as quantum memory for it slows or even stops the light propagation through the mechanism of electromagnetically induced transparency. the corresponding dispersion relation, we find the ensemble of the three-level atoms with $Λ$-type transitions may serve as quantum memory for it slows or even stops the light propagation through the mechanism of electromagnetically induced transparency.

quant-ph↗

arXiv2008

Simultaneous cooling of an artificial atom and its neighboring quantum system

We propose an approach for cooling both an artificial atom (e.g., a flux qubit) and its neighboring quantum system, the latter modeled by either a quantum two-level system or a quantum resonator. The flux qubit is cooled by manipulating its states, following an inverse process of state population inversion, and then the qubit is switched on to resonantly interact with the neighboring quantum system. By repeating these steps, the two subsystems can be simultaneously cooled. Our results show that this cooling is robust and effective, irrespective of the chosen quantum systems connected to the qubit.

cond-mat.supr-con↗

arXiv2007

Superconducting qubits can be coupled and addressed as trapped ions

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 SQUID). Both single-qubit rotations and qubit-LC-circuit couplings/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.

cond-mat.supr-con↗

arXiv2007

Cooling a Micromechanical Beam by Coupling it to a Transmission Line

We study a method to cool down the vibration mode of a micro-mechanical beam using a capacitively-coupled superconducting transmission line. The Coulomb force between the transmission line and the beam is determined by the driving microwave on the transmission line and the displacement of the beam. When the frequency of the driving microwave is smaller than that of the transmission line resonator, the Coulomb force can oppose the velocity of the beam. Thus, the beam can be cooled. This mechanism, which may enable to prepare the beam in its quantum ground state of vibration, is feasible under current experimental conditions.

cond-mat.mes-hall↗

arXiv2007

Teleportation of qubit states through dissipative channels: Conditions for surpassing the no-cloning limit

We investigate quantum teleportation through dissipative channels and calculate teleportation fidelity as a function of damping rates. It is found that the average fidelity of teleportation and the range of states to be teleported depend on the type and rate of the damping in the channel. Using the fully entangled fraction, we derive two bounds on the damping rates of the channels: one is to beat the classical limit and the second is to guarantee the non-existence of any other copy with better fidelity. Effect of the initially distributed maximally entangled state on the process is presented; and the concurrence and the fully entangled fraction of the shared states are discussed. We intend to show that prior information on the dissipative channel and the range of qubit states to be teleported is helpful for the evaluation of the success of teleportation, where success is defined as surpassing the fidelity limit imposed by the fidelity of 1-to-2 optimal cloning machine for the specific range of qubits.

quant-ph↗

arXiv2007

Two-Mode Squeezed States and Entangled States of Two Mechanical Resonators

We study a device consisting of a dc-SQUID with two sections of its loop acting as two mechanical resonators. An analog of the parametric down-conversion process in quantum optics can be realized with this device. We show that a two-mode squeezed state can be generated for two overdamped mechanical resonators, where the damping constants of the two mechanical resonators are larger than the coupling strengths between the dc-SQUID and the two mechanical resonators. Thus we show that entangled states of these two mechanical resonators can be generated.

quant-ph↗

arXiv2007

Switchable coupling between charge and flux qubits

We propose a hybrid quantum circuit with both charge and flux qubits connected to a large Josephson junction that gives rise to an effective inter-qubit coupling controlled by the external magnetic flux. This switchable inter-qubit coupling can be used to transfer back and forth an arbitrary superposition state between the charge qubit and the flux qubit working at the optimal point. The proposed hybrid circuit provides a promising quantum memory because the flux qubit at the optimal point can store the tranferred quantum state for a relatively long time.

cond-mat.supr-con↗