arXiv subjects

Cheng-Zu Li

Publications and source records attributed to Cheng-Zu Li.

At least 19 recordsLinked to original sources

arXiv2012

Classification of entanglement and quantum phase transition in XX model

We study the relation between entanglement and quantum phase transition (QPT) from a new perspective. Motivated by one's intuition: QPT is characterized by the change of the ground-state structure, while entangled states belong to different classes have different structures, we conjecture that QPT occurs as the class of ground-state entanglement changes and prove it in XX model. Despite the classification of multipartite entanglement is yet unresolved, we proposed a new method to judge whether two many-body states belong to the same entanglement class.

quant-ph↗

arXiv2011

Scalable one-way quantum computer using on-chip resonator qubits

We propose a scalable and robust architecture for one-way quantum computation using coupled networks of superconducting transmission line resonators. In our protocol, quantum information is encoded into the long-lived photon states of the resonators, which have a much longer coherence time than the usual superconducting qubits. Each resonator contains a charge qubit used for the state initialization and local projective measurement of the photonic qubit. Any pair of neighboring photonic qubits are coupled via a mediator charge qubit, and large photonic cluster states can be created by applying Stark-shifted Rabi pulses to these mediator qubits. The distinct advantage of our architecture is that it combines both the excellent scalability of the solid-state systems and the long coherence time of the photonic qubits. Furthermore, this architecture is very robust against the parameter variations.

quant-ph↗

arXiv2011

Large detuning limit for the multipartite systems interacting with electromagnetic fields

We study the dynamics of the multipartite systems nonresonantly interacting with electromagnetic fields and show that, since the coupling strength $g$ is collectively enhanced by the square root of the number of microparticles involved, the more rigorous large detuning condition for neglecting the rapidly oscillating terms for the effective Hamiltonian should be $Δ\gg\sqrt{N}g$, instead of $Δ\gg g$ usually used in the literature even in the case of multipartite systems, with $Δ$ the detuning and and $N$ the number of microparticles. This result is significant since merely the satisfaction of the original condition will result in the invalidity of the effective Hamiltonian and the errors of the parameters associated with the detuning in the multipartite case.

quant-ph↗

arXiv2011

Quantum superposition of localized and delocalized phases of photons

Based on a variant of 2-site Jaynes-Cummings-Hubbard model, which is constructed using superconducting circuits, we propose a method to coherently superpose the localized and delocalized phases of photons. In our model, two nonlinear superconducting stripline resonators are coupled by an interfacial circuit composed of parallel combination of a superconducting qubit and a capacitor, which plays the role of a quantum knob for the photon hopping rate: with the knob qubit in its ground/excited state, the injected photons tend to be localized/delocalized in the resonators. We show that, by applying a microwave field with appropriate frequency on the knob qubit, we could demonstrate Rabi oscillation between photonic localized phase and delocalized phase. Furthermore, this set-up offers advantages (e. g. infinite on/off ratio) over other proposals for the realization of scalable quantum computation with superconducting qubits.

quant-ph↗

arXiv2011

Quantum phase transition of light in a 1-D photon-hopping-controllable resonator array

We give a concrete experimental scheme for engineering the insulator-superfluid transition of light in a one-dimensional (1-D) array of coupled superconducting stripline resonators. In our proposed architecture, the on-site interaction and the photon hopping rate can be tuned independently by adjusting the transition frequencies of the charge qubits inside the resonators and at the resonator junctions, respectively, which permits us to systematically study the quantum phase transition of light in a complete parameter space. By combining the techniques of photon-number-dependent qubit transition and fast read-out of the qubit state using a separate low-Q resonator mode, the statistical property of the excitations in each resonator can be obtained with a high efficiency. An analysis of the various decoherence sources and disorders shows that our scheme can serve as a guide to coming experiments involving a small number of coupled resonators.

quant-ph↗

arXiv2010

Quantum repeaters based on Rydberg-blockade coupled atomic ensembles

We propose a scheme for realizing quantum repeaters with Rydberg-blockade coupled atomic ensembles, based on a recently proposed collective encoding strategy. Rydberg-blockade mediated two-qubit gates and efficient cooperative photon emission are employed to create ensemble-photon entanglement. Thanks to deterministic entanglement swapping operations via Rydberg-based two-qubit gates, and to the suppression of multi-excitation errors by the blockade effect, the entanglement distribution rate of the present scheme is higher by orders of magnitude than the rates achieved by other ensemble-based repeaters. We also show how to realize temporal multiplexing with this system, which offers an additional speedup in entanglement distribution.

quant-ph↗

arXiv2009

Experimental verification of statistical correlation for bosons: Another kind of Hong-Ou-Mandel interference

According to the identity principle in quantum theory, states of a system consisted of identical particles should maintain unchanged under interchanging between two of the particles. The whole wavefunction should be symmetrized or antisymmetrized. This leads to statistical correlations between particles, which exhibit observable effects. We design an experiment to directly observe such effects for bosons. The experiment is performed with two photons. The effect of statistical correlations is clearly observed when the wavepackets of two photons are completely overlapped, and this effect varies with the degree of overlapping. The results of our experiment substantiate the statistical correlation in a simple way. Experiment reported here can also be regarded as another kind of two-photon Hong-Ou-Mandel interference, occurs in the polarization degree of freedom of photon.

quant-ph↗

arXiv2009

Deterministic remote preparation of arbitrary photon polarization states

We propose a deterministic remote state preparation scheme for photon polarization qubit states, where entanglement, local operations and classical communication are used. By consuming one maximally entangled state and two classical bits, an arbitrary (either pure or mixed) qubit state can be prepared deterministically at a remote location. We experimentally demonstrate the scheme by remotely preparing 12 pure states and 6 mixed states. The fidelities between the desired and achieved states are all higher than 0.99 and have an average of 0.9947.

quant-ph↗

arXiv2009

Robust quantum repeater with atomic ensembles against phase and polarization instability

We propose an alternative scheme for quantum repeater without phase stabilization and polarization calibration of photons transmitted over long-distance channel. We introduce time-bin photonic states and use a new two-photon interference configuration to robustly generate entanglement between distant atomic-ensemble-based memory qubits. Our scheme can be performed with current experimental setups through making some simple adjustments.

quant-ph↗

arXiv2008

Quantum Coherence Effects in Four-level Diamond Atomic System

A symmetric four-level closed-loop $\diamondsuit$ type (the diamond structure) atomic system driven by four coherent optical fields is investigated. The system shows rich quantum interference and coherence features. When symmetry of the system is broken, interesting phenomena such as single and double dark resonances appear. As a result, the double electromagnetically induced transparency effect is generated, which will facilitate the implementation of quantum phase gate operation.

quant-ph↗

arXiv2008

Long distance quantum key distribution without decoy state

We present a method without decoy state to estimate faithfully and efficiently the yield and quantum bit error rate of single photon pulse for BB84 protocol. The numerical calculations show that key generation rate and maximal secure communication distance of our method are better than results of the decoy state method. As we know, it is the first time we present this method and the idea is very novel. Furthermore, our method can overcome the drawbacks of decoy state method. In our method, the maximal secure distance is insensitive to the deviation of dark count rate from exact value.

quant-ph↗

arXiv2007

Coherence Induced by Incoherent Pumping Field and Decay Process in Three-level $Λ$ Type Atomic System

Following the method of Victor V. Kozlov et al.[PhysRevA. 74. 063829],we inspect the coherence induced by incoherent pump and spontaneous decay process in $Λ$ type three-level atomic system with degenerated lower duplicate levels. The system shows a coherent population trapping state and multi-steady states characteristic in different conditions. Interestingly, two kinds of steady states generated by the system in different sets of pumping and decaying parameters, the "singlet" state and the "triplet" state, exhibit stable or unstable characteristics under the action of pumping field and vacuum reservoir, which promise fruitful applications to atomic coherence and interference in quantum optics.

quant-ph↗

arXiv2003

Local distinguishability of quantum states and the distillation of entanglement

This paper tries to probe the relation between the local distinguishability of orthogonal quantum states and the distillation of entanglement. An new interpretation for the distillation of entanglement and the distinguishability of orthogonal quantum states in terms of information is given, respectively. By constraining our discussion on a special protocol we give a necessary and sufficient condition for the local distinguishability of the orthogonal pure states, and gain the maximal yield of the distillable entanglement. It is shown that the information entropy, the locally distinguishability of quantum states and the distillation of entanglement are closely related.

quant-ph↗

arXiv2002

A Necessary And Sufficient Condition of Distillability with unite fidelity from Finite Copies of a Mixed State: The Most Efficient Purification Protocol

It is well known that any entangled mixed state in $2\otimes 2$ systems can be purified via infinite copies of the mixed state. But can one distill a pure maximally entangled state from finite copies of a mixed state in any bipartite system by local operation and classical communication? This is more meaningful in practical application. We give a necessary and sufficient condition of this distillability. This condition can be expressed as: there exists distillable-subspaces. According to this condition, one can judge whether a mixed state is distillable or not easily. We also analyze some properties of distillable-subspaces, and discuss the most efficient purification protocols. Finally, we discuss the distillable enanglement of two-quibt system for the case of finite copies.

quant-ph↗

arXiv2002

Criterion for distinguishability of arbitrary bipartite orthogonal states

In this paper we present a necessary and sufficient condition of distinguishability of bipartite quantum states. It is shown that the operators to reliably distinguish states need only rounds of projective measurements and classical comunication. We also present a necessary condition of distinguishability of bipartite quantum states which is simple and general. With this condition one can get many cases of indistinguishability. The conclusions may be useful in understanding the essence of nonlocality and calculating the distillable entanglement and the bound of distillable entanglement.

quant-ph↗

arXiv2002

Improved analysis for the baryon masses to order Lambda_{QCD}/m_Q from QCD sum rules

We use the QCD sum rule approach to calculate the masses of the Lambda_Q and Sigma_Q baryons to the Lambda_{QCD}/m_Q order within the framework of heavy quark effective theory. We compare the direct approach and the covariant approach to this problem. Two forms of currents have been adopted in our calculation and their effects on the results are discussed. Numerical results obtained in both direct and covariant approaches are presented. The splitting between spin 1/2 and 3/2 doublets derived from our calculation is {Σ_Q^*}^2-Σ_Q^2\simeq 0.35\pm 0.03 GeV^2 which is in good agreement with the experiment.

hep-ph↗

arXiv2001

Impossibility Criterion for Obtaining Pure Entangled States From Mixed States By Purifying Protocols

Purifying noisy entanglement is a protocol which can increase the entanglement of a mixed state (as a source)at expense of the entanglement of others(as an ancilla)by collective measurement. A protocol with which one can get a pure entangled state from a mixed state is defined as purifying mixed states. We address a basic question: can one get a pure entangled state from a mixed state? We give a necessary and sufficient condition of purifying a mixed state by fit local operations and classical communication and show that for a class of source states and ancilla states in arbitrary bipartite systems purifying mixed states is impossible by finite rounds of purifying protocols. For $2\otimes 2$ systems, it is proved that arbitrary states cannot be purified by individual measurement. The possible application and meaning of the conclusion are discussed.

quant-ph↗

arXiv2001

Reply to Comment

In the comment(quant-ph/0103003) Eggeling,Vollbrecht and Wolf suspect our method in quant-ph/0102133 is not practical. Here we explain our result and method and show that our example can tell one how to judge a separable state, and so our method is practical, at least for many mixed states.

quant-ph↗