arXiv subjects

Yu-Xiang Zhang

Publications and source records attributed to Yu-Xiang Zhang.

27 records · Page 2Linked to original sources

arXiv2019

Theory of Subradiant States of a One-Dimensional Two-Level Atom Chain

Recently, the subradiant states of one-dimensional two-level atom chains coupled to light modes were found to have decay rates obeying a universal scaling, and an unexpected fermionic character of the multiply-excited subradiant states was discovered. In this Letter, we theoretically obtain the singly-excited subradiant states, and by eliminating the superradiant modes, we demonstrate a relation between the multiply-excited subradiant states and the Tonks-Girardeau limit of the Lieb-Liniger model which explains the fermionic behavior. In addition, we identify a new family of states with correlations different from the fermionic ansatz.

quant-ph↗

arXiv2018

Monte-Carlo Simulations of Superradiant Lasing

We present simulations of the superradiant dynamics of ensembles of atoms in the presence of collective and individual atomic decay processes. We unravel the density matrix with Monte-Carlo wave-functions and identify the quantum jumps in a reduced Dicke state basis, which reflects the permutation symmetry of the identical atoms. While the number of density matrix elements in the Dicke representation increases polynomially with atom number, the quantum jump dynamics populates only a single Dicke state at the time and thus efficient simulations can be carried out for tens of thousands of atoms. The calculated superradiance pulses from initially excited atoms agree quantitatively with recent experimental results with strontium atoms but rapid atom loss in these experiments does not permit steady-state superradiance. By introducing an incident flux of new atoms, the system can maintain a large average atom number, and our theoretical calculations predict lasing with millihertz linewidth despite rapid atom number fluctuations.

physics.optics↗

arXiv2018

Dicke Phase Transition in a Disordered Emitter-Graphene Plasmon System

We study the Dicke phase transition in a disordered system of emitters coupled to the plasmonic modes of a graphene monolayer. This system has unique properties associated with the tunable, dissipative and broadband characters of the graphene surface plasmons, as well as the disorder due to the random spatial distribution and the inhomogeneous line-width broadening of the emitters. We apply the Keldysh functional-integral approach, and identify a normal phase, a superradiant phase and a spin-glass phase of the system. The conditions for these phases and their experimental signatures are discussed.

quant-ph↗

arXiv2016

Direct state reconstruction with coupling-deformed pointer observables

Direct state tomography (DST) using weak measurements has received wide attention. Based on the concept of coupling-deformed pointer observables presented by Zhang \emph{et al}.[Phys. Rev. A \textbf{93}, 032128 (2016)], a modified direct state tomography (MDST) is proposed, examined, and compared with other typical state tomography schemes. MDST has exact validity for measurements of any strength. We identify the strength needed to attain the highest efficiency level of MDST by using statistical theory. MDST is much more efficient than DST in the sense that far fewer samples are needed to reach DST's level of reconstruction accuracy. Moreover, MDST has no inherent bias when compared to DST.

quant-ph↗

arXiv2016

Ground-state cooling of dispersively coupled optomechanical system in unresolved sideband regime via dissipatively coupled oscillator

In the optomechanical cooling of a dispersively coupled oscillator, it is only possible to reach the oscillator ground state in the resolved sideband regime, where the cavity-mode line width is smaller than the resonant frequency of the mechanical oscillator being cooled. In this paper, we show that the dispersively coupled system can be cooled to the ground state in the unresolved sideband regime using an ancillary oscillator, which is coupled to the same optical mode via dissipative interaction. The ancillary oscillator has a resonant frequency close to that of the target oscillator; thus, the ancillary oscillator is also in the unresolved sideband regime. We require only a single blue-detuned laser mode to drive the cavity.

quant-ph↗

arXiv2016

Coupling-Deformed Pointer Observables and Weak Values

While the novel applications of weak values have recently attracted wide attention, weak measurement, the usual way to extract weak values, suffers from risky approximations and severe quantum noises. In this paper, we show that the weak-value information can be obtained exactly in strong measurement with postselections, via measuring the coupling-deformed pointer observables, i.e., the observables selected according to the coupling strength. With this approach, we keep all the advantages claimed by weak-measurement schemes and at the same time solve some widely criticized problems thereof, such as the questionable universality, systematical bias, and drastic inefficiency.

quant-ph↗

arXiv2015

The amplification of weak measurements under quantum noise

The influence of outside quantum noises on the amplification of weak measurements is investigated. Three typical quantum noises are discussed. The maximum values of the pointer's shifts decrease sharply with the strength of the depolarizing channel and phase damping. In order to obtain significant amplified signals, the preselection quantum systems must be kept away from the two quantum noises. Interestingly, the amplification effect is immune to the amplitude damping noise.

quant-ph↗

arXiv2014

Quantum Uncertainty and Error-Disturbance Tradeoff

The uncertainty principle is often interpreted by the tradeoff between the error of a measurement and the consequential disturbance to the followed ones, which originated long ago from Heisenberg himself but now falls into reexamination and even heated debate. Here we show that the tradeoff is switched on or off by the quantum uncertainties of two involved non-commuting observables: if one is more certain than the other, there is no tradeoff; otherwise, they do have tradeoff and the Jensen-Shannon divergence gives it a good characterization.

quant-ph↗

arXiv2013

Direct and Parallel Tomography of a Quantum Process

As the method to completely characterize quantum dynamical processes, quantum process tomography (QPT) is vitally important for quantum information processing and quantum control, where the faithfulness of quantum devices plays an essential role. Here via weak measurements, we present a new QPT scheme characterized by its directness and parallelism. Comparing with the existing schemes, our scheme needs a simpler state preparation and much fewer experimental setups. Furthermore, each parameter of the quantum process is directly determined from only five experimental values in our scheme, meaning that our scheme is robust against the accumulation of errors.

quant-ph↗