Physical Review A

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ISSN / EISSN : 2469-9926 / 2469-9934
Published by: American Physical Society (APS) (10.1103)
Total articles ≅ 14,416
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, Heng Wang, Yaodi Pi, Wei Huang, Yang Li, Jinlu Liu, Jie Yang, , Bingjie Xu
Published: 14 September 2021
Physical Review A, Volume 104; https://doi.org/10.1103/physreva.104.032608

Abstract:
The value of residual phase noise, after phase compensation, is one of the key limitations of performance improvement for continuous-variable quantum key distribution using a local local oscillator (LLO CV-QKD) system, since it is the major excess noise. However, due to the nonideality of the realistic devices implemented in practice, for example, imperfect lasers, detectors, and unbalanced interferometers, the value of residual phase noise in the current system is still relatively large. Here, we develop a phase noise model to improve the phase noise tolerance of the LLO CV-QKD schemes. In our model, part of the phase-reference measurement noise associated with detection efficiency and electronic noise of Bob's detector as well as a real-time monitored phase-reference intensity at Bob's side is considered trusted because it can be locally calibrated by Bob. We show that using our phase noise model can significantly improve the secure key rate and transmission distance of the LLO CV-QKD system. We further conduct an experiment to substantiate the superiority of the phase noise model. Based on experimental data of a LLO CV-QKD system in the 25-km optical fiber channel, we demonstrate that the secure key rate under our phase noise model is approximately 40% higher than that under the conventional phase noise model.
, Tomotaro Namba, , Kenji Ohmori
Published: 14 September 2021
Physical Review A, Volume 104; https://doi.org/10.1103/physreva.104.033107

Abstract:
We use nonlinear optimal control simulation to systematically examine how to suppress wave-packet spreading with strong nonresonant near-infrared (NIR) laser pulses through a case study of a vibrational wave packet in the B state of I2. As the degree of spreading of the vibrational wave packet is controllable by adjusting the pump pulse shape, the vibrational wave packet may be regarded as a prototype system to study spreading suppression by using nonresonant laser pulses. We quantitatively define spreading suppression in terms of the probability of finding the wave packet in the analytically defined, initially excited state at every vibrational period. From optimal control simulation and simulation by using model Gaussian pulse trains that mimic optimal pulses, we conclude that a simple periodic NIR pulse train without highly tuned temporal widths, amplitudes, or irradiation timings can almost perfectly stop the wave-packet spreading over a long control period, provided that the degree of spreading is not too large.
, R. Srinivas
Published: 14 September 2021
Physical Review A, Volume 104; https://doi.org/10.1103/physreva.104.032609

Abstract:
Using discrete and continuous variable subsystems, hybrid approaches to quantum information could enable more quantum computational power for the same physical resources. Here, we propose a hybrid scheme that can be used to generate the necessary Gaussian and non-Gaussian operations for universal continuous variable quantum computing in trapped ions. This scheme utilizes two linear spin-motion interactions to generate a broad set of nonlinear effective spin-motion interactions including one- and two-mode squeezing, beam splitter, and trisqueezing operations in trapped ion systems. We discuss possible experimental implementations using laser-based and laser-free approaches.
, Samudra Roy, Shailendra K. Varshney
Published: 14 September 2021
Physical Review A, Volume 104; https://doi.org/10.1103/physreva.104.033514

Abstract:
Understanding the intracavity field dynamics in passive microresonator systems has already been intriguing. It becomes fascinating when the system is complex, such as a concentric dual microring resonator that exhibits avoided mode crossing (AMC). In this work, we present a systematic study of intracavity oscillatory field dynamics near AMC in a concentric silicon-nitride microring resonator with the help of coupled Lugiato-Lefever equation. We identify two regions where the hybrid modes are strongly coupled and weakly coupled based on their eigenfrequency separation, which originate from mode coupling near AMC. We calculate the overlap integral to identify the cutoff pump wavelength region to observe the existence of the two hybrid modes. In a strongly coupled hybrid mode region, we observe intracavity power oscillation and transfer of energy between the two hybrid modes in a periodic manner. We also evaluate the nonidentical phase variation of these two modes. In a weakly coupled hybrid mode region, power oscillation and energy transfer between modes reduces significantly, whereas their phases vary in almost identical fashion. We validate our numerical findings with the semianalytical variational method, leading to an in-depth understanding of the mode-coupling-induced dynamics. We finally analyze the polarization properties of the field confined in the coupled system. Exploiting the Stokes parameters and Jones vector, we deduce a polarization-evolving state and a polarization-locked state in strongly coupled region and weakly coupled region, respectively.
K. H. Spicer, C. T. Plowman, , , , Sh. U. Alladustov
Published: 14 September 2021
Physical Review A, Volume 104; https://doi.org/10.1103/physreva.104.032818

Abstract:
In a series of papers we present a comprehensive investigation of the four-body proton-helium differential scattering problem using the two-center wave-packet convergent close-coupling approach in the intermediate energy region where coupling between various channels is important. The approach uses correlated two-electron wave functions for the helium target. For comparison, a recently developed method that reduces the target to an effective single-electron system is also used. In this paper we present calculations of angular differential cross sections for elastic-scattering, target excitation, and electron-capture processes. The results of the two-electron and effective single-electron methods exhibit a good level of agreement. They also agree well with available experimental data. It is concluded that both versions of the wave-packet convergent close-coupling approach are capable of providing a realistic differential picture of all interdependent and interconnected binary processes taking place in proton-helium collisions at intermediate energies.
, Mohit Lal Bera, Arnau Riera, Maciej Lewenstein,
Published: 13 September 2021
Physical Review A, Volume 104; https://doi.org/10.1103/physreva.104.032411

Abstract:
A reasonable quantum information theory for fermions must respect the parity superselection rule to comply with the special theory of relativity and the no-signaling principle. This rule restricts the possibility of any quantum state to have a superposition between even- and odd-parity fermionic states. thereby characterizing the set of physically allowed fermionic quantum states. Here we introduce the physically allowed quantum operations, in congruence with the parity superselection rule, that map the set of allowed fermionic states onto itself. We first introduce unitary and projective measurement operations of the fermionic states. We further extend the formalism to general quantum operations in the forms of Stinespring dilation, operator-sum representation, and axiomatic completely positive and trace-preserving maps. We explicitly show the equivalence between these three representations of fermionic quantum operations. We discuss the possible implications of our results in characterization of correlations in fermionic systems.
Q. Y. Cai,
Published: 13 September 2021
Physical Review A, Volume 104; https://doi.org/10.1103/physreva.104.033710

Abstract:
We investigate the single-photon scattering spectra of a giant atom coupled to a one-dimensional waveguide via multiple connection points or a continuous coupling region. Using a full quantum mechanical method, we obtain the general analytic expressions for the single-photon scattering coefficients, which are valid in both the Markovian and the non-Markovian regimes. We summarize the influences of the nondipole effects, mainly caused by the phases accumulated by photons traveling between coupling points, on the scattering spectra. We find that under the Markovian limit, the phase decay is detuning independent, resulting in Lorentzian line shapes characterized by the Lamb shifts and the effective decay rates, while in the non-Markovian regime, the accumulated phases become detuning dependent, giving rise to non-Lorentzian line shapes, characterized by multiple side peaks and total transmission points. Another interesting phenomenon in the non-Markovian regime is the generation of a broad photonic band gap by a single giant atom. We further generalize the case of discrete coupling points to the continuum limit with atom coupling to the waveguide via a continuous area, and analyze the scattering spectra for some typical distributions of coupling strength.
, Kevin Keiler, , Peter Schmelcher
Published: 13 September 2021
Physical Review A, Volume 104; https://doi.org/10.1103/physreva.104.033315

Abstract:
We demonstrate that an ultracold many-body bosonic ensemble confined in a one-dimensional double-well potential can exhibit chaotic dynamics due to the presence of a single impurity. The nonequilibrium dynamics is triggered by a quench of the impurity-Bose interaction and is illustrated via the evolution of the population imbalance for the bosons between the two wells. While the increase of the postquench interaction strength always facilitates the irregular motion for the bosonic population imbalance, it becomes regular again when the impurity is initially populated in the highly excited states. Such an integrability to chaos (ITC) transition is fully captured by the transient dynamics of the corresponding linear entanglement entropy, whose infinite-time-averaged value additionally characterizes the edge of the chaos and implies the existence of an effective Bose-Bose attraction induced by the impurity. To elucidate the physical origin for the observed ITC transition, we perform a detailed spectral analysis for the mixture with respect to both the energy spectrum as well as the eigenstates. Specifically, two distinguished spectral behaviors upon a variation of the interspecies interaction strength are observed. While the avoided level crossings take place in the low-energy spectrum, the energy levels in the high-energy spectrum possess a bandlike structure and are equidistant within each band. This leads to a significant delocalization of the low-lying eigenvectors, which, in turn, accounts for the chaotic nature of the bosonic dynamics. By contrast, those highly excited states bear a high resemblance to the noninteracting integrable basis, which explains the recovery of the integrability for the bosonic species. Finally, we discuss the induced Bose-Bose attraction as well as its impact on the bosonic dynamics.
Zhiqiang Huang,
Published: 13 September 2021
Physical Review A, Volume 104; https://doi.org/10.1103/physreva.104.032212

Abstract:
In this paper, we study measures of quantum non-Markovianity based on the conditional mutual information. We obtain such measures by considering multiple parts of the total environment such that the conditional mutual information can be defined in this multipartite setup. The benefit of this approach is that the conditional mutual information is closely related to recovery maps and Markov chains; we also point out its relations with the change in distinguishability. We study along the way the properties of leaked information, which is the conditional mutual information that can be back flowed, and we use this leaked information to show that the correlated environment is necessary for the nonlocal memory effect.
Yicheng Zhang, Lev Vidmar, Marcos Rigol
Published: 13 September 2021
Physical Review A, Volume 104; https://doi.org/10.1103/physreva.104.l031303

Abstract:
Generalized thermalization is a process that occurs in integrable systems in which unitary dynamics, e.g., following a quantum quench, results in states in which observables after equilibration are described by generalized Gibbs ensembles. Here we discuss an emergent eigenstate construction that allows one to built emergent local Hamiltonians of which one eigenstate captures the entire generalized thermalization process following a global quantum quench. Specifically, we study the emergent eigenstate that describes the quantum dynamics of hard-core bosons in one dimension for which the initial state is a density wave, and this state evolves under a homogeneous Hamiltonian.
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