Quantum Physics

Understanding and simulating how a quantum system interacts and exchanges information or energy with its surroundings is a ubiquitous problem, one which must be carefully addressed in order to establish a coherent framework to describe the dynamics and thermodynamics of quantum systems. Significant effort has been invested in developing various methods for tackling this issue and in this Perspective we focus on one such technique, namely collision models, which have emerged as a remarkably flexible approach. We discuss their application to understanding non-Markovian dynamics and to studying the thermodynamics of quantum systems, two areas in which collision models have proven to be particularly insightful. Their simple structure endows them with extremely broad applicability which has spurred their recent experimental demonstrations. By focusing on these areas, our aim is to provide a succinct entry point to this remarkable framework.

In a recent article García-Pintos et al. [Rev. Lett. 125, 040601 (2020)] studied the connection between the charging power of a quantum battery and the fluctuations of a "free energy operator" that characterizes the maximum extractable work of the battery. The authors claimed that for a battery to have a nonzero charging power there must exist fluctuations of the free energy operator, and the fluctuations upper bound the charging power of the battery. In this Comment, we point out that the analysis contains serious misconceptions that call into question the validity of the results and conclusions.

In the abstract of~[Phys. Rev. Lett. {\bf 125}, 040601 (2020)] one can read that: [...]{\it to have a nonzero rate of change of the extractable work, the state ? W of the battery cannot be an eigenstate of a "free energy operator", defined by F= H W + β ?? log ? W , where H W is the Hamiltonian of the battery and β is the inverse temperature} [...]. Contrarily to what is presented below Eq.~(17) of the paper, we observe that the above conclusion does not hold when the battery is subject to nonunitary dynamics.

The view exists that the Bell inequality is a mere inconsistent application of classical concepts to a well-established quantum world. In the article, "Nonlocality claims are inconsistent with Hilbert-space quantum mechanics" [Phys. Rev. A, 101, 022117, (2020)], Robert B. Griffiths advocates for quantum theory's locality. Although R. B. Griffiths presents valuable insights in favor of quantum mechanics' local character, he erroneously assumes the Bell inequality is an inconsistent application of classical physics to quantum mechanics. We explain why the Bell inequality correct assessment does not conflict with Griffiths's views of quantum locality. Hence, Bell inequality inconsistency is not necessary for Griffiths's quantum locality to hold.

The purpose of the present Comment is to point out the connection between an approach to spawning and regularization that was recently introduced by D. Mendive-Tapia and H.-D. Meyer [J. Chem. Phys. 153, 234114 (2020)] in the context of the Multiconfiguration Time-Dependent Hartree (MCTDH) method, and earlier work where adaptive variational quantum propagation based on the Local-in-Time Error (LITE) was introduced [R. Martinazzo and I. Burghardt, Phys. Rev. Lett. 124, 150601 (2020); arXiv:1907.00841 [quant-ph] (2019)]. Furthermore, we show that the LITE represents a gauge-invariant distance which provides a natural, physically sound tool for adaptive quantum dynamics.

In a recent interesting article Chris Nagele, Ebubechukwu O. IloOkeke, Peter P. Rohde, Jonathan P. Dowling, and Tim Byrnes discuss an entanglement swapping experiment using a setup where it is possible to switch the time ordering of measurements. I would like to draw your attention to the fact that the very same idea was introduced in two previous papers, and briefly address some important points related to the subject.

This comment on the Phys. Rev. A paper "Nonlinear quantum effects in electromagnetic radiation of a vortex electron" by Karlovets and Pupasov-Maximov [Phys. Rev. A 103, 12214 (2021)] addresses their criticism of the combined experimental and theoretical study "Observing the quantum wave nature of free electrons through spontaneous emission" by Remez et al, published in Phys. Rev. Lett. [Phys. Rev. Lett. 123, 060401 (2019)]. We show, by means of simple optical arguments as well as numerical simulations, that the criticism raised by Karlovets and Pupasov-Maximov regarding the experimental regime reported by Remez et al is false. Further, we discuss a necessary clarification for the theoretical derivations presented by Karlovets and Pupasov-Maximov, as they only hold for a certain experimental situation where the final state of the emitting electron is observed in coincidence with the emitted photon - which is not the common scenario in cathodoluminescence. Upon lifting the concerns regarding the experimental regime reported by Remez et al, and explicitly clarifying the electron post-selection, we believe that the paper by Karlovets and Pupasov-Maximov may constitute a valuable contribution to the problem of spontaneous emission by shaped electron wavefunctions, as it presents new expressions for the emission rates beyond the ubiquitous paraxial approximation.

It is shown that for the one-dimensional quantum quartic anharmonic oscillator the numerical results obtained by Okun-Burke in Ref.2 are easily reproduced and can be significantly improved in Lagrange Mesh Method (based on non-uniform lattice).

In the strive for scalable quantum processors, significant effort is being devoted to the development of cryogenic classical hardware for the control and readout of a growing number of qubits. Here we report on a cryogenic circuit incorporating a CMOS-based active inductor enabling fast impedance measurements with a sensitivity of 10 aF and an input-referred noise of 3.7 aF/sqrt(Hz). This type of circuit is especially conceived for the readout of semiconductor spin qubits. As opposed to commonly used schemes based on dispersive rf reflectometry, which require mm-scale passive inductors, it allows for a markedly reduced footprint (50 μ m ? 60 μ m), facilitating its integration in a scalable quantum-classical architecture. In addition, its active inductor results in a resonant circuit with tunable frequency and quality factor, enabling the optimization of readout sensitivity.

We prove that every GNS-symmetric quantum Markov semigroup on a finite dimensional matrix algebra satisfies a modified log-Sobolev inequality. In the discrete time setting, we prove that every finite dimensional GNS-symmetric quantum channel satisfies a strong data processing inequality with respect to its decoherence free part. Moreover, we establish the first general approximate tensorization property of relative entropy. This extends the famous strong subadditivity of the quantum entropy (SSA) of two subsystems to the general setting of two subalgebras. All the three results are independent of the size of the environment and hence satisfy the tensorization property. They are obtained via a common, conceptually simple method for proving entropic inequalities via spectral or L 2 -estimates. As applications, we combine our results on the modified log-Sobolev inequality and approximate tensorization to derive bounds for examples of both theoretical and practical relevance, including representation of sub-Laplacians on SU(2) and various classes of local quantum Markov semigroups such as quantum Kac generators and continuous time approximate unitary designs. For the latter, our bounds imply the existence of local continuous time Markovian evolutions on nk qudits forming ϵ -approximate k -designs in relative entropy for times scaling as O ? ( n 2 poly(k)) .