Geometric Discord Of the Jaynes-Cummings Model: Pure Dephasing Regime

In this paper, the dynamical behaviour of the geometric discord of a system consisting of a two-level atom interacting with a quantised radiation field described by the Jaynes-Cummings model has been studied. The evolution of the system has been considered in the pure dephasing regime when the field is initially in a general pure state and the atom is initially in a mixed state. Dynamics of the geometric discord, as a measure of non-classical correlation, has been compared with the dynamics of negativity, as a measure of quantum entanglement. In particular, the influence of different parameters of system such as detuning and mixedness of the initial atomic state on the dynamics of geometric discord has been evaluated for when the field is initially in coherent and number states. It is shown that for asymptotically large times, the steady state geometric discord of the system presents a non-zero optimum value at some intermediate value of detuning.Comment: 21 pages, 7 figures. appears in Eur. Phys. J. D 201

Thermodynamic fingerprints of non-Markovianity in a system of coupled superconducting qubits

The exploitation and characterization of memory effects arising from the interaction between system and environment is a key prerequisite for quantum reservoir engineering beyond the standard Markovian limit. In this paper we investigate a prototype of non-Markovian dynamics experimentally implementable with superconducting qubits. We rigorously quantify non-Markovianity highlighting the effects of the environmental temperature on the Markovian to non-Markovian crossover. We investigate how memory effects influence, and specifically suppress, the ability to perform work on the driven qubit. We show that the average work performed on the qubit can be used as a diagnostic tool to detect the presence or absence of memory effects.Comment: 9 page

Remote polarization entanglement generation by local dephasing and frequency upconversion

We introduce a scheme for remote entanglement generation for the photon polarization. The technique is based on transferring the initial frequency correlations to specific polarization-frequency correlations by local dephasing and their subsequent removal by frequency up-conversion. On fundamental level, our theoretical results show how to create and transfer entanglement, to particles which never interact, by means of local operations. This possibility stems from the multi-path interference and its control in frequency space. For applications, the developed techniques and results allow for the remote generation of entanglement with distant parties without Bell state measurements and opens the perspective to probe frequency-frequency entanglement by measuring the polarization state of the photons.Comment: 8 page

Finite-time quantum Stirling heat engine

We study the thermodynamic performance of a finite-time non-regenerative quantum Stirling-like cycle used as a heat engine. We consider specifically the case in which the working substance (WS) is a two-level system (TLS). The Stirling cycle is made of two isochoric transformations separated by a compression and an expansion stroke during which the WS is in contact with a thermal reservoir. To describe these two strokes we derive a non-Markovian master equation which allows to study the real-time dynamics of a driven open quantum system with arbitrary fast driving. Following the real-time dynamics of the WS using this master equation, the endpoints of the isotherms can deviate from the equilibrium thermal states. The role of this deviation in the performance of the heat engine is addressed. We found that the finite-time dynamics and thermodynamics of the cycle depend non-trivially on the different time scales at play. In particular, driving the WS at a time scale comparable to the resonance time of the bath enhances the performance of the cycle and allows for an efficiency higher than the efficiency of the quasistatic cycle, but still below the Carnot bound. However, by adding thermalization of the WS with the baths at the end of compression/expansion processes one recovers the conventional scenario in which efficiency decreases by speeding up the processes. In addition, the performance of the cycle is dependent on the compression/expansion speeds asymmetrically, which suggests new freedom in optimizing quantum heat engines. The maximum output power and the maximum efficiency are obtained almost simultaneously when the real-time endpoints of the compression/expansion processes are considered instead of the equilibrium thermal endpoint states. However, the net extractable work always declines by speeding up the drive.Peer reviewe

Experimental realization of high-fidelity teleportation via a non-Markovian open quantum system

Open quantum systems and study of decoherence are important for our fundamental understanding of quantum physical phenomena. For practical purposes, a large number of quantum protocols exist that exploit quantum resources, e.g., entanglement, which allows us to go beyond what is possible to achieve by classical means. We combine concepts from open quantum systems and quantum information science and give a proof-of-principle experimental demonstration—with teleportation—that it is possible to implement efficiently a quantum protocol via a non-Markovian open system. The results show that, at the time of implementation of the protocol, it is not necessary to have the quantum resource in the degree of freedom used for the basic protocol—as long as there exists some other degree of freedom or the environment of an open system, which contains useful resources. The experiment is based on a pair of photons, where their polarizations act as open system qubits and frequencies as their environments, while the path degree of freedom of one of the photons represents the state of Alice's qubit to be teleported to Bob's polarization qubit.</p

Broadband Continuous-Variable Entanglement Generation Using a Kerr-Free Josephson Metamaterial

| openaire: EC/H2020/862644/EU//QUARTET | openaire: EC/H2020/820505/EU//QMiCS | openaire: EC/H2020/820363/EU//OpenSuperQ | openaire: EC/H2020/824109/EU//EMP | openaire: EC/H2020/670743/EU//QuDeT Funding Information: We thank Alpo Ahonen, Paula Holmlund, and Harri Pohjonen for technical assistance and Terra Quantum AG for scientific support. K.V.P. is funded by the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 862644 (FET-Open project: Quantum readout techniques and technologies, QUARTET). The work at VTT is funded from the EU Flagship on Quantum Technology Grant No. H2020-FETFLAG-2018-03 Project Nos. 820363 OpenSuperQ and 820505 QMiCS. The contribution of M.R.P., I.L., M.W., and A.S. is supported by Grant Agreement No. 824109 (European Microkelvin Platform project, EMP), and ERC Grant Agreement No. 670743 (QuDeT). P.J.H. and V.V. acknowledge financial support from the Academy of Finland through Grants No. 314448 and No. 321700, respectively. The work of S.H.R. and P.J.H. is supported by a MATINE research grant. G.S.P. and K.V.P. thank Saab for scientific collaboration under a research agreement with Aalto University. This work is done under the “Finnish Center of Excellence in Quantum Technology QTF” of the Academy of Finland, Projects No. 312059, No. 312294, No. 312295, No. 336810, and No. 312296. Publisher Copyright: © 2022 American Physical Society.Entangled microwave photons form a fundamental resource for quantum information processing and sensing with continuous variables. We use a low-loss Josephson metamaterial comprising superconducting, nonlinear, asymmetric inductive elements to generate frequency-entangled photons from vacuum fluctuations at a rate of 2 giga entangled bits per second spanning over the 4-GHz bandwidth. The device is operated as a traveling-wave parametric amplifier under Kerr-relieving biasing conditions. Furthermore, we demonstrate single-mode squeezing in such devices - 3.1±0.7dB below the zero-point level at half of modulation frequency.Peer reviewe