Output enhanced quantum correlations using a coherence-controlled atomic reservoir

Chang-shui Yu, Yang Zhang, Haiqing Zhao

Siddhartha Dutta, Sujay Mondal, Ankush Das +2 more

We investigate the thermodynamic consequences of longitudinal-transverse cross-spectral reservoir correlations in a finite-time quantum Otto engine with a two-level working medium. Each reservoir couples through excitation-relaxation and dephasing channels whose fluctuations are characterized by a Hermitian positive-semidefinite spectral-density matrix, with the off-diagonal elements encoding their cross correlations. The finite-time isochoric dynamics is derived within the second-order time-convolutionless framework, without imposing the Markov limit at the outset, so that finite reservoir-memory effects can enter through time-dependent dissipative and reservoir-induced coherent contributions. The resulting dynamics is then recast in Bloch-vector form to construct the stroke-resolved cycle dynamics. At fixed auto-spectral densities, cross-spectral correlations modify the populations and coherences of the working medium and thereby its thermodynamic performance. Increasing the correlation strength can enhance the output power, with the enhancement controlled by the cross-spectral phase and characteristic frequency scale. The correlations also reshape the transient cycle-to-cycle evolution and the approach to periodic operation, while the limit-cycle efficiency remains fixed at the Otto value for the population-preserving unitary strokes considered here. These results establish off-diagonal reservoir spectra as an additional resource for controlling finite-time quantum thermal machines.

L. Davidovich, M. Brune, J. M. Raimond +1 more

Sabrina Burgardt, Julian Feß, Silvia Hiebel +2 more

The non-equilibrium properties of open quantum systems are determined by the microscopic laws governing energy exchange with their environment. In particular, an enhancement of the performance of quantum heat engines has been predicted by speeding up the dynamics through control of the system--bath interaction. However, direct microscopic control of heat transfer between the machine and the reservoir has remained elusive so far. Here, we experimentally demonstrate such control in a quantum Otto engine realized with ultracold Cs-133 atoms coupled to an atomic reservoir of ultracold Rb-87 atoms. Heat exchange between the two is mediated by inelastic s-wave collisions whose energy-dependent scattering cross sections lead to an asymmetric equilibration dynamics in the isochoric heating and cooling strokes. By tuning the kinetic temperature of the atomic reservoir, we modify the associated microscopic scattering rates, and thereby the heat transfer law, giving control over the time allocation within the engine cycle through control over the microscopic, multi-exponential relaxation dynamics. This enables power output optimization at fixed efficiency. Our results establish microscopic control of system-reservoir interactions as a tool for manipulating heat flow at the nanoscale and engineering the finite-time performance of quantum thermal machines.

Yashuai Han, Y.H. Wang

Coherent feedback control (CFC) has been proven to be a promising approach for manipulating nonclassical light beams. In this paper, a CFC scheme based on the spatial multiplexing cascaded four-wave mixing (FWM) process is proposed, employing a beam splitter as a feedback controller. Considering the absorption loss of the atomic vapor and the transmission loss of the feedback loop, the input–output relationship of the CFC system is derived. Based on this, the quadripartite quantum correlation of the CFC system versus the feedback strength, phase delay of the feedback loop, gain of the FWM process, and the losses is studied. It is found that the quadripartite quantum correlation can be enhanced within a certain range of feedback ratio and phase. Furthermore, there always exists an optimal feedback ratio to maximize the correlation. In addition, the bipartite correlation present in the multiple quantum correlated beams is also investigated. The numerical calculation based on feasible physical parameters of realistic systems provides direct references for the experimental design, which may find applications in quantum communication and quantum metrology.

Ana Palacios de Luis