edoc: No conditions. Results ordered -Date Deposited. 2024-06-23T05:15:20ZEPrintshttps://edoc.unibas.ch/images/uni-logo.jpghttps://edoc.unibas.ch/2022-04-12T10:04:19Z2022-04-12T10:04:19Zhttps://edoc.unibas.ch/id/eprint/87298This item is in the repository with the URL: https://edoc.unibas.ch/id/eprint/872982022-04-12T10:04:19ZControl of stochastic quantum dynamics by differentiable programmingControl of the stochastic dynamics of a quantum system is indispensable in fields such as quantum information processing and metrology. However, there is no general ready-made approach to the design of efficient control strategies. Here, we propose a framework for the automated design of control schemes based on differentiable programming. We apply this approach to the state preparation and stabilization of a qubit subjected to homodyne detection. To this end, we formulate the control task as an optimization problem where the loss function quantifies the distance from the target state, and we employ neural networks (NNs) as controllers. The system's time evolution is governed by a stochastic differential equation (SDE). To implement efficient training, we backpropagate the gradient information from the loss function through the SDE solver using adjoint sensitivity methods. As a first example, we feed the quantum state to the controller and focus on different methods of obtaining gradients. As a second example, we directly feed the homodyne detection signal to the controller. The instantaneous value of the homodyne current contains only very limited information on the actual state of the system, masked by unavoidable photon-number fluctuations. Despite the resulting poor signal-to-noise ratio, we can train our controller to prepare and stabilize the qubit to a target state with a mean fidelity of around 85%. We also compare the solutions found by the NN to a hand-crafted control strategy. Frank SchäferPavel SekatskiMartin KoppenhöferChristoph BruderMichal Kloc2022-04-12T08:56:35Z2022-04-12T08:56:35Zhttps://edoc.unibas.ch/id/eprint/87309This item is in the repository with the URL: https://edoc.unibas.ch/id/eprint/873092022-04-12T08:56:35ZSuperradiant Many-Qubit Absorption RefrigeratorWe show that the lower levels of a large-spin network with a collective antiferromagnetic interaction and collective couplings to three reservoirs may function as a quantum-absorption refrigerator. In appropriate regimes, the steady-state cooling current of this refrigerator scales quadratically with the size of the working medium, i.e., the number of spins. The same scaling is observed for the noise and the entropy production rate. Michal KlocKurt MeierKimon HadjikyriakosGernot Schaller2022-04-12T08:50:30Z2022-04-12T08:50:30Zhttps://edoc.unibas.ch/id/eprint/87301This item is in the repository with the URL: https://edoc.unibas.ch/id/eprint/873012022-04-12T08:50:30ZExcited-state quantum phase transitionsWe review the effects of excited-state quantum phase transitions (ESQPTs) in interacting many-body systems with finite numbers of collective degrees of freedom. We classify typical ESQPT signatures in the spectra of energy eigenstates with respect to the underlying classical dynamics and outline a variety of quantum systems in which they occur. We describe thermodynamic and dynamic consequences of ESQPTs, like those in microcanonical thermodynamics, quantum quench dynamics, and in the response to nearly adiabatic or periodic driving. We hint at some generalizations of the ESQPT concept in periodic lattices and in resonant tunneling systems. P. CejnarP. StranskyM. MacekM. Kloc2022-04-12T08:45:52Z2022-04-12T08:45:52Zhttps://edoc.unibas.ch/id/eprint/87300This item is in the repository with the URL: https://edoc.unibas.ch/id/eprint/873002022-04-12T08:45:52ZQuasiclassical approach to quantum quench dynamics in the presence of an excited-state quantum phase transitionThe dynamics of a quantum system following a sudden, highly nonadiabatic change of its control parameter (quantum quench) is studied with quasiclassical techniques. Recent works have shown, using exact quantum mechanical approach, that equilibration after quantum quench exhibits specific features in the presence of excited-state quantum phase transitions. In this paper, we demonstrate that these features can be understood from the classical evolution of the Wigner function in phase space. M. KlocD. SimsaF. HanakP. R. Kapralova-ZvdanskaP. StranskyP. Cejnar2021-04-14T12:08:11Z2021-04-14T12:08:11Zhttps://edoc.unibas.ch/id/eprint/80354This item is in the repository with the URL: https://edoc.unibas.ch/id/eprint/803542021-04-14T12:08:11ZA differentiable programming method for quantum controlOptimal control is highly desirable in many current quantum systems, especially to realize tasks in quantum information processing. We introduce a method based on differentiable programming to leverage explicit knowledge of the differential equations governing the dynamics of the system. In particular, a control agent is represented as a neural network that maps the state of the system at a given time to a control pulse. The parameters of this agent are optimized via gradient information obtained by direct differentiation through both the neural network and the differential equation of the system. This fully differentiable reinforcement learning approach ultimately yields time-dependent control parameters optimizing a desired figure of merit. We demonstrate the method`s viability and robustness to noise in eigenstate preparation tasks for three systems: a single qubit, a chain of qubits, and a quantum parametric oscillator. Frank SchäferMichal KlocChristoph BruderNiels Lörch2021-04-14T12:03:33Z2021-04-14T12:03:33Zhttps://edoc.unibas.ch/id/eprint/80356This item is in the repository with the URL: https://edoc.unibas.ch/id/eprint/803562021-04-14T12:03:33ZComplex Density of Continuum States in Resonant Quantum TunnelingWe introduce a complex-extended continuum level density and apply it to one-dimensional scattering problems involving tunneling through finite-range potentials. We show that the real part of the density is proportional to a real “time shift” of the transmitted particle, while the imaginary part reflects the imaginary time of an instantonlike tunneling trajectory. We confirm these assumptions for several potentials using the complex scaling method. In particular, we show that stationary points of the potentials give rise to specific singularities of both real and imaginary densities which represent close analogues of excited-state quantum phase transitions in bound systems. Pavel StránskýMilan ŠindelkaMichal KlocPavel Cejnar2020-04-21T14:09:24Z2020-04-21T14:09:24Zhttps://edoc.unibas.ch/id/eprint/74548This item is in the repository with the URL: https://edoc.unibas.ch/id/eprint/745482020-04-21T14:09:24ZCollective performance of a finite-time quantum Otto cycleWe study the finite-time effects in a quantum Otto cycle where a collective spin system is used as the working fluid. Starting from a simple one-qubit system we analyze the transition to the limit cycle in the case of a finite-time thermalization. If the system consists of a large sample of independent qubits interacting coherently with the heat bath, then the super-radiant equilibration is observed. We show that this phenomenon can boost the power of the engine. Mutual interaction of qubits in the working fluid is modeled by the Lipkin-Meshkov-Glick Hamiltonian. We demonstrate that in this case the quantum phase transitions for the ground and excited states may have a strong negative effect on the performance of the machine. Conversely, by analyzing the work output we can distinguish between the operational regimes with and without a phase transition. Michal KlocPavel CejnarGernot Schaller