81,432 research outputs found
Control of tetrahedral coordination and superconductivity in FeSe0.5Te0.5 thin films
We demonstrate a close relationship between superconductivity and the
dimensions of the Fe-Se(Te) tetrahedron in FeSe0.5Te0.5. This is done by
exploiting thin film epitaxy, which provides controlled biaxial stress, both
compressive and tensile, to distort the tetrahedron. The Se/Te height within
the tetrahedron is found to be of crucial importance to superconductivity, in
agreement with the theoretical proposal that (pi,pi) spin fluctuations promote
superconductivity in Fe superconductors
Entropy and specific heat for open systems in steady states
The fundamental assumption of statistical mechanics is that the system is
equally likely in any of the accessible microstates. Based on this assumption,
the Boltzmann distribution is derived and the full theory of statistical
thermodynamics can be built. In this paper, we show that the Boltzmann
distribution in general can not describe the steady state of open system. Based
on the effective Hamiltonian approach, we calculate the specific heat, the free
energy and the entropy for an open system in steady states. Examples are
illustrated and discussed.Comment: 4 pages, 7 figure
The driven-Markovian master equation based on the Lewis-Riesenfeld invariants theory
We derive a Markovian master equation for driven open quantum systems based
on the Lewis-Riesenfeld invariants theory, which is available for arbitrary
driving protocols.The role of the Lewis-Riesenfeld invariants is to help us
bypass the time-ordering obstacle in expanding the propagator of the free
dynamics, such that the Lindblad operators in our driven-Markovian master
equation can be determined easily. We also illustrate that, for the driven open
quantum systems, the spontaneous emission and the thermal excitation induce the
transitions between eigenstates of the Lewis-Riesenfeld invariant, but not the
system Hamiltonian's. As an example, we present the driven-Markovian master
equation for a driven two-level system coupled to a heat reservoir. By
comparing to the exactly solvable models, the availability of the
driven-Markovian master equation is verified. Meanwhile, the adiabatic limit
and inertial limit of the driven-Markovian master equation are also discussed,
which result in the same Markovian master equations as those presented before
in the corresponding limits.Comment: Including Erratum of Phys. Rev. A 106, 052217 (2022
Quantum state engineering with flux-biased Josephson phase qubits by Stark-chirped rapid adiabatic passages
In this paper, the scheme of quantum computing based on Stark chirped rapid
adiabatic passage (SCRAP) technique [L. F. Wei et al., Phys. Rev. Lett. 100,
113601 (2008)] is extensively applied to implement the quantum-state
manipulations in the flux-biased Josephson phase qubits. The broken-parity
symmetries of bound states in flux-biased Josephson junctions are utilized to
conveniently generate the desirable Stark-shifts. Then, assisted by various
transition pulses universal quantum logic gates as well as arbitrary
quantum-state preparations could be implemented. Compared with the usual
PI-pulses operations widely used in the experiments, the adiabatic population
passage proposed here is insensitive the details of the applied pulses and thus
the desirable population transfers could be satisfyingly implemented. The
experimental feasibility of the proposal is also discussed.Comment: 9 pages, 4 figure
Suppressing decoherence and improving entanglement by quantum-jump-based feedback control in two-level systems
We study the quantum-jump-based feedback control on the entanglement shared
between two qubits with one of them subject to decoherence, while the other
qubit is under the control. This situation is very relevant to a quantum system
consisting of nuclear and electron spins in solid states. The possibility to
prolong the coherence time of the dissipative qubit is also explored. Numerical
simulations show that the quantum-jump-based feedback control can improve the
entanglement between the qubits and prolong the coherence time for the qubit
subject directly to decoherence
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