54 research outputs found
Nonlinear Phenomena of Ultracold Atomic Gases in Optical Lattices: Emergence of Novel Features in Extended States
The system of a cold atomic gas in an optical lattice is governed by two
factors: nonlinearity originating from the interparticle interaction, and the
periodicity of the system set by the lattice. The high level of controllability
associated with such an arrangement allows for the study of the competition and
interplay between these two, and gives rise to a whole range of interesting and
rich nonlinear effects. This review covers the basic idea and overview of such
nonlinear phenomena, especially those corresponding to extended states. This
includes "swallowtail" loop structures of the energy band, Bloch states with
multiple periodicity, and those in "nonlinear lattices", i.e., systems with the
nonlinear interaction term itself being a periodic function in space.Comment: 39 pages, 21 figures; review article to be published in a Special
Issue of Entropy on "Non-Linear Lattice
Quantum fluctuation theorems and power measurements
Work in the paradigm of the quantum fluctuation theorems of Crooks and
Jarzynski is determined by projective measurements of energy at the beginning
and end of the force protocol. In analogy to classical systems, we consider an
alternative definition of work given by the integral of the supplied power
determined by integrating up the results of repeated measurements of the
instantaneous power during the force protocol. We observe that such a
definition of work, in spite of taking account of the process dependence, has
different possible values and statistics from the work determined by the
conventional two energy measurement approach (TEMA). In the limit of many
projective measurements of power, the system's dynamics is frozen in the power
measurement basis due to the quantum Zeno effect leading to statistics only
trivially dependent on the force protocol. In general the Jarzynski relation is
not satisfied except for the case when the instantaneous power operator
commutes with the total Hamiltonian at all times. We also consider properties
of the joint statistics of power-based definition of work and TEMA work in
protocols where both values are determined. This allows us to quantify their
correlations. Relaxing the projective measurement condition, weak continuous
measurements of power are considered within the stochastic master equation
formalism. Even in this scenario the power-based work statistics is in general
not able to reproduce qualitative features of the TEMA work statistics.Comment: 26 pages, 9 figure
Backaction-Driven Transport of Bloch Oscillating Atoms in Ring Cavities
We predict that an atomic Bose-Einstein condensate strongly coupled to an
intracavity optical lattice can undergo resonant tunneling and directed
transport when a constant and uniform bias force is applied. The bias force
induces Bloch oscillations, causing amplitude and phase modulation of the
lattice which resonantly modifies the site-to-site tunneling. For the right
choice of parameters a net atomic current is generated. The transport velocity
can be oriented oppositely to the bias force, with its amplitude and direction
controlled by the detuning between the pump laser and the cavity. The transport
can also be enhanced through imbalanced pumping of the two counter-propagating
running wave cavity modes. Our results add to the cold atoms quantum simulation
toolbox, with implications for quantum sensing and metrology.Comment: Published version: 5 pages, 4 figures; Supplementary Material
include
Quantum Performance of Thermal Machines over Many Cycles
The performance of quantum heat engines is generally based on the analysis of
a single cycle. We challenge this approach by showing that the total work
performed by a quantum engine need not be proportional to the number of cycles.
Furthermore, optimizing the engine over multiple cycles leads to the
identification of scenarios with a quantum enhancement. We demonstrate our
findings with a quantum Otto engine based on a two-level system as the working
substance that supplies power to an external oscillator.Comment: 5 pages, 3 figures; published in Phys. Rev. Lett. as an Editors'
Suggestio
Study of bounds on non-equilibrium fluctuations for asymmetrically driven quantum Otto engine
For a four-stroke asymmetrically driven quantum Otto engine with working
medium modeled by a single qubit, we study the bounds on non-equilibrium
fluctuations of work and heat. We find strict relations between the
fluctuations of work and individual heat for hot and cold reservoirs in
arbitrary operational regimes. Focusing on the engine regime, we show that the
ratio of non-equilibrium fluctuations of output work to input heat from the hot
reservoir is both upper and lower bounded. As a consequence, we establish
hierarchical relation between the relative fluctuations of work and heat for
both cold and hot reservoirs and further make a connection with the
thermodynamic uncertainty relations. We discuss the fate of these bounds also
in the refrigerator regime. The reported bounds, for such asymmetrically driven
engines, emerge once both the time-forward and the corresponding reversed
cycles of the engine are considered on an equal footing. We also extend our
study and report bounds for a parametrically driven harmonic oscillator Otto
engine.Comment: 14 pages, 6 figure
On-chip quantum interference of a superconducting microsphere
We propose and analyze an all-magnetic scheme to perform a Young's double slit experiment with a micron-sized superconducting sphere of mass amu. We show that its center of mass could be prepared in a spatial quantum superposition state with an extent of the order of half a micrometer. The scheme is based on magnetically levitating the sphere above a superconducting chip and letting it skate through a static magnetic potential landscape where it interacts for short intervals with quantum circuits. In this way, a protocol for fast quantum interferometry using quantum magnetomechanics is passively implemented. Such a table-top earth-based quantum experiment would operate in a parameter regime where gravitational energy scales become relevant. In particular, we show that the faint parameter-free gravitationally-induced decoherence collapse model, proposed by Diósi and Penrose, could be unambiguously falsified
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