369 research outputs found
Quantum thermodynamics in a multipartite setting: A resource theory of local Gaussian work extraction for multimode bosonic systems
Quantum thermodynamics can be cast as a resource theory by considering free
access to a heat bath, thereby viewing the Gibbs state at a fixed temperature
as a free state and hence any other state as a resource. Here, we consider a
multipartite scenario where several parties attempt at extracting work locally,
each having access to a local heat bath (possibly with a different
temperature), assisted with an energy-preserving global unitary. As a specific
model, we analyze a collection of harmonic oscillators or a multimode bosonic
system. Focusing on the Gaussian paradigm, we construct a reasonable resource
theory of local activity for a multimode bosonic system, where we identify as
free any state that is obtained from a product of thermal states (possibly at
different temperatures) acted upon by any linear-optics (passive Gaussian)
transformation. The associated free operations are then all linear-optics
transformations supplemented with tensoring and partial tracing. We show that
the local Gaussian extractable work (if each party applies a Gaussian unitary,
assisted with linear optics) is zero if and only if the covariance matrix of
the system is that of a free state. Further, we develop a resource theory of
local Gaussian extractable work, defined as the difference between the trace
and symplectic trace of the covariance matrix of the system. We prove that it
is a resource monotone that cannot increase under free operations. We also
provide examples illustrating the distillation of local activity and local
Gaussian extractable work.Comment: 22 pages, 5 figures, minor corrections to make it close to the
published version, updated list of reference
Partial order on passive states and Hoffman majorization in quantum thermodynamics
Passive states, i.e., those states from which no work can be extracted via
unitary operations, play an important role in the foundations and applications
of quantum thermodynamics. They generalize the familiar Gibbs thermal states,
which are the sole passive states being stable under tensor product. Here, we
introduce a partial order on the set of passive states that captures the idea
of a passive state being virtually cooler than another one. This partial order,
which we build by defining the notion of relative passivity, offers a
fine-grained comparison between passive states based on virtual temperatures
(just like thermal states are compared based on their temperatures). We then
characterize the quantum operations that are closed on the set of virtually
cooler states with respect to some fixed input and output passive states.
Viewing the activity, i.e., non-passivity, of a state as a resource, our main
result is then a necessary and sufficient condition on the transformation of a
class of pure active states under these relative passivity-preserving
operations. This condition gives a quantum thermodynamical meaning to the
majorization relation on the set of non-increasing vectors due to Hoffman. The
maximum extractable work under relative passivity-preserving operations is then
shown to be equal to the ergotropy of these pure active states. Finally, we are
able to fully characterize passivity-preserving operations in the simpler case
of qubit systems, and hence to derive a state interconversion condition under
passivity-preserving qubit operations. The prospect of this work is a general
resource-theoretical framework for the extractable work via quantum operations
going beyond thermal operations.Comment: 21 pages, 3 figure
Gaussian work extraction from random Gaussian states is nearly impossible
Quantum thermodynamics can be naturally phrased as a theory of quantum state
transformation and energy exchange for small-scale quantum systems undergoing
thermodynamical processes, thereby making the resource theoretical approach
very well suited. A key resource in thermodynamics is the extractable work,
forming the backbone of thermal engines. Therefore it is of interest to
characterize quantum states based on their ability to serve as a source of
work. From a near-term perspective, quantum optical setups turn out to be ideal
test beds for quantum thermodynamics; so it is important to assess work
extraction from quantum optical states. Here, we show that Gaussian states are
typically useless for Gaussian work extraction. More specifically, by
exploiting the ``concentration of measure'' phenomenon, we prove that the
probability that the Gaussian extractable work from a zero-mean energy-bounded
multimode random Gaussian state is nonzero is exponentially small. This result
can be thought of as an -no-go theorem for work extraction from
Gaussian states under Gaussian unitaries, thereby revealing a fundamental
limitation on the quantum thermodynamical usefulness of Gaussian components.Comment: 7+8 pages, 2 figures, close to the published versio
Fragmentation, domain formation and atom number fluctuations of a two-species Bose-Einstein condensate in an optical lattice
We theoretically study the loading of a two-species Bose-Einstein condensate
to an optical lattice in a tightly-confined one-dimensional trap. Due to
quantum fluctuations the relative inter and intra species phase coherence
between the atoms and the on-site atom number fluctuations are reduced in the
miscible regime. For the immiscible case the fluctuations are enhanced and the
atoms form metastable interleaved spatially separated domains where the domain
length and its fluctuations are affected by quantum fluctuations.Comment: 32 page
Collective molecule formation in a degenerate Fermi gas via a Feshbach resonance
We model collisionless collective conversion of a degenerate Fermi gas into
bosonic molecules via a Feshbach resonance, treating the bosonic molecules as a
classical field and seeding the pairing amplitudes with random phases. A
dynamical instability of the Fermi sea against association into molecules
initiates the conversion. The model qualitatively reproduces several
experimental observations {[Regal et al., Nature {\bf 424}, 47 (2003)]}. We
predict that the initial temperature of the Fermi gas sets the limit for the
efficiency of atom-molecule conversion.Comment: 4 pages, 3 figures, 10+ references, accepted to PR
Control of InGaAs facets using metal modulation epitaxy (MME)
Control of faceting during epitaxy is critical for nanoscale devices. This
work identifies the origins of gaps and different facets during regrowth of
InGaAs adjacent to patterned features. Molecular beam epitaxy (MBE) near SiO2
or SiNx led to gaps, roughness, or polycrystalline growth, but metal modulated
epitaxy (MME) produced smooth and gap-free "rising tide" (001) growth filling
up to the mask. The resulting self-aligned FETs were dominated by FET channel
resistance rather than source-drain access resistance. Higher As fluxes led
first to conformal growth, then pronounced {111} facets sloping up away from
the mask.Comment: 18 pages, 7 figure
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