1,541 research outputs found
Precision and Work Fluctuations in Gaussian Battery Charging
One of the most fundamental tasks in quantum thermodynamics is extracting
energy from one system and subsequently storing this energy in an appropriate
battery. Both of these steps, work extraction and charging, can be viewed as
cyclic Hamiltonian processes acting on individual quantum systems.
Interestingly, so-called passive states exist, whose energy cannot be lowered
by unitary operations, but it is safe to assume that the energy of any not
fully charged battery may be increased unitarily. However, unitaries raising
the average energy by the same amount may differ in qualities such as their
precision, fluctuations, and charging power. Moreover, some unitaries may be
extremely difficult to realize in practice. It is hence of crucial importance
to understand the qualities that can be expected from practically implementable
transformations. Here, we consider the limitations on charging batteries when
restricting to the feasibly realizable family of Gaussian unitaries. We derive
optimal protocols for general unitary operations as well as for the restriction
to easier implementable Gaussian unitaries. We find that practical Gaussian
battery charging, while performing significantly less well than is possible in
principle, still offers asymptotically vanishing relative charge variances and
fluctuations.Comment: 14+8 pages, 8 figures, accepted for publication in Quantu
Layered Quantum Key Distribution
We introduce a family of QKD protocols for distributing shared random keys
within a network of users. The advantage of these protocols is that any
possible key structure needed within the network, including broadcast keys
shared among subsets of users, can be implemented by using a particular
multi-partite high-dimensional quantum state. This approach is more efficient
in the number of quantum channel uses than conventional quantum key
distribution using bipartite links. Additionally, multi-partite
high-dimensional quantum states are becoming readily available in quantum
photonic labs, making the proposed protocols implementable using current
technology.Comment: 11 pages, 5 figures. In version 2 we extended section 4 about the
dimension-rate trade-off and corrected minor error
Quantifying high dimensional entanglement with two mutually unbiased bases
We derive a framework for quantifying entanglement in multipartite and high
dimensional systems using only correlations in two unbiased bases. We
furthermore develop such bounds in cases where the second basis is not
characterized beyond being unbiased, thus enabling entanglement quantification
with minimal assumptions. Furthermore, we show that it is feasible to
experimentally implement our method with readily available equipment and even
conservative estimates of physical parameters.Comment: 17 pages, 1 figur
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