347 research outputs found
On the von Neumann capacity of noisy quantum channels
We discuss the capacity of quantum channels for information transmission and
storage. Quantum channels have dual uses: they can be used to transmit known
quantum states which code for classical information, and they can be used in a
purely quantum manner, for transmitting or storing quantum entanglement. We
propose here a definition of the von Neumann capacity of quantum channels,
which is a quantum mechanical extension of the Shannon capacity and reverts to
it in the classical limit. As such, the von Neumann capacity assumes the role
of a classical or quantum capacity depending on the usage of the channel. In
analogy to the classical construction, this capacity is defined as the maximum
von Neumann mutual entropy processed by the channel, a measure which reduces to
the capacity for classical information transmission through quantum channels
(the "Kholevo capacity") when known quantum states are sent. The quantum mutual
entropy fulfills all basic requirements for a measure of information, and
observes quantum data-processing inequalities. We also derive a quantum Fano
inequality relating the quantum loss of the channel to the fidelity of the
quantum code. The quantities introduced are calculated explicitly for the
quantum "depolarizing" channel. The von Neumann capacity is interpreted within
the context of superdense coding, and an "extended" Hamming bound is derived
that is consistent with that capacity.Comment: 15 pages RevTeX with psfig, 13 figures. Revised interpretation of
capacity, added section, changed titl
Entropic bounds on coding for noisy quantum channels
In analogy with its classical counterpart, a noisy quantum channel is
characterized by a loss, a quantity that depends on the channel input and the
quantum operation performed by the channel. The loss reflects the transmission
quality: if the loss is zero, quantum information can be perfectly transmitted
at a rate measured by the quantum source entropy. By using block coding based
on sequences of n entangled symbols, the average loss (defined as the overall
loss of the joint n-symbol channel divided by n, when n tends to infinity) can
be made lower than the loss for a single use of the channel. In this context,
we examine several upper bounds on the rate at which quantum information can be
transmitted reliably via a noisy channel, that is, with an asymptotically
vanishing average loss while the one-symbol loss of the channel is non-zero.
These bounds on the channel capacity rely on the entropic Singleton bound on
quantum error-correcting codes [Phys. Rev. A 56, 1721 (1997)]. Finally, we
analyze the Singleton bounds when the noisy quantum channel is supplemented
with a classical auxiliary channel.Comment: 20 pages RevTeX, 10 Postscript figures. Expanded Section II, added 1
figure, changed title. To appear in Phys. Rev. A (May 98
Information-theoretic interpretation of quantum error-correcting codes
Quantum error-correcting codes are analyzed from an information-theoretic
perspective centered on quantum conditional and mutual entropies. This approach
parallels the description of classical error correction in Shannon theory,
while clarifying the differences between classical and quantum codes. More
specifically, it is shown how quantum information theory accounts for the fact
that "redundant" information can be distributed over quantum bits even though
this does not violate the quantum "no-cloning" theorem. Such a remarkable
feature, which has no counterpart for classical codes, is related to the
property that the ternary mutual entropy vanishes for a tripartite system in a
pure state. This information-theoretic description of quantum coding is used to
derive the quantum analogue of the Singleton bound on the number of logical
bits that can be preserved by a code of fixed length which can recover a given
number of errors.Comment: 14 pages RevTeX, 8 Postscript figures. Added appendix. To appear in
Phys. Rev.
Floquetifying the Colour Code
Floquet codes are a recently discovered type of quantum error correction
code. They can be thought of as generalising stabilizer codes and subsystem
codes, by allowing the logical Pauli operators of the code to vary dynamically
over time. In this work, we use the ZX-calculus to create new Floquet codes
that are in a definable sense equivalent to known stabilizer codes. In
particular, we find a Floquet code that is equivalent to the colour code, but
has the advantage that all measurements required to implement it are of weight
one or two. Notably, the qubits can even be laid out on a square lattice. This
circumvents current difficulties with implementing the colour code
fault-tolerantly, while preserving its advantages over other well-studied
codes, and could furthermore allow one to benefit from extra features exclusive
to Floquet codes. On a higher level, as in arXiv:2303.08829, this work shines a
light on the relationship between 'static' stabilizer and subsystem codes and
'dynamic' Floquet codes; at first glance the latter seems a significant
generalisation of the former, but in the case of the codes that we find here,
the difference is essentially just a few basic ZX-diagram deformations.Comment: 15 + 24 pages, 18 figures. Comments encouraged - email address in
paper
Code deformation and lattice surgery are gauge fixing
International audienceThe large-scale execution of quantum algorithms requires basic quantum operations to be implemented fault-tolerantly. The most popular technique for accomplishing this, using the devices that can be realized in the near term, uses stabilizer codes which can be embedded in a planar layout. The set of fault-tolerant operations which can be executed in these systems using unitary gates is typically very limited. This has driven the development of measurement-based schemes for performing logical operations in these codes, known as lattice surgery and code deformation. In parallel, gauge fixing has emerged as a measurement-based method for performing universal gate sets in subsystem stabilizer codes. In this work, we show that lattice surgery and code deformation can be expressed as special cases of gauge fixing, permitting a simple and rigorous test for fault-tolerance together with simple guiding principles for the implementation of these operations. We demonstrate the accuracy of this method numerically with examples based on the surface code, some of which are novel
Boolean Differential Operators
We consider four combinatorial interpretations for the algebra of Boolean
differential operators. We show that each interpretation yields an explicit
matrix representation for Boolean differential operators
Molecular study of drought response in the Mediterranean conifer Pinus Pinaster Ait. : differential transcriptomic profiling reveals constitutive water deficitâindependent drought tolerance mechanisms
Adaptation of longâliving forest trees to respond to environmental changes is essential to secure their performance under adverse conditions. Water deficit is one of the most significant stress factors determining tree growth and survival. Maritime pine (Pinus pinaster Ait.), the main source of softwood in southwestern Europe, is subjected to recurrent drought periods which, according to climate change predictions for the years to come, will progressively increase in the Mediterranean region. The mechanisms regulating pine adaptive responses to environment are still largely unknown. The aim of this work was to go a step further in understanding the molecular mechanisms underlying maritime pine response to water stress and drought tolerance at the whole plant level. A global transcriptomic profiling of roots, stems, and needles was conducted to analyze the performance of siblings showing contrasted responses to water deficit from an ad hoc designed fullâsib family. Although P. pinaster is considered a recalcitrant species for vegetative propagation in adult phase, the analysis was conducted using vegetatively propagated trees exposed to two treatments: wellâwatered and moderate water stress. The comparative analyses led us to identify organâspecific genes, constitutively expressed as well as differentially expressed when comparing control versus water stress conditions, in droughtâsensitive and droughtâtolerant genotypes. Different response strategies can point out, with tolerant individuals being preâadapted for coping with drought by constitutively expressing stressârelated genes that are detected only in latter stages on sensitive individuals subjected to drought
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