Limits to clock synchronization induced by completely dephasing communication channels

Clock synchronization procedures are analyzed in the presence of imperfect communications. In this context we show that there are physical limitations which prevent one from synchronizing distant clocks when the intervening medium is completely dephasing, as in the case of a rapidly varying dispersive medium.Comment: 6 Pages. Revised version as published in PR

Electromagnetic channel capacity for practical purposes

We give analytic upper bounds to the channel capacity C for transmission of classical information in electromagnetic channels (bosonic channels with thermal noise). In the practically relevant regimes of high noise and low transmissivity, by comparison with know lower bounds on C, our inequalities determine the value of the capacity up to corrections which are irrelevant for all practical purposes. Examples of such channels are radio communication, infrared or visible-wavelength free space channels. We also provide bounds to active channels that include amplification.Comment: 6 pages, 3 figures. NB: the capacity bounds are constructed by generalizing to the multi-mode case the minimum-output entropy bounds of arXiv:quant-ph/0404005 [Phys. Rev. A 70, 032315 (2004)

Improved transfer of quantum information using a local memory

We demonstrate that the quantum communication between two parties can be significantly improved if the receiver is allowed to store the received signals in a quantum memory before decoding them. In the limit of an infinite memory, the transfer is perfect. We prove that this scheme allows the transfer of arbitrary multipartite states along Heisenberg chains of spin-1/2 particles with random coupling strengths.Comment: 4 pages, 1 figure; added references to homogenization and asymptotic completenes

Full control by locally induced relaxation

We demonstrate a scheme for controlling a large quantum system by acting on a small subsystem only. The local control is mediated to the larger system by some fixed coupling Hamiltonian. The scheme allows to transfer arbitrary and unknown quantum states from a memory on the large system (``upload access'') as well as the inverse (``download access''). We study sufficient conditions of the coupling Hamiltonian and give lower bounds on the fidelities for downloading and uploading.Comment: 4 pages, 2 figure

A solution of the Gaussian optimizer conjecture

The long-standing conjectures of the optimality of Gaussian inputs for Gaussian channel and Gaussian additivity are solved for a broad class of covariant or contravariant Bosonic Gaussian channels (which includes in particular thermal, additive classical noise, and amplifier channels) restricting to the class of states with finite second moments. We show that the vacuum is the input state which minimizes the entropy at the output of such channels. This allows us to show also that the classical capacity of these channels (under the input energy constraint) is additive and is achieved by Gaussian encodings.Comment: 24 pages, no figures (minor typos corrected

Quantum MERA Channels

Tensor networks representations of many-body quantum systems can be described in terms of quantum channels. We focus on channels associated with the Multi-scale Entanglement Renormalization Ansatz (MERA) tensor network that has been recently introduced to efficiently describe critical systems. Our approach allows us to compute the MERA correspondent to the thermodynamic limit of a critical system introducing a transfer matrix formalism, and to relate the system critical exponents to the convergence rates of the associated channels.Comment: 4 pages, 2 figure

Quantum relative positioning in Hilbert space

A new class of state transformations that are quantum mechanically prohibited is introduced. These can be seen as the generalization of the universal-NOT transformation which, for all pure inputs state of a given Hilbert space produces pure outputs whose projection on the original state is fixed to a value smaller than one. The case of not pure output states is also addressed. We give an application of these transformations in the context of separability criteria.Comment: 5 pages, 1 figure; new material added: in particular we present an application of quantum movers in the context of separability criteria. Typos corrected. Phys. Rev. A, accepted for publicatio

High-temperature, high-pressure spherical segment valve Patent

High-temperature, high-pressure spherical segment valv

Energy upper bound for structurally-stable N-passive states

Passive states are special configurations of a quantum system which exhibit no energy decrement at the end of an arbitrary cyclic driving of the model Hamiltonian. When applied to an increasing number of copies of the initial density matrix, the requirement of passivity induces a hierarchical ordering which, in the asymptotic limit of infinitely many elements, pinpoints ground states and thermal Gibbs states. In particular, for large values of N the energy content of a N-passive state which is also structurally stable (i.e. capable to maintain its passivity status under small perturbations of the model Hamiltonian), is expected to be close to the corresponding value of the thermal Gibbs state which has the same entropy. In the present paper we provide a quantitative assessment of this fact, by producing an upper bound for the energy of an arbitrary N-passive, structurally stable state which only depends on the spectral properties of the Hamiltonian of the system. We also show the condition under which our inequality can be saturated. A generalization of the bound is finally presented that, for sufficiently large N, applies to states which are N-passive, but not necessarily structurally stable

A Lorentz-invariant look at quantum clock synchronization protocols based on distributed entanglement

Recent work has raised the possibility that quantum information theory techniques can be used to synchronize atomic clocks nonlocally. One of the proposed algorithms for quantum clock synchronization (QCS) requires distribution of entangled pure singlets to the synchronizing parties. Such remote entanglement distribution normally creates a relative phase error in the distributed singlet state which then needs to be purified asynchronously. We present a fully relativistic analysis of the QCS protocol which shows that asynchronous entanglement purification is not possible, and, therefore, that the proposed QCS scheme remains incomplete. We discuss possible directions of research in quantum information theory which may lead to a complete, working QCS protocol.Comment: 5 pages; typeset in RevTe