Comment on "Resilient Quantum Computation in Correlated Environments: A Quantum Phase Transition Perspective" and "Fault-tolerant Quantum Computation with Longe-range Correlated Noise"

Critical comments on the recent papers supporting the idea of resilient quantum computations are presented.Comment: 2 pages, no figure

Breaking information-thermodynamics link

The information-thermodynamics link is revisited, going back to the analysis of Szilard's engine. It is argued that instead of equivalence rather complementarity of physical entropy and information theoretical one is a correct concept. Famous Landauer's formula for a minimal cost of information processing is replaced by a new one which takes into account accuracy and stability of information encoding. Two recent experiments illustrating the information-energy conversion are critically discussed.Comment: 5 pages, 3 figures. arXiv admin note: text overlap with arXiv:1402.241

Quantum decay cannot be completely reversed. The 5% rule

Using an exactly solvable model of the Wigner-Weisskopf atom it is shown that an unstable quantum state cannot be recovered completely by the procedure involving detection of the decay products followed by creation of the time reversed decay products state, as proposed in \cite{Son}. The universal lower bound on the recovery error is approximately equal to $5$ of the \emph{error per cycle} - the dimensionless parameter characterizing decay process in the Markovian approximation. This result has consequences for the efficiency of quantum error correction procedures which are based on syndrome measurements and corrective operations.Comment: 3 pages, no figure

A search for a border between classical and quantum worlds

The effects of environmental decoherence on a mass center position of a macroscopic body are studied using the linear quantum Boltzmann equation. The border between the classical world and the quantum one is discussed and the results are illustrated by the recent experiments od Zeilinger group involving C_60 and C_70 fullerenes.Comment: 3 page

Quantum error correction fails for Hamiltonian models

It is argued that the existing schemes of fault-tolerant quantum computation designed for discrete-time models and based on quantum error correction fail for continuous-time Hamiltonian models even with Markovian noise.Comment: 8 page

Model of quantum measurement and thermodynamical cost of accuracy and stability of information processing

The quantum measurement problem is revisited and discussed in terms of a new solvable measurement model which basic ingredient is the quantum model of a controlled single-bit memory. The structure of this model involving strongly coupled spin and quantum harmonic oscillator allows to define stable pointer states as well-separated Gaussian states of the quantum oscillator and analyze the transition from quantum to classical regime. The relations between accuracy of measurement, stability of pointer states, effective temperature of joint thermal and quantum noise and minimal work needed to perform the bit-flip are derived. They differ from those based on the Landauer principle and are used to analyze thermodynamic efficiency of quantum Szilard engine and imply more realistic estimations of minimal amount of work needed to perform long computations.Comment: 13 pages, 2 figure

BCS model of Cooper Pair Box

The standard phenomenological Hamiltonian of a small superconducting Josephson junction in the charge regime (Cooper Pair Box) produces a model of the effective charge qubit with possible applications to quantum information processing. In this note a new model based on the BCS Hamiltonian with individual tunneling yields an effective multi-level picture with a highly degenerated level placed between the ground state and the excited state. Unlike in the standard approach, the excited Cooper pairs play here an important role. For such a system coupled to a zero temperature bath the additional levels act as a probability sink. In contrast to the standard large-spin model the coupling to phonons can be an effective source of dissipation. This model provides also alternative explanations of various effects observed in experiments and sheds new light on the issue of Josephson junctions as macroscopic quantum systems.Comment: 8 pages, text amended, 1 figure adde

Comments on the paper "Foundation of statistical mechanics from symmetries of entanglement" by S. Deffner and W. Zurek

The authors of the recent paper [1] boldly claim to discover a new fully quantum approach to foundation of statistical mechanics: "Our conceptually novel approach is free of mathematically ambiguous notions such as probability, ensemble, randomness, etc." The aim of this note is to show that this approach is neither specific for quantum systems nor really conceptually different from the standard textbook arguments supporting microcanonical or canonical ensembles in statistical mechanics.Comment: 2 pages, no figure

False qubits II. Entanglement of Josephson junctions

The recent experimental evidence for entangled states of two Josephson junction qubits is briefly discussed. It is argued that the interpretation of the experimental data strongly depends on the assumed theoretical model. Namely, the qubit states are supposed to be the lowest lying eigenstates of a certain effective Hamiltonian and hence automatically orthogonal, while the simple analysis within a more fundamental many-particle model shows that those states should strongly overlap. This makes the standard interpretation of the measurement procedure questionable.Comment: 2 page

Remarks on the nature of quantum computation

Two models of computer, a quantum and a classical "chemical machine" designed to compute the relevant part of Shor's factoring algorithm are discussed. The comparison shows that the basic quantum features believed to be responsible for the exponential speed-up of quantum computations possess their classical counterparts for the hybrid digital-analog computer. It is argued that the measurement errors which cannot be fully corrected make the computation not efficient for both models.Comment: 8 page