arXiv subjects

D. B. Saakian

Publications and source records attributed to D. B. Saakian.

28 records · Page 2Linked to original sources

arXiv1998

About optimal measurements in quantum hypothesizes testing

We consider the problem of a state determination for a two-level quantum system which can be in one of two nonorthogonal mixed states. It is proved that for the two independent identical systems the optimal combined measurement (which considers the pair as one system) cannot be less optimal than the corresponding sequential one (local measurements, accompanying by transfer of classical information). The case of equality is achieved only when the mixed states have the same eigenvalues or the same eigenvectors. Further, we consider a case then the two systems are entangled: measurement of one system induces a reduction of the another one's state. The conclusion about optimal character of combined measurement takes place again, and conditions where the above-mentioned methods coincide are derived.

quant-ph↗

arXiv1998

Spin-glass model with partially annealed asymmetric bonds

We have considered the two-spin interaction spherical spin-glass model with asymmetric bonds (coupling constants). Besides the usual interactions between spins and bonds and between the spins and a thermostat with temperature $T_σ$ there is also an additional factor: the bonds are not assumed random {\it a priori} but interact with some other thermostat at the temperature $T_{J}$. We show that when the bonds are frozen with respect to the spins a first order phase transition to a spin-glass phase occurs, and the temperature of this transition tends to zero if $T_J$ is large. Our analytical results show that a spin-glass phase can exist in mean-field models with nonrelaxational dynamics.

cond-mat.dis-nn↗

arXiv1998

Strengthened Lindblad inequality: applications in non equilibrium thermodynamics and quantum information theory

A strengthened Lindblad inequality has been proved. We have applied this result for proving a generalized $H$-theorem in non equilibrium thermodynamics. Information processing also can be considered as some thermodynamic process. From this point of view we have proved a strengthened data processing inequality in quantum information theory.

cond-mat.stat-mech↗

arXiv1998

The broadcast quantum channel for classical information transmission

It is well known that quantum theory forbids the exact copying of an unknown quantum state. Therefore in broadcasting of classical information by a quantum channel an additional contribution to the error in the decoding is expected. We consider the optimal copying transformation which is adapted to classical information transmission by two linearly independent quantum states, and show that there is no additional contribution to the error. Instead the clones are correlated, and this breaks their usefulness: The entanglement increases with non-orthogonality of the states. The capacity of the corresponding quantum channel is considered also.

quant-ph↗

arXiv1997

Multi-access channels in quantum information theory

The multi-access channels in quantum information theory are considered. Classical messages from independent sources, which are represented as some quantum states, are transported by a channel to one address. The messages can interact with each other and with external environment. After statement of problem and proving some general results we investigate physically important case when information is transported by states of electromagnetic field. One-way communication by noisy quantum channels is also considered.

quant-ph↗

arXiv1997

Variational approach to interfaces in random media: negative variances and replica symmetry breaking

A Gaussian variational approximation is often used to study interfaces in random media. By considering the 1+1 dimensional directed polymer in a random medium, it is shown here that the variational Ansatz typically leads to a negative variance of the free energy. The situation improves by taking into account more and more steps of replica symmetry breaking. For infinite order breaking the variance is zero (i.e. subextensive). This situation is reminiscent of the negative entropies in mean field spin glass models, which were also eliminated by considering infinite order replica symmetry breaking.

cond-mat.dis-nn↗

arXiv1997

Converse coding theorems for quantums source and noisy channel

The weak converse coding theorems have been proved for the quantum source and channel. The results give the lower bound for capacity of source and the upper bound for capacity of channel. The monotonicity of mutual quantum information have also been proved. PACS numbers: 03.65.Bz

quant-ph↗

arXiv1997

Data processing in quantum information theory

The strengthened data processing inequality have been proved. The general theory have been illustrated on the simple example.

quant-ph↗

arXiv1996

Spherical P-spin glass at $P\to\infty$ and the information storing by continuous spins

The $P\to\infty$ limit was considered in the spherical P-spin glass. It is possible to store information in the vacuum configuration of ferromagnetic phase. Maximal allowed level of noise was calculated in ferromagnetic phase.

cond-mat↗

arXiv1996

Finite Size Effects for the Dilute Coupling Derrida Model

We consider paramagnetic, spin-glass and ferromagnetic phases. At $T=0$ model gives for the some values of connectivity (near the critical) extremal suppression of finite size effects (decoding error probability).

cond-mat↗