Hidetoshi Nishimori

Statistical mechanics of image restoration and error-correcting codes

We develop a statistical-mechanical formulation for image restoration and error-correcting codes. These problems are shown to be equivalent to the Ising spin glass with ferromagnetic bias under random external fields. We prove that the quality of restoration/decoding is maximized at a specific set of parameter values determined by the source and channel properties. For image restoration in a mean-field system a line of optimal performance is shown to exist in the parameter space. These results are illustrated by solving exactly the infinite-range model. The solutions enable us to determine how precisely one should estimate unknown parameters. Monte Carlo simulations are carried out to see how far the conclusions from the infinite-range model are applicable to the more realistic two-dimensional case in image restoration.

Mathematical foundation of quantum annealing

Quantum annealing is a generic name of quantum algorithms that use quantum-mechanical fluctuations to search for the solution of an optimization problem. It shares the basic idea with quantum adiabatic evolution studied actively in quantum computation. The present paper reviews the mathematical and theoretical foundations of quantum annealing. In particular, theorems are presented for convergence conditions of quantum annealing to the target optimal state after an infinite-time evolution following the Schrodinger or stochastic (Monte Carlo) dynamics. It is proved that the same asymptotic behavior of the control parameter guarantees convergence for both the Schrodinger dynamics and the stochastic dynamics in spite of the essential difference of these two types of dynamics. Also described are the prescriptions to reduce errors in the final approximate solution obtained after a long but finite dynamical evolution of quantum annealing. It is shown there that we can reduce errors significantly by an ingenious choice of annealing schedule (time dependence of the control parameter) without compromising computational complexity qualitatively. A review is given on the derivation of the convergence condition for classical simulated annealing from the view point of quantum adiabaticity using a classical-quantum mapping.

Ground State of Antiferromagnetic Quantum Spin Systems on the Triangular Lattice

The spin-1/2 antiferromagnetic Heisenberg and X Y models are investigated on the triangular lattice by numerically diagonalizing finite size systems (up to 21 spins). The ground state energies of both models are found to be close to other theoretical predictions except those of Marland and Betts who performed similar analysis to ours. We have numerically constructed a resonating valence bond state of Anderson. It has a relatively large projection on the ground state of the finite Heisenberg model, although the absolute value of the projection decreases linearly with the system size. The trial function proposed by Miyashita for the X Y model does not have an appreciable projection on the numerically exact state. We present evidence for the absence of a sublattice long range order in the Heisenberg model and the existence of the same quantity in the X Y model.

Phase diagram and critical exponents of the ±J ising model in finite dimensions by Monte Carlo renormalization group

We have performed large-scale numerical simulations for the ± J Ising model on the simple cubic and the square lattices with asymmetric weight of ferromagnetic and antiferromagnetic bonds. The Monte Carlo renormalization group method is used to estimate T c , γ/ν, β/ν and ν at various points along the phase boundary between ferromagnetic and paramagnetic phases including the tricritical point. The obtained values of T c and the resulting phase diagrams of the simple cubic and the square lattices are much more accurate than those estimated so far. In the simple cubic lattice, we find evidence for the possibility that universality of the ferromagnetic critical exponents holds along the critical line except at the tricritical point, while weak universality holds even at the tricritical point. Weak universality seems to hold also in the square lattice.

Comment on “Statistical mechanics of CDMA multiuser demodulation” by T. Tanaka

In a recent paper, Tanaka formulated and solved a model of CDMA multiuser demodulation by applying the theory of spin glasses, the replica method in particular. It is shown in the present comment that some of his results can be derived without recourse to the replica method; conclusions from the somewhat tricky replica method are justified rigorously.

Modified Spin Wave Theory of the Two-Dimensional Frustrated Heisenberg Model

The modified spin wave theory of Takahashi is applied to the antiferromagnetic Heisenberg model with next nearest neighbor interactions on the square lattice in the ground state. The result indicates existence of ordered states for any value of the ratio of the nearest and next nearest neighbor interactions in contrast to some of the other theories.

Retrieval dynamics of associative memory of the Hopfield type

The authors have investigated the dynamics of retrieval processes of an associative memory of the Hopfield type (1982). For synchronous dynamics, they have generalized the theory of Amari and Maginu (1988), which enables them to treat the intermediate processes of memory retrieval in terms of a few simple macrovariables to the finite-temperature case. The resulting phase diagram in the equilibrium limit agrees qualitatively well with that from equilibrium statistical mechanics. They have carried out Monte Carlo simulations to clarify the limit of applicability of their theory. They have found that their basic assumption, an independent Gaussian distribution of the noise term with a time-dependent variance, is satisfied if the network succeeds in retrieval. When retrieval fails, the distribution of noise is non-Gaussian from very early stages of time development. For asynchronous dynamics, they propose a time-dependent Ginzburg-Landau approach, which simply expresses a downhill motion of the network in the free energy landscape. The resulting flow diagram in a phase space describes the behaviour of the network when it is close to equilibrium.

Phase Diagram of the ± J Ising Model in Two Dimensions

The ± J Ising model with asymmetric probability weight of ± bonds in two dimensions is studied in the p - T plane by the numerical transfer-matrix method. We calculate finite-width susceptibilities and finite-width correlation lengths of long strips of various sizes up to 14×10 5 . Finite-size scaling analysis suggests that the random antiphase state exists adjacent to the ferromagnetic phase. The boundary separating these phases is vertical (parallel to the temperature axis) in the phase diagram. The weak universality seems to hold along the ferromagnetic critical line.

Phase diagram of gauge glasses

The authors introduce a general class of random spin systems which are symmetric under local gauge transformations. Their model is a generalization of the usual Ising spin glass and includes the Zq, XY, and SU (2) gauge glasses. For this general class of systems, the internal energy and an upper bound on the specific heat are calculated explicitly in any dimensions on a special line in the phase diagram. Although the line intersects a phase boundary at a multicritical point, the internal energy and the bound on the specific heat are found to be written in terms of a simple function. They also show that the boundary between the ferromagnetic and nonferromagnetic phases is parallel to the temperature axis in the low-temperature region of the phase diagram. This means the absence of re-entrant transitions. All these properties are derived by simple applications of gauge transformations of spin and randomness degrees of freedom.

Duality and multicritical point of two-dimensional spin glasses

Determination of the precise location of the multicritical point and phase boundary is a target of active current research in the theory of spin glasses. In this short note we develop a duality argument to predict the location of the multicritical point and the shape of the phase boundary in models of spin glasses on the square lattice.