16,487 research outputs found

### Stationary quantum Markov process for the Wigner function

As a stochastic model for quantum mechanics we present a stationary quantum
Markov process for the time evolution of the Wigner function on a lattice phase
space Z_N x Z_N with N odd. By introducing a phase factor extension to the
phase space, each particle can be treated independently. This is an improvement
on earlier methods that require the whole distribution function to determine
the evolution of a constituent particle. The process has branching and
vanishing points, though a finite time interval can be maintained between the
branchings. The procedure to perform a simulation using the process is
presented.Comment: 12 pages, no figures; replaced with version accepted for publication
in J. Phys. A, title changed, an example adde

### Optimal estimation of a physical observable's expectation value for pure states

We study the optimal way to estimate the quantum expectation value of a
physical observable when a finite number of copies of a quantum pure state are
presented. The optimal estimation is determined by minimizing the squared error
averaged over all pure states distributed in a unitary invariant way. We find
that the optimal estimation is "biased", though the optimal measurement is
given by successive projective measurements of the observable. The optimal
estimate is not the sample average of observed data, but the arithmetic average
of observed and "default nonobserved" data, with the latter consisting of all
eigenvalues of the observable.Comment: v2: 5pages, typos corrected, journal versio

### Unitary-process discrimination with error margin

We investigate a discrimination scheme between unitary processes. By
introducing a margin for the probability of erroneous guess, this scheme
interpolates the two standard discrimination schemes: minimum-error and
unambiguous discrimination. We present solutions for two cases. One is the case
of two unitary processes with general prior probabilities. The other is the
case with a group symmetry: the processes comprise a projective representation
of a finite group. In the latter case, we found that unambiguous discrimination
is a kind of "all or nothing": the maximum success probability is either 0 or
1. We also closely analyze how entanglement with an auxiliary system improves
discrimination performance.Comment: 9 pages, 3 figures, presentation improved, typos corrected, final
versio

### Distinct doping dependences of the pseudogap and superconducting gap La$_{2-x}$Sr$_{x}$CuO$_4$ cuprate superconductors

We have performed a temperature-dependent angle-integrated photoemission
study of lightly-doped to heavily-overdoped La$_{2-x}$Sr$_{x}$CuO$_4$ and
oxygen-doped La$_2$CuO$_{4.10}$. We found that both the magnitude $\Delta$* of
the (small) pseudogap and the temperature \textit{T}* at which the pseudogap is
opened increases with decreasing hole concentration, consistent with previous
studies. On the other hand, the superconducting gap $\Delta_{sc}$ was found to
remain small for decreasing hole concentration. The results can be explained if
the superconducting gap opens only on the Fermi arc around the nodal
(0,0)-($\pi,\pi$) direction while the pseudogap opens around $\sim$($\pi$, 0).Comment: 4 pages, 3 figure

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