arXiv subjects

Hiroki Okada

Publications and source records attributed to Hiroki Okada.

2 recordsLinked to original sources

arXiv2017

Heat and charge current fluctuations and the time dependent coefficient of performance for a nanoscale refrigerator

We theoretically investigate the coefficient of performance (COP) of a mesoscopic thermoelectric refrigerator realized by using a tunnel junction. We analyze the influence of particle and heat current fluctuations on the COP out of the equilibrium regime. We calculate the average COP by using full counting statistics and find that it depends on the measurement time $τ$. The deviation from the macroscopic COP value can be expressed with the Skellam distribution at all times. This result enables us to improve the Gaussian approximation valid within the linear response regime, which cannot predict the average COP in the limit of $τ\to 0$. We illustrate the time dependence of the average COP and find that in the short-time regime, the average COP possesses a minimum. In order to confirm the physical consistency far from equilibrium, we propose checking the correlation coefficient between the particle and the heat currents in addition to the positivity of the entropy production rate.

cond-mat.mes-hall↗

arXiv2017

Randomness Evaluation with the Discrete Fourier Transform Test Based on Exact Analysis of the Reference Distribution

In this paper, we study the problems in the discrete Fourier transform (DFT) test included in NIST SP 800-22 released by the National Institute of Standards and Technology (NIST), which is a collection of tests for evaluating both physical and pseudo-random number generators for cryptographic applications. The most crucial problem in the DFT test is that its reference distribution of the test statistic is not derived mathematically but rather numerically estimated, the DFT test for randomness is based on a pseudo-random number generator (PRNG). Therefore, the present DFT test should not be used unless the reference distribution is mathematically derived. Here, we prove that a power spectrum, which is a component of the test statistic, follows a chi-squared distribution with 2 degrees of freedom. Based on this fact, we propose a test whose reference distribution of the test statistic is mathematically derived. Furthermore, the results of testing non-random sequences and several PRNGs showed that the proposed test is more reliable and definitely more sensitive than the present DFT test.

cs.CR↗