Approximation Methods

Jean-Louis Basdevant

Wen Li Yu, Hui Qiu Du

Wenzel-Kramers-Brillouin (WKB) method being an approximation method in quantum mechanics has been applied widely. For the potential problems WKB approximation method is taken as an effective technology. With the help of the WKB approximation method the quantum conditions of bound States particle being in three different typical one dimension (1D) potential well models are derived in this paper. In addition, the idea of the WKB approximation method and the procedures dealing with the potential problems are demonstrated clearly, which are very helpful for understanding the quantum conditions of 1D potential well models deeply and mastering the WKB approximation method to solve some related problems in quantum mechanics.

Łukasz Pawela, Przemysław Sadowski

We study three methods of obtaining an approximation of unitary evolution of a quantum system under decoherence. We use three methods of optimizing the control pulses: genetic optimization, approximate evolution method and approximate gradient method. To model the noise in the system we use the Lindblad equaiton. We obtain results showing that genetic optimization may give a better approximation of a unitary evolution in the case of high noise.

J. Daniel Kelley, Jacob J. Leventhal

Hirofumi Nishi, Taichi Kosugi, Yu‐ichiro Matsushita

Abstract The imaginary-time evolution method is a well-known approach used for obtaining the ground state in quantum many-body problems on a classical computer. A recently proposed quantum imaginary-time evolution method (QITE) faces problems of deep circuit depth and difficulty in the implementation on noisy intermediate-scale quantum (NISQ) devices. In this study, a nonlocal approximation is developed to tackle this difficulty. We found that by removing the locality condition or local approximation (LA), which was imposed when the imaginary-time evolution operator is converted to a unitary operator, the quantum circuit depth is significantly reduced. We propose two-step approximation methods based on a nonlocality condition: extended LA (eLA) and nonlocal approximation (NLA). To confirm the validity of eLA and NLA, we apply them to the max-cut problem of an unweighted 3-regular graph and a weighted fully connected graph; we comparatively evaluate the performances of LA, eLA, and NLA. The eLA and NLA methods require far fewer circuit depths than LA to maintain the same level of computational accuracy. Further, we developed a “compression” method of the quantum circuit for the imaginary-time steps to further reduce the circuit depth in the QITE method. The eLA, NLA, and compression methods introduced in this study allow us to reduce the circuit depth and the accumulation of error caused by the gate operation significantly and pave the way for implementing the QITE method on NISQ devices.

Jean-Louis Basdevant