Examenspresentation Erika Magnusson

Combining the Transcorrelated Method with Adaptive Ansatzes for NISQ Computation. 

TC-AVQITE - Transcorrelated Adaptive Variational Quantum Imaginary Time Evolution 

Abstract: 

The fundamental problem of quantum chemistry is that the cost of solving the electronic Schrödinger equation scales exponentially with size. One way to potentially circumvent this scaling is by using quantum computers, whose properties allow an exponential speedup. However, near term quantum computers struggle to compete with conventional quantum chemistry methodology. Near term quantum processors are both small in terms of number of available qubits, and the number of operations that can be run on them is limited by systematic noise. Despite this, there are various methods that attempts to find quantum advantage in the current noisy intermediate-scale quantum (NISQ) regime, like variational quantum imaginary time evolution, which iteratively moves toward the energy ground state by evolving the quantum state in imaginary time. By splitting the evolution into steps and iterating, the necessary circuit depth is limited. This thesis evaluates the combination of variational quantum imaginary time evolution with two approaches aimed at decreasing quantum costs: the transcorrelated (TC) method of Boys and Handy and the adaptive ansatzes of Grimsley et al. The resulting method, transcorrelated adaptive variational quantum imaginary time evolution (TC-AVQITE), is evaluated through simulations of near-term quantum devices. The results obtained for small systems (H2, quadratic H4, and lithium hydride) indicate that the combination works well, especially for increasing system sizes. This development takes us a step closer to chemically relevant calculations on quantum computers.