Quantum Optics Seminar by Kaur Kristjuhan


Solving constrained electronic structure problems with quantum computers


One of the most investigated applications of quantum computers is the simulation of electronic structure problems in chemistry and materials science. To solve these problems effectively, it is crucial to represent the system of interest, such as a molecule, on the quantum computer accurately and faithfully. Commonly used tools for this task are fermion-qubit mappings such as the Jordan-Wigner or Bravyi-Kitaev mapping, which create a direct correspondence between the electronic orbitals of atoms and the qubits of quantum computers. Unfortunately, these methods lack a way of incorporating information about known physical quantities such as the total number of electrons in the system, which results in the quantum computers spending unnecessary time and resources representing and exploring physically irrelevant states.

In this talk, we present a classical method which enables us to include physical constraints directly within the mapping procedure. Benefits of this approach include a lower number of required qubits, more freedom in designing quantum circuits, and higher resilience to various errors caused by hardware. We will explain the theory behind the method:


...and also present recent experimental results obtained from calculations performed on gate-based superconducting quantum computers provided by IBM. We will discuss what this study has taught us about using real quantum hardware and outline a methodology for benchmarking related methods on such machines in the future. We also touch upon how classical and quantum computations can aid each other even further and describe our efforts in employing tensor networks for this task.