Efficient Multiphoton Sampling of Molecular Vibronic Spectra on a Superconducting Bosonic Processor

Circuit quantum electrodynamics, in which microwave cavity modes are coupled to “artificial atoms” realized with Josephson junction qubits, has allowed for a variety of investigations in quantum optics and quantum information. In recent years, our team at Yale has focused on a hardware efficient approach, where high-Q microwave cavities serve as quantum memories. When dispersively coupled to transmon qubits, complex non-classical states can be created in these cavities, and operations between cavities can be enacted through parametric driving. For instance, we have recently shown high-quality cavity-cavity swaps via a beam-splitter or conversion operation, single and two-mode squeezing, and engineered cross and self-Kerr interactions. Finally, one can perform strong projective measurements of the photon number, the photon parity, or indeed any other binary-valued operator within the multi-dimensional Hilbert space. This system therefore has all of capabilities of linear optical systems, but with the addition of deterministic state preparation, measurement, and nonlinear interactions. One way to employ these capabilities is to directly simulate problems which are “naturally” bosonic in nature. In this talk, I will present our results utilizing these capabilities for determining Franck-Condon factors in the vibronic spectra of simple molecules [1], in which a microwave cavity can directly emulate the multiple excitations of a vibrational mode. With this programmable simulator, we obtain good agreement with classical calculations for simple molecules, and employ a novel photon number resolving detection that allows efficient sampling. Finally, we can compare this approach to using a traditional qubit-based simulation, which would require eight or more qubits and thousands of gates, illustrating some of the advantages of direct bosonic emulation.