Quantum Photonic Simulation of Molecular Spectra

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Project summary: We will develop a quantum-enhanced simulator of molecular vibronic spectra, based on the interference of quantum light. Photonic processing capabilities are currently far from meeting the stringent requirements for universal quantum computation; however, photonics does provide unique, and powerful, approaches to limited-purpose processors. For example, the Boson Sampling (BS) computing problem, which concerns the statistics of single photons exiting a multi-port interferometer, is recognized as a promising framework for the experimental demonstration of quantum supremacy in computing. The project we propose here builds on a BS variation that identifies a connection to molecular transitions, recently put forth by Alán Aspuru-Guzik (Harvard) and collaborators (10.1038/nphoton.2015.153). In this scheme, the interference of quantum light naturally emulates the electronic-vibronic transitions in complex molecules. The statistics from such an interferometer will reveal the spectrum of the molecule- a key task in theoretical chemistry that is currently limited by the power of classical computation. This student project aims to achieve the first demonstration of this type of simulator. We will also develop a theoretical connection of the simulator to Gaussian Boson Sampling, clarify the consequences of experimental imprecision, and formulate a route to molecule simulation that will surpass the current capacity of classical computation. This project falls within the EPSRC Quantum Technologies research area, as it is one of the studentship projects associated with the NQIT Hub. The impact of this project will include its contribution toward NQIT goals: building a photonic simulator is NQIT deliverable D23 (Month 58). In current plans, this consists of a large-scale BS machine (based on the "scattershot" variation) and a small-scale co-processor for DMFT calculation (with the quantum simulation research group in NQIT). The proposed device is a new concept for a photonic simulator. If successful, this device would have impact beyond the academic race to quantum supremacy by connecting the computational promise of BS to a task of wide interest in chemistry and molecular biology. One full-time NQIT researcher is working towards the photonic simulator. To date, this has primarily involved developing technical components for the simulator through collaborative projects with researchers working on non-NQIT projects. The proposed project aligns closely with this ongoing work, and critically, it provides a dedicated researcher needed to investigate this promising new concept. The molecular simulator is an example of a purpose-built quantum processor that runs without error correction. The photonic simulator will stimulate NQIT activity in this type of computation, which is likely to be the first achieved at an interesting scale with an NQIT ion-trap processor. Important theoretical problems to investigate with NQIT researchers include verification and error tolerance. The technical requirements to build the simulator have complete overlap with the light sources, interferometers, and detectors developed and used for other aspects of NQIT photonics. Beyond NQIT, we will collaborate with Aspuru-Guzik to compile a test set of molecules that are interesting from the perspective of a computational chemist, and understand how a realistic environment (e.g. collisions in solution) is simulated by optical loss and dephasing.