Matt L. Goh

selected publications

1. arXiv

   Algorithmic Shadow Spectroscopy

   Chan, Hans Hon Sang, Meister, Richard,  Goh, Matthew L., and Koczor, Bálint

   arXiv preprint arXiv:2212.11036 Dec 2022

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   Finding energy differences between eigenstates of a quantum system, which contains key information about its properties, is a central task in many-body physics. Quantum computers promise to perform this task more efficiently than classical hardware; however, extraction of this information remains challenging. Non-trivial protocols for this task require either a substantial increase in circuit complexity (phase estimation) or a large number of circuit repetitions (variational quantum eigensolvers). Here we present shadow spectroscopy, a novel simulator-agnostic quantum algorithm which extracts energy gaps using an extremely low number of circuit repetitions (shots) and no extra resources (ancilla qubits) beyond performing time evolution and measurements. The approach builds on the fundamental feature that every observable property of a quantum system must evolve according to the same harmonic components, whose frequencies can be extracted from classical shadows of time-evolved quantum states. Classical post processing of the large set of resulting time-periodic signals directly reveals Hamiltonian energy differences with Heisenberg-limited precision. While the classical computational complexity is linear, the number of circuit repetitions (quantum resources) required is only logarithmic in the number of analysed observables. Moreover, applying shadow spectroscopy numerically to probe model systems and CH2 excited states verifies that the approach is intuitively easy to use in practice, very robust against gate noise, amiable to a new type of algorithmic-error mitigation technique, and uses orders of magnitude fewer number of shots than typical near-term quantum algorithms – as low as 10 shots per timestep is sufficient.

2. arXiv

   Feedback cooling Bose gases to quantum degeneracy

   Goh, Matthew L., Mehdi, Zain, Taylor, Richard L., Thomas, Ryan J., Bradley, Ashton S., Hush, Michael R., Hope, Joseph J., and Szigeti, Stuart S.

   Jun 2022

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   Degenerate quantum gases are instrumental in advancing many-body quantum physics and underpin emerging precision sensing technologies. All state-of-the-art experiments use evaporative cooling to achieve the ultracold temperatures needed for quantum degeneracy, yet evaporative cooling is extremely lossy: more than 99.9% of the gas is discarded. Such final particle number limitations constrain imaging resolution, gas lifetime, and applications leveraging macroscopic quantum coherence. Here we show that atomic Bose gases can be cooled to quantum degeneracy using real-time feedback, an entirely new method that does not suffer the same limitations as evaporative cooling. Through novel quantum-field simulations and scaling arguments, we demonstrate that an initial low-condensate-fraction thermal Bose gas can be cooled to a high-purity Bose-Einstein condensate (BEC) by feedback control, with substantially lower atomic loss than evaporative cooling. Advantages of feedback cooling are found to be robust to imperfect detection, finite resolution of the control and measurement, time delay in the control loop, and spontaneous emission. Using feedback cooling to create degenerate sources with high coherence and low entropy enables new capabilities in precision measurement, atomtronics, and few- and many-body quantum physics.

3. PhysRevLett

   Improved Phase Locking of Laser Arrays with Nonlinear Coupling

   Mahler, Simon,  Goh, Matthew L., Tradonsky, Chene, Friesem, Asher A., and Davidson, Nir

   Physical Review Letters Apr 2020

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   An arrangement based on a degenerate cavity laser for forming an array of nonlinearly coupled lasers with an intracavity saturable absorber is presented. More than 30 lasers were spatially phase locked and temporally Q switched. The arrangement with nonlinear coupling was found to be 25 times more sensitive to loss differences and converged five times faster to the lowest loss phase locked state than with linear coupling, thus providing a unique solution to problems that have several near-degenerate solutions.

   @article{mahler_improved_2020,
     abbr = {PhysRevLett},
     bibtex_show = {true},
     selected = {true},
     html = {https://link.aps.org/doi/10.1103/PhysRevLett.124.133901},
     title = {Improved {Phase} {Locking} of {Laser} {Arrays} with {Nonlinear} {Coupling}},
     volume = {124},
     doi = {10.1103/PhysRevLett.124.133901},
     number = {13},
     urldate = {2022-01-23},
     journal = {Physical Review Letters},
     author = {Mahler, Simon and Goh, Matthew L. and Tradonsky, Chene and Friesem, Asher A. and Davidson, Nir},
     month = apr,
     year = {2020},
     note = {Publisher: American Physical Society},
     pages = {133901}
   }