publications

2022

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. APS

   Improving the sensitivity of cold-atom inertial sensors using robust control

   Anderson, Russell P., Perunicic, Viktor S., Stuart, Szigeti S.,  Goh, Matthew L., Saywell, Jack C., Light, Philip S., Wilson, Nathaniel, Milne, Alastair R., and Biercuk, Michael J.

   Bulletin of the American Physical Society Mar 2022

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   Cold-atom sensors have demonstrated state-of-the-art inertial measurements in laboratory environments. Future generations of atomic inertial sensors aim to achieve this performance in compact and rugged form factors, enabling new capabilities in navigation, hydrology, and space-based gravimetry. However, there are significant challenges to achieving laboratory performance in real-world environments. In particular, deploying a cold-atom sensor onboard a moving platform (e.g. a ship or plane) degrades measurement sensitivity by many orders of magnitude, or even prevents sensor operation altogether. Here we show that error-robust quantum control can suppress dominant noise sources due to platform motion, reducing the performance gap between laboratory operation and real-world field deployment. We use a custom-built flexible optimization package, which exploits automated analytic differentiation and stochastic optimization, to efficiently create broadband robust beamsplitter and mirror pulses. Through simulated operation under realistic vehicle motion, we verify that these robust control solutions suppress the effect of transverse accelerations by considerably in a single-axis sensor.

2020

1. 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}
   }

2019

1. ANU

   Feedback control of atomic Fermi gases

   Goh, Matthew L.

   ANU Open Research Repository Nov 2019

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   Ultracold atomic Fermi gases are the leading platform for analogue quantum simulation, and provide a promising avenue to study the origin of high-temperature superconductivity in cuprates. However, current experimental approaches to cooling Fermi gases use evaporative cooling, which is limited by poor thermalisation properties of fermions and is non-number-conserving. This prevents the creation of useful analogue simulators of many collective phenomena. This thesis is the first theoretical investigation into the use of continuous-measurement feedback control as an alternative means of cooling an atomic Fermi gas. Since tractable simulation of Fermi gas dynamics requires simplifications to the full quantum field theory, we derive and simulate a fermionic equivalent to the Gross-Pitaevskii equation, generalising a model of feedback-controlled BECs by Haine et al. to multimode ultracold atomic Fermi gases. We demonstrate that in the absence of measurement effects, a suitable control can drive an interacting Fermi gas arbitrarily close to its ground state. However, although control schemes based upon damping spatial density fluctuations work well for single-spatial-mode BECs, we show that they perform poorly for Fermi gases with a large number of atoms due to counter-oscillation of multiple spatial modes, which must exist due to Pauli exclusion. We generalise a feedback-measurement model of BECs by Szigeti et al. to a multimode atomic Fermi gas, and perform stochastic simulations of measured, feedback-controlled fermions in the single-atom and many-atom mean-field limits. The effects of measurement backaction are an important consideration, since in a realistic experiment knowledge of the system state used for feedback must be obtained from measurement, leading to competition between measurement-induced heating and feedback cooling. We show that weaker and less precise measurements cool the system to a lower equilibrium excitation energy, but are unable to place practical lower bounds on measurement strength due to the lack of a system-filter separation. When measurement-induced heating is accounted for, we find that the equilibrium energy per particle scales superlinearly, suggesting that existing control schemes which work well for bosons would not be effective for fermions. In light of this, we propose several avenues of future investigation to overcome this limitation, leaving open the possibility of feedback control of atomic Fermi gases as a pathway to analogue quantum simulation.