Quantinuum uses the extremely high precision of the H2-1 quantum computer to take a step forward in the race to understand exotic physics

A joint team of scientists at Quantinuum, NIST, and the University of Maryland have measured the Loschmidt amplitude of the Fermi-Hubbard model, demonstrating a vital tool for understanding enigmatic phases of matter such as superconductivity

Some of the more pressing and intractable problems in physics may be closer to being answered, such as the nature of superconductivity and other exotic properties, thanks to work done by a team at Quantinuum using the H2-1 trapped-ion quantum computer.  

Detailed in a scientific paper available on arXiv, the team used the H2-1 device to measure the “Loschmidt amplitude”, which quantifies how much a quantum system has changed after some time has passed (for the experts: this is the inner product between the time-evolved state and the initial state). Measuring the Loschmidt amplitude is central to several proposed quantum computing algorithms, including one described in the seminal work of Lu, Banuls and Cirac (2019). Their algorithm is a non-variational, hybrid quantum-classical scheme aimed at obtaining equilibrium properties of quantum systems. This is the first experimental demonstration of the quantum computation required for this algorithm.

To sweeten the pot, the research team measured the Loschmidt amplitude of a beloved, much-studied, and not-fully-understood model called the “Fermi-Hubbard” model. The Fermi-Hubbard model is used, among other things, to help scientists understand superconductivity, which is very challenging to explore fully with classical computing methods. When Richard Feynman “launched” the field of quantum computing with a famous talk in 1981, it was exactly this type of system he proposed we study with quantum computers: large quantum-mechanical systems that are difficult or impossible to effectively simulate classically. Using quantum computers to gain greater insights into the Fermi-Hubbard model could take us one step closer to understanding the behavior of high-temperature superconductors, a valuable goal with the potential to transform multiple industries.

A measurement of the Loschmidt amplitude is difficult because it is a “global observable”, meaning that any error in the quantum calculation will have an impact on the final results. This work highlights the outstanding precision of Quantinuum’s System Model H2 quantum computers. In particular, the trapped ion architecture allows for almost perfect state preparation and measurement, which is a necessary condition for such kind of calculations. Until now, this model had not been simulated with more than 16 qubits, in part because the gate operations applied are so complex. This paper explores the model on 32 qubits and includes a number of difficult elements; such as Schrodinger cat states, deep circuits, and complex Hamiltonians, making for a powerful demonstration of the H2-1 system capabilities. 

While this work is certainly a “NISQ”-era result, it shows that quantum computing can achieve interesting milestones without error correction – highlighting the fact that quantum methods may offer real advantages over classical methods in the near future. In addition, the team noted that while analog quantum simulators have made substantial progress in the study of exotic systems over the past decade, using a quantum computer to study these same systems allows for a wider exploration of the parameter space than Nature herself allows in laboratory simulations.

A more complex version of the algorithm will need to be implemented in the future to unlock the secrets of materials like superconductors, but in the meantime this work highlights the fact that Quantinuum is closing in on the answer to extremely relevant open questions, so far intractable with existing classical methods.