Performing induced quantum phase measurements on a trapped ion quantum computer

The quantum computer used in this study at the University of Maryland. Credit: Noel et al

Trapped ion quantum computers are quantum devices in which trapped ions vibrate together and are completely isolated from the external environment. These computers can be particularly useful for studying and realizing various states of quantum physics.

Researchers from NIST/University of Maryland and Duke University recently used a trapped ion quantum computer to realize two measurement-induced quantum phases, namely the pure phase and the mixed or coding phase during a transition from purification stage. Their findings, published in an article in the Natural Physicscontribute to the experimental understanding of quantum many-body systems.

“Our methods were based on the work of Michael Gullans and David Huse, who identified a measurement-induced purification transition in random quantum circuits,” said Crystal Noel, one of the researchers who conducted the study, at Phys.org. “The main objective of our paper was to experimentally observe this critical phenomenon using a quantum computer.”

To measure the purification phase transition first described by Gullans and Huse, the researchers had to average data collected across multiple random circuits. Additionally, the measurements they collected included both unitary and projective measurements.

“By starting in a mixed state with high entropy, or information, and then evolving the circuits, the entropy at the end of the circuit indicates whether that information has been lost, or in other words, the system is getting lost. is purified,” Noel explained. “We measured the entropy of the system after the circuit evolved while adjusting the measurement rate throughout the transition.”

According to theoretical predictions, the purification phase transition probed by the team should have appeared at a critical point, resembling a fault tolerance threshold. Noel and his colleagues conducted their experiments on random circuits optimized to work well with their ion trap quantum computer. This allowed them to observe the different phases of purification using a relatively small system.

“Critical phenomena of this nature are difficult to observe due to the need for large systems, mid-circuit measurements, and averaging over many random circuits that take significant computation time,” Noel said. “We found a way to adapt the model we studied to the system we had, and show that with a minimal model, critical phenomena can still be observed.”

Using their trapped ion quantum computer, the team was able to probe both the pure phase of the purification phase transition and the mixed or encoding phase. In the first of these states, the system is quickly projected to a pure state, which is related to the results of the measurement. In the second, the initial state of the system is partly encoded in a quantum error correction coding space, which retains the memory of the system from its original conditions for longer.

Performing induced quantum phase measurements on a trapped ion quantum computer

The new Duke Quantum Center team. Credit: Noel et al

Noel and his colleagues’ successful realization of these two phases of the purification transition in their ion-trap quantum computer could inspire other teams to use similar systems to probe other quantum phases of matter. In their next work, the researchers will continue to use the same computer, which has now been transferred to the New Duke Quantum Center, to study other physical phenomena. Chris Monroe, the principal investigator of the recent study, is now director of this center and will lead further studies using the trapped ion quantum computer.

“We now plan to continue studying critical phenomena in random circuits using our trapped ion quantum computer. We will add more qubits and mid-circuit measurements to increase hardware capabilities. We will work to find new new observables and interesting transitions similar to the one observed here in order to better understand quantum computing and open quantum systems more generally.”


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More information:
Crystal Noel et al, Measurement-Induced Quantum Phases Performed in a Trapped Ion Quantum Computer, Natural Physics (2022). DOI: 10.1038/s41567-022-01619-7

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