NQCP Life in Quantum - Senior Research Scientist Amir Karamlou, Google Quantum AI

Observation of constructive interference at the edge of quantum ergodicity

Quantum observables in the form of few-point correlators are the key to characterizing the dynamics of quantum many-body systems. In dynamics with fast entanglement generation, quantum observables generally become insensitive to the details of the underlying dynamics at long times due to the effects of scrambling. In experimental systems, repeated time-reversal protocols have been successfully implemented to restore sensitivities of quantum observables. Using a 103-qubit superconducting quantum processor, we characterize ergodic dynamics using the second-order out-of-time-order correlators, OTOC(2). In contrast to dynamics without time reversal, OTOC(2) are observed to remain sensitive to the underlying dynamics at long time scales. Furthermore, by inserting Pauli operators during quantum evolution and randomizing the phases of Pauli strings in the Heisenberg picture, we observe substantial changes in OTOC(2) values. This indicates that OTOC(2) is dominated by constructive interference between Pauli strings that form large loops in configuration space. The observed interference mechanism endows OTOC(2) with a high degree of classical simulation complexity, which culminates in a set of large-scale OTOC(2) measurements exceeding the simulation capacity of known classical algorithms. Further supported by an example of Hamiltonian learning through OTOC(2), our results indicate a viable path to practical quantum advantage. 

Bio: Amir is a Senior Research Scientist at Google Quantum, where he specializes in quantum simulation with superconducting qubits. He completed his Ph.D. and bachelor's degrees at MIT. His work explores how quantum processors can serve as a powerful tool for understanding physics in ways that are beyond the reach of classical computers. In addition to his research, he is dedicated to sharing his expertise and passion for the field through various teaching and outreach opportunities.