Phase transitions and critical states of monitored quantum systems

Prof Ehud Altman, Professor of Physics, Berkleley University of California

Catch-up: watch online

Phase transitions emerging in monitored (observed) quantum systems force intriguing phase transitions that can affect the behaviour of the quantum circuits that make up quantum computers.

The unique interplay of unitary gates, entanglement, measurement and noise in systems driven out of equilibrium gives rise to phase transitions detrimental to the circuit’s performance as a platform for quantum information processing, as information becomes coded into highly non-local degrees of freedom, where the information becomes irretrievable.

Generic unitary evolution of a quantum state, as affected for example by non-integrable Hamiltonian dynamics or by random gates in a quantum circuit, leads to encoding of information into highly non-local degrees of freedom, where the information becomes irretrievable. Recent work on quantum circuits began to explore how this kind of non-local encoding is affected by an external observer that can perform measurement on the quantum state as it evolves. It was found that a large system undergoes a phase transition, from a state with non-local encoding, evidenced by volume law entanglement, at a small enough measurement rate to an area law state above a threshold rate. Prof Altman will review how such transitions in the dynamics of quantum information can be understood in terms of effective statistical mechanics models, which describe a self-organized quantum error correcting code in random circuits. Prof Altman will use these models or random circuits to predict new universal phenomena in the dynamics of quantum information in monitored systems and discuss possible tests of these predictions using quantum hardware.

Prof. Altman’s group is interested in the theory of quantum matter, which broadly refers to matter in which quantum entanglement between many particles leads to unexpected emergent phenomena, such as unconventional magnetism, superconductivity and topological states. A recent focus has been to understand what types of universal phenomena can emerge in the dynamics of quantum systems, when they are driven strongly out of equilibrium. More reseach highlights from the Altman group can be found here.

This talk is part of an ongoing series of talks by US and Australian researchers presenting novel developments in condensed matter and cold atomic physics, enriching connections between the two physics communities. Co-presented by FLEET, Monash School of Physics and Astronomy, and the Joint Quantum Institute.

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