Measurement-induced quantum phases realized in a trapped-ion quantum computer

• Published in: Nature physics Vol. 18; no. 7; pp. 760 - 764
• Main Authors: Noel, Crystal, Niroula, Pradeep, Zhu, Daiwei, Risinger, Andrew, Egan, Laird, Biswas, Debopriyo, Cetina, Marko, Gorshkov, Alexey V., Gullans, Michael J., Huse, David A., Monroe, Christopher
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.07.2022; Nature Publishing Group; Nature Publishing Group (NPG)
• Subjects: 639/766/119/2795; 639/766/483/481; Atomic; Circuits; Classical and Continuum Physics; Complex Systems; Condensed Matter Physics; Critical point; Error correction; Fault tolerance; Initial conditions; Letter; Mathematical and Computational Physics; Molecular; Optical and Plasma Physics; Phase transitions; Phases; Physics; Physics and Astronomy; Purification; Quantum computers; Quantum computing; Theoretical
• DOI: 10.1038/s41567-022-01619-7
• ISSN: 1745-2473

Many-body open quantum systems balance internal dynamics against decoherence and measurements induced by interactions with an environment 1 , 2 . Quantum circuits composed of random unitary gates with interspersed projective measurements represent a minimal model to study the balance between unitary dynamics and measurement processes 3 – 5 . As the measurement rate is varied, a purification phase transition is predicted to emerge at a critical point akin to a fault-tolerant threshold 6 . Here we explore this purification transition with random quantum circuits implemented on a trapped-ion quantum computer. We probe the pure phase, where the system is rapidly projected to a pure state conditioned on the measurement outcomes, and the mixed or coding phase, where the initial state becomes partially encoded into a quantum error correcting codespace that keeps the memory of initial conditions for long times 6 , 7 . We find experimental evidence of the two phases and show numerically that, with modest system scaling, critical properties of the transition emerge. Many-body open quantum systems are predicted to undergo a phase transition towards a pure state through frequent projective measurements. The phases separated by this transition have now been observed with random circuits on a trapped-ion computer.