Theoretical Physics Seminar

Quantum computers, e.g., made of many superconducting qubits, are also complex quantum many-body systems. However, unlike conventional solid-state systems, these many-body assemblies, while evolving under various unitary gate operations, can also be frequently monitored by an external observer through repeated quantum measurements and feedback. Thus, quantum computers are specific types of open quantum systems, where repeated monitoring can be used as engineered bath, in addition to the usual sources of environmental dissipation and decoherence. How does such an 'observer bath' differ from a usual thermal bath, and can this bath induce interesting non-equilibrium steady states and phase transitions between them? After a broad introduction, I will discuss two examples. Starting from a generic model of continuously monitored or weakly measured chain of coupled anharmonic oscillators with feedback, I will show that the quantum dynamics maps to a stochastic Langevin dynamics with noise strength controlled by measurement strength in the semiclassical limit. I will show that such interacting stochastic system generically undergoes a phase transition analogous to well-known stochastic synchronization transition. I will discuss a similar measurement model for Josephson junction arrays (JJAs) and show that it undergoes an unusual inverse superconductor-insulator transition. I will contrast the phase diagram of monitored JJA with the well-studied case of dissipative JJA. 

References:

1. S. Ruidas and S. Banerjee, Phys. Rev. Lett. 132, 030402 (2024).

2. P. Das and S. Banerjee, arXiv:2412.04556.