Measurements and open quantum systems

The inherent probabilistic nature of quantum mechanics places particular importance on the concept of quantum measurements. My interests lie in the interplay of the effect of measurements on a quantum state and unitary quantum dynamics. Measuring a quantum degree of freedom generally collapses its state to one of the eigenstates of the observable being measured with the Born rule probability. This disetangles the measured degree of freedom from the rest of the system and leads to a loss of coherence in the dynamics. The competition between entangling unitary dynamics and dientangling measurements is an interesting problem in statistical mechanics as well as in quantum information theory. Another setting is to consider a measurement apparatus weakly coupled to the system, and measuring the apparatus after decoupling it from the system. However, since the quantum dynamics prior to the measurement entangles the apparatus with the system, measuring the former non-trivially affects the state of the latter. This seemingly undesirable affect can be turned to an advantage by designing appropriate protocols that allow us to steer quantum systems towards correlated states or realise novel phases via measurements. In the time-continuum limit, systems undergoing continuous measurements can be described using a Lindblad equation which arises also in the context of open quantum systems in contact with Markovian environments.

Measurement-induced entanglement transition

The competition between entangling unitary dynamics and disentangling projective measurements leads to an entanglement phase transition. In the entangled phase, the system retains coherence and stays volume-law entangled for an exponentially (in system size) long time. Beyond a critical rate of measurement, the system transitions to a disentangled phase with area law entanglement. We proposed an exact theory for this phase transition in the setting of all-to-all random unitary circuits punctuated with measurements.

Adam Nahum, Sthitadhi Roy, Brian Skinner, and Jonathan Ruhman, "Measurement and entanglement phase transitions in all-to-all quantum circuits, on quantum trees, and in Landau-Ginsburg theory",
[PRX Quantum 2, 010352 (2021)][arXiv:2009.11311]

Measurement-induced quantum steering

We set out a general protocol for measurement-induced steering of quantum systems. The protocol requires multiple repetitions of an elementary step: during each step the system evolves for a fixed time while coupled to auxiliary degrees of freedom (which we term 'detector qubits') that have been prepared in a specified initial state. The detectors are discarded at the end of the step, or equivalently, their state is determined by a projective measurement with an unbiased average over all outcomes. The steering harnesses back-action of the detector qubits on the system, arising from entanglement generated during the coupled evolution. We illustrate our general ideas using a many-body example of a spin-1 chain steered to the Affleck-Kennedy-Lieb-Tasaki state.

Sthitadhi Roy, J. T. Chalker, I. V. Gornyi, Yuval Gefen, "Measurement-induced steering of quantum systems", [Phys. Rev. Research 2, 033347 (2020)] [arXiv:1912.04292]

Dissipative time-crystal

One route to protecting driven quantum systems from a heat death is to couple them to environments that dissipative energy from the systems. Naively, this suggests that a time-crystal can therefore be stabilised by dissipation. We however show another key issue, noise induced by the environment can jeopardise the stability of time-crystal by decohering the subharmonic oscillations. We show that this indeeds leads to the death of time-crystal in 1D but a 2D system is resilient to the noise and realises a stable dissipative time-crystal.

Achilleas Lazarides, Sthitadhi Roy, Francesco Piazza, Roderich Moessner, "Time crystallinity in dissipative Floquet systems" [Phys. Rev. Research 2, 022002(R) (2020)][arXiv:1904.04820]