Speaker
Description
I present the recent concept of first-order dissipative phase transitions, that can occur in meso- and even microscopic quantum systems.
One of the first examples of this phenomenon was the photon-blockade breakdown (PBB) effect, occurring most simply in the driven-dissipative Jaynes-Cummings model that contains only a single qubit. For PBB, an abstract thermodynamic limit has been identified [1] where the coupling between the qubit and the mode goes to infinity without affecting the system size (a.k.a. zero-dimensional or finite-component phase transition). This limit was studied in a finite-size scaling approach [2], with scaling exponents determined numerically. PBB and its thermodynamic limit were observed in circuit QED systems [3,4]. For the highest realized coupling strength, the system alternates with a characteristic timescale as long as 6s – exceeding the microscopic timescales by 6 orders of magnitude – between a bright coherent state with approximately 10000 intracavity photons and the vacuum state.
In the second half of the talk, I describe analogous effects observed in a rubidium cold-atom cavity-QED setup [5]. Atoms in an optical cavity can manifest a first-order dissipative phase transition dubbed transmission-blockade breakdown. The stable coexisting phases are quantum states with high quantum purity, that include atomic hyperfine ground states and coherent states of electromagnetic field modes [6]. Similarly to PBB, the phases become perfectly pure states in the thermodynamic limit, making the phenomenon relevant for quantum technologies.
[1] Carmichael, Howard J. "Breakdown of photon blockade: A dissipative quantum phase transition in zero dimensions." Physical Review X 5, no. 3 (2015): 031028.
[2] Vukics, András, András Dombi, Johannes M. Fink, and Péter Domokos. "Finite-size scaling of the photon-blockade breakdown dissipative quantum phase transition." Quantum 3 (2019): 150.
[3] Fink, Johannes M., András Dombi, András Vukics, Andreas Wallraff, and Peter Domokos. "Observation of the photon-blockade breakdown phase transition." Physical Review X 7, no. 1 (2017): 011012.
[4] Sett, Riya, Farid Hassani, Duc Phan, Shabir Barzanjeh, Andras Vukics, and Johannes M. Fink. "Emergent macroscopic bistability induced by a single superconducting qubit." PRX Quantum 5, no. 1 (2024): 010327.
[5] Dombi, András et al. "Collective self-trapping of atoms in a cavity." New J. Phys. 23 (2021): 083036
[6] Gábor, Bence et al. "Ground-state bistability of cold atoms in a cavity." Physical Review A 107.2 (2023): 023713; Gábor, Bence et al. "Quantum bistability in the hyperfine ground state of atoms." Physical Review Research 5.4 (2023): L042038.
