Doctorate hsnResearch Projects
Excited-State Quantum Phase Transitions in Spinor-BECs

Excited-State Quantum Phase Transitions in Spinor-BECs

Phase space and trajectories for different external parameters.
Led by:  Carsten Klempt
Team:  Bernd Meyer
Year:  2020

At ultra-cold temperatures and low densities, neutral atoms can form a Bose-Einstein condensate (BEC). When a BEC is trapped in an optical potential, the spin is a degree of freedom (Spinor-BEC) and controlled collisions between the atoms can lead to versatile quantum dynamics.

Interestingly, the competing energy contributions from external magnetic fields and the collisional interactions open up a rich phase diagram for such magnetic superfluids. In our case of a ferromagnetic condensate of Rubidium-87 in the state F=1, three phases are separated by quantum phase transitions and distinguished by an order parameter. Studying quantum phase transitions and the corresponding order parameter in ground-state Spinor-BECs enables deterministic generation of highly entangled twin-Fock states. This technique has been successfully implemented experimentally [1] and is the basis for a protocol to generate entangled momentum modes [2], a major step towards quantum-enhanced gravimetry. 

To further understand fundamental characteristics of Spinor-BECs, we want to study quantum phase transitions of excited states. Only recently, an experimentally accessible order parameter for those excited-state quantum phase transitions (ESQPTs) was presented [3]. This order parameter can be investigated using interferometric measurements in the F=1 hyperfine manifold of Rb-87 atoms, a ferromagnetic spin-1 system. Establishing the concept of excited-state quantum phase transitions by the measurement of order parameters would provide a substantial contribution to the field due to its applicability to completely different quantum many-body systems.

[1] Luo et al., Deterministic entanglement generation from driving through quantum phase transitions, Science 355, 620-623 (2017).
[2] Anders et al., Momentum Entanglement for Atom Interferometry, arXiv:2010.15796 (2020).
[3] Feldmann et al., Excited-state quantum phase transitions in spinor Bose-Einstein condensates, arXiv:2011.02823 (2020).