Density-controlled Crossing of a Quantum Phase Transition in a Spin-1 BEC

We explore the crossing of a quantum phase transition in a spin-1, ferromagnetic BEC. By changing the ratio of the quadratic Zeeman energy and the collisional energy of the BEC, we can control the spin dynamics across the critical point between the polar and broken-axis phases. Typical experimental procedures traverse the two quantum phases through sudden reduction of the magnetic field strength, which has potential experimental drawbacks originating from field instabilities. Our technique instead replaces this magnetic field “quench” with a change in the BEC’s density, inducing the necessary change in the atomic interactions. When allowed to evolve, the resultant dynamics produce spin-nematic squeezing. Correcting for the detection noise, we observed a squeezing of -8dB using this procedure. This demonstrates a new method by which to cross a quantum phase transition and points the way toward further work employing density-induced quenches to traverse a quantum critical point and engineer other exotic quantum phenomena, such as parametric squeezing and Dicke states.