We study the dynamics of an initially degenerate homogeneous Bose gas after an interaction quench to the unitary regime at a magnetic Feshbach resonance. As the cloud decays and heats, it exhibits a crossover from degenerate-to thermal-gas behaviour, both of which are characterised by universal scaling laws linking the particle-loss rate to the total atom number N . In the degenerate and thermal regimes the per-particle loss rate is ∝ N 2/3 and N 26/9 , respectively. The crossover occurs at a universal kinetic energy per particle and at a universal time after the quench, in units of energy and time set by the gas density. By slowly sweeping the magnetic field away from the resonance and creating a mixture of atoms and molecules, we also map out the dynamics of correlations in the unitary gas, which display a universal temporal scaling with the gas density, and reach a steady state while the gas is still degenerate.Strong interactions and correlations are at the heart of the most interesting many-body quantum phenomena. The possibility to control the interaction strength via Feshbach resonances [1] makes ultracold atomic gases an excellent setting for studies of strongly correlated behaviour. On resonance, the s-wave scattering length a, which characterises two-body contact interactions, diverges. In this so-called unitary regime the interactions are as strong as allowed by quantum mechanics, and the physics cannot explicitly depend on a, leading to the possibility of new types of universal behaviour [2][3][4][5][6].Of particular interest are the interaction-dominated degenerate unitary gases. Within the 'universality hypothesis', they have only one relevant lengthscale -the average interparticle spacing, given by the density n, which also sets the natural energy and time scales [2]:where m is the atom mass. These 'Fermi energy' and 'Fermi time' scales are applicable to both Fermi and Bose gases. In Bose gases, however, the universality can be broken by Efimov physics [7][8][9][10][11][12][13][14][15][16][17]. The Feshbach dimer molecular state, responsible for the resonance, is of size a and becomes unbound as a → ∞, but the infinite series of Efimov trimer states, each of a size 22.7 times larger than the previous one, can introduce new lengthscales into the problem. While unitary Fermi gases have been extensively explored [3][4][5], the experimental [15,16,[18][19][20][21] and theoretical [22][23][24][25][26][27][28][29][30][31][32][33][34] studies of unitary Bose gases are only recently emerging. An experimental challenge is that they exhibit rapid threebody loss and heating, which also raises fundamental questions about the extent to which they have well defined equilibrium properties [20], but the loss dynamics also offer a valuable probe of the unitary behaviour [16,[18][19][20][21]. While coherent three-body correlations [15] and Efimov trimers [16] have been observed, the decay dynamics [16,[18][19][20][21] have been consistent with universal scalings (see also [35]). All experiments so far were performe...
