We present photoexcitation of ultra-long-range Rydberg molecules as a probe of spatial correlations in bosonic and fermionic quantum gases. Rydberg molecules can be created with well-defined internuclear spacing, set by the radius of the outer lobe of the Rydberg electron wave function
Rn. By varying the principal quantum number
n of the target Rydberg state, the molecular excitation rate can be used to map the pair-correlation function of the trapped gas
g(2)(Rn). We demonstrate this with ultracold Sr gases and probe pair-separation length scales in the range
Rn=1400−−3200 a0, which are on the order of the thermal de Broglie wavelength for temperatures around 1 $\ensuremath{\mu}\mathrm{K}$. We observe bunching for a single-component Bose gas of
84Sr and antibunching due to Pauli exclusion at short distances for a polarized Fermi gas of
87Sr, revealing the effects of quantum statistics.
Laser-driven Rydberg gases are many-body quantum systems that exhibit a pronounced collective excitation behavior due to the strong interaction between atoms in high-lying electronic states. Atoms located within a so-called blockade volume share a single Rydberg excitation, which is dynamically created and annihilated. For sufficiently long times, this driven system approaches a steady state, which lends its properties from a maximum entropy state of a Tonks gas. Using this connection, we show that spatial correlations between Rydberg atoms are controlled by the number of atoms contained within a blockade volume. For a small number, the system favors a disordered arrangement of Rydberg atoms, whereas in the opposite limit, Rydberg atoms tend to arrange in an increasingly ordered configuration. We argue that this is an entropic effect which is observable in current experiments. Our work demonstrates how ordered structures can spontaneously emerge in a strongly interacting, closed many-body system out of equilibrium.
Molecular-dynamics simulations have been performed for frozen Rydberg gases in a strong magnetic field of B = 2.9 T, up to a microsecond time scale. We found that a small number of spontaneously ionized electrons may de-excite a large portion of Rydberg atoms, while the free electrons leave the frozen Rydberg gases along the B-field. Such de-excitations as well as Rydberg–Rydberg atom interactions can induce significant heating to strongly magnetized Rydberg gases. Kinetic energy increase by 2–3 orders of magnitude has been observed for the initially frozen (~0.2 mK) Rydberg gases.