Ultracold molecules for quantum simulation: rotational coherences in CaF and RbCs

Abstract

Polar molecules offer a new platform for quantum simulation of systems with long-range interactions, based on the electrostatic interaction between their electric dipole moments. Here, we report the development of coherent quantum state control using microwave fields in 40 Ca 19 F and 87 Rb 133 Cs molecules, a crucial ingredient for many quantum simulation applications. We perform Ramsey interferometry measurements with fringe spacings of ∼ 1 kHz and investigate the dephasing time of a superposition of N = 0 and N = 1 rotational states when the molecules are confined. For both molecules, we show that a judicious choice of molecular hyperfine states minimises the impact of spatially varying transition-frequency shifts across the trap. For magnetically trapped 40 Ca 19 F we use a magnetically insensitive transition and observe a coherence time of 0.61(3) ms. For optically trapped 87 Rb 133 Cs we exploit an avoided crossing in the AC Stark shifts and observe a maximum coherence time of 0.75(6) ms.

Ultracold polar molecules as qudits

“…For 87 Rb 133 Cs molecules the quadrupole terms contribute around 0.5 MHz for N=1, although there are other alkali dimers such as 6 Li 85 Rb for which the contributions are an order of magnitude larger [60]. The splittings are two orders of magnitude smaller for 87 projection quantum numbers are conserved at sufficiently high magnetic fields, but the hyperfine part of the Hamiltonian is off-diagonal and introduces mixings. Transitions between many hyperfine levels are allowed [55,56].…”

“…Molecules formed by associating an alkali atom and a closed-shell atom [79][80][81] will also have 2 Σ ground states. To illustrate our discussion, we consider the specific cases of 40 Ca 19 F and 87 Rb 133 Cs molecules.…”

“…The difference between the hyperfine splittings of the N and N±1 rotational manifolds depends largely on two terms: (i) the interaction between the electron spin and the nuclear rotation and (ii) the dipolar interaction between the electron and nuclear spins [87]. For 40 Ca 19 F, the contributions of these two terms to the splitting are ∼60 MHz and ∼20 MHz respectively [61].…”

“…The results of such searches for N=0 and N=1 rotational manifolds of 87 Rb 133 Cs are discussed below.…”

“…In this section, we discuss the main decoherence mechanisms for a qudit formed from a single ultracold molecule. For the states of 87 Rb 133 Cs considered here, the decoherence rate due to spontaneous emission and room-temperature blackbody radiation will be less than 10 −5 Hz [94] and can be ignored. For 40 Ca 19 F the excitation from room temperature blackbody radiation results in a decoherence rate of ∼0.2Hz, which can be reduced to∼10 −5 Hz at a temperature of 77 K [94].…”