Loss of Ultracold Rb87Cs133 Molecules via Optical Excitation of Long-Lived Two-Body Collision Complexes

Abstract

We show that the lifetime of ultracold ground-state 87 Rb 133 Cs molecules in an optical trap is limited by fast optical excitation of long-lived two-body collision complexes. We partially suppress this loss mechanism by applying square-wave modulation to the trap intensity, such that the molecules spend 75% of each modulation cycle in the dark. By varying the modulation frequency, we show that the lifetime of the collision complex is 0.53 ± 0.06 ms in the dark. We find that the rate of optical excitation of the collision complex is 3 +4 −2 × 10 3 W −1 cm 2 s −1 for λ = 1550 nm, leading to a lifetime of < 100 ns for typical trap intensities. These results explain the two-body loss observed in experiments on nonreactive bialkali molecules.

Robust storage qubits in ultracold polar molecules

“…We see no evidence for collisional energy shifts, which would be observed by a change in the energies of the states when the density reduces over the course of each Ramsey measurement (see Supplementary Information). This is consistent with previous observations [25], and the absence of collisional energy shifts or decoherence may be expected as short-range collisions in the gas lead to loss of molecules with high probability [35][36][37][38]. Measurements of the coherence out to longer times will require confinement of the molecules to a 3D optical lattice [39] , optical tweezers [40][41][42], or the use of alternative trapping techniques such as a blue-detuned optical trap [43] to avoid losses from the optical excitation of two-molecule collision complexes [36,44].…”

“…This is consistent with previous observations [25], and the absence of collisional energy shifts or decoherence may be expected as short-range collisions in the gas lead to loss of molecules with high probability [35][36][37][38]. Measurements of the coherence out to longer times will require confinement of the molecules to a 3D optical lattice [39] , optical tweezers [40][41][42], or the use of alternative trapping techniques such as a blue-detuned optical trap [43] to avoid losses from the optical excitation of two-molecule collision complexes [36,44]. The creation of controlled arrays of molecules is also a key component of the proposed quantum computing protocols where storage qubits have applications; our method of using a magic-polarisation trap is compatible with the confinement of molecules to arrays of optical tweezers or a 3D optical lattice [45].…”

“…3. The maximum Ramsey time available is limited by collisional loss of the molecules [35,36] with T 1 = 0.61(4) seconds, which reduces our signal at long times. We measure Ramsey fringes out to T = 1.2 s with no evidence of decoherence.…”

Controlling the ac Stark effect of RbCs with dc electric and magnetic fields

“…The transitions to vibronic levels of the b 3 state in this wavelength range are suppressed by negligible Franck-Condon factors [78,101]. Nevertheless, during initial attempts to load RbCs molecules into an ODT of λ ≈ 1064 nm, we observed a loss of groundstate molecules that was orders of magnitude faster than the near-universal collisional losses that typically dominate our experiments [102,103].…”

“…We can suppress the resonant loss, such that it becomes unobservable in our experiments, by tuning the laser frequency several GHz away from the transition. By doing this, we have been able to load RbCs molecules into an ODT made with 1064 nm light, with the lifetime limited by optical excitation of complexes formed in bimolecular collisions [102,103,105].…”