Single-photon loading of polar molecules into an optical trap

TL;DR

The paper presents a single-photon loading scheme that uses a state-dependent Stark landscape and an irreversible $\ket{i}\to\ket{e}\to\ket{t}$ transfer to load slow molecular beams into an optical lattice trap without requiring optical cycling. Using BaF as a prototype, it numerically evaluates the loading efficiency under realistic parameters, finding typical efficiencies around $\sim0.04\%$, with potential increases to $\sim0.52\%$ (≈$10^4$ molecules per shot) by synchronizing the pump with arrival times, and possible further gains from modulating the electric field. The method leverages an enhancement-cavity–based 1D lattice at $\lambda=1064\,\text{nm}$ to achieve trap depths of about $7.3\ \text{mK}$, and it avoids cycling requirements, enabling access to polyatomic and other challenging molecular species for precision measurements, quantum information, and cold chemistry. Practical considerations include photon-scattering rates, blackbody-induced rovibrational losses, and choices of trap wavelength and polarization to mitigate undesired transitions.

Abstract

We propose a scheme to transfer molecules from a slow beam into an optical trap using only a single photon absorption and emission cycle. The efficiency of such a scheme is numerically explored for BaF using realistic experimental parameters. The technique makes use of the state-dependent potential in an external electric field to trap molecules from an initial velocity of order 10 m/s. A rapid optical transition at the point where the molecules come to a standstill in the electric field potential irreversibly transfers them into a ~7 mk optical lattice trap. For a pulsed Stark decelerated beam, we estimated the per-shot efficiency to be ~0.52% or up to ~10$^4$ molecules, with a potential factor 2 improvement when the fields are synchronously modulated with the arriving velocity components. The irreversibility of the scheme allows for larger numbers to be built up over time. Since this scheme does not rely on a closed cycling transition for laser cooling, it broadens the range of molecules that can be used for research on cold molecular chemistry, quantum information, and fundamental interactions in optical traps.

Figures (3)

• Figure 1: A schematic of the single-photon loading scheme. Molecules approach from the left whilst in some initial state $\ket{i}$ and fly along the $+\hat{z}$ axis. At their turning point in the potential, they are pumped to an excited state $\ket{e}$, from which there is a chance to decay to the trapped state $\ket{t}$. The center of the optical trap is located at $z=0$. A fraction of the molecules remain trapped in this state by the optical trap.
• Figure 2: a) The Stark curves of the lowest rotational levels (notation $\ket{N;m_N}$) in the $X^2\Sigma^+\;v=0$ vibronic state of $\text{BaF}$; b) The lowest electronic states in $\text{BaF}$. The dashed line indicates the wavelength used for the optical trap.
• Figure 3: a) Top: phase space acceptance of molecules arriving in $\ket{i}$ into the trapped state $\ket{t}$. The solid red curve represents the potential following the AC and DC Stark effects, and effectively shows the position $z$ where molecules with a given initial velocity will come to a standstill. Bottom: position-dependent transition energy for $\ket{i}\to\ket{e}$. Further details are elaborated in the main text; b) (Not to scale.) Modulating the external fields with time allows different velocity components to be captured, as the relative height of the trap in $\ket{i}$ decreases.