Extended Rydberg Lifetimes in a Cryogenic Atom Array

TL;DR

The work addresses the limitation of two-qubit gate errors set by $T_1$ relaxation in neutral-atom platforms by suppressing blackbody radiation with a cryogenic environment. The authors realize a Cs optical tweezer array inside a 4 K radiation shield and perform single-photon ground–Rydberg control at $319\,$nm, achieving a $55P_{3/2}$ lifetime of $406(36)\,\mu$s, corresponding to an effective BBR temperature of $<25$ K. They also measure a small differential dynamic polarizability (light shifts) and demonstrate coherent operation of the ground–Rydberg qubit with $\Omega=2\pi\times1.35\,$MHz and $T_2^*=6.2(4)\,\mu$s, indicating suppressed dephasing due to intensity fluctuations. The extended lifetimes and reduced BBR-induced transitions pave the way for higher two-qubit gate fidelities and robust operation in cryogenic, scalable neutral-atom processors, with potential extensions to circular Rydberg states and broad applicability across Rydberg levels.

Abstract

We report on the realization of a $^{133}$Cs optical tweezer array in a cryogenic blackbody radiation (BBR) environment. By enclosing the array within a 4K radiation shield, we measure long Rydberg lifetimes, up to $406 (36)\,μ$s for the $55 P_{3/2}$ Rydberg state, a factor of 3.3(3) longer than the room-temperature value. We employ single-photon coupling for coherent manipulation of the ground-Rydberg qubit. We measure a small differential dynamic polarizability of the transition, beneficial for reducing dephasing due to light intensity fluctuations. Our results pave the path for advancing neutral-atom two-qubit gate fidelities as their error budgets become increasingly dominated by $T_1$ relaxation of the ground-Rydberg qubit.

Figures (6)

• Figure 1: (a) UHV cryostat with 4K and 35K radiation shields inside a room-temperature vacuum chamber. The atomic beam (not shown) is directed along the $y$ axis. (b) Assembly inside the 4K radiation shield, integrating aspheric lenses, electrodes and MOT coils. Inset: averaged fluorescence image of a $7\times7$ atom array, with a tweezer spacing of 12 $\mu$m. (c) Vacuum trapping lifetime measurement.
• Figure 2: Coherent control of the ground-Rydberg qubit. (a) Relevant energy levels and decay pathways. Atoms are initialized into $\ket{g} = \ket{6S_{1/2}, F=4, m_F= 4}$ via optical pumping and excited to the Rydberg state $\ket{r} = \ket{nP_{3/2}, m_J =1/2}$ using single-photon excitation at 319 nm. The Rydberg state can undergo BBR-induced transitions to other Rydberg states $\ket{r'}$ or spontaneous decay to low-lying states, indicated by curvy arrows. (b) Configuration of the UV excitation and optical pumping (OP) beams relative to the atom array. The UV and OP beams have $\pi$ and $\sigma^+$ polarizations, respectively. The quantization axis is defined by the magnetic field $B$ along the $y$ axis. (c) Rabi oscillation between $\ket{g}$ and $\ket{r}$ for the $55P_{3/2}$ Rydberg state, averaged over atoms in the central column along the $z$ direction. $P_g$ denotes the ground-state fraction. (d) Ramsey measurement for a single atom at the center of the array, yielding a fitted coherence time $T_2^*$ = 6.2(4)$\mu$s.
• Figure 3: Extended $T_1$ lifetimes. (a) Experimental sequence and lifetime measurement of the $55P_{3/2}$ Rydberg state. $P_g(t)$, with $t$ being the gap time between two UV $\pi$ pulses, is fitted to an exponential decay with no offset. The fitted $1/e$ lifetime is 406(36) $\mu$s. (b) Lifetimes of different $nP_{3/2}$ Rydberg states at different BBR temperatures. Green circles with error bars ($1\sigma$) denote experimental values measured for $n=46, 50, 55$. Dashed lines serve as guides to the eye connecting results calculated using ARC arc_sibalic2017.
• Figure 4: Light shift of the $55P_{3/2}$ Rydberg transition as a function of the square of the Rabi frequency $\Omega^2$. A linear fit (dashed line) to the data gives a light-shift coefficient $\kappa = 29(15)\,$kHz/MHz$^2$.
• Figure S1: (a) Atom loss after different release times. When the release time is less than 24 $\mu$s, there is essentially no loss during the release-recapture sequence. The approximately $0.5\%$ loss observed is consistent with imaging loss. (b) Pushout of atoms in the ground states. The atoms are not optically pumped into the stretched state before the pushout. The experimental protocol used is the same as that shown in Fig. 3 (a) of the main text, except that the Rydberg beam remains off.