Fast Preparation and Detection of a Rydberg Qubit Using Atomic Ensembles

We demonstrate a new approach for fast preparation, manipulation, and collective readout of an atomic Rydberg-state qubit. By making use of Rydberg blockade inside a small atomic ensemble, we prepare a single qubit within $3\mu \mathrm{s}$ with a success probability of $F_p=0.93\pm 0.02$, rotate it, and read out its state in $6\mu \mathrm{s}$ with a single-shot fidelity of $F_d=0.92\pm 0.04$. The ensemble-assisted detection is ${10}^3$ times faster than imaging of a single atom with the same optical resolution, and enables fast repeated nondestructive measurement. We observe qubit coherence times of $15\mu \mathrm{s}$, much longer than the $\pi$ rotation time of 90 ns. Potential applications ranging from faster quantum information processing in atom arrays to efficient implementation of quantum error correction are discussed.

Figure 1

Fast collective detector of a single Rydberg atom. (a) State initialization. An atom is prepared in the Rydberg state $|r^{\prime }⟩$ through a three-photon process involving the preparation beam (${\mathrm{\Omega }}_p$, orange), the control beam (${\mathrm{\Omega }}_c$, blue), and a microwave field (${\mathrm{\Omega }}_{MW}$, grey). The detunings from the two intermediate states are ${\mathrm{\Delta }}_e={\delta }_r=2\pi \times 100\mathrm{MHz}$. The preparation of a single atom in $|r^{\prime }⟩$ is ensured by the strong interaction between two atoms in $|r^{\prime }⟩$ [1]. (b) A probe field (orange, waist size $w_p=4.5\mu \mathrm{m}$) in combination with the control field ($w_c=12.5\mu \mathrm{m}$) couples atoms to the Rydberg state $|r⟩$. Under conditions of EIT (${\mathrm{\Delta }}_e={\delta }_r=0$), high transmission through the atomic medium results in a large number of detected photons (left). On the other hand, if the Rydberg state $|r^{\prime }⟩$ is populated by an atom (right), then the strong interaction between $|r⟩$ and $|r^{\prime }⟩$ removes the EIT condition, resulting in a significant reduction of transmitted photon number due to absorption by the ensemble. The interaction $V_{r{r}^{\prime }}$ contains both dipolar-exchange ($V_{\mathrm{ex}}$) and van der Waals components ($V_{\mathrm{vdW}}$) (see SM).

Figure 2

(a) Histogram of the transmitted probe photon number for state detection performed in $6\mu \mathrm{s}$. Grey and orange histograms correspond to the presence and absence of an atom in Rydberg state $|r^{\prime }⟩\equiv |\uparrow ⟩$, respectively. The solid lines in (a), (b) indicate a theoretical model that for the presence (absence) of an atom in $|\uparrow ⟩$ assumes random sudden loss of the Rydberg atom in $|\uparrow ⟩$ (sudden decay of the slow-light polariton into a Rydberg state) at a rate $0.035\mu {\mathrm{s}}^{-1}$ ($0.015\mu {\mathrm{s}}^{-1}$). The dashed line indicates the detection threshold that gives us the highest fidelity of differentiating two distributions. The control Rabi frequency is ${\mathrm{\Omega }}_c/(2\pi )=25\mathrm{MHz}$, and the probabilities for collecting and detecting a transmitted probe photon are 0.90 and 0.47, respectively. (b) Time-resolved photon count rate during detection. (c) Correlation plot of number of detected-photon counts in two consecutive $6\mu \mathrm{s}$ measurements in the same run of the experiment. Gray (orange) points represent transmission data when we prepare (do not prepare) the $|\uparrow ⟩$ state. Vertical and horizontal lines represent threshold counts for state discrimination.

Figure 3

A microwave field at a frequency $f_0=4814.2\mathrm{MHz}$ is applied to drive Rabi oscillations between $|\uparrow ⟩$ and $|\downarrow ⟩$ at an oscillation frequency $\mathrm{\Omega }/(2\pi )=5.3\mathrm{MHz}$. Each point is an average of $\sim 150$ repetitions. The error bars are the standard deviation of the mean. The fitted contrast loss per $2\pi$ pulse is $\delta C<2\times {10}^{-3}$. The relevant energy level diagrams are shown on the right.

Figure 4

Ramsey measurements consisting of two $\pi /2$ pulses separated by a time $\tau$. The phase of the second $\pi /2$ pulse is scanned to obtain a Ramsey fringe. The contrast of the Ramsey fringe is plotted as a function of $\tau$. The solid curve is a fit to a Gaussian decay function $A{e}^{-(\tau /T_2^{*}{)}^2}$, yielding a dephasing time $T_2^{*}=(15\pm 5)\mu \mathrm{s}$ and $A=0.88\pm 0.04$. Inset: Ramsey fringe at $\tau =1000\mathrm{ns}$.