Fast quantum state control of a single trapped neutral atom

We demonstrate the initialization, readout, and high-speed manipulation of a qubit stored in a single ${}^{87}\mathrm{Rb}$ atom trapped in a submicrometer-sized optical tweezer. Single-qubit rotations are performed on a time scale below $100\mathrm{ns}$ using two-photon Raman transitions. Using the spin-echo technique, we measure an irreversible dephasing time of $34\mathrm{ms}$. The readout of the single atom qubit is at the quantum projection noise limit when averaging up to 1000 individual events.

Figure 1

Experimental setup. A high-performance objective lens creates a tightly focused optical dipole trap, which also acts as one of the Raman beams. The second Raman beam is generated using two additional diode lasers, and is superimposed with the trapping beam on a polarizing beam splitter (PBS). A single polarization-maintaining fiber carries both beams to the experiment. Inset: relevant energy levels of ${}^{87}\mathrm{Rb}$. The quantization axis is defined by a $0.36\mathrm{mT}$ magnetic field along the $x$ axis.

Figure 2

Single-atom Rabi oscillations. We measure the fraction of atoms in $F=1$ as a function of the Raman pulse length, at low (a) and high (b) intensity. We observe damped Rabi oscillations between the two qubit states with Rabi frequencies of $\Omega =2\pi \times 18\mathrm{kHz}$ (a) and $\Omega =2\pi \times 6.7\mathrm{MHz}$ (b). In (b) we could not observe the first $400\mathrm{ns}$ due to the response time of the acousto-optic modulator. The error bars correspond to the quantum projection noise.

Figure 3

Ramsey fringes recorded with a $\pi ∕2$ pulse length of $1.2\mu \mathrm{s}$ and a detuning $\delta =2\pi \times 20.8\mathrm{kHz}$. The solid line is a fit using the model presented in [23], which yields a dephasing time $T_2^{*}=370\mu \mathrm{s}$. The dotted line is the envelope of this fit.

Figure 4

(a) Example of the echo signal. We fix the time between the first $\pi ∕2$ pulse and the $\pi$ pulse at $T=5$ (left) and $15\mathrm{ms}$ (right), and vary the time of the second $\pi ∕2$ pulse around $t=2T$. The trap depth is $U=0.4\mathrm{mK}$, and the magnetic field is $B=0.18\mathrm{mT}$. (b) Echo signal contrast as a function of the total time between the $\pi ∕2$ pulses with $U=1.2\mathrm{mK}$ and $B=0.36\mathrm{mT}$ (open squares) and $U=0.4\mathrm{mK}$ and $B=0.18\mathrm{mT}$ (filled circles). The dashed and solid lines are exponential fits with $1∕e$ decay times of $13\pm 2$ and $34\pm 5\mathrm{ms}$, respectively.