Reversible State Transfer between Light and a Single Trapped Atom

We demonstrate the reversible mapping of a coherent state of light with a mean photon number $\overline{n}\simeq 1.1$ to and from the hyperfine states of an atom trapped within the mode of a high-finesse optical cavity. The coherence of the basic processes is verified by mapping the atomic state back onto a field state in a way that depends on the phase of the original coherent state. Our experiment represents an important step toward the realization of cavity QED-based quantum networks, wherein coherent transfer of quantum states enables the distribution of quantum information across the network.

Figure 1

Illustration of the protocol of Ref. 6 for quantum state transfer and entanglement distribution from system $A$ to system $B$. By expanding to a larger set of interconnected cavities, complex quantum networks can be realized.

Figure 2

(a) Schematic of the experiment. The probe $\lambda (t)$ resonantly drives the cavity through input mirror $M_{\mathrm{in}}$; the classical field $\Omega (t)$ excites the atom transverse to the cavity axis. Photons emitted from the output mirror $M_{\mathrm{out}}$ are directed to a pair of avalanche photodiodes. (b) Atomic level diagram. Double arrow $g$ indicates the coherent atom-cavity coupling, and $\Omega (t)$ is the classical field. The cavity and $\Omega$ field are blue-detuned from atomic resonance by $\Delta$.

Figure 3

Timing diagram: the upper curve shows the ${\Omega }_1$ and ${\Omega }_2$ pulses; the lower curve shows the ${\lambda }_1$ and ${\lambda }_2$ pulses. Each of these pulses can be turned on or off independently. Here $\Delta t$ is the delay between the falling edge of ${\Omega }_1$ and the rising edge of ${\Omega }_2$. By enabling various combinations of these pulses, and/or varying the relative phase $\theta$ between ${\lambda }_1$ and ${\lambda }_2$, we perform different measurements on the atom [28].

Figure 4

Ratio $r$ of adiabatic transfer probability to incoherent transfer probability versus arrival time $t_1$ for the incident coherent pulse ${\lambda }_1$. Red data points (○): $r$ versus $t_1$ (experiment). Solid red curve: $r$ vs $t_1$ (computer simulation). Dotted black curve: coherent component $r^c$ vs $t_1$ (simulation). Dashed blue curve: incoherent component $r^i$ vs $t_1$ (simulation).

Figure 5

Ratios $R_a(\theta ),R_i(\theta )$ for photon generation as a function of the relative phase $\theta$ between the ${\lambda }_{1,2}$ fields. Red data points (○): $R_a(\theta )$ for adiabatic state transfer with ${\Omega }_1$ on. Blue points (□): $R_i(\theta )$ for the incoherent process with ${\Omega }_1$ off. The full curve is a fit to obtain the fringe visibility $v_a\simeq 0.46\pm 0.03$. On average, each point represents about 130 atoms. The error bars represent statistical fluctuations from atom to atom.