Dynamics of a pulsed continuous-variable quantum memory

We study the transfer dynamics of nonclassical fluctuations of light to the ground-state collective spin components of an atomic ensemble during a pulsed quantum memory sequence, and evaluate the relevant physical quantities to be measured in order to characterize such a quantum memory. We show in particular that the fluctuations stored into the atoms are emitted in temporal modes which are always different from those of the readout pulse, but which can nevertheless be retrieved efficiently using a suitable temporal mode-matching technique. We give a simple toy model—a cavity with variable transmission—that accounts for the behavior of the atomic quantum memory.

Figure 1

(Color online) Quantum memory setup: field $A^{\mathrm{in}}$ is sent into a cavity containing an ensemble of three-level atoms. The coupling of the intracavity field $A\left(t\right)$ with the atomic ground-state variables is dynamically controlled by a pulsed coherent field $\Omega \left(t\right)$. The fluctuations of the output field $A^{\mathrm{out}}$ are measured by homodyning with a local oscillator $E\left(t\right)$, the temporal profile of which may be adjusted so as to optimize the detection efficiency.

Figure 2

(Color online) “Write” efficiency ${\eta }_w\left(t\right)$ (normalized to $\eta$) vs time $t∕T$, when the write pulse “area” increases [Gaussian envelopes with (a) $a\left(\infty \right)=0.1$, (b) 0.5, (c) 1, (d) 5, (e) 10]. The dashed curve represents the write pulse envelope $\xi \left(t\right)$.

Figure 3

(Color online) Radiated pulse (plain), read pulse (dotted), and delayed read pulse (dashed) envelopes vs time $t∕T$, for ${\tilde{\gamma }}_{0}T=5$.

Figure 4

(Color online) Readout efficiency ${\eta }_r$ (normalized by the cw efficiency $\eta$) vs pulse “area” $a\left(\infty \right)={\tilde{\gamma }}_{0}T$ when the LO is either the retarded readout pulse (lower curve) or a pulse temporally matched with the atomic emission (upper curve).

Figure 5

(Color online) Cavity with variable transmission.