Reversible Quantum Interface for Tunable Single-Sideband Modulation

Using electromagnetically induced transparency in a cesium vapor, we demonstrate experimentally that the quantum state of a light beam can be mapped into the long-lived Zeeman coherences of an atomic ground state. Two noncommuting variables carried by light are simultaneously stored and subsequently read out, with no noise added. We compare the case where a tunable single sideband is stored independently of the other one to the case where the two symmetrical sidebands are stored using the same electromagnetically induced transparency window.

Figure 1

Experimental setup. DL1 and DL2: pumping and repumping diode lasers; HD: homodyne detection; Ph: photodiode; EOM: electro-optical modulator; AOM: acousto-optical modulator; PBS: polarizing beam splitter. The control field intensity is controlled by an AOM, in double pass configuration in the 0th order (1:200 extinction). All laser beams have a $1/e^2$ diameter of 14 mm in the cell. Insets: Experimental sequence and Cs transitions involved.

Figure 2

Time-dependent mean values of the amplitude (a) and phase (b) quadratures, measured from a typical 2000-sequence run. Cell temperature $T=40°\mathrm{C}$, control field power 10 mW, input pulse duration $5\mu \mathrm{s}$, input intensity 0.1 nW.

Figure 3

(a) Ratio of the amplitudes of the input and output states, as a function of the storage time. $T=40°\mathrm{C}$. The points indicated by ■ (◆) correspond to a signal pulse duration of $6.4\mu \mathrm{s}$ ($1.6\mu \mathrm{s}$) with a control field power of 10 mW (140 mW). (b) Ratio of the amplitudes of the input and output states, as a function of the modulation frequency $\Omega /2\pi$. The bar indicates the spectral width of the input pulse.

Figure 4

Phase ${\phi }_r$ of the retrieved pulse (a) as a function of the input pulse phase ${\phi }_i$ with a constant ${\Omega }_L$, and (b) as a function of ${\Omega }_L$, with a fixed input phase. The storage time is $20\mu \mathrm{s}$, and $\Omega /4\pi =625\mathrm{kHz}$.

Figure 5

(a) Variance of the amplitude quadrature for the signal. (b) Variance of the amplitude quadrature for vacuum. The phase quadratures are similar.

Figure 6

(a) FWHM of the EIT transparency window (MHz) as a function of the control field power (mW). The control and signal beams’ waists are 7.2 mm. (b) Ratio of the output and input states amplitudes, as a function of the control field power, for a single sideband of frequency 1.25 MHz (squares) and a dual sideband modulation of frequency 400 kHz (circles). $T=50°\mathrm{C}$. Pulse duration $5\mu \mathrm{s}$; storage time $15\mu \mathrm{s}$.