Photon Echoes Produced by Switching Electric Fields

We demonstrate photon echoes in ${\mathrm{Eu}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ by controlling the inhomogeneous broadening of the ${\mathrm{Eu}}^{3+}{}^7\mathrm{F}_0\leftrightarrow {}^5\mathrm{D}_0$ optical transition. This transition has a linear Stark shift, and we induce inhomogeneous broadening by applying an external electric field gradient. After optical excitation, reversing the polarity of the field rephases the ensemble, resulting in a photon echo. This is the first demonstration of such a photon echo, and its application as a quantum memory is discussed.

Figure 1

Energy level diagram of ${\mathrm{Eu}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ and the arrangement of the electrodes around the sample. The experiment was carried out on the transition labeled ${\omega }_p$. ${\omega }_r$ and ${\mathrm{RF}}_r$ were used to optically pump the desired ions into the ground state $\pm 3/2$ hyperfine level.

Figure 2

Broadening of a narrow spectral feature with the electric field gradient. The dotted line shows the FID from a prepared 25 kHz wide feature. The solid line shows how this feature is broadened (and hence its FID is shortened) by the application of the field gradient. The solid trace was taken with $\pm 4.5\mathrm{V}$ on the electrodes.

Figure 3

Stark echoes produced by switching the polarity of the electric field. The top trace is the voltage on the electrodes, and the lower traces show the echoes. The signal from the transmitted input pulses saturated the detector and so are shown only schematically. The input pulses were $1\mu s$ square shaped pulses, and a ratio of 60 dB between the input pulse and the largest echo was measured using a neutral density filter after the sample. The output pulses are broader in time than the input pulses because of the finite bandwidth of the memory. The voltages on the electrodes were $\pm 25\mathrm{V}$.

Figure 4

The solid line represents the FID of a $3\mu \mathrm{s}$ pulse absorbed over a 25 kHz wide spectral feature. The crosses show the intensity of the Stark echo as a function of total delay. The dephasing mechanism in both cases is the same, the spread of frequencies in the original feature. Therefore the FID provides an envelope for the amplitude of the Stark echoes.

Figure 5

The energy of the Stark echo as a function of the area of the input pulse. The response is linear until it saturates as the input pulse becomes comparable to a $\pi /2$ pulse.