Gradient Echo Memory (GEM) and quantum optical pulse sequencing.

Storage of quantum states of light requires a coherent optical memory. One possible method is to use atoms.

GEM is a coherent light storage technique based on photon echoes. The idea was first demonstrated in a 2-level atomic system by the solid state group at the ANU.

Our GEM experiment is based on a 3-level system in hot rubidium vapour. Our first results are shown on the right. The input pulse (i) is partially transmitted (ii) through the gas cell. After switching the atomic frequency gradient, we observed a photon echo (iii). The strength of the echo varies as a function of the storage time. See here for details.

The storage mechanism works as shown in the figure. An ensemble of two level atoms (a) is frequency shifted (b) to create a gradient of atomic frequencies along the length of the ensemble. The frequency width of the atomic sample is adjusted to capture the entire bandwidth of the optical pulse. After flipping the frequency gradient at time t_s, a photon echo is generated at time 2t_{s (c).}

In our case the two levels we use in this system are the hyperfine ground states of rubidium that form a quasi-2-level system under the influence of a strong control field. The atomic frequency gradient is formed using a magnetic field. See here for details.

The storage of a light in this system can be understood using a normal mode description. This is a combined oscillation of the atomic polarisation and optical field.

This figure shows the real parts of the optical field (left) and atomic polarisation (middle) and the magnitude of the normal mode (right). The normal mode of our system is described in the spatial Fourier domain. Whenever the normal mode reaches k=0, light can be coupled out of our memory. The motion of the normal mode is controlled by the frequency gradient in our atomic sample.
For details see here.

A movie showing pulse storage with GEM. The atomic polarisation along the length of the storage medium is the seen to be the Fourier transform of the pulse shape.

A coherent optical pulse sequencer.

Our 3-level GEM scheme can also be used to recall stored optical information on demand, split stored information over multiple recall events and control the temporal width of a recalled information. There are two important control knobs that help us to do this. The first is the atomic frequency gradient that moves the k-space normal mode in the t-k plane. Every time the normal mode passes through the k=0 point, the atoms are rephased and a photon echo can be produced. The second knob is the control field that couples the two atomic ground states. Only when the control field is on can a photon echo be produced. In this way we can control exactly when and how much of each stored pulse is recalled. An example of this capability is shown below (click to enlarge).

One way to understand this system is to imagine the normal mode as an optical conveyer belt to which the pulses of light are stuck. By changing the magnetic field we can control the direction and speed of the belt. With careful control of the magnetic field, therefore, we can move the pulses back and forth through the k=0 point. Every time the pulses pass through this point, we can use the control field to push them off the belt and out of the memory.

The same data can be animated to show how the atomic polarisation is sucked out of the storage medium as the various light pulses are recalled. In this example we recall some pulses in reverse order (first in last out, or FILO) and some in normal order (first in first out or FIFO). Pulses 1 and 2 are recalled in 3 different parts as well as being stretched and compressed.

In an experiment, it looks like this. Here we take pulses in order 1,2,3,4 and release them in order 4,3,1,2

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A top view of the experiment.