Slowing and Stopping Images
February 6, 2008
Associate Professor John Howell reported in January of 2007 that his group showed how to slow images down to "300 times lower than the speed of light" and preserve the amplitude and phase of the image. He also stated that, "we're working on systems that slow images down to 10 million times lower than the speed of light." Howell and his Quantum Optics team of Ryan Camacho, Curtis Broadbent, and Irfan Ali Khan used a technique known as slow light. When close to a narrow resonance feature, the group velocity of the light can be very slow. His team used naturally-occurring resonances in a cesium vapor to precisely slow images and delayed them for about 10 nanoseconds while retaining their properties.
Now the group (above from left to right: Ryan Camacho, Praveen VudyaSetu, and John Howell) has stopped images in a hot gas of Rubidium atoms for about 10 microseconds and is working toward a goal of a millisecond (Phys. Rev. Lett. 100, 123903). The new process changes the light field into an atomic excitation, then reads out that atomic excitation and converts it back into a light field. This differs from the method used in January of 2007, in which the light propagated slowly through a dilute vapor. In the stored light technique, the light field is interconverted into a coherence in the atoms and then read out at a later time. Remarkably, the storage process remains robust even given the diffusion of the rapidly moving atoms.
Rubidium 85, one of the two most common isotopes of the element, has two hyperfine ground states that are shifted slightly in energy from one to another. A relatively strong laser beam of a few mW of light prepares the hot vapor so that atoms are in a single ground state. A weak pulse of light carrying the image then puts the atoms into a superposition of both ground states. The strong pump beam is then turned off, which causes the coherence setup by the two lasers to be "frozen" or stored in the medium. Each atom carries the local image phase and amplitude.
At some later time, the strong pump beam is turned on in the reverse fashion in which it was turned off. The strong pump beam reads out the coherence in the atoms. The weak pulse is then regenerated with the phase and amplitude of the image in tact. Howell and his students, Praveen VudyaSetu and Ryan Camacho, then read out the image with varying time delays. The greater the time between turning off the pump beam and turning it on again, the more the image was attenuated. However, the contrast in the image is not corrupted as might be expected with atoms moving very rapidly in a hot gas.
The surprising aspect of the work is that the image remains robust to strong diffusion. The Fourier transform of the image is stored in the atoms and has phase oscillations in a 2-dimensional cross section. The robustness occurs because the atoms store the local phase of the light field. As atoms move, they cancel their effect with atoms of different phase as they pass through zero phase points in the Fourier transform plane. The preservation of the zero phase points result in high contrast image readout long after the image would have washed out. (lhg)