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ScienceWeek
QUANTUM OPTICS: ON STANDING LIGHT
The following points are made by Marlan O. Scully (Nature 2003 426:610):
1) Frozen light is now a reality. Bajcsy et al(1) have trapped, and held, a pulse of light for a few hundredths of a millisecond, which is a long time in optical terms. To those unfamiliar with the realm of quantum optics, the notion of stationary light may seem rather strange. But ultraslow light -- travelling at just a few, instead of 300 million, meters per second -- is easily available in many labs these days. So bringing a light pulse to a full stop was the next logical step.
2) Frozen light is of interest for many reasons. This is the latest development in a continuing paradigm shift in optics, occasioned by the marriage of quantum coherence in atoms and molecules with coherent light(2-5). The ability to trap and hold light also holds promise for application to such diverse areas as quantum informatics, nonlinear optics, and even the foundations of quantum mechanics. Perhaps most important of all, it is simply fascinating science.
3) The landmark slow-light experiment -- down to a speed of just 17 meters per second -- was carried out in an ultracold gas. The advantage of such low temperatures is that they eliminate the spread in atomic frequencies that is caused by atomic motion: this optical Doppler effect could otherwise obscure, even spoil, the measurements of slow light. It is not necessary, however, to use trapped ultracold atoms and get rid of the Doppler effect to slow light down: this has been proved by the slowing of light to less than 100 meters per second in a hot gas of rubidium atoms.
4) It could be argued that the know-how to make slow light has been around since the late 19th century. The velocity of the peak of an optical pulse is called the "group velocity" because it takes a "group" of two or more frequencies of light to make a pulse. Atoms and molecules tend to interact with light more strongly at certain frequencies, and this is manifest in a variation of the index of refraction of the material, which causes different waves to move with different velocities. Of course, this affects the group velocity of the pulse -- and is the reason that light is dispersed by a prism or water droplets to make a rainbow.
5) Strong dispersion is easy to achieve: the light frequency should be tuned close to the resonant frequency of the atoms, and then the group velocity may be slowed by a factor of a thousand or more. But minimum pulse velocity also means maximum absorption of the light by the material: the pulses move very slowly, but they are so strongly absorbed that they penetrate only a distance of a few wavelengths through the material.
6) However, the last decade of the 20th century saw revolutionary changes in optical science. We now have "phase-coherent" materials, which give the huge dispersion associated with resonance but do not absorb light. These materials are truly a new state of matter, aptly called "phaseonium", and their electromagnetically induced transparency has proved useful in slowing light down to just a few meters per second.
References (abridged):
1. Bajcsy, M., Zibrov, A. S. & Lukin, M. D. Nature 426, 638-641 (2003)
2. Kocharovskaya, O. Phys. Rep. 219, 175-190 (1992)
3. Arimondo, E. Prog. Optics 35, 257-354 (1996)
4. Harris, S. Phys. Today 50, 36-42 (1997)
5. Scully, M. O. & Zubairy, M. S. Quantum Optics (Cambridge Univ. Press, 1997)
Nature http://www.nature.com/nature
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PHYSICS: ULTRASLOW LIGHT AND ATOMIC COHERENCE
The following points are made by M.O. Scully and M.S. Zubairy (Science 2003 301:181):
1) Pulses of light travel through a vacuum at the speed of ~100 million miles per hour (mph). In glass and other transparent matter, light moves somewhat slower, at ~50 million mph. But the speed of light pulses has recently been slowed to a few meters per second. The quantum state of such pulses has been stored (stopped light) and even reversed in time.
2) Ultraslow and frozen light are just one of many manifestations of atomic coherent effects. For the first 25 years of quantum physics, the founding developers of the field struggled to break away from classical physics (where energy comes in arbitrarily small packages) to the quantum world (where energy is parceled out in quantum lumps and atoms and molecules have discreet energy states).
3) The next quarter century was devoted largely to applying quantum mechanics to problems such as calculating scattering rates and absorption cross sections. These studies involved quantum probabilities, which require knowledge of the amplitude of the wave function but not of its phase.
4) In the second half of the 20th century, the phase of the wave function held center stage. For example, the phase of the electron pairs governs much of the physics of superconductivity, and in the gauge field theories of modern particle physics, the phase of a particle (such as an electron or quark) is intimately tied to its gauge quanta (photons and gluons).
5) More recently, phase-coherent ensembles of atoms have yielded a different state of matter. Often called "phaseonium", this state has many novel properties, such as the ability to slow or even freeze light. Other examples of novel applications of phase-coherent matter include lasing without inversion, electromagnetically induced transparency, and new sensitive anthrax detectors.
Science http://www.sciencemag.org
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ON ULTRA-SLOW AND STORED LIGHT PULSES
An experimental "light pulse" has a finite duration, and in theory (the so-called "bandwidth theorem") this requires an infinite number of waves of different frequency to be added together. The shorter the desired pulse, the larger the bandwidth of frequencies that must be used. Theoretically, all light pulses are therefore formed by a packet of waves of different frequency, each of which has a different amplitude and phase. The speed of individual waves is called the "phase velocity", and the velocity at which the peak of the wave packet propagates is called the "group velocity". In a vacuum, the phase and group velocities are identical, but in a highly absorbing or dispersive medium the phase and group velocities are usually different.
Experiments involving the control of light pulses in a quantum mechanical regime hold great promise for a future technology of quantum computing involving optical information storage and transmission. In 1999, L.V. Hau et al succeeded in slowing light to a velocity of 30 meters per second in an ultracold sodium gas. In 2001, the same laboratory reported effectively stopping light completely for an interval of 1 millisecond before releasing the pulse to resume normal velocity. Essentially, the phenomenon involves "storing" the light pulse in the quantum states of the atoms, with the light pulse reconstituted for propagation at a later time.
The following points are made by A.V. Turukhin et al (Phys. Rev. Lett. 2002 88:023602):
1) Since the first observations of ultraslow light (1,2), there has been substantial interest in its potential applications. For example, it was proposed that slowing the group velocity of a laser pulse down to the speed of sound in a material can produce strong coupling between acoustic waves and the electromagnetic field (3). This might be utilized for efficient multiwave mixing and quantum nondemolition measurements (4), as well as for novel acousto-optical devices. Ultraslow light might even allow nonlinear interactions down to a single photon level (5). Finally, it has been suggested and experimentally demonstrated that the group velocity of light can be decelerated to zero, effectively trapping or "stopping" the light pulse. Light stored by this technique is potentially important for quantum computing applications because it is relatively easy to implement and can be accomplished with near 100% fidelity in principle.
2) For many potential applications of slow light, a solid-state medium is preferred. However, most solid materials have relatively broad optical linewidths, which limits the achievable light-speed reduction. A notable exception to this general rule is a class of materials consisting of rare-earth doped insulators that exhibit spectral hole burning. These materials are generally used for ultrahigh density optical memories and processors. In one such material, Pr doped Y(sub2)SiO(sub5) (Pr:YSO), efficient, narrow-linewidth electromagnetically induced transparency has also been demonstrated. This is significant because narrow band electromagnetically induced transparency enabled the demonstration of ultraslow and "stopped" light in atomic vapors.
3) The authors report ultraslow group velocities of light in an optically dense crystal of Pr doped Y(sub2)SiO(sub5). Light speeds as slow as 45 meters per second were observed, corresponding to a group delay of 66 microseconds. Deceleration and "stopping" or trapping of the light pulse was also observed. These reductions of the group velocity are accomplished by using a sharp spectral feature in absorption and dispersion that is produced by resonance Raman excitation of a ground-state spin coherence.
References (abridged):
1. L. V. Hau, S. E. Harris, Z. Dutton, and C. Behroozi, Nature (London) 397, 594 (1999).
2. M. Kash, V. Sautenkov, A. Zibrov, L. Hollberg, G. Welch, M. Kukin, Y. Rostovsev, E. Fry, and M. Scully, Phys. Rev. Lett. 82, 5229 (1999).
3. A. Matsko, Y. Rostovtsev, H. Cummins, and M. Scully, Phys. Rev. Lett. 84, 5752 (2000).
4. V. Braginsky and F. Khalili, Quantum Measurement (Cambridge University Press, Cambridge, United Kingdom, 1992).
5. M. Lukin and A. Imamoglu, Phys. Rev. Lett. 84, 1419 (2000).
Phys. Rev. Lett. http://prl.aps.org
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