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APPLIED PHYSICS: ON LASER COOLING OF MACROSCOPIC OBJECTS

The following points are made by P.W. Milonni and B.M. Chernobrod (Nature 2004 432:965):

1) Microfabricated cantilevers have permitted by far the most sensitive studies of small forces at tiny length scales. They are the basis of, among other things, the atomic force microscope and the magnetic-resonance force microscope. Microlevers are increasingly of interest as tools for the study of materials and the development of quantum microscopes for such fundamental investigations as detection of the presence of a single unpaired electron spin[1]. To reduce thermal fluctuations -- and therefore increase the sensitivity of these techniques -- it is generally desirable to work at low temperatures; experiments in single-spin detection, for example, are typically performed at 1.6 K. Other possible applications, such as the detection of gravitational waves or the study of the quantum superposition of states of photons or macroscopic objects, also require very low temperatures. Whereas the laser cooling of atomic gases is now commonplace, progress in the laser cooling of macroscopic objects has been less dramatic.

2) New work[2] builds on earlier studies of radiation forces acting on macroscopic objects, in particular a mechanical effect of radiation[3] that is used to cool one of the mirrors of an optical resonator, using feedback control[4]. With a resonator consisting of two parallel and highly reflecting mirrors, the phase of the laser light that is reflected from the resonator varies markedly with changes in the separation of the mirrors. Measurement of the phase variation in this laser light allows detection of the thermal (brownian) motion of a mirror if it is sufficiently light-weight. It can also provide the signal for a feedback loop that controls the motion of the mirror, by adjusting the force, resulting from radiation pressure, that is exerted on it by a second laser beam: if the force is adjusted so that it opposes the motion of the mirror, this results in a reduction (cooling) of the mirror's brownian motion.

3) But the microlever cooling technique reported by Höhberger Metzger and Karrai[2] does not use a feedback loop or radiation pressure from a second laser. Instead, it relies on the force on a resonator mirror that is generated by the light inside the resonator. This force could be simply radiation pressure, or a photo-thermal stress resulting from the absorption of radiation by different parts of the mirror possessing different coefficients of thermal expansion (such as the bulk of the microlever and a metal coating). In either case, the force is proportional to the intensity of light stored in the cavity between the mirrors, and is greatest when the separation of the mirrors is such that the laser wavelength matches a cavity (Fabry-Pérot) resonance.

4) Because the mirror separation is changed by the force, it is strongly coupled to the light intensity inside the cavity. Changes in this intensity do not occur instantaneously with mirror separation. Therefore, the delay in the response of the intensity to a change in the mirror separation leads to a force that can enhance or oppose the motion of a mirror, depending on whether the optical frequency is higher or lower than that of the cavity resonance. The motion of the mirror can be modelled using Newton's second law, having a total force that includes this intensity-dependent force, as well as a fluctuating thermal force. Such a model accounts accurately for Höhberger Metzger and Karrai's results[2].

References:

1. Rugar, D. et al. Nature 430, 329-332 (2004)

2. Höhberger Metzger, C. & Karrai, K. Nature 432, 1002-1005 (2004)

3. Braginsky, V. B. & Manukin, A. B. Sov. Phys. JETP 25, 653-655 (1967)

4. Cohadon, P. F., Heidmann, A. & Pinard, M. Phys. Rev. Lett. 83, 3174-3177 (1999)

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QUANTUM OPTICS: ON CAVITY COOLING OF SINGLE ATOMS

The following points are made by P. Maunz et al (Nature 2004 428:50):

1) All conventional methods to laser-cool atoms rely on repeated cycles of optical pumping and spontaneous emission of a photon by the atom. Spontaneous emission in a random direction provides the dissipative mechanism required to remove entropy from the atom. However, alternative cooling methods have been proposed(1,2) for a single atom strongly coupled to a high-finesse cavity. In such a method, the role of spontaneous emission is replaced by the escape of a photon from the cavity. Application of such cooling schemes would improve the performance of atom-cavity systems for quantum information processing(3,4). Furthermore, as cavity cooling does not rely on spontaneous emission, it can be applied to systems that cannot be laser-cooled by conventional methods; these include molecules(2) (which do not have a closed transition) and collective excitations of Bose condensates(5), which are destroyed by randomly directed recoil kicks.

2) The basic idea behind cavity cooling can be understood from a simple classical picture based on the notion of a refractive index. Consider a standing-wave optical cavity resonantly excited by a weak probe laser blue-detuned from the atomic resonance. For strong atom-cavity coupling, even one atom can significantly influence the optical path length between the cavity mirrors. Consequently, the intracavity intensity is strongly affected by the atom. For example, at a node of the standing wave the atom is not coupled to the cavity, and thus the intracavity intensity is large. An atom at an antinode, in contrast, shifts the cavity to a higher frequency because the atom's refractive index is smaller than unity above its resonance. This tunes the cavity out of resonance from the probe laser and leads to a small intracavity intensity.

3) However, in a high-finesse cavity the intensity cannot drop instantaneously when the atom moves away from a node. Instead, the blue-shift of the cavity frequency leads to an increase of the energy stored in the field. The photons finally escaping from the cavity are therefore blue-shifted from the photons of the probe laser. This occurs at the expense of the atom's kinetic energy. The reverse effect, namely the acceleration of an atom approaching an antinode, is much smaller, as here the cavity is initially out of resonance with the probe laser and consequently the intracavity intensity is small.

4) Note that the cooling process does not require atomic excitation. Indeed, the atomic excitation is low at all times, as the atom is not coupled to the light at a node while the intracavity intensity is very low for an atom near an antinode. It follows that the lowest attainable temperature is not limited by the atomic linewidth as for free-space Doppler cooling but by the linewidth of the cavity, which can be much smaller. Therefore temperatures below the Doppler limit can be reached. An upper limit on the velocity of the atom to be cooled is given by the requirement that the atom must not move farther than about one-quarter of a wavelength during the lifetime of a photon in the cavity.

5) In summary: The authors demonstrate cavity cooling of single rubidium atoms stored in an intracavity dipole trap. The cooling mechanism results in extended storage times and improved localization of atoms. The authors estimate that the observed cooling rate is at least five times larger than that produced by free-space cooling methods for comparable excitation of the atom.

References (abridged):

1. Horak, P., Hechenblaikner, G., Gheri, K. M., Stecher, H. & Ritsch, H. Cavity-induced atom cooling in the strong coupling regime. Phys. Rev. Lett. 79, 4974-4977 (1997)

2. Vuleti, V. & Chu, S. Laser cooling of atoms, ions, or molecules by coherent scattering. Phys. Rev. Lett. 84, 3787-3790 (2000)

3. Kuhn, A., Hennrich, M. & Rempe, G. Deterministic single-photon source for distributed quantum networking. Phys. Rev. Lett. 89, 067901 (2002)

4. Monroe, C. Quantum information processing with atoms and photons. Nature 416, 238-246 (2002)

5. Horak, P. & Ritsch, H. Dissipative dynamics of Bose condensates in optical cavities. Phys. Rev. A 63, 023603 (2001)

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QUANTUM ENCOUNTERS OF THE COLD KIND

The following points are made by K. Burnett et al (Nature 2002 416:225):

1) Since the introduction of laser-cooling techniques for neutral atoms in the early 1980s, the study of collisional interactions between atoms and molecules has been extended to the regime of ultracold temperatures. With nanokelvin temperatures now attainable, our ability to probe the interactions, both experimentally and theoretically, has also progressed. Understanding of the subtle and often highly quantum-mechanical effects that are manifest at such low energies has advanced to the point where new precision measurements are matched by highly accurate theoretical calculations. Low-energy phenomena such as Bose Einstein condensation and the photoassociation of atoms into bound molecules are now accurately described with no free parameters.

2) The behavior of atoms and their interactions at ultracold temperatures is a fascinating area of study. These interactions and their effects distinguish them from those encountered in collisions at room temperature. The realization that these interactions would be both subtle and interesting began in the 1970s with studies(1) of spin-polarized hydrogen and long-range molecules, and expanded as laser cooling(2-4) reached temperatures in the millikelvin and then microkelvin ranges. With the advent of evaporative cooling(5) and the production of atomic Bose Einstein condensates (BECs), we now require a detailed understanding of atomic interactions at nanokelvin temperatures. These applications have driven a tremendous growth of interest in the field.

3) It was realized early on that the quantal nature of ultracold atomic interactions would have a profound role and provide a challenge to theorists and experimentalists in the field of collision dynamics. At the low energies involved, the precise nature of the interatomic forces -- at ludicrously large distances from the point of view of "normal molecular physics" --would have to be determined, requiring new ways to examine them. It was also evident that nuclear spin dynamics, usually irrelevant to collision dynamics, would complicate the analysis enormously. In fact, the term complicated does not do service to the range of new physics that the hyperfine interactions bring up in this regime.

References (abridged):

1. Weiner, J., Zilio, S., Bagnato, V. S. & Julienne, P. S. Experiments and theory in cold and ultracold collisions. Rev. Mod. Phys. 71, 1-85 (1999)

2. Cohen-Tannoudji, C. Manipulating atoms with photons. Rev. Mod. Phys. 70, 707-719 (1997)

3. Chu, S. The manipulation of neutral particles. Rev. Mod. Phys. 70, 685-706 (1997)

4. Phillips, W. D. Laser cooling and trapping of neutral atoms. Rev. Mod. Phys. 70, 721-741 (1997)

5. Ketterle, W. & Van Druten, N. J. Evaporative cooling of trapped atoms. Adv. At. Mol. Opt. Phys. 37, 181-236 (1996)

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