We demonstrate that structures made of light can be used to coherently control the motion of complex molecules. In particular, we show diffraction of the fullerenes C(60) and C(70) at a thin grating based on a standing light wave. We prove experimentally that the principles of this effect, well known from atom optics, can be successfully extended to massive and large molecules which are internally in a thermodynamic mixed state and which do not exhibit narrow optical resonances. Our results will be important for the observation of quantum interference with even larger and more complex objects.
We demonstrate that structures made of light can be used to coherently control the motion of complex molecules. In particular, we show diffraction of the fullerenes C(60) and C(70) at a thin grating based on a standing light wave. We prove experimentally that the principles of this effect, well known from atom optics, can be successfully extended to massive and large molecules which are internally in a thermodynamic mixed state and which do not exhibit narrow optical resonances. Our results will be important for the observation of quantum interference with even larger and more complex objects.
Summary form only given, as follows. In a recent experiment we were able to demonstrate the de Broglie wave character of the fullerene molecule C/sub 60/ by diffraction at a free standing material grating. This proves that it is possible to preserve quantum coherence almost completely in the external degrees of freedom of a hot, massive and complex object over a long time and distance. This may seem astonishing in the light of the fact that the fullerenes were produced in a thermal source which was kept at about 900 K. At such a high temperature the de Broglie wavelength is about 400 times smaller than the size of the molecule (/spl lambda//sub dB//spl sim/25 pm), all internal degrees of freedom (174 different vibrational modes, and rotational modes up to J>100) are excited and the molecules may emit a few infrared photons (blackbody radiation and vibrational quanta) during their time of flight through the apparatus. Our published measurements so far have been essentially consistent with the absence of decoherence during the time of flight of the molecules. The interference contrast in all experiments has however so far been limited by the thermal velocity spread and the finite collimation of the fullerene beam.
Wave–particle duality is frequently the first topic students encounter in elementary quantum physics. Although this phenomenon has been demonstrated with photons, electrons, neutrons, and atoms, the dual quantum character of the famous double-slit experiment can be best explained with the largest and most classical objects, which are currently the fullerene molecules. The soccer-ball-shaped carbon cages C60 are large, massive, and appealing objects for which it is clear that they must behave like particles under ordinary circumstances. We present the results of a multislit diffraction experiment with such objects to demonstrate their wave nature. The experiment serves as the basis for a discussion of several quantum concepts such as coherence, randomness, complementarity, and wave–particle duality. In particular, the effect of longitudinal (spectral) coherence can be demonstrated by a direct comparison of interferograms obtained with a thermal beam and a velocity selected beam in close analogy to the usual two-slit experiments using light.
We present a theory for the diffraction of large molecules or nanoparticles at a standing light wave. Such particles can act as a genuine photon absorbers due to their numerous internal degrees of freedom effecting fast internal energy conversion. Our theory incorporates the interplay of three light-induced properties: the coherent phase modulation due to the dipole interaction, a nonunitary absorption-induced amplitude modulation described as a generalized measurement, and a coherent recoil splitting that resembles a quantum random walk in steps of the photon momentum. We discuss how these effects show up in near-field and far-field interference schemes, and we confirm our effective description by a dynamic evaluation of the grating interaction, which accounts for the internal states.
We study the theory behind a Talbot–Lau interferometer. This consists of three gratings in each other's Fresnel near field and accepts spatially incoherent illumination. Our formalism gives a clear physical picture and permits efficient numerical simulations. We concentrate on the case of matter waves and provide an adequate description of recent fullerene experiments taking into account the Casimir–Polder interaction between a molecular beam and mechanical gratings. For more massive molecules, the influence of this interaction is more drastic and leads to a forbiddingly narrow velocity distribution requirement for future experiments with very massive molecules. This problem can be avoided by configurations where at least one mechanical grating is replaced with an optical grating, i.e. a standing light wave. Such interferometers show improved scaling behaviour. Magnifying or demagnifying interferometer variants are also discussed.
Two of the major classes of atom optical elements are microfabricated structures and light beams. For instance, a standing light wave can act as a phase grating for atoms, as each absorption and emission process is accompanied by a coherent atomic recoil. After a simple discussion of the mechanical effects of light on atoms, we use a band-theoretical approach to categorize the main resonances (Bragg and Doppleron) that can be exploited to operate such a grating effectively. We discuss the incoherent effects of spontaneous emission, showing when and how they lead from a diffractive to a diffusive regime of atom–field interactions. We turn next to the diffraction of atoms by quantized fields and show the sensitivity of the resulting atomic diffraction pattern to the field photon statistics. In this quantized regime, the atom–field interaction leaves the system in an entangled state. We conclude by reviewing some proposals that exploit this property to perform measurements on the field (atom) to prepare and/or monitor specific atomic (field) states.
In this thesis, the use of pulsed laser fields to control the center-of-mass motion of neutral atoms and molecules is studied both numerically and experimentally. In particular, we consider the use of a pulsed standing wave for the deflection of atomic beams, and develop novel types of mirrors for applications in atom and molecule optics. The research work shows that pulsed laser fields can be applied to form efficient mirrors for neutral particles regardless of the details of their internal energy-level structure. Moreover, it is shown that pulsed standing waves can also be utilized to efficiently deflect atomic beams. In the first part of the thesis, we study the effects of phase fluctuations of the laser field on the momentum distribution of atoms after interaction with a standing wave. Our numerical simulations show that the coherence properties of the laser field play a major role in diffraction of atoms, especially when using pulsed lasers. It turns out that the effects caused by the limited coherence time of the optical field are clearly observable as a transfer of atoms in momentum space to the region around zero momentum rather than to high momentum states. The numerical analysis of the pulsed standing-wave diffraction was complemented by experimental measurements of the deflection profiles of a beam of thermal sodium atoms. The experimental distributions were measured for a value of the coherence time intermediate between the Rabi period and the interaction time. A comparison of the results with an expected distribution calculated for a fully coherent interaction shows that the phase fluctuations of the light field drastically affect the achievable transverse momentum values. Based on our theoretical and experimental analysis of the diffraction profiles, we conclude that large deflection angles can be reached only when laser pulses with a Fourier-limited bandwidth are applied. The second part of the thesis concentrates on the development of pulsed mirrors for neutral atoms and molecules. In particular, we consider the use of an evanescent wave, formed in total internal reflection of light on a vacuum-dielectric interface, and of a standing wave as possible mirror configurations. Our numerical simulations of the pulsed evanescent-wave mirror show that atoms with initial velocities up to several tens of meters per second can be reflected already at the moderate field intensities of a few tens of MW/cm2. This allows pulsed evanescent waves to be applied in a compact setup to control the motion of thermal atomic beams, for which the traditional atom mirrors are suited only at grazing incidence. However, since the effective interaction volume of the atomic beam and the laser field is limited by the short decay length of the evanescent wave, only a relatively small number of atoms can be reflected with a single laser pulse. Furthermore, the reflection is always inelastic, since the force exerted on the atoms varies as a function of the initial position of the particles. To solve the above problem we consider the use of a standing wave as a pulsed mirror for atoms. Our numerical simulations show that with a proper choice of the pulse duration and field intensity, each period of the standing-wave pattern functions as an elastic mirror for particles with an initial kinetic energy below the depth of the standing-wave potential. Hence, the standing-wave pattern acts as a long row of independent atom mirrors, which provides a novel route to controlling large volumes of fast atoms even with a single laser pulse. As a second application for the pulsed standing-wave mirror we consider the reflection of slow molecules, for which no efficient mirror configuration previously has been available. It is shown that pulsed standing waves of nonresonant frequencies can be used to efficiently manipulate large volumes of rotationally cold molecules. With a suitable choice of the parameter values, the molecules can be reflected nearly elastically, or with a tailored momentum distribution. This opens up a possibility to use the pulsed standing-wave mirror to control the motion of particles in various applications of atomic and molecular physics. Such a mirror could be applied, for example, to steer and image molecular beams, separate molecular species, and to form pulsed molecular beams.