Quantum superposition lies at the heart of quantum mechanics and gives rise to many of its paradoxes. Superposition of de Broglie matter waves' has been observed for massive particles such as electrons, atoms and dimers, small van der Waals clusters, and neutrons. But matter wave interferometry with larger objects has remained experimentally challenging, despite the development of powerful atom interferometric techniques for experiments in fundamental quantum mechanics, metrology and lithography. Here we report the observation of de Broglie wave interference of C(60) molecules by diffraction at a material absorption grating. This molecule is the most massive and complex object in which wave behaviour has been observed. Of particular interest is the fact that C(60) is almost a classical body, because of its many excited internal degrees of freedom and their possible couplings to the environment. Such couplings are essential for the appearance of decoherence, suggesting that interference experiments with large molecules should facilitate detailed studies of this process.

The phenomenon of quantum interrogation allows one to optically detect the presence of an absorbing object, without the measuring light interacting with it. In an application of the quantum Zeno effect, the object inhibits the otherwise coherent evolution of the light, such that the probability that an interrogating photon is absorbed can in principle be arbitrarily small. We have implemented this technique, demonstrating efficiencies exceeding the 50% theoretical-maximum of the original "interaction-free" measurement proposal. We have also predicted and experimentally verified a previously unsuspected dependence on loss; efficiencies of up to 73% were observed and the feasibility of efficiencies up to 85% was demonstrated. (EV) showed that the wave-particle duality of light could allow "interaction-free" quantum interrogation of classical objects, in which the presence of a non-transmitting object is ascertained seemingly without interacting with it [3], i.e., with no photon absorbed or scattered by the object. In the basic EV technique, an interferometer is aligned to give complete destructive interference in one output port -the "dark" output -in the absence of an object. The presence of an opaque object in one arm of the interferometer eliminates the possibility of interference so that a photon may now be detected in this output. If the object is completely non-transmitting, any photon detected in the dark output port must have come from the path not containing the object. Hence, the measurements were deemed "interaction-free", though we stress that this term is sensible only for objects that completely block the beam. For measurements on partially-transmitting (and quantum) objects, we suggest the more general terminology "quantum interrogation". In any event there is necessarily a coupling between light and object (formally describable by some interaction Hamiltonian) -somewhat paradoxically, in the high-efficiency schemes discussed below, it is crucial that the possibility of an interaction exist, in order to reduce the probability that such an interaction actually occurs.The EV gedanken experiment has been realized using true single-photon states [4] and with a classical light beam attenuated to the single-photon level [5], as well as in neutron interferometry [6]. This methodology has even been employed to investigate the possibility of performing "absorption-free" imaging [7]. The EV technique suffers two serious drawbacks, however. First, the measurement result is ambiguous at least half of the timea photon may be detected in the non-dark output port whether or not there is an object. Second, at most half of the measurements are interaction-free [4,7]. Following Elitzur and Vaidman [3], we define a figure of merit η = P(QI)/[P(QI) + P(abs)] to characterize the "efficiency" of a given scheme, where P(QI) is the probability that the photon is detected in the otherwise dark port, and P(abs) is the probability that the object absorbs or scatters the photon. Physically, η is the fraction of measurements...

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 C 60 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 ͑spectral͒ 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 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 C60 and C70 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.

Using the complementary wave-and particle-like natures of photons, it is possible to make "interaction-free" measurements where the presence of an object can be determined with no photons being absorbed. We investigated several "interaction-free" imaging systems, i.e. systems that allow optical imaging of photosensitive objects with less than the classically expected amount of light being absorbed or scattered by the object. With the most promising system, we obtained high-resolution (10 µm), one-dimensional profiles of a variety of objects (human hair, glass and metal wires, cloth fibers), by raster scanning each object through the system. We discuss possible applications and the present and future limits for interaction-free imaging.PACS number(s): 03.65.Bz, 42.25.Hz