Phase-dispersion optical tomography

We report on phase-dispersion optical tomography, a new imaging technique based on phase measurements using low-coherence interferometry. The technique simultaneously probes the target with fundamental and second-harmonic light and interferometrically measures the relative phase shift of the backscattered light fields. This phase change can arise either from reflection at an interface within a sample or from bulk refraction. We show that this highly sensitive 5 phase technique can complement optical coherence tomography, which measures electric field amplitude, by revealing otherwise undetectable dispersive variations in the sample

Cavity ring-down technique and its application to the measurement of ultraslow velocities

We have developed a new ring-down technique that does not require a shutter to turn a probe laser on and off. With a rapid cavity scan we can measure a simple exponential cavity decay from which a cavity finesse can be found. When the cavity is scanned slowly, the cavity decay exhibits an amplitude modulation, and an analytic expression is derived for this modulation. With this new technique we measured the ultraslow relative velocity of the mirrors (of the order of micrometers per second) as well as the linewidth (~100 kHz) of the probe laser

Measurement of angular distributions by use of low-coherence interferometry for light-scattering spectroscopy

We present a novel interferometer for measuring angular distributions of backscattered light. The new system exploits a low-coherence source in a modified Michelson interferometer to provide depth resolution, as in optical coherence tomography, but includes an imaging system that permits the angle of the reference field to be varied in the detector plane by simple translation of an optical element. We employ this system to examine the angular distribution of light scattered by polystyrene microspheres. The measured data indicate that size information can be recovered from angular-scattering distributions and that the coherence length of the source influences the applicability of Mie theory

Measuring the Generalized Friendship Paradox in Networks with Quality-dependent Connectivity

The friendship paradox is a sociological phenomenon stating that most people have fewer friends than their friends do. The generalized friendship paradox refers to the same observation for attributes other than degree, and it has been observed in Twitter and scientific collaboration networks. This paper takes an analytical approach to model this phenomenon. We consider a preferential attachment-like network growth mechanism governed by both node degrees and `qualities'. We introduce measures to quantify paradoxes, and contrast the results obtained in our model to those obtained for an uncorrelated network, where the degrees and qualities of adjacent nodes are uncorrelated. We shed light on the effect of the distribution of node qualities on the friendship paradox. We consider both the mean and the median to measure paradoxes, and compare the results obtained by using these two statistics

Direct Observation of Second Order Atom Tunnelling

Tunnelling of material particles through a classically impenetrable barrier constitutes one of the hallmark effects of quantum physics. When interactions between the particles compete with their mobility through a tunnel junction, intriguing novel dynamical behaviour can arise where particles do not tunnel independently. In single-electron or Bloch transistors, for example, the tunnelling of an electron or Cooper pair can be enabled or suppressed by the presence of a second charge carrier due to Coulomb blockade. Here we report on the first direct and time-resolved observation of correlated tunnelling of two interacting atoms through a barrier in a double well potential. We show that for weak interactions between the atoms and dominating tunnel coupling, individual atoms can tunnel independently, similar to the case in a normal Josephson junction. With strong repulsive interactions present, two atoms located on one side of the barrier cannot separate, but are observed to tunnel together as a pair in a second order co-tunnelling process. By recording both the atom position and phase coherence over time, we fully characterize the tunnelling process for a single atom as well as the correlated dynamics of a pair of atoms for weak and strong interactions. In addition, we identify a conditional tunnelling regime, where a single atom can only tunnel in the presence of a second particle, acting as a single atom switch. Our work constitutes the first direct observation of second order tunnelling events with ultracold atoms, which are the dominating dynamical effect in the strongly interacting regime. Similar second-order processes form the basis of superexchange interactions between atoms on neighbouring lattice sites of a periodic potential, a central component of quantum magnetism.Comment: 18 pages, 4 figures, accepted for publication in Natur

Interferometric phase-dispersion microscopy

We describe a new scanning microscopy technique, phase-dispersion microscopy (PDM). The technique is based on measuring the phase difference between the fundamental and the second-harmonic light in a novel interferometer. PDM is highly sensitive to subtle refractive-index differences that are due to dispersion (differential optical path sensitivity, 5 nm). We apply PDM to measure minute amounts of DNA in solution and to study biological tissue sections. We demonstrate that PDM performs better than conventional phase-contrast microscopy in imaging dispersive and weakly scattering samples

Spatial coherence of forward-scattered light in a turbid medium

We study spatially coherent forward-scattered light propagating in a turbid medium of moderate optical depth (0-9 mean free paths). Coherent detection was achieved by using a tilted heterodyne geometry, which desensitizes coherent detection of the attenuated incident light. We show that the degree of spatial coherence is significantly higher for light scattered only once in comparison with that for multiply scattered light and that it approaches a small constant value for large numbers of scattering events