Applied Physics / Physics Colloquium

Exciton-Polaritons in semiconductor microcavities are hybrid states of light and matter exhibiting a mix of electronic and photonic properties [1], including strong nonlinearity, low dephasing, ultrafast dynamics, sensitivity to electric and magnetic fields and a rich spin dynamics. These properties are ideal for the construction of a new generation of polaritonic information processing devices, such as exciton-polariton based circuits [2].

For such applications, attention is needed to overcome disorder in the system. Here, advances in the patterning of polariton potentials [3] are theoretically expected to allow for topological polaritons [4]. In analogy to photonic topological insulators [5], propagation immune to scattering with disorder is predicted.

In this presentation, I will also show preliminary work in the application of exciton-polaritons in unconventional circuit architectures such as neural networks [6].

Finally, I will consider the enhancement of nonlinear polaritonic effects at the quantum level with a viewpoint toward triggered single-photon emission [7].

Sketched out in 1992, selected by ESA in 1996, launched in 2009, Planck delivered a first set of results on March 21, 2013, in particular a "definitive" map of the anisotropies of the Cosmic Microwave Background (CMB). The later displays minuscule variations as a function of the observing direction of the temperature of the fossil radiation around its mean temperature of 2.725K. These CMB anisotropies, of rms ~100microK, reveal the imprint of the primordial fluctuations which initiate the growth of the large scale structures of the Universe, as transformed by their evolution, in particular during the first 370 000 years. Since 2013, we analyzed twice more data and in particular the polarization information we gathered over the full course of the mission. I will describe the new results we just obtained, and in particular confront what temperature and polarization anisotropies teach us, both in terms of content of the universe and of characteristics of the primordial fluctuations.

Held Tuesdays at 4:15 pm, in the William R. Hewlett Teaching Center, room 200 (see map). Refreshments in the lobby of Varian Physics at 4:00 pm.

Autumn 2014/15, Committee: A. Linde (Chair), L. Hollberg, B. Macintosh & Young Lee

Learning how sensory signals are transformed into perception is a challenge. In vitro preparations offer access to single cells as well as to an ever-expanding arsenal of tools to measure physiology, but come at the cost of perception. Living animals impose limitations on access (controlling the retinal stimulation, for example) and can only offer primitive measures of perception. By combining adaptive-optics micro-optical-stimulation with high-speed eye tracking in humans, we believe we can offer the best of both worlds. Adaptive optics and high-speed eye tracking allow us to control light stimulation down to the single receptor level and, since it is done in humans, we can use that in concert with sophisticated psychophysical tasks. This talk will describe the evolution of the technology that we've developed in our lab followed by an update on our ongoing efforts to learn about the neural circuits that underlie human spatial and color vision.

Held Tuesdays at 4:15 pm, in the William R. Hewlett Teaching Center, room 200 (see map). Refreshments in the lobby of Varian Physics at 4:00 pm.

Autumn 2014/15, Committee: A. Linde (Chair), L. Hollberg, B. Macintosh & Young Lee

Shock waves are ubiquitous in astrophysical environments and are tightly connected with magnetic-field amplification and particle acceleration. The fast progress in high-power laser technology is bringing the study of high Mach number shocks into the realm of laboratory plasmas, where in situ measurements can be made helping us understand the fundamental kinetic processes behind shocks. I will discuss the recent progress in laser-driven shock experiments at state-of-the-art facilities like NIF and Omega and the important role that ab initio massively parallel simulations are playing in the design of these experiments and in our understanding of the plasma microphysics involved in particle acceleration and radiation emission. Finally, I will show that by controlling the laser and plasma conditions it is possible to explore different particle acceleration mechanisms, from Fermi-like processes, which are relevant in astrophysics, to the generation of high-quality ion beams for radiotherapy.

Held Tuesdays at 4:15 pm, in the William R. Hewlett Teaching Center, room 200 (see map). Refreshments in the lobby of Varian Physics at 4:00 pm.

Autumn 2014/15, Committee: A. Linde (Chair), L. Hollberg, B. Macintosh & Young Lee

Photons are boring: they all move at one speed and do not interact with one another. I will present an unusual optical medium that is nonlinear at the quantum scale: In this medium, photons travel slowly, acquire mass, and exhibit strong mutual attraction, so strong that two photons can even form a two-body bound state. The medium can also be made to transmit one, but absorb two photons. This and other progress in the field enables novel quantum optical devices, such as a strongly interacting quantum gas of photons, an optical transistor gated by just one photon, or a device that can detect and count optical photons without destroying them.

Held Tuesdays at 4:15 pm, in the William R. Hewlett Teaching Center, room 200 (see map). Refreshments in the lobby of Varian Physics at 4:00 pm.

Autumn 2014/15, Committee: A. Linde (Chair), L. Hollberg, B. Macintosh & Young Lee

Recent advances in fields ranging from cosmology to computer science have hinted at a possible deep connection between intelligence and entropy maximization, but no formal physical relationship between them has yet been established. In this talk, we present recent advances toward such a relationship in the form of a causal generalization of entropic forces that we find can cause a variety of rich cognitive adaptive behaviors -- including the passing of multiple animal intelligence tests, human game playing, and profitable financial trading -- to spontaneously emerge in simple physical and digital systems. Our results suggest a potentially general thermodynamic model of adaptive behavior as a nonequilibrium process in open systems.

Held Tuesdays at 4:15 pm, in the William R. Hewlett Teaching Center, room 200 (see map). Refreshments in the lobby of Varian Physics at 4:00 pm.

Autumn 2014/15, Committee: A. Linde (Chair), L. Hollberg, B. Macintosh & Young Lee

Ongoing surveys of nearby stars have revealed an amazing diversity of planetary systems, many of which have characteristics that differ substantially from those of the solar system planets. Perhaps one of the biggest surprises to come out of these surveys was the discovery that "super-Earths" (planets between 1-10 times the mass of the Earth) are in fact the most common type of extrasolar planet. Despite the name we actually know very little about the compositions of these mysterious planets, and it has been suggested that this mass range may include both "water worlds" and "mini-Neptunes" with thick hydrogen envelopes in addition to more Earth-like terrestrial planets. In my talk I will explore current constraints on the compositions of planets with masses ranging from that of Neptune down into the super-Earth regime, and discuss the corresponding implications for our understanding of planet formation and evolution.

Twenty years after the discovery of Shor's factoring algorithm, I'll survey what we now understand about the structure of problems that admit quantum speedups. I'll start with the basics, discussing the hidden subgroup, amplitude amplification, adiabatic, and linear systems paradigms for quantum algorithms. Then I'll move on to some general results, obtained by Andris Ambainis and myself over the last few years, about quantum speedups in the black-box model. These results include the impossibility of a superpolynomial quantum speedup for any problem with permutation symmetry, and the largest possible separation between classical and quantum query complexities for any problem.

Held Tuesdays at 4:15 pm, in the William R. Hewlett Teaching Center, room 200 (see map). Refreshments in the lobby of Varian Physics at 4:00 pm.

Autumn 2014/15, Committee: A. Linde (Chair), L. Hollberg, B. Macintosh & Young Lee

A longstanding goal of violin research has been to establish objectively measurable parameters for violin quality. Doing so would presumably substantiate one of the violin world's most passionately held beliefs: Violins made by Stradivari and his contemporaries in 18th Century Italy sound better than any made elsewhere or since. Over the past five years, a team of researchers led by Claudia Fritz and Joseph Curtin have shown that under double-blind conditions neither professional violinists nor experienced listeners can tell Old Italian violins from new ones at better than chance levels. Moreover, both players and listeners tend to prefer new instruments. Violin-maker, researcher, and 2005 MacArthur Fellow Joseph Curtin will discuss recent developments in violin science and his own interest in measuring violin sound. He will also preview the team's upcoming paper: "Objective parameters for violin quality."

Held Tuesdays at 4:15 pm, in the William R. Hewlett Teaching Center, room 200 (see map). Refreshments in the lobby of Varian Physics at 4:00 pm.

Autumn 2014/15, Committee: A. Linde (Chair), L. Hollberg, B. Macintosh & Young Lee