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Migratory birds travel spectacular distances, navigating and orienting by a variety of means, most of which are poorly understood. Among them is a remarkable ability to perceive the intensity and direction of the Earth's magnetic field. Biologically credible mechanisms for the sensing of such weak fields (25-65 microtesla) are scarce and in recent years just two proposals have emerged as frontrunners. One involves biogenic iron-containing nanoparticles; the other relies on the magnetic sensitivity of short-lived photochemical intermediates known as radical pairs. The latter began to attract attention following the proposal 15 years ago that the necessary physics and chemistry could take place in the bird's retina in specialised photoactive proteins called cryptochromes. The coherent dynamics of the electron-nuclear spin systems of pairs of photo-induced radicals is conjectured to form the basis of the sensing mechanism even though the interaction of an electron spin with the geomagnetic field is six orders of magnitude smaller than the thermal energy. The possibility that slowing decohering, entangled electron spins could form the basis of an important sensory mechanism has qualified radical pair magnetoreception for a place under the umbrella of ``Quantum Biology.'' In this talk, I will introduce the radical pair mechanism, comment on the roles of entanglement and quantum coherence, outline some of the experimental evidence for the cryptochrome hypothesis, and summarize what still needs to be done to determine whether birds (and maybe other animals) really do use a chemical compass to find their way around.

Does it quantum compute?
October 9, 2015

Moderator
Jeremy Levy, PhD
 
Panelists
Michael Hatridge, PhD, Daniel Lambrecht, PhD, Peyman Givi, PhD
 

A panel discussion of faculty members from the Pittsburgh Quantum Institute (http://www.pqi.org) was organized during Science 2015 to learn about the new field of quantum computing.  The panel includes experts from physics, chemistry and engineering disciplines, and will be moderated by the Director of PQI.  What is a quantum computer?  How are they different from ordinary computers?  What kinds of disciplines can a quantum computer impact?  Why don’t we have quantum computers already?

We set out to study a system which couples a nanomechanical harmonic oscillator to a single spin. While individual spins are intrinsically quantum objects, mechanical resonators are usually observed as classical systems. Not coincidentally, spins are usually largely isolated from their environment, while nanomechanical devices excel at coupling to almost everything. In our system, a spin and a ​nanomechanical resonator interact such that they perform quantum non-demolition (QND) measurements on each other, enabling a bridge between the quantum and classical worlds. The strength of the coupling is enhanced by utilizing an avoided level crossing of the coupled spin-resonator system. The sensitivity is maximized by minimizing the mass of the oscillator, leading us to explore graphene resonator and trap-based implementations. Diamond nitrogen vacancy centers are chosen as the source of a spin due to their exceptional spin state coherence times, large zero-field splitting, and optical addressability. Progress towards an experimental realization of this system has further lead us to improve graphene growth techniques, develop novel fabrication methods, and create magnetic traps for diamond nanocrystals.

In transport through nanostructures connected to two semi-infinite leads, the transmission probability calT (E) as a function of the energy E of the incoming electron plays a central role in the Landauer calculation of the electrical conductance G. A quantum dragon nanostructure is one which when connected to appropriate leads has total electron transmission for all energies, calT (E) =1. In two-lead measurements of single-channel quantum dragons, the quantum of conductance, G0 = 2e2 / h , should be observed. A quantum dragon may have strong scattering. In the disorder was along the axis of electron propagation, the z axis. We show that quantum dragon nanostructures can be found for strong disorder perpendicular to the z axis. In select types of nanostructures, we find the ratio of the dimension of the parameter space where quantum dragons exist to that of the complete parameter space. The results use the single-band tight-binding model, and are for the case with only one open channel and homogeneous leads. One type of nanostructure with calT (E) =1 has completely disordered slices perpendicular to the z axis, but identical slices along the z direction.

Electronic confinement at nanoscale dimensions remains a central means of science and technology.  I will describe a novel method for producing electronic nanostructures at the interface between two normally insulating oxides, LaAlO3 and SrTiO3.  Conducting nanostructures are written, erased and reconfigured under ambient conditions at room temperature, similar to the operation of an etch-a-sketch toy.  A wide variety of devices can be created, including nanowires, tunnel junctions, diodes, field-effect transistors, single-electron transistors, superconducting nanowires, and nanoscale THz emitters and detectors.   After an introduction, I will focus on two recent results: the discovery of a novel phase in which electrons form pairs without becoming superconducting, and the discovery of electronically controlled ferromagnetism at room temperature.  Both phenomena occur in the same family of LaAlO3/SrTiO3 heterointerfaces.

Ed Gerjuoy gives a colloquim on the history of Physics.

Alexandre is an undergraduate senior who majors in physics at the University of Pittsburgh. He began his research with Professor Jeremy Levy when he was a freshman. Gauthier’s research focuses on the production of an advanced canvas analyzer, used to measure the electrical properties of multiterminal devices, and a low temperature scanning probe microscope, used to study electromechanical properties of single-electron transistors. He was awarded the Goldwater Scholarship for his innovations which was described by Chancellor Mark A. Nordenberg as “the highest national honor that can be won by undergraduate students studying science, math, or engineering, which makes the entire Pitt community particularly proud of Alexandre's selection.” 

Cong Wang is a graduate student in the Department of Physics and Astronomy at Pitt.

He works in the Petek lab where he does research in ultrafast surface phenomena.

Cong was a PQI Graduate Student Research Fellow in 2014/2015 for his work on “Three-Dimensional Coherent Photoelectron Spectroscopy".

Michelle Tomczyk is a graduate student in the Department of Physics and Astronomy at Pitt.

She works in the Levy lab where she studies quantum transport phenomena at the lanthanum aluminate and strontium titanate interface. She creates nanostructures at the interface using an innovative AFM lithography technique in order to study emergent phenomena like superconductivity and magnetism. This information can benefit classical as well as quantum computing.

Michelle won a travel award at the Science 2014 poster session for her poster on “Electron Pairing Without Superconductivity”.

Mathew Daniels is a graduate student in the Department of Physics at Carnegie Mellon University.

He works in the Xiao group on magnonics and antiferromagnets.

Mathew won a travel award at the Science 2014 poster session for his poster on “Spin-Transfer Torque Induced Spin Waves in Antiferromagnetic Insulators”.

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