Seminar

Status of the Search for Majorana Fermions in Semiconductor Nanowires

Speaker(s): 
Dr. Sergey Frolov
Dates: 
Monday, August 27, 2018 - 4:00pm

Majorana fermions are non-trivial quantum excitations that have remarkable topological properties and can be used to protect quantum information against decoherence. Tunneling spectroscopy measurements on one-dimensional superconducting hybrid materials have revealed signatures of Majorana fermions which are the edge states of a bulk topological superconducting phase. We couple strong spin-orbit semiconductor InSb nanowires to conventional superconductors (NbTiN, Al) to obtain additional signatures of Majorana fermions and to explore the  topological phase transition. A potent alternative explanation for many of the recent experimental Majorana reports is that a non-topological Andreev state localizes near the end of a nanowire. We compare Andreev and Majorana modes and investigate ways to clearly distinguish the two phenomena. We are also exploring how Andreev states can be chained together along the nanowire to realize the one-dimensional Kitaev model, a discrete way of generating Majorana modes.

Chemical and Physical Considerations in the Production of a Cup of Coffee

Speaker(s): 
Christopher Hendon
Dates: 
Monday, February 11, 2019 - 4:00pm

Despite coffee’s ubiquity and tremendous economic value (~1.5% of the USA GDP), there remains very little research in the field. Yet, numerous physical and chemical processes play a determining role in cup quality, ranging from agricultural practices, to roasting and brewing. This talk canvases the landscape of coffee research to date, detailing areas that require further study, as well as discussing our early efforts to better understand the key factors that determine cup quality and reproducibility.

Theory of strong driving of silicon quantum dot qubits

Speaker(s): 
Yuan-Chi Yang
Dates: 
Friday, August 31, 2018 - 12:00pm

Quantum computation is a promising way to expand computational power as well as perform quantum simulations. There are many proposals on implementing quantum computation, including topological materials, trapped ions, superconducting circuits as well as semiconductor quantum dots. Semiconductor quantum dot qubits are promising candidates for quantum information processing and have recently made substantial experimental progress. One challenge for qubits without topological protection, however, is to suppress decoherence. Performing qubit gate operations as quickly as possible can be important to minimize the effects of decoherence. For resonant gates, this requires applying a strong ac drive. However, strong driving can present control challenges because of the strong driving effects that cannot be described using the rotating-wave approximation. Here we analyze resonant X rotations of a silicon double quantum dot hybrid qubit within a dressed-state formalism. We show that the strong driving effects can be suppressed to the point that gate fidelities above 99.99% are possible, in the absence of decoherence. When coupled to 1/f charge noise typical to our device, we further show that, by applying strong driving, gate fidelities can be above 99.9%. This shows that the quantum operations on silicon quantum dot hybrid qubits can be above the error-correction threshold, which is an important step towards realizing quantum computation.

Lorentzian symmetry predicts universality beyond scaling laws

Speaker(s): 
Stephen J. Watson
Dates: 
Wednesday, August 15, 2018 - 11:00am to 12:00pm

We present a covariant theory for the ageing characteristics of phase-ordering systems that possess dynamical symmetries beyond mere scalings. A chiral spin dynamics which conserves the spin-up (+) and spin-down (−) fractions, $\mu_+$  and $\mu_-$ , serves as the emblematic paradigm of our theory. Beyond a parabolic spatio-temporal scaling, we discover a hidden Lorentzian dynamical symmetry therein, and thereby prove that the characteristic length L of spin domainsgrows in time t according to $L = \frac{\beta}{\sqrt{1 - \sigma^2}}t^{\frac{1}{2}}$ , where $\sigma:= \mu_+ - \mu_-$  (the invariant spin-excess) and βis a universal constant. Furthermore, the normalised length distributions of the spin-up and the spin-down domains each provably adopt a coincident universal (σ-independent) time-invariant form, and this supra-universal probability distribution is empirically verified to assume a form reminiscent of the Wigner surmise.

Mechanism of metal-like transport in bacterial protein nanowires

Speaker(s): 
Dr. Nikhil Malvankar & Sibel Yalcin
Dates: 
Thursday, September 20, 2018 - 4:00pm

A cornerstone of quantum physics is the interference of electron waves arising from the superposition principle. Metallic conductivity is an effect of interference of partial electron waves multiply scattered at the ion cores of the crystal lattice. But proteins are generally insulators. Electron transfer in proteins occurs through either tunneling or hopping a few nanometers via inorganic cofactors. However, the common soil bacteriaGeobacter sulfurreducens transfer electrons over hundreds of micrometers, to insoluble electron acceptors1 or syntrophic partner species2 for...

Building a quantum computer using silicon quantum dots

Speaker(s): 
Dr. Sue Coppersmith
Dates: 
Thursday, October 11, 2018 - 4:00pm

In principle, quantum computers that exploit the nature of quantum physics can solve some problems much more efficiently than classical computers can.  Motivated by the tremendous scalability of classical silicon electronics, we are working to build a large-scale quantum computer using silicon technology similar to that used to build current classical computers.  This talk will discuss the fundamental physics and materials science challenges that arise, and how close coupling between theory and experiment has enabled substantial progress towards the goal of high fidelity qubits.  Prospects...

Orbital selective pairing in Fe-based superconductors

Speaker(s): 
Dr. Peter Hirschfeld
Dates: 
Thursday, October 18, 2018 - 4:00pm

Iron-based superconductors are unconventional superconductors with relatively high Tc that derive from metallic parent compounds with several Fe d-states dominant at the Fermi level. This gives rise to a number of novel effects based on differentiated degree of correlation of the different orbital states. I discuss the influence on spin-fluctuation pairing theory of orbital selective strong correlation effects in Fe-based superconductors, particularly Fe chalcogenide systems. This paradigm yields remarkably good agreement with the experimentally observed anisotropic gap structures in both...

Coupling a Superconducting Qubit to a Metamaterial Resonator

Speaker(s): 
Dr. Britton Plourde
Dates: 
Tuesday, November 27, 2018 - 4:00pm

Superconducting metamaterials formed from arrays of thin-film lumped circuit elements provide a route for implementing novel dispersion relations and band structure in a circuit QED environment. We have implemented metamaterial resonators from left-handed transmission lines and characterized their dense spectrum of modes through a combination of microwave transmission measurements and laser scanning microscopy imaging of the standing-wave structure on the various resonances. By appending a segment of a conventional transmission line on one end of our metamaterial, we have coupled a flux-...

Are we quantum computers, or merely clever robots?

Speaker(s): 
Matthew Fisher
Dates: 
Monday, October 15, 2018 - 4:00pm

Of course quantum information processing is not possible in the warm wet brain. There is, however, one \loophole" - oered by nuclear spins - that must be closed before acknowledging that we are merely clever robots. Putative neural quantum processing with nuclear spins seemingly requires fulllment of many unrealizable conditions: for example, a common biological element with a very isolated nuclear spin to serve as a qubit, a mechanism for quantum entangling qubits, a mechanism for quantum memory storage and processing, a quantum to biochemical transduction that modulates neuron ring rates...

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