Seminar

The Physics of Brain Science: Quasicriticality, An Organizing Principle?

Speaker(s): 
Dr. Rashid Williams-Garcia
Dates: 
Thursday, November 8, 2018 - 4:00pm

Empirical evidence suggests that living neural networks operate near a continuous phase transition, conjectured to be an optimal point for information storage and processing. Applying theoretical approaches, however, is challenging since vital features of neural networks present numerous obstacles to the applicability of traditional statistical physics tools, many of which have not yet been adapted to neuroscience. I will describe a simple cellular automaton model which allows for the characterization of the out-of-equilibrium transition and demonstrates an explicit symmetry breaking due...

Using Interfacial Electric Fields at Domain Walls to Stabilize Novel Ground States

Speaker(s): 
Dr. Julia Mundy
Dates: 
Thursday, September 27, 2018 - 4:00pm

Interfaces between two distinct complex oxide materials can display ground states which diverge greatly from the parent compounds, making them a playground to establish emergent phenomena. Particularly intriguing are the so-called polar interfaces where a diverging electrostatic potential leads to charge transfer. The canonical polar interface between two insulating oxides, LaAlO3/SrTiO3, forms a two-dimensional electron liquid which superconductors at low-temperature and where the conductivity can be manipulated by changing the film surface. Here, I will demonstrate novel functionality at...

Reciprocal and nonreciprocal amplification at the quantum level

Speaker(s): 
Dr. Anja Metelmann
Dates: 
Wednesday, November 28, 2018 - 2:00pm

Preserving the quantum coherence of signals is of paramount importance for components utilized in quantum information processing, quantum computation and quantum measurement setups. In recent years a tremendous progress has been made in the development of quantum-limited components, such as reciprocal and nonreciprocal amplifiers, circulators and isolators. A promising way to design these devices is based on parametric modulation of coupled modes, where the required mode-mixing processes are realized by utilizing Josphson junction-based tunable couplers or via coupling to mechanical...

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...

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