|USC Quantum Information and Condensed Matter Physics Seminars|
For more information contact Stephan Haas (condensed matter) (213) 740-4528, or Ben Reichardt (quantum information) (213) 740-4683.
Stephen Jordan (NIST)
Jan 25, 2pm
Plasmon excitations of metal nanoclusters and their assemblies
Guanghou Wang (Nanjing University)
I will discuss plasmon excitation and detection of metal nanoclusters by electron beam and their size effect. Furthermore, I will talk about plasmon coupling in nanocluster dimers and cluster chains and anisotropic transportation. Finally, I will discuss surface enhancement of Raman 'scattering for metal cluster arrays.
Feb 1, 2pm
Quantum quenches and crossovers in quantum dot systems: a form factor approach
Romain Vasseur (Saclay)
In this talk, I will discuss a special class of quantum quenches in impurity systems, for which the interaction between the impurity and the electron bath can be suffenly turned on (or off). A particularly interesting quantity for such quenches is the so-called Loschmidt echo which measures the overlap between the wave function of the system at a given time and the initial state of the system before the quench. I will focus on the Resonant Level Model and show how the dynamics of the quench can be understood thanks to a mapping onto the classical Ising model. Using the integrability of the system and boundary form factor techniques, both the Loschmidt echo and the distribution of the work done during the quench can be computed exactly, in very good agreement with numerical results. I will also discuss how this relates to the well known Anderson orthogonality catastrophe, and describe finite temperature effects as well.
Feb 15, 2pm
Hamed Sadeghi (CSULB)
March 8, 11am, EEB 248
Excursions through flatland: braiding interactions of anyons Gavin Brennen (Macquarie University)
In systems with physics constrained to two dimensions, point like particles named anyons can occur which have more general exchange statistics than bosons or fermions. While these particles emerge as quasiparticle excitions from strongly correlated ``vacuum" states, they are long lived and robust to perturbations. I will describe the transport properties of anyons in the presence of ordered and random topological environments. Using both a discrete time quantum walk model and a continuous time Hubbard model we find very distinct behaviours for Abelian and non-Abelian anyons which could be observed in experiment.
March 29, 2pm
Possible spin liquid and valence bond solid on honeycomb lattice J1-J2 model through DMRG study
Donna Sheng (CSU Northridge)
Recent numerical simulations have led to the finding of a few possible spin liquid states on kagome and square lattice J1-J2 models. However, the fate of the spin state in honeycomb lattice model with competing J1-J2 coupling remains controversy. We will present our results based on SU(2) invariant density matrix renormalization group and the variational Monte Carlo simulations. We determine a quantum phase diagram and find a Neel phase for small J2/J1<0.22 and a staggered dimer phase for large J2/J1>0.35. In the intermediate region accurate DMRG calculations find a disordered phase with a vanishing magnetization. The plaquette and columnar valence bond order appear to be week in the region where J2 <0.26, while they grow stronger at larger J2 side for our large system sizes. The spin gap is determined to be finite in this phase. Furthermore, we establish a positive topological entanglement entropy and observing striking matchings of the spin and dimer correlations between the DMRG results and VMC results using a projected slave-particle Z2 spin liquid wave function. These findings support that spin liquid phase may be still there. We will discuss the effect of a ferromagnetic J3, and outline the difficulty in fully pinning down the quantum phases in such a system. Future direction will also be addressed.
April 5, 2pm
Engineering Peptides for Electronic Applications
Nurit Ashkenasy (Ben Gurion University)
In recent years there is a great interest in the integration of proteins within electronic devices. Since proteins did not evolve to carry out such tasks, the targeted design of proteins for these specific applications is of great importantce. Our research focuses on the design and preparation of simple artificial protein systems for molecular electronics applications. This approach will be presented here using common protein motifs such as a-helices and b-sheets that are adopted to function in molecular junction configurations. In addition, I will show how inorganic peptide binders, which were selected by biological combinatorial libraries screening tests, can be used to control and modify the electronic properties of semiconductor surfaces. Our studies demonstrate the great potential of the use of specifically designed proteins as components of electronic devices. Moreover, I will show that these systems can be used as simple platforms for understanding the role of different charge transfer mechanisms in protein.
April 12, 10am at SSC 319
How Hard is it to Decide if a Quantum State is Separable or Entangled?
Mark M. Wilde (McGill University)
Suppose that a physical process, described as a sequence of local interactions that can be executed in a reasonable amount of time, generates a quantum state shared between two parties. We might then wonder, does this physical process produce a quantum state that is separable or entangled? Here, we give evidence that it is computationally hard to decide the answer to this question, even if one has access to the power of quantum computation. In order to address this question, we begin by demonstrating a two-message quantum interactive proof system that can decide the answer to a promise version of this problem. We then prove that this promise problem is hard for the class ``quantum statistical zero knowledge'' (QSZK) by demonstrating a polynomial-time reduction from the QSZK-complete promise problem ``quantum state distinguishability'' to our quantum separability problem. Thus, the quantum separability problem (as phrased above) constitutes the first nontrivial promise problem decidable by a two-message quantum interactive proof system while being hard for both NP and QSZK. This is joint work with Patrick Hayden and Kevin Milner, it will be presented at the 2013 IEEE Conference on Computational Complexity, and it is available as arXiv:1211.6120.
April 12, 2pm
Robust quantum self-testing and binary nonlocal XOR games
A quantum input-output device is "self-testing" if the internal behavior of the device (i.e., its initial state and measurements) can be verified based only on the correlation between its classical inputs and outputs. Results on self-testing, which began with the work of D. Mayers and A. Yao in 1998, are crucial building blocks in proofs of security for quantum cryptography. Past results have shown that certain nonlocal games, such as the Greenberger-Horne-Zeilinger game, can serve as self-tests for quantum devices. The work discussed in this talk (arXiv:1207.1819) attempts to begin a systematic classification of quantum self-tests. We prove a necessary and sufficient criterion for self-testing within the class of binary nonlocal XOR games. Our methods invite generalization to larger classes of games. This is joint work with Yaoyun Shi.
April 19, 2pm
First-Principles Studies of Atomic-Scale Engineered Spin Chains
We have studied the unusual charge and spin properties of magnetic atoms (Mn, Co, Fe, Ti, Gd) on a complex surface as constructed by STM. This surface, a lattice of N atoms on Cu(100), was designed to be insulating in order to inhibit the Kondo effect. However, the magnetic adatom may be drawn down into the surface, or stay high above and attract surface atoms to it, with very different resulting properties. We show illustrations from our electronic structure calculations of these systems. The various magnetic atoms exhibit behavior ranging from spin chains to large-anisotropy atomic-scale molecular magnets to a Kondo effect for Co and Ti. Finally, when two magnetic atoms are close to one another, their magnetic spins can interact, with complex and interesting results. We calculate the behavior of dimers, with an excellent match to experimental values of the exchange coupling. We moreover show that the coupling can be decomposed into three different pairing interactions and how to extract the values of each separately, with unexpected contrasts between two binding sites. I will conclude with some comments about the role of first-principles calculations for nanostructures.
May 3, 2pm
The Conditional Entropy Power Inequality for Gaussian Quantum States
Robert Koenig (University of Waterloo)
The classical entropy power inequality, originally proposed by Shannon, is a powerful tool in multi-user information theory. We have recently found a quantum generalization which lower bounds the output entropy as two independent signals combine at a beamsplitter. This yields upper bounds on the capacity of additive bosonic noise channels. In this talk, I summarize these results and propose a generalization of the quantum entropy power inequality involving conditional entropies. I discuss some implications for entanglement-assisted classical communication over additive bosonic noise channels. For the special case of Gaussian states, a proof can be given based on perturbation theory for symplectic spectra. This is based on joint work with Graeme Smith.
May 10, 2pm
Progress on error suppression and error correction in adiabatic quantum computation
Kevin Young (Sandia)
Adiabatic quantum computation (AQC) is often praised for its inherent robustness to decoherence, but I'll open the talk by discussing the modes and mechanisms by which AQC can fail. I'll then discuss how many of these failure mechanisms can be suppressed by exploiting properties of quantum stabilizer codes. Suppression, however, is unlikely to be sufficient for large-scale, fault-tolerant AQC, and some form of error correction will likely be necessary. I'll review the major barriers to error correction in AQC, highlighting key differences with the circuit model, and suggest a few paths forward.
May 31, 2pm
A Return to the Optimal Detection of Quantum Information
Min-Hsiu Hsieh (Centre for Quantum Computation and Intelligent Systems, University of Technology, Sydney, Australia)
In 1991, Asher Peres and William Wootters wrote a seminal paper on the nonlocal processing of quantum information [Phys. Rev. Lett. 66 1119 (1991)]. We return to their classic problem and solve it in various contexts. Specifically, for discriminating the .double trine. ensemble with minimum error, we prove that global operations are more powerful than local operations with classical communication (LOCC). Even stronger, there exists a finite gap between the optimalLOCC probability and that obtainable by separable operations (SEP). Additionally we prove that a two-way, adaptive LOCC strategy can always beat a one-way protocol. Our results provide the first known instance of .nonlocality without entanglement. in two qubit pure states.
June 14, 2pm
Sebestiano Peotta (UCSD)
June 20, 2pm
Optics with a single atom
What is the smallest object that can function as an optical component? We have shown the first absorption imaging by a single isolated atom (174Yb+) and subsequently used our technique to induce and measure a large optical phase shift of 1.3(0.1) radians in light scattered by the atom. The observed image contrast of 3.1(3)% achieved the maximum aloowed by atomic theory for our setup. Imaging resolution of the order of the 370 nm illumination wavelength allowed us to perform spatial interferometry between the scattered light and unscattered illumination light, enabling us to isolate the phase shift in the scattered component. Our results point to new opportunities in microscopy, quantum information, and nanophotonics.