## Abstracts for Condensed Matter Seminars

### Vanita Srinivasa, September 8, 2016, Abstract:

Many proposed realizations of quantum information processing rely on rapid and robust entanglement of coherent qubits over a wide range of distances. While solid-state implementations based on electron spin qubits are potentially scalable, spin manipulation and coupling methods that take advantage of rapid control of the electron charge are often limited in range and remain susceptible to charge noise and relaxation. I will describe our theoretical approaches to addressing these challenges for spin qubits encoded in multiple electrons within systems of coupled quantum dots. We analyze a new regime for capacitive coupling of two-electron spin qubits that leads to high theoretical fidelities for entangling gates within silicon-based implementations in the presence of charge-based decoherence. We also show that the three-electron resonant exchange qubit provides both a protected operating point for rapid single-qubit manipulation and an electric dipole moment that enables multiple approaches for long-range entangling gates via a superconducting microwave resonator. These methods are inspired by techniques from circuit quantum electrodynamics, Hartmann-Hahn double resonance in NMR, and the Cirac-Zoller gate for trapped ions.

### Adam Nahum, September, 29, 2016, Abstract:

A quantum many-body system, prepared initially in a state with low entanglement, will entangle distant regions dynamically. How does this happen? I will discuss entanglement entropy growth for quantum systems subject to random unitary dynamics — i.e. Hamiltonian evolution with time-dependent noise, or a random quantum circuit. I will show how entanglement growth in this ‘noisy’ situation exhibits remarkable universal structure, which in 1D is related to the Kardar—Parisi—Zhang equation. I will argue that understanding this structure leads us to heuristic pictures for entanglement growth which may be useful for more general (non-noisy) dynamics.

### Chandra Varma, October 3, 2016, Abstract:

Models for metallic anti-ferromagnets map to the dissipative XY model as do XY ferromagnets, the superconductor-insulator transition, and the model forloop-current order in Cuprates. The spectral function of the quantum-critical fuctuations for this model in 2D for a range of parameters is determined by topological defects - instantons and 2D vortices; it is a separable function of space and time, with a 1= dependence at criticality. The marginal fermi-liquid properties for the fermions follow from coupling to such fluctuations.

These fluctuations are directly measured by inelastic neutron scattering in the quantum-critical region of the new Fe- based compounds, in a 2D ferromagnet and in heavy-Fermions, and deduced in cuprates through analysis of ARPES. ARPES also reveals the coupling function of the fermions to the fluctuations so as to give nearly angle-independent normal self-energy but d-wave pairing energy.

### Nuh Gedik, October 10, 2016, Abstract:

The coherent optical manipulation of solids is emerging as a promising way to engineer novel quantum states of matter. The strong time-periodic potential of intense laser light was predicted to generate hybrid photon–electron states named Floquet–Bloch states. In this talk, I will report on first experimental observation of these states in topological insulators (TI). Using time- and angle-resolved photoemission spectroscopy, we show that an intense ultrashort mid-infrared pulse with energy below the bulk band gap of TI hybridizes with the surface Dirac fermions of a topological insulator to form Floquet-Bloch bands. The photon-dressed surface band structure is composed of a manifold of Dirac cones evenly spaced by the photon energy and exhibits polarization-dependent band gaps at the avoided crossings of the Dirac cones. Circularly polarized photons induce an additional gap at the Dirac point, which is a signature of broken time-reversal symmetry on the surface. Beyond topological insulators, manipulation of electronic bands via light matter interaction holds great promise in many other systems as well. I will illustrate this with our recent measurements on monolayer semiconducting transition-metal dichalcogenides in which we were able to break valley degeneracy using off-resonant circularly polarized light.

### Victor Albert, October 13, 2016, Abstract:

Lindbladians, one of the simplest extensions of Hamiltonian-based quantum mechanics, are used to describe “drainage” (i.e., decay) and decoherence of a quantum system induced by the system's environment. While traditionally viewed as detrimental to fragile quantum properties, a tunable environment offers the ability to drive the system toward exotic phases of matter, which may be difficult to stabilize in nature, or toward protected subspaces, which can be used to store and process quantum information. An important property of Lindbladians is their behavior in the limit of infinite time, and in this talk I will discuss a formula for the map corresponding to infinite-time Lindbladian evolution. This formula allows us to determine to what extent decay affects a system's linear or adiabatic response. It also allows us to determine geometrical structures (holonomy, curvature, and metric) associated with adiabatically deformed steady-state subspaces.

### Andy Lucas, October 17, 2016, Abstract:

Although hydrodynamics is believed to be a universal limit for many interacting quantum systems, observing this regime is hard in ordinary metals. Thanks to recent developments in materials physics, graphene is a material where we should hope to observe multiple interesting hydrodynamic regimes. I will first discuss the indirect observation of electron fluid vorticity via negative nonlocal resistances in the Fermi liquid regime of doped graphene. I will follow up with a complementary experiment which discovers exotic thermal transport at the charge neutrality point, and argue that this is a signature of quasirelativistic hydrodynamics in the electron-hole plasma.

### Arbel Haim, October 19, 2016, Abstract:

Recent experiments have provided mounting evidence for the existence of Majorana bound states (MBSs) in condensed-matter systems. Until the long-term goal of braiding MBSs is achieved, one is prompted to ask: what is the next step in the study of topological superconductivity and MBSs? In my talk I will discuss two topics relating to this question. In the first part I will examine the possibility of, not only detecting the Majoranas, but also witnessing some of their exotic properties. In particular their non-local nature, or in other words, the fact that the MBS is half a fermion whose occupation is encoded in a nonlocal way. I will show that current cross correlations in a T-junction with a single MBS exhibit universal features, related to the Majorana nonlocality. This will be contrasted with the case of an accidental low-energy Andreev bound state. In the second part I will discuss the possibility of realizing a different topological phase hosting MBSs in currently available experimental platforms. This will be a topological superconducting phase which is protected by time-reversal symmetry, and which is characterized by having a Kramers’ pair of MBSs at each end. As I will discuss, repulsive interactions are a necessary ingredient for the realization of this phase. I will present a mechanism, based on the interplay between repulsive interactions and proximity to a conventional superconductor, which drives the system into the topological phase. The effect of interactions is studied analytically using both a mean-field approach and the renormalization group. We corroborate our conclusions numerically using DMRG.

[1] Arbel Haim, Anna Keselman, Erez Berg, Yuval Oreg, Phys. Rev. B, 89, 220504(R) (2014)

[2] Arbel Haim, Erez Berg, Felix von Oppen, Yuval Oreg, Phys. Rev. B, 92, 245112 (2015)

[3] Arbel Haim, Erez Berg, Felix von Oppen, Yuval Oreg, Phys. Rev. Lett. 114, 166406 (2015)

[4] Arbel Haim, Konrad Wölms, Erez Berg, Yuval Oreg, Karsten Flensberg, Phys. Rev. B 94, 115124 (2016)

[5] Arbel Haim, Erez Berg, Karsten Flensberg, Yuval Oreg, arXiv:1605.07179

### Andy Mackenzie, November 7, 2016, Abstract:

The delafossites are a series of layered compounds with triangular lattices similar to that of NaCoO_{2} but with a different stacking sequence along the *c* axis. They are host to intriguing magnetic insulators and semimetals, as well as metals such as PdCoO_{2}, PtCoO_{2}, PdCrO_{2} and PdRhO_{2}. The properties of these metals are remarkable. Although they are strongly two-dimensional, their room temperature electrical conductivity is higher per carrier than that of any elemental metal, and PdCoO_{2} crystals can have a low temperature resistivity of only a few n Ω cm, corresponding to mean free paths of tens of microns. On the one hand, our group is attempting to accept this huge conductivity and profit from it, for example by investigating whether we can enter the hydrodynamic regime of electronic transport. On the other hand we are trying to understand why the conductivity is so high, combining spectroscopic properties and electronic structure calculations. I will report on our progress on both fronts.

### Erik Nielsen, November 8, 2016, Abstract:

Research in quantum computing is a still-growing field, fueled by interest in both it’s powerful applications and connection to fundamental science. Qubits, the building blocks of future quantum computers, must both manipulate and protect the quantum information they hold – a task fraught with challenges. In this talk, I will describe some of the techniques used at Sandia National Laboratories to model and design silicon qubit devices. Results showing that some aspects of a device’s behavior can be predicted well, while others require further work, will be presented. About mid-way through the talk, I will switch gears and speak about the distinct but related area of quantum characterization. I will give a brief overview of this field and describe in more detail the Gate Set Tomography (GST) protocol. GST is designed to aid in debugging qubit devices, and will be related to the microscopic device modeling techniques presented in the first half of the talk.

### Brian Swingle, November 14, 2016, Abstract

Out-of-time-order correlation functions are of theoretical interest for diagnosing the scrambling of quantum information in black holes and strongly interacting quantum systems generally. I will describe a general protocol for measuring them which requires an echo-type sequence in which the sign of a many-body Hamiltonian is reversed. I will discuss an implementation employing cold atoms and cavity quantum electrodynamics to realize the chaotic kicked top model and will analyze the effects of dissipation to verify its feasibility with current technology. Following some comments on subsequent proposals and on two preliminary experiments, I will conclude with a survey of some recent calculations of out-of-time-order correlators and what I think we might learn from experiments.

### Andrea Liu, November 21, 2016, Abstract

When we first learn the physics of solids, we are taught the theory of perfect crystals. Only later do we learn that in the real world, all solids are imperfect. The perfect crystal is invaluable because we can describe real solids by perturbing around this extreme limit by adding defects. But such an approach fails to describe a glass, another ubiquitous form of rigid matter. I will argue that the jammed solid is an extreme limit that is the anticrystal--an opposite pole to perfect order. Like the perfect crystal, it is an abstraction that can be understood in depth and used as a starting point for understanding the mechanical properties of solids with surprisingly high amounts of order. Unlike the crystal, however, the jammed state becomes rigid via a critical phase transition, which can be described by a critical scaling ansatz, paving the way for a renormalization group analysis.

### AndrewJordan, December 12, 2016, Abstract:

Recent theoretical and experimental progress in continuously monitored quantum systems has permitted the real-time tracking of the quantum state during the measurement process. I will discuss (1) how to predict the most likely path between two boundary conditions in time, (2) the probability distribution of entanglement created by the measurement process in two remote qubits, and (3) how to rapidly estimate parameters of the system from the time continuous output. Comparisons of some of these ideas with experimental data will be presented.