Virtual Seminar Series
Virtual Seminar Series
Click here to view the schedule for upcoming seminars.
Starting in June 2020, we are hosting a series of online talks about topics related to Quantum Information Sciences in its various forms, including (but not limiting to):
- Quantum computers
- Measuring the elusive Majorana fermion
Talks will be given by senior researchers as well as students and postdocs.
Tuesday, January 26 | 11:00 a.m. CST
Saikat Guha Quantum enhancements in photonic information processing
The ultimate limits in photonic based information processing---be it the maximum rate of reliable and secure communications, resolution of an optical imager or sensor, or the computational power of an optical computer---are all ultimately governed by the laws of quantum mechanics, since light is fundamentally quantum. Most current-day systems, which do not exploit the manifestly quantum properties of light, are limited to classical (often purportedly "fundamental") performance limits that can be far inferior to the truly-fundamental quantum-mechanics-mandated performance limits of such systems in their encoding, transmitting, processing or extracting information in the photon. In this seminar, I will discuss a few illustrative problems and applications in quantum communications and networking, and in optical sensing and imaging, where exploiting quantum effects at the transmitter or the receiver measurements could lead to performance enhancements. Interspersed with my talk, I will also describe what new capabilities in experiments and in theory would enable those various applications.
Tuesday, December 8 | 11:00 a.m. CST
Lilian Childress Coupling diamond defects to high finesse optical microcavities
Defect centers in diamond can offer atomic-like optical transitions and long-lived spin degrees of freedom. Integrating them into high quality optical resonators opens a route toward realizing a cavity quantum electrodynamics system combining atomic-like coherence with a robust solid-state platform. While approaches based on diamond nanophotonics have been pursued for more than a decade, Fabry-Perot microcavities present a complementary approach that has recently received significant attention. This talk will consider the potential benefits and challenges to open micro-cavities, examine progress toward coupling them to diamond defect centers, and discuss our development of a passively and actively stabilized cryogenic system.
Tuesday, December 1 | 11:00 a.m. CST
Felix Leditzky Optimality of the pretty good measurement for port based teleportation
Port-based teleportation (PBT) is a protocol in which Alice teleports an unknown quantum state to Bob using measurements on a shared entangled multipartite state called the port state and forward classical communication. We give an explicit proof that the so-called pretty good measurement, or square-root measurement, is optimal for the PBT protocol with independent copies of maximally entangled states as the port state. We then show that the very same measurement remains optimal even when the port state is optimized to yield the best possible PBT protocol. Hence, there is one particular pretty good measurement achieving the optimal performance in both cases. The following well-known facts are key ingredients in the proofs of these results: (i) the natural symmetries of PBT, leading to a description in terms of representation-theoretic data; (ii) the operational equivalence of PBT with certain state discrimination problems, which allows us to employ duality of the associated semidefinite programs. Along the way, we rederive the representation-theoretic formulas for the performance of PBT protocols proved in [Studzinski et al., 2017] and [Mozrzymas et al., 2018] using only standard techniques from the representation theory of the unitary and symmetric groups. Providing a simplified derivation of these beautiful formulas is one of the main goals of this work. Based on arXiv:2008.11194
Tuesday, November 17 | 11:00 a.m. CST
Peter Shor Quantum Money
Quantum money is a quantum cryptographic protocol that allows for the creation of verifiable but uncopyable states. The requirements are
A) One player (the mint) must be able to produce a quantum money state, along with a serial number.
B) The serial number gives a verification test, and the quantum money state must pass this test with very high probability.
C) If some aspiring counterfeiter has the quantum money state and knows the verification test, they cannot create two quantum states that both pass the verification test.
Quantum money was first proposed in 2009. Since then, several protocols for quantum money have been proposed. We will discuss these protocols and the underlying mechanisms by which they operate.
Tuesday, November 10 | 11:00 a.m. CST
Wolfgang Pfaff Distributing quantum information in superconducting circuits through parametric conversion
Quantum circuits provide a promising path for engineering tailored quantum systems from the ground up. However, the strong light-matter interactions that enable high-fidelity gate operations and measurements in these circuits make it difficult to maintain good isolation of memories and simultaneously realize good connectivity. In this talk I will review ongoing efforts to achieve modular scaling using microwave photons as information carriers. In particular, I will focus on the use of nonlinear mixing in Josephson junctions to realize microwave photonic interconnects that allow connecting individual qubits and cavities on-demand. Recent experiments in this area have enabled the realization of prototypical quantum networks, and ongoing work shows great potential for achieving multi-node networks with high degrees of connectivity.
Tuesday, October 27 | 11:00 a.m. CST
Ashwin Nayak Applications of the information-theoretic method in quantum computation
Quantum phenomena offer the possibility of more efficient computation in a host of information processing scenarios. At the same time, their unusual properties also make it challenging for us to characterize potential gains in efficiency. In this talk, we will review recent results in a few different settings: the streaming model, distributed computation, and learning theory. All of these results are based on the information-theoretic method, which provides an intuitive approach for understanding highly counter-intuitive behavior.
Tuesday, October 13 | 11:00 a.m. CST
Hannes Bernien Building quantum processors and quantum networks atom-by-atom
The realization of large-scale controlled quantum systems is an exciting frontier in modern physical science. Such systems can provide insights into fundamental properties of quantum matter, enable the realization of exotic quantum phases, and ultimately offer a platform for quantum information processing. Recently, reconfigurable arrays of neutral atoms with programmable Rydberg interactions have become promising systems to study such quantum many-body phenomena, due to their isolation from the environment, and high degree of control. I will show how these techniques can be used to study quantum phase transitions in spin models with system sizes up to 51 qubits and to create a 20 qubit GHZ entangled state. Prospects for scaling this approach beyond hundreds of qubits and the implementation of quantum algorithms will be discussed.
An alternative, hybrid approach for engineering interactions is the coupling of atoms to nanophotonic structures in which photons mediate interactions between atoms. Such a system can function as the building block of a large-scale quantum network. In this context, I will present a novel quantum network node architecture that is capable of long-distance entanglement distribution.
Tuesday, October 13 | 11:00 a.m. CST
Liang Jiang Quantum Error Correction for Sensing and Simulation
Quantum error correction is a powerful method for protecting a quantum system from the damaging effects of noise. Besides computation and communication, quantum error correction can also improve the performance of quantum sensing and quantum simulation. We study how measurement precision can be enhanced through quantum error correction by identifying a necessary and sufficient condition for achieving the Heisenberg limit using quantum probes subject to Markovian noise. We develop a new class of bosonic encoding that can preserve the bosonic nature at the logical level while correcting excitation loss error, which will enable error-corrected quantum simulation. The talk will provide a perspective on using quantum error correction for various applications.
Tuesday, September 22 | 11:00 a.m. CST
Jelena Vuckovic Connecting and scaling semiconductor quantum systems
At the core of most quantum technologies, including quantum networks, quantum computers and quantum simulators, is the development of homogeneous, long lived qubits with excellent optical interfaces, and the development of high efficiency and robust optical interconnects for such qubits. To achieve this goal, we have been studying color centers in diamond (SiV, SnV) and silicon carbide (VSi in 4H SiC), in combination with novel fabrication techniques, and relying on the powerful and fast photonics inverse design approach that we have developed. We illustrate this with a number of demonstrated devices, including efficient quantum emitter-photon interfaces for color centers in diamond and in SiC.
Tuesday, September 15 | 11:00 a.m. CST
Alicia Kollár Band engineering for quantum simulation in circuit QED
The field of circuit QED has emerged as a rich platform for both quantum computation and quantum simulation. Lattices of coplanar waveguide (CPW) resonators realize articial photonic materials in the tight-binding limit. Combined with strong qubit-photon interactions, these systems can be used to study dynamical phase tran sitions, many-body phenomena, and spin models in driven-dissipative systems. I will show that these waveguide cavities are uniquely deformable and can produce lattices and networks which cannot readily be obtained in other systems, including periodic lattices in a hyperbolic space of constant negative curvature. Furthermore, Alicia will show that the one-dimensional nature of CPW resonators leads to degenerate flat bands and that criteria for when they are gapped can be derived from graph-theoretic techniques. The resulting gapped at-band lattices are dicult to realize in standard atomic crys tallography, but readily realizable in superconducting circuits.
Tuesday, September 8 | 11:00 a.m. CST
Tracy Northup Linking up trapped-ion quantum computers
Entanglement-based quantum networks hold out the promise of new capabilities for secure communication, distributed quantum computing, and interconnected quantum sensors. However, only a handful of elementary quantum networks have been realized to date. Trapped ions are among the most promising platforms for quantum information science, and I will review the state of the art in linking together trapped-ion quantum computers. Optical cavities provide efficient quantum interfaces between ions and photons, and I will present ongoing work to link two cavity-coupled ion traps in Innsbruck across a 400 m channel, including recent measurements of Hong-Ou-Mandel interference. Finally, we will consider the role that these prototype experiments can play in a systematic blueprint for a quantum internet.
Tuesday, September 1 | 11:00 a.m. CST
Nicole Yunger Halpern Quantum steampunk: Quantum information meets thermodynamics
Thermodynamics has shed light on engines, efficiency, and time’s arrow since the Industrial Revolution. But the steam engines that powered the Industrial Revolution were large and classical. Much of today’s technology and experiments are small-scale, quantum, far from equilibrium, and processing information. Nineteenth-century thermodynamics requires updating for the 21st century. Guidance has come from the mathematical toolkit of quantum information theory. Applying quantum information theory to thermodynamics sheds light on fundamental questions (e.g., how does entanglement spread during quantum thermalization? How can we distinguish quantum heat from quantum work?) and practicalities (e.g., nanoscale engines and the thermodynamic value of quantum coherences). I will overview how quantum information theory is being used to modernize thermodynamics for quantum-information-processing technologies. I call this combination quantum steampunk, after the steampunk genre of literature, art, and cinema that juxtaposes futuristic technologies with 19th-century settings.
- NYH, “Quantum steampunk: Quantum information, thermodynamics, their intersection, and applications thereof across physics,” California Institute of Technology (2018) https://arxiv.org/abs/1807.09786.
- NYH, “Quantum Steampunk,” Scientific American 322, 5 (2020) https://www.scientificamerican.com/article/quantum-steampunk-19th-century-science-meets-technology-of-today/.
Tuesday, August 18 | 11:00 a.m. CST
Simeon Bogdanov Plasmon-enhanced quantum emitters for ultrafast quantum photonics
Nearly all existing applications of quantum photonics are limited by our ability to generate and manipulate single indistinguishable photons deterministically at high repetition rates. Solid-state quantum emitters are excellent sources of single photons for applications in quantum networks. Metal-based nanostructures made from low-loss plasmonic materials allow a targeted and strong enhancement of light-matter interaction in a broad wavelength range. As a result, the far-field single-photon emission rates from solid-state quantum defects could overcome both the rate of dipole dephasing and that of plasmon absorption in metals. Integrated plasmon-enhanced devices can be used as a platform for cryogen-free high-speed integrated quantum photonics. We establish simple and intuitive fundamental enhancement limits for plasmonic systems coupled to quantum emitters and present practical methods for achieving these advantageous regimes. We also discuss methods for the on-chip integration of such single-photon sources and related opportunities for the readout of solid-state spins.
Tuesday, July 28 | 11:00 a.m. CST
Eric Chitambar Quantum resource theories
A quantum resource theory is a broad framework for studying some particular features of quantum mechanics under a restricted class of physical operations. A paradigm example is the resource theory of quantum entanglement, which characterizes the behavior of multi-party entanglement under the restriction of local dynamics and classical communication. When viewed through the lens of a quantum resource theory, seemingly different quantum phenomena often emerge as having many formal similarities.
In this seminar, Eric provides a survey of quantum resource theories and some of its applications in quantum information science. We first motivate the topic by considering some well-known results in thermodynamics and statistical decision problems. We then discuss some of the basic elements and common structural properties found in most resource theories. To see this formalism in action, we will consider in some detail the resource theories of coherence, incompatibility, and nonlocality. Elements of this talk will be taken from [Rev. Mod. Phys 91, 25001 (2019)]; [Phys. Rev. Lett. 124, 120401 (2020)]; [Phys. Rev. Res. 2, 23298 (2020)].
Tuesday, July 21 | 11:00 a.m. CST
Elizabeth Goldschmidt Quantum light-matter interfaces
Elizabeth gives an overview of recent, ongoing, and future work using coherent atomic and atom-like optical emitters to build quantum light-matter interfaces. Optical fields play an important role in virtually all schemes for interconnected quantum information systems since only optical photons are well-suited for carrying quantum information at room temperature. I will discuss different physical platforms that can form the basis for quantum light-matter interfaces, different modalities of light-matter entanglement for various applications in quantum information science, and the tradeoffs related to these different systems. She includes recent experimental results efficiently generating high-fidelity single photons, investigating the role of inhomogeneity in ensemble-based quantum memory, and developing a new integrated photonic platform with highly coherent emitters.
Tuesday, July 7 | 11:00 a.m. CST
Edgar Solomonik Tensor software and algorithms for quantum simulation
Tensor computations are central numerical primitives for computational modeling of quantum systems. These include standard linear algebra operations on multidimensional data (tensors) as well as specialized methods for decomposition of tensors and for optimization of tensor networks. I will discuss challenges and opportunities in this area and present our work on algorithms and software in this domain. Specifically, I will introduce new methods for (1) handling symmetries in tensors, (2) numerical optimization for tensor decomposition, and (3) contraction of 2D tensor networks. On the software side, I will also describe three efforts: (1) Cyclops, a library for distributed-memory tensor computations, (2) AutoHOOT, tensor-algebra-centric library for automatic differentiation, and (3) Koala, a library for simulation of 2D quantum systems. Special focus will be given to the Koala library, which provides efficient parallel algorithms for approximate simulation of 2D quantum circuits (https://arxiv.org/abs/2006.15234).
Tuesday, August 4 | 11:00 a.m. CST
Marius Junge Decay estimates for continuous quantum systems
We study quantum dynamical systems and their return and decoherence times. It turns out that estimating these times theoretically requires deep tools from geometry and mathematical physics. Our main goal is to obtain decay estimates for relative entropy. We will indicate how these return time estimates can be used to simulate the channel behavior of black holes with the help of random gates.
This is joint work with Haojian Li and Nick LaRacuente.
Tuesday, June 23 | 11:00 a.m. CST
Vidya Madhavan Creating and measuring the elusive Majorana fermion
In 1937, Ettore Majorana predicted the existence of a special class of fermions where the particle and the anti-particle are identical. However, with the possible exception of neutrinos, there are no known fundamental particles that belong to this class. The potential realization of Majorana fermions as quasiparticle excitations in solids has rekindled interest in these particles, especially since Majorana states in solids may be useful as fault tolerant qubits for quantum information processing. While most studies have focused on Majorana bound states which can serve as topological qubits, more generally, akin to elementary particles, Majorana fermions can propagate and display linear dispersion. This talk is focused on recent work in realizing Majorana modes in condensed matter systems. I will first describe in detail the conditions under which such states can be realized and what their signatures are. I will then show scanning tunneling microscopy data on 1D domain walls and step edges in two different superconductors, which might potentially be the first realizations of dispersing Majorana states in 1D.
Tuesday, June 16 | 10:00 a.m. CST
Benjamin Villalonga A Look Into the Strong Quantum Advantage Experiment
The quantum supremacy experiment announced last year aimed at providing the first practical demonstration of a quantum computer performing a task that is out of reach for classical computers at that date. Behind this demonstration is the problem of random circuit sampling, i.e., a task that (1) shows a separation in the amount of computing resources needed to be carried out by the quantum computer and its classical counterpart and (2) whose output can be verified. Classical simulations of the quantum computer have a dual role: on the one hand they serve as a competitor for the quantum computer to beat, while on the other hand they are an essential tool to verify that the quantum hardware is operating as expected. In this talk I will first give a broad overview of the experiment and the task of random circuit sampling; I will then focus on my fractional contribution to the simulation side of this large effort. Finally, I will briefly talk about how this demonstration might be a stepping stone towards useful applications in the near term, as well as convey the idea that the quantum supremacy frontier is not a fixed target but rather one that moves with classical hardware and algorithmic improvements.
Tuesday, June 2 | 10:00 a.m. CST
Taylor Hughes Melting a Topological Quantum Computer
After an elementary introduction to some unique properties of topological phases of matter, I will discuss the possibility of realizing bound topological qubits on semiclassical defects in topological phases coexisting with a conventional ordered phase. These defects take the form of vortices, dislocations, or disclinations, and there is a rich interplay between topology and symmetry which will be outlined