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, April 20 | 11:00 a.m. CST
Christine Silberhorn Non-linear integrated quantum optics with pulsed light
Quantum technologies promise a change of paradigm for many fields of application, for
example in communication systems, in high-performance computing and simulation of
quantum systems, as well as in sensor technology. They can shift the boundaries of today’s
systems and devices beyond classical limits and seemingly fundamental limitations. Photonic
systems, which comprise multiple optical modes as well as many nonclassical light quantum
states of light, have been investigated intensively in various theoretical proposals over the last
decades. However, their implementation requires advanced setups of high complexity, which
poses a considerable challenge on the experimental side. The successful realization of
controlled quantum network structures is key for many applications in quantum optics and
quantum information science.
Here we present three differing approaches to overcome current limitations for the experimental
implementation of multi-dimensional quantum networks: non-linear integrated quantum optics,
pulsed temporal modes and time-multiplexing. Non-linear integrated quantum devices with
multiple channels enable the combinations of different functionalities, such as sources and fast
electro-optic modulations, on a single compact monolithic structure. Pulsed photon temporal
modes are defined as field orthogonal superposition states, which span a high dimensional
system. They occupy only a single spatial mode and thus they can be efficiently used in singlemode
fibre communication networks. Finally, time-multiplexed quantum walks are a versatile
tool for the implementation of a highly flexible simulation platform with dynamic control of
the underlying graph structures and propagation properties.
Tuesday, April 6 | 11:00 a.m. CST
Ewin Tang On quantum linear algebra for machine learning
We will discuss quantum singular value transformation (QSVT), a simple unifying framework for quantum linear algebra algorithms developed by Gilyén et al. QSVT is often applied to try to achieve quantum speedups for machine learning problems. We will see the typical structure of such an application, the barriers to achieving super-polynomial quantum speedup, and the state of the literature that's attempting to bypass these barriers. Along the way, we'll also see an interesting connection between quantum linear algebra and classical sampling and sketching algorithms (explored in the form of "quantum-inspired" classical algorithms).
Tuesday, March 30 | 11:00 a.m. CST
Johannes Bausch Recurrent Quantum Neural Networks
Recurrent neural networks are the foundation of many sequence-to-sequence models in machine learning, such as machine translation and speech synthesis. In this work we construct a quantum recurrent neural network (QRNN) with demonstrable performance on non-trivial tasks such as sequence learning and integer digit classification. The QRNN cell is built from parametrized quantum neurons, which, in conjunction with amplitude amplification, create a nonlinear activation of polynomials of its inputs and cell state, and allow the extraction of a probability distribution over predicted classes at each step. To study the model's performance, we provide an implementation in pytorch, which allows the relatively efficient optimization of parametrized quantum circuits with thousands of parameters. We establish a QRNN training setup by benchmarking optimization hyperparameters, and analyse suitable network topologies for simple memorisation and sequence prediction tasks from Elman's seminal paper (1990) on temporal structure learning. We then proceed to evaluate the QRNN on MNIST classification, both by feeding the QRNN each image pixel-by-pixel; and by utilizing modern data augmentation as preprocessing step. Finally, we analyse to what extent the unitary nature of the network counteracts the vanishing gradient problem that plagues many existing quantum classifiers and classical RNNs.
Tuesday, March 23 | 11:00 a.m. CST
Konrad Lehnert A new science of quantum sound
The mechanical vibrations of man-made and macroscopic objects have recently come under quantum control. It is now possible to resolve the individual quantum particles of sound and to measure, prepare, and manipulate the quantum state of vibrating membranes and acoustical resonators. I will describe how these fragile quantum states were tamed and how this new capability might be applied. Unlike electrical and optical systems, which are governed by fundamental equations of electromagnetism, acoustical phenomena are described by the equations of elastic waves in solid bodies. They are subject to different limitations and can reach different regimes of behavior. The speed of sound in a solid material is 100,000-fold slower than light, elastic waves do not propagate through vacuum, and they couple to atom-like defects through strain rather than electrical or magnetic dipole interactions. These facts have consequences for quantum information science that we have yet to fully understand.
Tuesday, March 9 | 11:00 a.m. CST
Tzu-Chieh Wei Measurement and quantum computation
Quantum computation has the potential to perform certain computational tasks more efficiently than classical computation. In the past decade, there has been impressive progress in building quantum devices with many qubits. I will discuss the role of measurement in several frameworks of quantum computation. First, it is needed to readout the outcome in the computation. In some implementations, such as in the superconducting qubits, readout error can be as large as the two-qubit gate error. I will show that the so-called detector tomography can be used to correct the readout distribution and illustrate this with experiments on cloud quantum computers. Next, I will describe a simple iterative scheme with quantum circuits to measure a system in its energy eigenstate basis. This can be used to implement a Zeno approach that achieves adiabatic quantum computation. In the last part, I will discuss how single-qubit measurements by themselves can drive universal quantum computation, exploiting entanglement as a resource. If these entangled resource states emerge as unique ground states of nearest-neighbor interacting Hamiltonians, then they may be created by cooling the engineered Hamiltonians. A nonzero gap that separates the ground state from all excited states is thus a desired feature. I will show that some of the Affleck-Kennedy-Lieb-Tasaki (AKLT) models indeed possess these properties that they are gapped and their ground states can be used for universal quantum computation by measuring spins locally.
Tuesday, March 2 | 11:00 a.m. CST
Mikael Backlund Quantum tools for molecular microscopy
We are a new physical chemistry lab at UIUC broadly interested in developing quantum sensing methods for applications in the molecular sciences. In this talk I will primarily discuss two studies from my previous work that set the stage for my independent lab’s ongoing and future research. First, I will discuss how framing single-molecule fluorescence microscopy as an exercise in quantum parameter estimation can yield surprising insights. Then I will describe how we leverage nitrogen-vacancy color centers in diamond to perform micro- and nanoscopic magnetometry.
Tuesday, February 23 | 11:00 a.m. CST
Mohammad Hafezi Topological physics: from photons to electrons
There are many intriguing physical phenomena that are associated with topological features --- global properties that are not discernible locally. The best-known examples are quantum Hall effects in electronic systems, where insensitivity to local properties manifests itself as conductance through edge states which are insensitive to defects and disorder. In the talk, we first discuss how similar physics can be explored with photons; specifically, how various topological models can be simulated in various photonics systems, from ring resonators to photonic crystals. We then discuss that the integration of strong optical nonlinearity can lead to unique bosonic phenomena, such as a topological source of quantum light and chiral quantum optics. These results may enable the development of classical and quantum optical devices with built-in protection for next-generation optoelectronic and quantum technologies. In the end, we discuss an emerging field at the interface of quantum optics and correlated electron systems, with the goal of creating and manipulating many-body states of light-matter hybrids with new functionalities, such as high-Tc superconductors.
Tuesday, February 16 | 9:30 a.m. CST
Alexander Ling 600 days of the SpooQy-1 mission
The SpooQy-1 satellite was envisioned as an in-orbit experiment for critical components that will contribute to an entanglement distribution demonstration using smaller satellites. It carries an entangled photon-pair source, and single photon detectors, within a shoe-box sized satellite. In this seminar, we will discuss the mission concept and milestones as well as the latest set of observations that have been collected from the satellite. Challenges and projects for future missions will be shared as well.
Tuesday, February 9 | 9:30 a.m. CST
Lijian Zhang Precision of weak measurement
Weak measurement reveals partial information about a quantum system without collapsing it. The measurement outcome, weak value, may lie outside the spectrum of the measurement operator and can even be complex. This phenomenon has been used to amplify small effects in precision metrology, as well as to reconstruct the wavefunctions of quantum systems directly. Yet the amplification effect of weak measurement comes at the cost of a reduction in the rate at which data can be acquired, due to the requirement to select almost orthogonal pre- and post-selected states. Therefore, whether weak measurement can really enhance the measurement precision or even beat the classical limit has been under debate for a long time. Here I will talk about a proper analysis of the precision of weak measurement using the tools of quantum metrology and give examples for the practical advantages of weak measurement in precision metrology. I will also talk about the applications of weak measurement to directly characterize a quantum detector and its precision.
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.