Events

Lindbladians Obeying Local Conservation Laws and Showing Thermalization

This talk will investigate the possibility of a Markovian quantum master equation (QME) that consistently describes a finite-dimensional system, a part of which is weakly coupled to a thermal bath. For physical consistency, we will demand that the QME should preserve local conservation laws and be able to show thermalization. After providing some background on QMEs, I will present our three main results: 

1. The microscopically derived Redfield equation (RE), which is known to preserve local conservation laws and show thermalization, necessarily violates complete positivity except in extremely special cases. These special cases can be easily identified.
2. I will then turn to Lindblad QMEs and show that imposing complete positivity and demanding preservation of local conservation laws enforces the Lindblad operators and the lamb-shift Hamiltonian to be `local', i.e. to be supported only on the part of the system directly coupled to the bath.
3. Finally, I will show how the problem of finding 'local' Lindblad QME, which can show thermalization, can be turned into a semidefinite program (SDP). This SDP can be solved numerically for any specific example, and its solution conclusively shows whether the desired type of QME is possible up to a given precision. Whenever a QME is possible, it also outputs a form for such a QME.

Taken together, our results indicate that the possibility of a Markovian QME with the desired properties must be taken on a case-by-case basis, since there are setups where such a QME is impossible.

This talk is based on https://arxiv.org/abs/2301.02146.

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Driving-enhanced Qudit-Oscillator Interactions

Classical drives on a qudit have been extensively used to create, control and read out quantum states. We consider a qudit-oscillator system where the qudit is continuously driven. We show that strong driving allows for qudit-conditional operations on the oscillator such as displacement, squeezing and higher order effects. We discuss the case of a driven qubit with linear or quadratic coupling to the oscillator, and we generalize the scheme to multi-qubit and qudit (d>2) systems. We discuss the use of driven qudit-oscillator systems for encoding and performing operations on bosonic codes.

Optimal Bounds for Quantum Learning via Information Theory

I will discuss our recent work on finding lower bounds to solve three problems in Quantum Learning Theory: Quantum PAC learning, Quantum Agnostic Learning and Quantum Coupon Collector. Our main goal was to use tools from Quantum Information Theory, specifically the data processing inequality, to obtain these results, instead of going for more exotic ones. We succeed in doing so for the first two problems, and we show concretely that it doesn't work for the last problem, due to an inherent loss of information that is possible even for valid learning algorithms, for which we give a bound using an alternate method that utilizes the analysis we went through previously. We hope that these tools are broadly applicable to other quantum learning problems.

IQC Colloquium - Nir Bar-Gill, Applied Physics and Physics, The Hebrew University

In this talk I will address these topics through the platform of nitrogen-vacancy (NV) spins in diamond, in the context of purification (or cooling) of a spin bath as a quantum resource and for enhanced metrology and sensing.

IQC Seminar - Yujie Zhang, University of Illinois at Urbana-Champaign

A global quantum network stands at the frontier of the ongoing technological revolution and has led to both theoretical discussion on designing new network quantum protocols, and experimental works in their implementation.  In this talk, we will explore applications of the quantum network with novel theoretical proposals and their table-top experimental demonstrations including quantum enhanced multiple-access communications and astronomical sensing with quantum telescopy.

Some Learning Bounds and Guarantees for Testing (Quantum) Hypotheses

Machine learning is a powerful tool, yet we often do not know how well a learning algorithm might perform on any given task. One standard approach to bound the accuracy of a learning algorithm is to reduce the learning task to hypothesis testing. Fano's inequality then states that a large amount of mutual information between the learner's observations and the set of unknown parameters is a necessary condition for success.

In this talk, I will describe how such a condition is also sufficient for succeeding at some learning task, thereby providing a purely information-theoretic guarantee for learning. Noting that this guarantee has an immediate extension to quantum information theory, I will then introduce the task of "testing quantum hypotheses", in which the unknown parameters of the learning task are prepared in a quantum register in superposition (rather than being sampled stochastically) and the learner's success at this task is measured by their ability to establish quantum correlations with that register. I will discuss ongoing attempts to characterize this scenario.

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CS/Math Seminar - Rajesh Pereira, the University of Guelph

We explore two convex subsets of the sets of quantum channels. The set of mixed unitary channels and the set of entanglement breaking channels. We study the geometry of these sets, and show how certain geometric and spectral properties of these sets can be studied using positive definite functions, stochastic matrices and other mathematical tools.

Control and Readout of High-Dimensional Trapped Ion Qudits

The trapped ion platform is one of the quantum computing platforms that is at the forefront for realizing large-scale quantum information processing, which is crucial for practically actualizing the advantages of quantum algorithms. Scaling up the trapped ion quantum computing architecture remains a challenge. We explore an alternative avenue in a trapped ion system for increasing the computational Hilbert space other than trapping more ions, which is by increasing the qudit dimension of an ion. Our ion of choice is 137Ba+, which has a rich energy level structure for high-dimensional qudit encoding. Utilizing the additional energy states found in 137Ba+ also comes with non-trivial complexities that require careful considerations, which we have solved and report in this work. We report on a single-shot state measurement protocol which allows qudit encoding in 137Ba+ of up to 25 levels, and demonstrate state preparation and measurement of up to 13 levels, which is unprecedented in a trapped ion system. This PhD defense presentation also covers some other interesting topics within the thesis, which include our experimental setup, barium ion loading via laser ablation, and detailed studies of some experimental observations that may not be intuitively clear.