Events

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.

Positivity and Sum-of-Squares in Quantum Information

A multivariate polynomial is said to be positive if it takes only non-negative values over reals. Hilbert's 17th problem concerns whether every positive polynomial can be expressed as a sum of squares of other polynomials. In general, we say a noncommutative polynomial is positive (resp. matrix positive) if plugging operators (resp. matrices) always yields a positive operator. Many problems in math and computer science are closely connected to deciding whether a given polynomial is positive and finding certificates (e.g., sum-of-squares) of positivity.

In the study of nonlocal games in quantum information, we are interested in tensor product of free algebras. Such an algebra models a physical system with two spatially separated subsystems, where in each subsystem we can make different quantum measurements. The recent and remarkable MIP*=RE result shows that it is undecidable to determine whether a polynomial in a tensor product of free algebras is matrix positive. In this talk, I'll present joint work with Arthur Mehta and William Slofstra, in which we show that it is undecidable to determine positivity in tensor product of free algebras. As a consequence, there is no sum-of-square certificate for positivity in such algebras.

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Ablation Loading and Qudit Measurements with Barium Ions

Barium is one of the best ions for performing quantum information in a trapped-ion system. Its long-lived metastable D5/2 state allows for some interesting quantum operations, including the current best state preparation and measurement fidelity in qubits. This metastable state also opens up the possibility of implementing higher dimensional qudits instead of qubits. However, installing a barium metal source in a vacuum chamber has shown to be somewhat of a challenge. Here, we present a loading technique which uses a barium chloride source instead, making it much easier to install. Laser ablation with a high-energy pulsed laser is used to generate neutral atoms, and a two-step photoionization technique is used to selectively load different isotopes of barium in our ion trap. The process of laser ablation and the plume of atoms it generates are characterized, informing us on how to best load ions. Loading is achieved, and selectivity of our method is demonstrated, giving us a reliable way to load ba138 and ba137 ions. The quadrupole transition into the metastable D5/2 state is investigated, with all of the individual transitions successfully found and characterized for ba138 and ba137. Coherent operations are performed on these transitions, allowing us to use them to define a 13-level qudit, on which we perform a state preparation and measurement experiment. The main error source in operations using this transition is identified to be magnetic field noise, and so we present attempts at mitigating this noise. An ac-line noise compensation method is used, which marginally improved the coherence time of the quadrupole transitions, and an additional method of using permanent magnets is proposed for future work. These efforts will help to make trapping barium more reliable, making it an even more attractive option for trapped ion systems. The state preparation and measurement results using the quadrupole transition to the long-lived metastable D52 state establish barium as an interesting platform for performing high-dimensional qudit quantum computing.

IQC Seminar - Sahel Ashhab, National Institute of Information and Communications, Japan

Superconducting qubits are based on nonlinear electric circuits that support multiple quantum states. Although only two states are used to realize a qubit, it is possible to utilize more states and realize qudits (d-level quantum systems). We have proposed and experimentally demonstrated optimized implementations of qutrit gates. We have also investigated the time-optimal implementation of two-qubit gates in weakly anharmonic superconducting circuits.