Future graduate students

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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The Cooling and Manipulation of Ultra-Cold Atoms

Ultra-cold atoms have been used to simulate phenomena in condensed matter physics as well as in cosmology such as black holes. In this talk, we will give an overview of the field of ultra-cold atoms by discussing the physics behind the cooling and the manipulation of these atoms. Aimed to show the beautiful physics behind this process, this talk will be easy and general, requiring just an undergraduate level of physics understanding. 

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Near-Unity Entanglement from an Indium-Rich Nanowire Quantum Dot Source Compatible with Efficient Quantum Key Distribution

Thus far, the workhorse platform for generating entangled photons for many quantum information experiments has been spontaneous parametric down conversion (SPDC). However, due to their Poissonian photon statistics, these sources cannot operate in the high efficiency limit without a significant reduction in the degree of entanglement. In contrast, there are no such limits placed on semiconductor quantum dots (QDs) embedded in photonic nanostructures. To date, near-unity entanglement fidelity has not yet been measured from indium-rich QDs, which are promising candidates for realizing such a source. We performed quantum state tomography using single-photon detectors with ultra-low timing jitter and employed two-photon resonant excitation. We measured a raw peak concurrence and fidelity of 95.3 +/- 0.5% and 97.5 +/- 0.8%, respectively, as well as lifetime-weighted average concurrence and fidelity of 0.90 +/- 0.04% and 0.94 +/- 0.04%, respectively. These results conclusively demonstrate that most of the degradation from unity-measured entanglement fidelity in earlier studies was not due to spin dephasing. Additionally, we show that the exciton fine structure splitting, contrary to common understanding, is not in principle a fundamental barrier to implementing QKD with semiconductor QD entangled photon sources.

Exploring Wigner Negativity of Pure Spin States on a Spherical Phase Space

The past two decades have largely vindicated the long-held belief that Wigner negativity is an indicator of genuine nonclassicality in quantum systems.  Here we will discuss how Wigner negativity manifests in pure spin-j systems using the spherical Wigner function.  Common symmetric multi-qubit states are studied and compared, including Bell, W and GHZ states.  Spin coherent states are shown to never have vanishing Wigner negativity, in contrast to other phase spaces.  Pure states that maximize negativity are determined and analyzed using the Majorana stellar representation.  Time permitting, these results will be contrasted with similar works on symmetric state entanglement and other forms of phase-space nonclassicality.

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Graphical CSS Code Transformation Using ZX Calculus