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Tuesday, May 23, 2023 12:00 pm - 1:00 pm EDT (GMT -04:00)

IQC Student Seminar featuring Sarah Meng Li

Graphical CSS Code Transformation Using ZX Calculus

Abstract: In this work, we present a generic approach to transform CSS codes by building upon their equivalence to phase-free ZX diagrams. Using the ZX calculus, we demonstrate diagrammatic transformations between encoding maps associated with different codes. As a motivating example, we give explicit transformations between the Steane code and the quantum Reed-Muller code, since by switching between these two codes, one can obtain a fault-tolerant universal gate set. To this end, we propose a bidirectional rewrite rule to find a (not necessarily transversal) physical implementation for any logical ZX diagram in any CSS code.

Then we focus on two code transformation techniques: code morphing, a procedure that transforms a code while retaining its fault-tolerant gates, and gauge fixing, where complimentary codes (such as the Steane and quantum Reed-Muller codes) can be obtained from a common subsystem code. We provide explicit graphical derivations for these techniques and show how ZX and graphical encoder maps relate several equivalent perspectives on these code transforming operations.

Wednesday, May 24, 2023 1:30 pm - 2:30 pm EDT (GMT -04:00)

A tale of communication, entanglement and graphs

Math/CS Seminar - Featuring Olivier Lalonde Université de Montréal

Quantum communication complexity, which concerns itself with determining how much communication is required by two participants having access to quantum resources to compute a boolean function of their inputs, has long been a lively subfield of quantum information science. The topic of this talk will be the power of shared prior entanglement relative to quantum communication without prior entanglement, which, despite having been studied for more twenty years, remains rather mysterious. After a quick review of the bare bones of classical communication complexity, I will proceed to discuss the model of entanglement-assisted communication complexity. 

Wednesday, May 31, 2023 12:00 pm - 1:00 pm EDT (GMT -04:00)

IQC Student Seminar featuring Sainath Motlakunta

Preserving a Qubit During Adjacent Measurements at a Few Micrometers Distance  

Abstract:

Protecting a quantum object against irreversible accidental measurements from its surroundings is necessary for controlled quantum operations. This becomes especially challenging or unfeasible if one must simultaneously measure or reset a nearby object's quantum state, such as in quantum error correction. 

In atomic systems - among the most established quantum information processing platforms - current attempts to preserve qubits against resonant laser-driven adjacent measurements, waste valuable experimental resources such as coherence time or extra qubits and introduce additional errors. We preserve the quantum state of an 'asset' ion qubit with high fidelity, while a neighbouring qubit at a few microns distance is reset/measured. We achieve < 1 x 10-3 probability of accidental measurement of the asset qubit during a neighbouring qubit reset and < 4 x 10-3 while applying a detection beam on the same neighbour, for 11 μs, at a distance of 6 μm or 4 times the addressing Gaussian beam waist (permitted by the numerical aperture).

These low probabilities correspond to the preservation of the quantum state of the qubit with fidelities above 99.90% (state-reset) and 99.6% (state-measurement). Our results are enabled by precise wavefront control of the addressing optical beams, while utilizing a single ion as a quantum sensor of optical aberrations.

Our work demonstrates the feasibility of in-situ state-reset and measurement operations, building towards enhancements in the speed and capabilities of quantum processors such as in simulating measurement-driven quantum phases and realizing quantum error correction.

Thursday, June 1, 2023 3:30 pm - 4:30 pm EDT (GMT -04:00)

An Optimal Oracle Separation of Classical and Quantum Hybrid Schemes

Math/CS Seminar - Atsuya Hasegawa (University of Tokyo)

Recently, Chia, Chung and Lai (JACM 2023) and Coudron and Menda (STOC 2020) have shown that there exists an oracle $\mathcal{O}$ such that $\mathsf{BQP}^\mathcal{O} \neq (\mathsf{BPP^{BQNC}})^\mathcal{O} \cup (\mathsf{BQNC^{BPP}})^\mathcal{O}$. In fact, Chia et al. proved a stronger statement: for any depth parameter $d$, there exists an oracle that separates quantum depth $d$ and $2d+1$, when polynomial-time classical computation is allowed.

Tuesday, June 6, 2023 12:00 pm - 1:00 pm EDT (GMT -04:00)

IQC Student Seminar featuring Jack Davis

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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Wednesday, June 14, 2023 12:00 pm - 1:00 pm EDT (GMT -04:00)

IQC Student Seminar featuring Matteo Pennacchietti

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.

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Thursday, June 15, 2023 3:30 pm - 4:30 pm EDT (GMT -04:00)

Impure Quantum Codes: Applications and Bounds

CS/Math Seminar - Andrew Nemec, Duke University

Impure quantum error-correcting codes display interesting properties not found among classical codes, such as having multiple low-weight errors map to the same syndrome. In this talk, we will look at how these codes can be used to design good variants of quantum codes, such as quantum-classical hybrid codes and quantum data-syndrome codes.

Tuesday, June 20, 2023 12:00 pm - 1:00 pm EDT (GMT -04:00)

IQC Student Seminar featuring Omar Hussein

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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Monday, June 26, 2023 10:00 am - 11:00 am EDT (GMT -04:00)

Jack Davis PhD Thesis Defence

Wigner negativity on the sphere

The rise of quantum information theory has largely vindicated the long-held belief that Wigner negativity is an indicator of genuine nonclassicality in quantum systems.  This thesis explores its manifestation in spin-j systems using the spherical Wigner function.  Common symmetric multi-qubit states are studied and compared.  Spin coherent states are shown to never have vanishing Wigner negativity.  Pure states that maximize negativity are determined and analyzed using the Majorana stellar representation.  The relationship between negativity and state mixedness is discussed, and polytopes characterizing unitary orbits of lower-bounded Wigner functions are studied.  Results throughout are contrasted with similar works on symmetric state entanglement and other forms of phase-space nonclassicality.