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Wednesday, May 17, 2023 12:00 pm - 1:00 pm EDT (GMT -04:00)

IQC Student Seminar featuring Brendan Bramman

13-level Qudit Measurement Demonstrated in Trapped Ions

Abstract: Qudits are an interesting alternative to qubits for a number of algorithmic reasons, but for trapped ions they could be a path for scaling. Ion traps are running into limitations on the number of qubits they can confine in a single trap, and using more of the computational space available in the ions to make qudits is an attractive solution. We have proposed using trapped ion qudits in a previous paper, developing all of the necessary quantum information protocols for their implementation. Here, we present an experimental result of a 13-level qudit measurement with a fidelity of 91.3%. The protocol can be used to measure up to a 25-level qudit in barium. The error scaling is not inherent to the dimension of the qudit, so we can envision going to higher dimensions without a significant increase in error.
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 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.

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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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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Wednesday, June 28, 2023 12:00 pm - 1:00 pm EDT (GMT -04:00)

IQC Student Seminar featuring Devashish Tupkary

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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Tuesday, July 4, 2023 12:00 pm - 1:00 pm EDT (GMT -04:00)

IQC Student Seminar featuring Mohammad Ayyash

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.

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

IQC Student Seminar Featuring Pulkit Sinha

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.

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Thursday, July 20, 2023 11:00 am - 12:00 pm EDT (GMT -04:00)

Virtual IQC PhD Candidate Seminar Featuring Jamal Busnaina

Analog Quantum Simulations using a Parametric Multimode Cavity

While universal quantum computers are still years away from being used for simulating complicated quantum systems, analog quantum simulators have become an increasingly attractive approach to studying classically intractable quantum systems in condensed matter physics, chemistry, and high-energy physics.  

We propose a programmable platform based on a superconducting multimode cavity. The unique design of the cavity allows us to program arbitrarily connected lattices where the coupling strength and phase of each individual coupling are highly programmable via parametrically activated interactions. The effectiveness of the cavity-based AQS platform was demonstrated by the experimental simulation of two interesting models. First, we simulated the effect of a fictitious magnetic field on a 4-site plaquette of a bosonic Creutz ladder. We observed topological features such as emergent edge states and localized soliton states. The platform's ability is further explored by introducing pairing (downconversion) terms to observe features of the Bosonic Kitaev chain (BKC), such as chiral transport and sensitivity to boundary conditions.