Events

IQC Special Seminar Featuring Gerardo Adesso, The University of Nottingham

What makes quantum technologies tick? Advantages in communication, computation, metrology and other applications can be traced back to distinct manifestations of quantum theory, such as coherence and entaglement.

IQC Colloquium - Dvira Segal, University of Toronto

Rethinking the operation principles of thermal machines in the nanoscale and quantum domain, we focus on a continuous machine operating in steady state, the quantum absorption refrigerator (QAR), and examine three key questions: (i) How does the strong system-bath interaction affect the device's operation? (ii) What can we learn about the machine from current noise? (iii) What is the impact of coherences within the working fluid on the performance of the quantum machine? 

13-level Qudit Measurement Demonstrated in Trapped Ions

CS/Math seminar - Igor Klep, University of Ljubljana

The talk will discuss state polynomials, i.e., polynomials in noncommuting variables and formal states of their products. The motivation behind this theory arises from the study of correlations in quantum networks. We will give a state analog of Artin's solution to Hilbert's 17th problem showing that state polynomials, positive over all matrices and matricial states, are sums of squares with denominators.

Graphical CSS Code Transformation Using ZX Calculus

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. 

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.

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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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.