Canada

Physics is widely recognised as a subject which often does not recognise the diversity of the societies in which it is practised. There are differences between countries, but there are many parts of the world where female participation at all levels in Physics is 20% or below. In the UK specifically, there is also low participation by some groups either ethnic groups or lower socioeconomic groups. It is clear that for Physics to be a success and to build teams to solve the key challenges in Physics, we need the participants to fully represent the diversity of the society in which they are based. 

I will present some of the activities of the Department of Physics at the University of York in the UK, in attempting to open up Physics to all. In particular, I will focus on actions intended to address gender imbalance and the recognised higher drop-out rate of women at all levels in Physics. At York, we hold Athena Swan Silver and Project Juno Champion status which are two schemes in the UK to recognise Physics departments in their equality activities. I will outline key aspects of our action plan to ensure a level playing field. These have been welcomed by all staff as they recognise issues such as work/life balance and part-time working which are of relevance to all. I will review aspects of recruiting and how avoiding unconscious bias is being put at the centre of our approach. I will review the data which shows the significance of this work and the impacts that it can achieve. 

While there are many avenues to take in tackling challenges of inclusion, outreach can play a very strong role. I will discuss some aspects of our outreach programme. We engage with many different groups, at different levels and different ages. This starts with hands-on astronomy for schoolchildren at primary level and for local scouting groups such as cubs and brownies. Our outreach programme covers more advanced material for those in the final years of high school. For example, an online nuclear physics masterclass which has proven very popular and with audiences significantly more diverse than our own student cohort. I will also provide examples like the Physics outreach stand taken to the York Pride event. Finally, I will discuss a knowledge exchange programme with two historically disadvantaged universities in South Africa, supported by the UK Global Challenges Research Fund, which has seen many students from SA visit York for hands-on training and allowed the setting up of two detector development laboratories in South Africa.

CAP Virtual Congress 2021

Equality and diversity in Physics: A UK perspective (flipped)

equality

physics

diversity

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The National Research Council Canada (NRC) was contracted by Infrastructure Canada and the City of Toronto to improve the understanding of the performance of various catch basin covers under various conditions. A full scale model roadway was built 10.7 m long and 2.6 m wide in the NRC's Coastal Wave Basin and the water depth in front of the catch basin varied from 0.5 - 15 cm, the road grade was varied from 0.5 - 10.0% and the cross slope was varied from 2.0 - 4.0%. Early in the test protocol it was noted that the capacitance wire wave gauges used to measure the water depth on the roadway provided inconsistent results. In this work we will compare water depth measurements from a manual point gauge, an acoustic sensor and the capacitance wave probes. We will examine in which way each of the sensors is biased and examine the impact of those results.

Measuring water depth on an inclined road

Quantum dots embedded in photonic nanowires are highly efficient single photon generators. Integrating such sources on-chip offers enhanced stability and miniaturization; both of which are important in many applications involving quantum information processing. We demonstrate the efficient coupling of quantum light generated in a III-V photonic nanowire to a silicon-based photonic integrated circuit. This hybrid quantum photonic integrated circuit is assembled through a “pick & place” approach using a nanomanipulator in a scanning electron microscope where the nanowires are transferred individually from the growth substrate and carefully placed onto the photonic integrated circuit. The emission properties of on-chip nanowire QDs were measured using an all-fibre pump and collection technique. We demonstrate detected count rates of a million counts per second with single photon purities higher than 95 percent thus showing that using nanowires with embedded QDs coupled to on-chip photonic structures is a viable route for the fabrication of stable single photon sources.

Hybrid Integration of III-V Nanowires Embedded with Quantum Dots on Photonic Integrated Circuits

Among solid state quantum emitter systems, semiconductor quantum dots are particularly attractive due to their high radiative quantum efficiencies [1], their strong optical coupling enabling fast [2] and arbitrary [3] qubit rotations, and their tunable emission in the range of standard telecommunication wavelengths. For applications such as quantum light sources and quantum nodes, it is essential to maximize the fidelity of the optical control process governing quantum state initialization and control. While the dephasing time tied to radiative relaxation is many orders of magnitude longer than control times achievable with subpicosecond laser pulses, resonant coupling of the electron-hole pair to phonons in the solid-state environment can still contribute to decoherence during the optical control process [2]. This decoherence channel is often referred to as excitation-induced dephasing since the impact on fidelity is dictated in part by the characteristics of the driving laser field. Here we report the demonstration of suppression of phonon-mediated decoherence through the application of frequency-swept laser pulses via adiabatic rapid passage in the strong-driving regime [4]. We also investigate the dependence of the threshold for decoherence suppression on the size and shape of the quantum dot. Our findings indicate that the use of telecom-compatible quantum dots leads to decoherence suppression at pulse areas comparable to a single Rabi oscillation period.

[1] Atature et al. Nat. Rev. Mater. 3, 38 (2018).

[2] Mathew et al. Phys. Rev. B 90, 035316 (2014).

[3] Mathew et al. Phys. Rev. B 84, 205322 (2011); Gamouras et al. J. Appl. Phys. 112, 014313 (2012); Gamouras et al. Nano Letters 13, 4666 (2013); Mathew et al. Phys. Rev. B 92, 155306 (2015).

[4] Ramachandran et al. Opt. Lett. 45, 6498 (2020).

Suppression of Phonon-Mediated Decoherence using Frequency-Swept Laser Pulses in Optically-Driven Semiconductor Quantum Emitters

In addition to being extremely sensitive sensors, nitrogen vacancies (NV) centers in diamond are an ideal showcase of quantum technologies as they work in ambient conditions. Experiments with NV centers usually involve a bulky optical system, together with a wide assortment of signal generators and samplers, which is challenging to synchronize together. Here, we perform quantum control experiments on NV centers which are much more accessible to a broader community. We achieve this by (i) miniaturizing hardware components into a magnetometer the size of Rubik’s cube and (ii) leveraging a commercial platform for control and readout. We interfaced all the quantum magnetometers signal generation and readout components with a modular control platform, thus allowing it to fully operate the sensor. We will present room-temperature results including optically detected magnetic resonance, Rabi and Ramsey oscillations of an ensemble of NV centers. In addition to democratizing complex experiments in quantum physics, our work paves the way for efficient prototyping of quantum sensors with commercial control solutions.

Les centres azote-lacune (NV) dans le diamant sont une vitrine idéale des technologies quantiques car ils fonctionnent dans des conditions ambiantes. Les expériences avec les centres NV impliquent généralement un système optique volumineux, ainsi qu'un large assortiment de générateurs de signaux et d'échantillonneurs, qu'il est difficile de synchroniser ensemble. Ici, nous réalisons des expériences de contrôle quantique sur des centres NV qui sont beaucoup plus accessibles à une plus large communauté. Nous y parvenons (i) en miniaturisant l’électronique dans un magnétomètre de la taille d'un cube Rubik et (ii) en exploitant une plateforme commerciale pour le contrôle et la lecture. Nous avons interfacé tous les composants de génération de signaux et de lecture du magnétomètre quantique avec une plateforme de contrôle modulaire, lui permettant ainsi d’opérer le capteur quantique. Nous présenterons les résultats obtenus à température ambiante, notamment la résonance magnétique détectée optiquement et les oscillations de Rabi et de Ramsey d'un ensemble de centres NV. En plus de démocratiser les expériences complexes en physique quantique, notre travail ouvre la voie à un prototypage efficace des senseurs quantiques avec des solutions de contrôle commerciales.

A portable diamond-based quantum demonstrator based on a quantum control and readout platform - Démonstrateur quantique portable à base de diamant, basé sur une plateforme de contrôle et de lecture quantique

Quantum confinement and manipulation of charge carriers are critical for achieving devices practical for various quantum technologies such as quantum sensing. Atomically thin transition metal dichalcogenides (TMDCs) have attractive properties such as spin-valley locking, large spin-orbit coupling and high confinement energies which provide a promising platform for novel quantum technologies. In this talk, we present the design and fabrication of electrostatically gated quantum structures based on fully encapsulated monolayer tungsten diselenide (WSe2) aimed at probing and measuring the properties of the confined single and few-hole states in these structures. Furthermore, we successfully demonstrate that local control gates successfully pinch-off the current across the device with gate voltages consistent with their lithographic widths. Finally, we discuss the origins of the observed mesoscopic transport features related to the quantum dots through the WSe2 channel.

Gated quantum dots in van der Waals materials

In microscopy, the imaging of light-sensitive materials has been a persistent problem, as the sample being studied may be altered or damaged by the illumination itself. Naturally, to overcome over-illuminating the sample, one can reduce the intensity of the classical light source; however, reducing the source intensity comes with a trade-off which affects noise and image quality. In recent years, it has been shown that using quantum illumination as a source for imaging schemes significantly reduces photon illumination of the sample while maintaining image quality. In fact, in our previous work [1] we combine two quantum imaging and detection schemes in a technique coined “Interaction-free-ghost-imaging” and achieve results with low photon number and high contrast images. 
A further limitation of all direct imaging schemes, whether they be quantum or classical, is the so-called diffraction limit. When measuring intensity directly, the maximal resolution is dictated by the size of optical apertures in the imaging system. Recent results [2] [3] have shown that performing phase-sensitive measurements, as opposed to intensity measurements, could increase resolution by several orders of magnitude. 
We propose an experiment implementing the super-resolution technique in a quantum ghost-imaging scheme. We aim to show that the added benefits of low photon counts along with increased resolution show promise for imaging small light-sensitive objects such as biological cells, and further challenges the notion of how many photons are needed to form a visible image.

[1] Zhang,Y., Sit, A., Bouchard, F., Larocque, H., Grenapin, F., Cohen, E., Elitzur, A., Harden, J. Boyd,R., Karimi, E, “Interaction-free-ghost-imaging of structured objects”, Optics Express 27, 2212-2224 (2019). 
[2] Tsang, M., Nair, R., Lu X, “Quantum Theory of Superresolution for two Incoherent Optical Point Sources”, Physical Review X 6, 031033 (2016). 
[3] Tham, W., Ferretti, H., Steinberg, A, “Beating Rayeigh’s curse by Imaging Using Phase”, Physical Review Letters 118, 070801 (2017).

Super-resolution Ghost-Imaging

Optically active defects in solids---colour centres---are one of the most promising platforms for implementing quantum technologies. Their spin degrees of freedom serve as quantum memories that in some cases can operate at room temperature. Their control can be achieved with microwave spin control and resonant optical excitation but is hindered by the broadening of optical transitions from thermal phonons and spectral diffusion. Furthermore, spin-qubit optical transitions are often outside the telecommunications wavelength band required for long-distance fiber optic transmission. Harnessing the coupling between mechanical degrees of freedom and spins has emerged as an alternative route for controlling spin-qubits. However, connecting spin-mechanical interfaces to optical links to realize a spin-photon interface has remained a challenge. Here we demonstrate such an interface using a diamond optomechanical cavity that does not depend on optical transitions and can be applied to a wide range of spin qubits.
Our device consists of a diamond microdisk resonator studied in [Optica 3, 963-970 (2016)]. The microdisk is fabricated from optical grade diamond that contains ensembles of NV centres. We use an optical mode at 1564 nm with the quality factor Qo = 150k. The mechanical mode which we use to couple to the NV spin state is a radial breathing mode with a frequency of around 2.1 GHz with the quality factor Qm = 4k. The device operates in the sideband resolved regime enabling optomechanical self-induced oscillations for sufficiently high optical input power of a blue detuned laser. These oscillations can produce the stress of a few MPa, large enough to drive the electronic spins of NV centers. We use a standard diamond NV confocal microscope to initialize and readout the NV state. The MW pulses transfer the population between |-1⟩<->|0⟩ and |+1⟩<->|0⟩ state. We wait for 0.7 us with the mechanical drive that drives the |-1⟩<->|+1⟩ transition. 
In our measurements, we observe a coinciding dip in the |+1⟩ population and a peak |-1⟩ population, that verifies that the spins are being optomechanically driven. On calibrating this signal with the MW Rabi contrast and the background signal, we estimate a driving rate of 2pix170kHz and ~45% transfer of spin population between |±1⟩ states. Feasible improvements in device geometry will increase the optomechanically-induced driving rate by a few orders of magnitude allowing for coherent control of NV spins using an optomechanical resonator.

Optomechanical interface between telecom photons and spin quantum memory

Single acceptor dopants in Si along with dangling bonds are enabling technologies for atomic scale charge and spin-based devices.[1] Additionally, recent advances in hydrogen lithography have enabled the patterning of quantum dot based circuit elements with atomic precision.[2] We engineered a single acceptor coupled to a dangling bond wire on highly doped p-type H-Si(100) and characterized its electronic properties with scanning tunneling spectroscopy. The coupled entity has an electronic structure that behaves as a conductive wire from which the charge state of the dopant can be accessed and has a complex dependence on the dangling bond wire length. In addition, dI/dV mapping reveals features reminiscent of charging rings that are centered over the dopant and overlap with the wire.[3] This overlap varies with electric field and its tunability may augment the functionality of dangling bond based quantum devices.

References:
[1] A. Laucht et al., "Roadmap on quantum nanotechnologies", Nanotechnology, vol. 32, no. 16, p. 162003, 2021. Available: 10.1088/1361-6528/abb333 
[2] T. Huff et al., "Binary atomic silicon logic", Nature Electronics, vol. 1, no. 12, pp. 636-643, 2018. Available: 10.1038/s41928-018-0180-3
[3] N. Turek, S. Godey, D. Deresmes and T. Mélin, "Ring charging of a single silicon dangling bond imaged by noncontact atomic force microscopy", Physical Review B, vol. 102, no. 23, 2020. Available: 10.1103/physrevb.102.235433 

![Figure 1. a) (1.8 V 50 pA) STM image of a 7 dangling bond wire fabricated next to a subsurface Boron dopant on highly doped P type H-Si (100) with individual spectroscopy points denoted by arrows. b) dI/dV spectroscopy taken over the dopant and wire shown in a), exhibiting previously unobserved peaks at -1.4 V and 0.9 V.][1]


[1]: https://i.ibb.co/X355XS2/fig1.png

Towards the atomic scale readout of single acceptor states in p-doped Si

In the past 20 years, large-scale datasets of cell phone traces have emerged as a key proxy to study human movement patterns, and how those patterns change in response to exogenous events such as natural disasters or terrorist attacks. These data have proven especially useful to understand the effects of public policy in the current COVID-19 pandemic, where "lockdowns" and other mobility-based restrictions have served as the main intervention deployed by governments in the absence of a vaccine. In this talk, I will give an overview of how tools from information theory and statistical physics have been applied to high-resolution mobile phone data to understand the (in)effectiveness of physical distancing measures in reducing human mobility over the past year. In particular, I will discuss our own efforts to quantify heterogenity in human contact patterns and so-called "superspreading events"--both outsize drivers of epidemic spread.

Statistical Physics & Human Mobility in COVID-19

We numerically analyze the spectrum of the Laplacian on the moduli space of a genus zero Riemann surface with four punctures via a perturbative expansion of the path integral of Liouville theory. Our results furnish evidence that the eigenvalues obey the statistics of a random matrix in the Gaussian Orthogonal Ensemble. We comment on possible implications for the quantum geometry of Riemann surfaces and quantum gravity in anti–de Sitter space. Based on work with A. Maloney and T. Numasawa.

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