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Hydrogen terminated silicon has seen a recent resurgence in popularity due to several works demonstrating its use for ultra-dense memory, atomic electronics, and quantum devices. On this surface, individual hydrogen atoms can be removed with atomic precision through STM pulses, leaving a dangling bond (DB) behind. DBs are quantum dot-like entities that can hold either 0, 1, or 2 electrons, with their discrete energy levels in the bandgap. We are studying DBs to achieve true random number generators (RNG) on the atomic scale. Two DBs patterned in close proximity to each other on a highly n-doped crystal host a net extra electron, which can quantum mechanically tunnel to reside on either side. Through measurement of the extra electrons’ spatial location in the pair, it is expected that the electron will be found on either side with equal probability in the absence of any biasing effects. This opens the possibility of using the system as a true RNG, where the bits are generated by probing the electrons’ spatial location. Quantum processes are most desired for RNG because the randomness can be such that no common naturally occurring or imposed noise will alter the number generation. We study these DB pairs by DFT methods to establish the effect of geometry, external fields, and nearby dopants on charge localization and consequent RNG bit generation.
![Image abstract.][1]


[1]: https://i.ibb.co/jhvjz07/Image-Abstract.png

CAP Virtual Congress 2021

A Study of Silicon Dangling Bond Pairs in Search of a True Random Number Generator

atomic force microscope

scanning tunneling microscope

random number generator

dangling bond

silicon

density functional theory

quantum

**WELCOME TO THE 2021 CAP VIRTUAL CONGRESS UNDERLINE PLATFORM**

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The Institute of Physics and IOP Publishing, in partnership with the Chinese Physical Society (CPS) and the University of Science and Technology of China (USTC) are proud to have brought together the international quantum science community online for this inaugural conference.


Early career and leading researchers from universities, industry and government worldwide came together to share, learn and collaborate on the latest research and emerging areas of high interest across quantum science and technology. The broad themes covered by Quantum 2020 included:

Quantum communication;
Quantum computing and quantum simulation;
Quantum sensing, metrology and imaging;
Quantum information theory, software and algorithms;
Quantum foundations;
Quantum technology in industry.
The four-day event featured talks from world-renowned scientists, interactive poster sessions and two exciting panel discussions on the themes of Quantum Technology in Industry and Quantum Technology Programmes Around the World.


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Quantum 2020

poster

Autopsy studies indicate that the distribution of axons throughout the brain could differ in brains with and without schizophrenia. MRI has inferred axon diameters and distribution in the brain, typically for axons larger than 5 μm in diameter. The goal of this work is to modify these methods to adapt oscillating gradients (OG) to target small axons (1 to 2 μm range) which constitute the majority of cortical connections and shorten the data acquisition time so that the method can be used to measure axon diameters in *vivo*.
Methods to determine micron-sized axon radii using oscillating gradients were too time-consuming for in *vivo* mouse imaging. The methods use many gradient frequencies, and a geometric model called AxCaliber which uses intracellular and extracellular compartments. Through simulations, we found that fewer gradient frequencies for data provided a good fit to the intracellular model, and in some cases, fewer frequencies led to a better fit to the model than more frequencies. A moderate correlation was found between the predicted axonal radius from the model and the actual axonal radius used in the simulations. In conclusion, reducing the number of gradient frequencies and gradient strengths appears to be possible, in theory, to reduce the imaging time by a factor of 4.16 without a significant change in the precision of the inferred axon radii. The method proposed here requires *a priori* knowledge of the desired cell sizes to be inferred. Imaging data, as well as electron microscopy data, need to be collected after reopening the lab after the pandemic to verify the theoretical predictions. This work is the first step to reducing the imaging time so that OG can be used with the AxCaliber model to infer 1-2 μm axon sizes.
The authors wish to acknowledge funding from Mitacs and NSERC.

Moving toward faster measurements of micron-sized axon diameters in vivo

A prototype medical device has been developed and built at Western University to image adolescents with hemophilic arthropathy. The device consists of a plastic cylindrical tub able to rotate freely about a base plate. A 10 MHz linear array ultrasound transducer by Canon Medical Systems is mounted to the inside of the tub pointed toward its center. The tub is filled with water to act as a medium between the ultrasound transducer and the skin. The leg or arm is fixed, while a clinical ultrasound probe rotates over 360 degrees. 2D US images are recorded every 0.5 degrees, and reconstructed into a 3D volume in an inverse fan geometry. The 3D image provides a viewpoint that is unreachable in conventional 2D ultrasonography. The prototype design, study protocol workflow, and preliminary results will be presented; specifically showing corresponding anatomical landmarks between 3D ultrasound and MR images. The future work for the study is a clinical trial at SickKids Hospital in Toronto; whereby, ultrasound / MRI data from each subject will be viewed by two experienced musculoskeletal radiologists who are blinded to the other aspects of the study. Interpretations from both modalities will gauge the feasibility of this approach for use in imaging of hemophilic arthropathy in target joints.

Multi-Modality Comparison of Wrist and Ankle joints: A Feasibility Study

**Effect of Lung Surfactant Protein B Fragment, SP-B1-9 on Model Lipid Bilayer.**
Abinu, MHK, ISV, MRM

Lung surfactant is a mixture of protein and lipid that reduces surface tension at the air-water interface in the lungs and thus reduces the work needed to breathe. Two hydrophobic proteins, SP-B and SP-C, are thought to facilitate the re-spreading of fresh and recycled surfactant material from bilayer and multilayer reservoirs. In an earlier deuterium NMR study of bilayer model membranes containing either the SP-B fragment SP-B(1-25, 63-78) or the fragment SP-B(8-25, 63-78), it was found that lipid acyl chain orientational order was perturbed more strongly by SP-B(1-25, 63-78). Both fragments contain the first and last SP-B helices but differ in whether or not the insertion motif, SP-B1-7, is present, suggesting that the insertion motif might play a role in the capacity of SP-B to promote the bilayer reorganization implicit in lung surfactant function. To gain more insight into the interaction of the insertion motif with surfactant lipids, we have studied the effect of SP-B1-9 on lipid acyl chain order in DPPC-d62 /POPG(7:3) bilayers using deuterium NMR and GROMACS molecular dynamics simulations.
Using deuterium NMR at a peptide: lipid ratio of 0.065, we find that the peptide tends to disorder acyl chains in the liquid crystalline bilayer phase suggesting that it promotes an increase in average lipid headgroup separation. MD simulation of a bilayer model containing SP-B1-9 at a peptide: lipid ratio of 0.031 also shows a decrease in chain orientational order. The MD simulations also provide information about the average peptide orientation and conformation. These findings may provide a better understanding of the extent to which the insertion motif may contribute to the capacity of SP-B to promote reorganization of bilayer surfactant material.

Supported by NSERC Discovery Grants to MRM and ISV.

Effect of Lung Surfactant Protein B Fragment, SP-B1-9 on Model Lipid Bilayer.

With advances in nanofabrication, modelling the optical properties of nanoscale systems is critical.

The optical response of nanostructures can be simulated using the bulk permittivity of the constituent materials (1). The permittivity is generally assumed time-invariant and spatially dispersionless. While these approximations are enough for the simulations of most systems, in some cases the physics must be modelled more precisely.

We present three recent projects on the modelling of complex optical properties with implications in active nanophotonics (2), ultrafast physics and nonlinear plasmonics. The algorithms we developed are unavailable in commercial or open-source software.

1.) Nonlocal models: for small nanostructures (< 10 nm) spatial dispersion cannot be neglected, and we need to account for the electron degeneracy pressure (3).

2.) Nonlinear hydrodynamics: The hydrodynamic plasma model is required to properly model conduction electron dynamics in strong optical fields (4).

3.) Time-variant permittivity: Electrodynamics combined with two-temperature modelling is required to calculate the optical response induced by nonlinearity in ITO under strong optical fields (5).

Our models combined with high-performance computing allow us to achieve accurate results and significant agreement to experiments.

References

1. Pernice, W. H. P. Finite-Difference Time-Domain Methods and Material Models for the Simulation of Metallic and Plasmonic Structures. J. Comput. Theor. Nanosci. 7, 1–14 (2010).

2. Shaltout, A. M., Shalaev, V. M. & Brongersma, M. L. Spatiotemporal light control with active metasurfaces. Science 364, eaat3100 (2019).

3. Baxter, J., Lesina, A. C. & Ramunno, L. Parallel FDTD modelling of nonlocality in plasmonics. ArXiv200506997 Phys. (2020).

4. Bin-Alam, M. S. et al. Hyperpolarizability of plasmonic meta-atoms in metasurfaces. ArXiv200705142 Phys. (2020).

5. Alam, M. Z., Schulz, S. A., Upham, J., De Leon, I. & Boyd, R. W. Large optical nonlinearity of nanoantennas coupled to an epsilon-near-zero material. Nat. Photonics 12, 79–83 (2018)

Advanced modelling in computational nanophotonics

Strong field physics, femtosecond (1 fs = $10^{-15}$), and attosecond (1 as = $10^{-18}$) science require intense ultrashort laser pulses for excitation and measurement. Although fs pulses can be generated directly from a laser amplifier, durations necessary for ultrafast science require spectral broadening and compression. The most common spectral broadening technique exploits the nonlinear interaction of intense pulses focused into gas-filled hollow-core fibres [1]. These fibres allow for a long interaction length to generate octave-spanning spectra while maintaining a near Gaussian spatial distribution. However, these fibres can be 1 – 3 metres on an optical table with additional 1 m focusing and collimating optics, and require continuous backfilled gas flow. More recently, the self-phase modulation in thin crystals broadens the spectrum and uses a self-focusing relay to maintain the beam quality [2]. A combination of solids and gases to generate a supercontinuum has been used to generate attosecond extreme ultraviolet (XUV) pulses [3]. Ultrashort pulses can have a wide variety of applications, including disrupting biological processes for medical procedures, observation of electronic processes on an attosecond scale, and generating XUV and soft X-ray pulses via high harmonic generation or terahertz via difference frequency generation [4]. Here, we compare numerically and experimentally the spectral broadening in solvents and crystals to demonstrate the potential for a compact setup for generating octave spanning pulses for ultrafast experiments. 

We find that passing a beam through several cm of methanol produces a supercontinuum spectrum that can support few-fs pulses. Similar to the multi-plate thin crystal method, we place a series of 1 cm cuvettes of methanol around two foci in order to reduce self-focussing effects and to avoid filamentation within the material. We compress these pulses using dispersion compensating mirrors, and characterize the resulting pulses using frequency resolved optical gating (FROG) and a novel measurement scheme. Our simulations and models predict a sub-10 fs bandwidth-limited pulse resulting from two subsequent compressions using a total of 4 cm of methanol to generate a supercontinuum. Using this method, experimentally, we have achieved a broad spectrum capable of supporting sub-30 fs pulses.


[1] E. Haddad, R. Safaei, A. Leblanc, R. Piccoli, Y.G. Jeong, H. Ibrahim,B.E. Schmidt, R. Morandotti, L. Razzari, F. Légaré, P. Lassonde. "Molecular gases for pulse compression in hollow core fibers," Optics Express (2018).

[2] Y.-C. Cheng, C.-H. Lu, Y.-Y. Lin, and A. H. Kung, "Supercontinuum generation in a multi-plate medium," Optics Express (2016).

[3] T. J. Hammond, S. Monchocé, C. Zhang, G. Vampa, D. Klug, A. Y. Naumov, D. M. Villeneuve, and P. B. Corkum "Integrating solids and gases for attosecond pulse generation," Nature Photonics (2017).

[4] M. Cheng, A. Reynolds, H. Widgren, and M. Khalil, "Generation of tunable octave-spanning mid-infrared pulses by filamentation in gas media," Optics Letters (2012).

Novel Methods for Pulse Compression

From cytoskeleton to centriole, we investigate the photonic inner workings of the living cell. The cytoskeleton provides the structural organization of the cell, as the supporting architecture for all known forms of life. A major structural unit of this architecture is the cellular “microtubule,” a hollow tubular body composed of individual units of the protein “tubulin” organized in an array of spiral-shaped components. Recently, it was proposed that microtubules may exhibit useful photonic properties, as well as a myriad of known structural features. In this work, we explore the role that photonic super-radiance may play in facilitating excitonic transport and communication within the cellular architecture, from the cytoskeleton to the eukaryotic centriole—and beyond.

Photonic Cooperativity and Coherence in Tubulin Architectures

When a chemical bond breaks, all valence electrons in the bond will re-arrange and transfer as the nuclei are separating. The dynamical information of such electronic re-arrangement processes will be encoded in the kinetic energy of the ions if the breaking neutral molecule is ionized, allowing us to follow the entire electronic re-arrangement process by only observing the ions. Here, using Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) and a pump-probe experiment in the bromine molecule. we show how the single ionization reveals the bond breaking while the double ionization measures the separation distance of the neutral dissociating bromine molecule. In addition, we also studied the vibrational wave packet dynamics of bromine single ion, which is pumped by a strong 800nm pulse and probed by another weak 400nm pulse.

Light-induced dissociation dynamics of Br2

The connection between quantum entanglement and classical chaos has puzzled physicists for decades. To understand chaos in the quantum context, it is necessary to explore signatures of chaos in the deep quantum regime, where the quantum-classical correspondence cannot be invoked. A common approach is to study the quantum kicked top, which is a finite-dimensional quantum system that displays chaotic dynamics in the classical limit. Few experimental realizations of the quantum kicked top have been achieved for small number of time steps and low chaoticity parameters. However, the problem of accurate experimental realization of long-term dynamics and large range of chaoticity parameter in the quantum kicked top is still unsolved. In this work we propose and demonstrate an exact simulation of the 2-qubit quantum kicked top on a universal quantum computer. Our proposal allows for arbitrary time steps and chaoticity parameter without compromising experimental fidelity. Using the IBM 5-qubit quantum chip we physically demonstrate the theoretical connection between delocalization and entanglement. We obtain a phase space plot of time-averaged entanglement in the deep quantum regime, which surprisingly reflects the classical phase space structure. Since chaos is known to generate entanglement in the quantum kicked top, our proposed method could be useful in Quantum Information Processing.

Quantum Simulation of the Quantum Kicked Top

Simple Measurement for Field Reconstruction

The temporal resolution of pump-probe experiments is determined by the duration of the excitation and measurement pulses. A major constraint on femtosecond ($1$ fs = $10^{-15}$ s) and attosecond ($1$ as = $10^{-18}$ s) science is how well we can control and compress these pulses. However, such ultrashort pulses require a broad spectrum with careful phase control across its bandwidth to minimize the duration. The temporal characterization of these pulses is crucial in establishing ultrafast experiments. For this reason, several pulse characterizing techniques have been developed, each with their own strengths and weaknesses [1].

In this paper, we discuss a new optical measurement technique that can directly measure the electric field that constitutes the laser pulse. We find that compared to other techniques for electric field measurements, our method is relatively inexpensive and robust in characterizing pulses up to 100 fs in duration. This method requires only a commercially available CCD camera as opposed to spectrometers or other expensive instruments and provides the ability to reconstruct the field without ambiguities [2]. In this way, we can directly measure the temporal evolution of the field amplitude and phase [3]. We find that the measurement is in good agreement with established characterization techniques.

References

[1] J. C. Diels and W. Rudolph. Ultrashort Laser Pulse Phenomena, chapter Diagnostic Techniques,pages 452–466. Elsevier, 2006.

[2] T. J. Hammond, A. Korobenko, Y. Naumov, M. Villeneuve, P. B. Corkum and D. H. Ko. Near-field imaging for single-shot wave form measurements.Journal of Physics, 51, February 2018

[3] E. Goulielmakis, M. Uiberacker, R. Kienberger, A. Baltuska, V. Yakovlev, A. Scrinzi, T. West-erwalbesloh, U. Kleineberg, U. Heinzmann, M. Drescher and F. Krausz. Direct measurement of light waves.Science, 305(5688):1267–1269, August 2004.

Simple Measurement for Field Reconstruction

Trapped ions can offer a functional platform for quantum simulation of many-body Hamiltonians modeled by spin-systems. Programmable, arbitrary and precise control over each ion is required in order to tune ion-ion interactions, which translate to diverse parameters of the system under study. Current technologies suffer from scalability issues to large ion chains, and from ``cross-talk'' due to micron-level inter-ion separation. Here, we report our development of a holographic optical ion-addressing setup for Yb$^+$ ions ($\lambda = 369.5$ nm) using a Digital Micromirror Device (DMD). This technique uses a reprogrammable hologram to modulate the wavefront of the addressing beam and thus engineer the amplitude and phase profile of light across the ion ensemble to better than $\lambda/20$. We implement a novel Iterative Fourier Transform Algorithm (IFTA) to compute the desired hologram. This algorithm efficiently compensates for optical aberrations and produces $<10^{-4}$ intensity cross-talk error in arbitrary pair-wise addressing profiles, suitable for over fifty ions. This scheme relies on standard commercial hardware, can be readily extended to over a hundred ions, and adapted to other ion-species and quantum platforms. Such high-precision optical control will enable the simulation of arbitrary and dynamic lattice geometries of spins to be realized in a 1D chain of ions. This technique will also allow us to investigate problems in quantum quench, quantum phase transitions, and physics of higher dimensional systems such as the creation of topological states. 

We acknowledge financial support from University of Waterloo, US ARO, NSERC Discovery and NFRF grants, TQT (CFREF), and the Ontario Government.

A Study of Silicon Dangling Bond Pairs in Search of a True Random Number Generator

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