## About Summit

# Physics - Theses, Dissertations, and other Required Graduate Degree Essays

Receive updates for this collection## Towards optical readout of Si:Se+

The demonstration of a qubit system in silicon, with efficient optical control and readout of robust electronic and nuclear spin states, would change the current dominant industrial trends in quantum devices. Singly ionized deep double donors in silicon (Si:Se+) have shown promise as examples of such industry-changing qubit candidates. The (Si:Se+) system possesses a long-lived spin qubit with photonic access through a spin-selective optical transition. Under the assumption that this optical transition is radiatively efficient, it has been proposed that this optical transition be exploited for direct emission-based spin-state readout, or alternatively used as a much-sought-after silicon-integrated single-photon source. In the first part of this thesis, we present the measurement of the T1 lifetime of the optically excited state which in turn allowed us to determine a natural radiative efficiency of 0.80(1)%. Fortunately, this spin-photon interface can be coupled to photonic cavity modes for indirect spin-state read-out or to improve the emission rate through the Purcell effect. In the second part of this thesis, we present the hardware and software details of an adaptable automated photonics testing system that can be used to characterize integrated photonic devices.

## Mid-IR waveguides and grating couplers for 2.7-2.9 μm

Integrated silicon photonics strip waveguides and grating couplers are developed for mid-infrared (mid-IR) wavelengths at 2.7 μm and 2.9 μm. Waveguide loss is measured as a function of width for 2.7 μm light in the vicinity of a prominent OH absorption band, and a loss of less than 2 dB/cm is recorded for the 1.0 μm width waveguide. A fabrication bias metric is determined for accurately developing grating couplers at 2.9 μm on a 500 nm silicon-on-insulator (SOI), 3 μm buried-oxide substrate. A high resolution measurement scheme is motivated and measurements indicate that these devices will be capable of studying the Se+ donor spin qubit cavity coupling platform proposed by researchers at Simon Fraser University.

## Spin transport in an ultra-cold trapped non-condensed 87rb gas

Ultra-cold trapped atoms, with their high degree of tunability, provide ideal model systems to study physical phenomena with applications in many different fields of research. This thesis describes studies on spin transport phenomenon in a trapped ultra-cold spin-polarized 87Rb gas at temperatures above quantum degeneracy. This work is focused on the less studied regime of cross-over between classical and quantum transport. Diffusion is a fundamental dissipative process that tends to relax any system towards a state of minimum inhomogeneity. In this work we study longitudinal spin diffusion as a special case of spin transport. The system studied here consists of two anti-parallel longitudinal spin domains separated by a helical domain-wall. We report that the diffusion process manifests a significant deviation from classical diffusion due to purely quantum mechanical modifications. The two-domain spin textures are prepared using optical and microwave pulse techniques, and the dynamics of the spin structure is studied as it relaxes towards the final equilibrium state. Generally, there is a wide range of parameter space that could be studied. In this work we focused our studies on the effects related to the degree of coherence in the domainwall as well as the effective magnetic field acting on the spins. By controllably tuning these experimental parameters, we studied in detail how the spatiotemporal behaviour of the diffusion dynamics is modified. Our results show that the longitudinal spin diffusion time scales depend sensitively on the domain-wall degree of coherence. External magnetic field gradients also alter the dynamics noticeably, manifesting a significant dependence on the sign of the applied field gradients.

## Optical characterization of the Si:Se+ spin-photon interface

The combination of both matter qubits and photonic qubits presents a very promising method for generating entanglement between qubits in order to scale up both quantum computing and quantum communication platforms. Hosting both qubits in silicon would be a favourable approach as silicon has not only the most mature microelectronics industry but also the most mature photonics industry. Singly ionized selenium donors (Si:Se+) have recently been identified as a possible candidate. Si:Se+ possess the excellent coherence lifetimes of conventional donor spin qubits in silicon but additionally has photonic access to the spin states at a convenient wavelength, 2.9 um. The spin-photon interface of Si:Se+ has the potential to be the basis for an integrated, all silicon, quantum computer. For this work we made custom samples with the specific purpose of measuring the crucial optical properties that determine the viability of the Si:Se+ spin-photon interface as a basis for a quantum architecture. We present photoluminescence, absorption, hole burning, and magnetic resonance experiments towards the characterization of the Si:Se+ spin-photon interface. We determined a transition dipole moment of 1.96 +- 0.08 Debye, a lower bound for the zero-phonon line fraction of > 15.1 +- 0.3 %, and a lower bound for the radiative efficiency of > 0.75 +- 0.08 %. Further results of the peak and area conversion factors and the dependence of peak energy and linewidth on electrically neutral impurities are also presented.

## Cascade of magnetic transitions in the frustrated antiferromagnetic CePtPb

CePtPb is an antiferromagnetic, heavy-fermion, metallic compound that crystallizes in the ZrNiAl-type structure with space group P-62m, where the Ce-ions form a quasi-Kagome lattice in the ab-plane. Other compounds in this family with a quasi-Kagome magnetic lattice such as CePdAl and YbAgGe have shown a complex temperature-versus-magnetic field (T-H) phase diagram with multiple magnetically-ordered phases. In this thesis, a T-H phase diagram for single crystal CePtPb is constructed from electric resistivity and specific heat measurements. The constructed phase diagram also shows multiple magnetically-ordered phases as the Neel temperature T_N=0.9 K is suppressed continuously to T=0.4 K by applied field with an extrapolated zero-temperature critical field H_c=7 kOe. In zero-field, muon spin relaxation measurements show residual spin dynamics at 25 mK, consistent with the magnetic structure proposed for CePdAl, where 2/3 of the Ce-4f spins order antiferromagnetically and the other 1/3 of the Ce-4f spins remain fluctuating. From a power-law analysis of the electrical resistivity p=p_0+AT^n), neither Fermi-liquid (n=2) nor non-Fermi-liquid (n<2) behaviour have been observed down to T=0.4 K for H>=H_c. Instead, there is an anomalous evolution of n that increases from n=2.5 at H=H_c to n=4.1 at H=90 kOe, with a tendency towards saturation near the value n~4 for H>30 kOe. The phase diagram and the measurements are compared to CePdAl and YbAgGe.

## Slow dipole dynamics in organic charge transfer salts

At temperatures around 60 K, measurements of the dielectric constant in the κ-(BEDT-TTF)_2 X family of organic charge transfer salts have indicated the emergence of glassy dynamics in a relaxor ferroelectric phase. We propose that an extended Hubbard model on a triangular lattice of dimers is a minimal model for these systems to capture glassy dynamics. Allowing for disorder, we use a strong coupling expansion to second order in the Hubbard interaction to obtain a low-energy effective model in terms of spin and dipole degrees of freedom. By focusing on classical terms in the effective model we obtain a model amenable to classical Monte Carlo simulations. We perform equilibrium and out-of-equilibrium Monte Carlo simulations and calculate an analog of the Edwards-Anderson order parameter for dipoles and the two-time auto-correlation function for charge degrees of freedom. For appropriate parameters we find evidence for aging dynamics and a non-zero Edwards-Anderson order parameter, implying the emergence of glassiness in the charge degrees of freedom at low temperatures, as would be expected for a relaxor ferroelectric.

## Investigating a model lipid nanoparticle release system with 2H NMR and SAXS

Lipid Nanoparticles (LNPs) are an attractive way of delivering of short interfering RNA (siRNA) for cancer therapeutics. Their release method relies on protonation of an ionisable amino-lipid (XTC2) in acidic endosomes. Hypothetically, the protonated XTC2 and anionic lipids in endosomal membranes interact to form non-lamellar phases, releasing the siRNA. In this project, a model release system consisting of XTC2 and anionic distearoylphosphatidylserine (DSPS-d70) at pH 4.7 was investigated with deuterium nuclear magnetic resonance (2H NMR) and small angle x-ray scattering (SAXS) to determine the lipid phases which form as a function of temperature and their structural parameters. Since cholesterol is an important structural component in LNPs, increasing amounts of cholesterol were added to the system to determine its effect. Non-lamellar phases were observed for each sample particularly at high-temperatures, though interestingly the specific phase observed by each technique was not always in complete agreement.

## Thermodynamic and transport properties of single crystal YbNi4Cd

The single crystal growth and the physical properties of the rare-earth based ternary intermetallic compounds RNi4Cd (R = Y and Yb) will be presented. The powder X-ray diffraction measurement reveals that these compounds crystallize in the face-centered cubic (fcc), MgCu4Sn-type structure (space group F-43m). Magnetization, electrical resistivity, and specific heat measurements are used to study thermodynamic and transport properties of YbNi4Cd. The magnetic susceptibility shows that 4f electrons of Yb3+ ions are well localized. The electrical resistivity and specific heat measurements show antiferromagnetic ordering below T_N = 0.97 K. Applying a field along the [111] direction results in the suppression of T_N below 0.4 K at the critical field Hc ~ 4.5 kOe. No non-Fermi liquid behavior is observed in the vicinity of Hc. Above Hc, the magnetoresistivity shows an unconventional temperature dependence rho(T) = rho_0 + AT^n with n > 2, suggesting that an additional scattering mechanism in the resistivity needs to be considered. Based on the analysis of experimental results, we conclude that the Yb3+ moments and conduction electrons are weakly coupled. Despite the antiferromagnetic ordering below T_N, YbNi4Cd exhibits a large frustration parameter |theta_p/T_N| ~ 16, where the magnetic Yb3+ ions occupy the tetrahedra on the fcc lattice.

## Predictive models for chromatin folding: connecting sequence to structure

The DNA packaged inside a nucleus shows complex structures stabilized by a host of DNA-bound factors. This combination of DNA and bound factors is known as chromatin. Both the distribution of bound factors and the contacts between different locations of the DNA can be now measured on a genome-wide scale. Nevertheless, to what extent is the likelihood of contact between sites in the genome encoded by the spatial sequence of bound factors? Current approaches at addressing this question primarily use simulations of heterogeneous polymers to generate structures using the locations of bound factors. In contrast, here we develop novel predictive models for connecting chromatin sequence to structure using statistical physics, information theory and machine learning. Since our methods do not require costly polymer simulations they can quickly predict the effect on structure due to changes in the distribution of bound factors. In addition, our methods are formulated in a manner that allows us to solve the inverse problem: namely, given just structural data, predict the likely sequence of bound factors. We show that the models developed can make biologically meaningful predictions, highlighting key features of the mechanisms through which the three-dimensional conformation of DNA is coordinated by the interactions between DNA-bound factors.

## Observation of critical spin dressing

It has long been proposed that spin dressing could be employed to realize a highly effective helium-3 nuclear precession co-magnetometer for a neutron electric dipole moment (nEDM) search. The proposal is to apply an intense, continuous, and far off-resonant oscillating magnetic field, called a dressing field, in such a way that the apparent Larmor precession frequencies of the helium-3 and the neutron are modified. Under appropriateconditions a desirable situation known as critical spin dressing (CSD) is anticipated: the neutron and the helium-3 nucleus (or more generally, any two spin species) are expected to behave as if they had the same gyromagnetic ratio and hence should precess at the same rate in a static magnetic field. Spin dressing has been studied in the context of the neutron, helium-3, and a variety of other systems. Critical spin dressing, however, has not previously been demonstrated. In this thesis I report the first experimental demonstration of pulsed CSD in which simultaneous spin dressing of 1H and 19F nuclei is achieved and studied. I also demonstrate that CSD can be performed using variety of different dressing field waveforms, a consideration that until now has received little or no attention. Examples of parameters studied include the role of phase and amplitude modulation on spin dressing. Of particular note is a significant improvement in reproducibility achieved by alternating the phase of successive cycles of the dressing field waveform by pi radians. Such innovations may prove useful in an eventual nEDM search where demands on precession stability are anticipated to be extreme. To enable my study of CSD I developed a simple and robust apparatus. The central innovation was the first use of Magneto-Impedance (MI) sensors to detect weak magnetic fields associated with the precession of nuclear magnetic moments. The thesis thus begins with summaries of experiments to characterise and validate the use of MI sensors for ultra-low field (ULF) nuclear magnetic resonance. I then describe a refined version of the ULF NMR apparatus, and the manner in which it is used to investigate CSD.