arXiv subjects

A. M. Martin

Publications and source records attributed to A. M. Martin.

At least 19 recordsLinked to original sources

arXiv2026

Measurement of a quantum system using spin-mechanical conversion

Levitated macroscopic particles exhibiting quantum mechanical effects are garnering increased attention as a means for precision sensing and testing quantum mechanics. Defects in diamond, such as the nitrogen-vacancy (NV) centre possess optically-addressable spins with long coherence times at room temperature and offer an intriguing system to examine quantum spin dynamics coupled to a macroscopic classical particle. In this work, we convert the outcome of a quantum measurement on an ensemble of spins into a macroscopic rotation of the host particle via spin-mechanical coupling. Following a sequence of green laser and microwave control pulses, spin-mechanical coupling between the final qubit spin state and the host particle -- an electrically-levitated diamond -- exerts a torque on the particle that deflects a weak near-infra-red laser beam. We measure spin readout contrast in excess of 70\%, and demonstrate pulsed mechanical detection of coherent Rabi oscillations, spin-echo interferometry and $T_1$-induced relaxation. We directly measure with temporal resolution the particle reorientation from a 60\,attonewton-metre spin torque induced by flipping the spins. Our results open up interesting new opportunities for levitated spin-mechanical systems using pulsed control, from improved sensing to the prospect of realising macroscopic quantum superposition states.

quant-ph↗

arXiv2025

Photoelectric detection of single spins in diamond by optically controlled discharge of long-lived trap states

Electrical detection methods for solid-state spins are attractive for quantum technologies, being readily chip-scalable and not subject to the small photon budgets of single emitters. However, realising electrical spin readout in wide-bandgap materials with similar fidelity and bandwidth to optical approaches remains challenging. Here, we introduce a photoelectrical spin readout scheme that detects spin information stored long-term as trapped electrical charges. Using nitrogen-vacancy (NV) centres in diamond as a model system, spin-dependent photoionisation generates charge carriers that are stored in long-lived trap states at a diamond-metal Schottky junction. On-demand illumination of the junction under electrical bias releases stored charge, yielding a photocurrent transient proportional to the amount of trapped charge and hence spin state. Spin readout after coherent control of single NVs is demonstrated using charge readout in a protocol we call charge-capture detected magnetic resonance (CCDMR), and we use charge-based imaging to identify charge carrier generation and trapping processes. Our results establish CCDMR as a new technique for solid-state spin qubit readout, combining attaractive features of electrical detection with the stability of long-lived charge traps in wide-bandgap materials.

quant-ph↗

arXiv2024

Quantum electrometry of non-volatile space charges in diamond

The microscopic electric environment surrounding a spin defect in a wide-bandgap semiconductor plays a determining role in the spin coherence and charge stability of a given qubit and has an equally important role in defining the electrical properties of the host material. Here, we use electrometry of quantum defects embedded within a diamond to observe stable, micron-scale space charge distributions formed from trapped photogenerated charges. These space charges grow under optical illumination in the presence of an applied electric field, eventually screening the applied electric field entirely over a spatial extent of tens of microns due to charge carrier drift and capture. Our measurements suggest that these space charge fields originate from widely-dispersed spatial configurations of nitrogen charges. Our results have important consequences for electrometry and photoelectric detection using qubits in wide-bandgap semiconductors.

cond-mat.mes-hall↗

arXiv2024

3D-mapping and manipulation of photocurrent in an optoelectronic diamond device

Characterising charge transport in a material is central to the understanding of its electrical properties, and can usually only be inferred from bulk measurements of derived quantities such as current flow. Establishing connections between host material impurities and transport properties in emerging electronics materials, such as wide bandgap semiconductors, demands new diagnostic methods tailored to these unique systems, and the presence of optically-active defect centers in these materials offers a non-perturbative, in-situ characterisation system. Here, we combine charge-state sensitive optical microscopy and photoelectric detection of nitrogen-vacancy (NV) centres to directly image the flow of charge carriers inside a diamond optoelectronic device, in 3D and with temporal resolution. We optically control the charge state of background impurities inside the diamond on-demand, resulting in drastically different current flow such as filamentary channels nucleating from specific, defective regions of the device. We then optically engineered conducting channels that control carrier flow, key steps towards optically reconfigurable, wide bandgap designer optoelectronics. We anticipate our approach might be extended to probe other wide-bandgap semiconductors (SiC, GaN) relevant to present and emerging electronic technologies.

cond-mat.mtrl-sci↗

arXiv2024

Wavelength dependence of nitrogen-vacancy center charge cycling

Optically-active spin qubits in wide-bandgap semiconductors exist in several charge states, though typically only specific charge states exhibit desirable spin or photonic properties. An understanding of how interconversion between different charge states occurs is important for most applications seeking to employ such defects in quantum sensing and information processing, and additionally serves as a means of testing and verifying models of the defect electronic structure. Here, we use charge-sensitive confocal imaging to study the wavelength dependence of optical carrier generation in diamonds hosting nitrogen-vacancy (NV) centers, silicon vacancy (SiV) centers and substitutional nitrogen (N). We study the generation of distinctive charge-capture patterns formed when photogenerated charge carriers are captured by photoluminescent defects, using light spanning 405-633\,nm (1.96-3.06\,eV). We observe distinct regimes where one- or two-photon ionization or recombination processes dominate, and a third regime where anti-Stokes mediated recombination drives weak NV charge cycling with red light. We then compare red-induced charge cycling to fast charge carrier transport between isolated single NV centers driven with green and blue light. This work reports new optically-mediated charge cycling processes of the NV centers, and has consequences for schemes using charge transfer to identify non-luminescent defects and photoelectric detection, where ambiguity exists as to the source of photocurrent.

physics.app-ph↗

arXiv2023

Observing coherence in an incoherent paramagnetic nitrogen spin bath

The unpolarized spin environment surrounding a central spin qubit is typically considered as an incoherent source of dephasing, however, precise characterization and control of the spin bath can yield a resource for storing and sensing with quantum states. In this work, we use nitrogen-vacancy (NV) centers in diamond to measure the coherence of optically-dark paramagnetic nitrogen defects (P1 centers) and detect coherent interactions between the P1 centers and a local bath of $^{13}$C nuclear spins. The dipolar coupling between the P1 centers and $^{13}$C nuclear spins is identified by signature periodic collapses and revivals in the P1 spin coherence signal. We then demonstrate, using a range of dynamical decoupling protocols, that the probing NV centers and the P1 spins are coupled to independent ensembles of $^{13}$C nuclear spins. Our work illustrates how the optically-dark P1 spins, despite being unpolarized, can be used to extract information from their local environment and offers new insight into the interactions within a many-body system.

quant-ph↗

arXiv2023

Effects of Efimov states on quench dynamics in a three-boson trapped system

We investigate the effects of Efimov states on the post-quench dynamics of a system of three identical bosons with contact interactions, in a spherically-symmetric three-dimensional harmonic trap, which undergoes a quench in interaction strength. Using known hyperspherical solutions to the static three-body problem we calculate semi-analytic results for the Ramsey signal and particle separation as functions of time after the system is quenched. We consider the quench from the non-interacting to strongly interacting and vice versa for a variety of possible Efimov state energies.

cond-mat.quant-gas↗

arXiv2022

Energetics of three interacting mass-imbalanced bodies in a three-dimensional spherical harmonic trap

We consider a system of three particles, either three identical bosons or two identical fermions plus an impurity, within a three-dimensional isotropic trap interacting via a contact interaction. Using two approaches, one using an infinite sum of basis states for the wavefunction and the other a closed form wavefunction, we calculate the allowable energy eigenstates of the system as a function of the interaction strength, including the strongly and weakly interacting limits. For the fermionic case this is done while maintaining generality regarding particle masses. We find that the two methods of calculating the spectrum are in excellent agreement in the strongly interacting limit. However the infinite sum approach is unable to uniquely specify the energy of Efimov states, but in the strongly interacting limit there is, to a high degree of accuracy, a correspondence between the three-body parameter required by the boundary condition of the closed form approach and the summation truncation order required by the summation approach. This specification of the energies and wavefunctions forms the basis with which thermodynamic variables such as the virial coefficients or Tan contacts, or dynamic phenomena like quench dynamics can be calculated.

cond-mat.quant-gas↗

arXiv2022

Quench dynamics of mass-imbalanced three-body fermionic systems in a spherical trap

We consider a system of two identical fermions of general mass interacting with a third distinguishable particle via a contact interaction within an isotropic three-dimensional harmonic trap. We calculate time-dependent observables of the system after it is quenched in s-wave scattering length. To do this we use exact closed form mass-imbalanced hyperspherical solutions to the static three-body problem. These exact solutions enable us to calculate two time-dependent observables, the Ramsey signal and particle separation, after the system undergoes a quench from non-interacting to the unitary regime or vice-versa.

cond-mat.quant-gas↗

arXiv2021

Tunable Gyromagnetic Augmentation of Nuclear Spins in Diamond

Nuclear spins in solids exhibit long coherence times due to the small nuclear gyromagnetic ratio. This weak environmental coupling comes at the expense of slow quantum gate operations, which should be as fast as possible for many applications in quantum information processing and sensing. In this work, we use nitrogen-vacancy (NV) centers in diamond to probe the nuclear spins within dark paramagnetic nitrogen defects (P1 centers) in the diamond lattice. The gyromagnetic ratio of the P1 nuclear spin is augmented by hyperfine coupling to the electron spin, resulting in greatly enhanced coupling to radiofrequency control fields. We then demonstrate that this effect can be tuned by variation of an external magnetic field. Our work identifies regimes in which we are able to implement fast quantum control of dark nuclear spins, and lays the foundations for further inquiry into rapid control of long-lived spin qubits at room temperature.

quant-ph↗

arXiv2021

Analytical Investigation of Focusing Bose-Einstein condensates

The focusing of a propagating untrapped Bose-Einstein condensate is studied theoretically. We use a scaling solution method comprising a time-dependent scaling function to analytically examine the dynamics of a falling Bose-Einstein condensate in different regimes of propagation including the expansion and compression zones. Our model is based on the Gross-Pitaevskii equation which involves the interparticle interactions between atoms, and consequently their influence on the focused structures. We investigate the focused profile characteristic factors such as the resolution and peak density for various cases of the focusing optical potential parameters as well as the factors associated with the moving cloud. Our results are compared with numerical solutions of the Gross-Pitaevskii equation.

cond-mat.quant-gas↗

arXiv2021

Focusing Atom Laser Beams

We theoretically study the focusing of a quasi-continuous atom laser beam of rubidium-85 ($^{85}$Rb). A two-sate model analysis based on the Gross-Pitaevskii equation is used which comprises the effects of two-body atom-atom interactions and three-body recombination losses. Utilizing optical focusing potentials such as harmonic potentials, the essential factors such as the width, peak density and atom loss rate of the focused atom laser beam profile are investigated. Our analysis predicts that using an atom laser offers a dramatic improvement in resolution of up to $8$ nm.

quant-ph↗

arXiv2021

Arbitrary-angle rotation of the polarization of a dipolar Bose-Einstein condensate

We have employed the theory of harmonically trapped dipolar Bose-Einstein condensates to examine the influence of a uniform magnetic field that rotates at an arbitrary angle to its own orientation. This is achieved by semi-analytically solving the dipolar superfluid hydrodynamics of this system within the Thomas-Fermi approximation and by allowing the body frame of the condensate's density profile to be tilted with respect to the symmetry axes of the nonrotating harmonic trap. This additional degree of freedom manifests itself in the presence of previously unknown stationary solution branches for any given dipole tilt angle. We also find that the tilt angle of the stationary state's body frame with respect to the rotation axis is a nontrivial function of the trapping geometry, rotation frequency and dipole tilt angle. For rotation frequencies of at least an order of magnitude higher than the in-plane trapping frequency, the stationary state density profile is almost perfectly equivalent to the profile expected in a time-averaged dipolar potential that effectively vanishes when the dipoles are tilted along the `magic angle', $54.7 °$. However, by linearizing the fully time-dependent superfluid hydrodynamics about these stationary states, we find that they are dynamically unstable against the formation of collective modes, which we expect would result in turbulent decay.

cond-mat.quant-gas↗

arXiv2021

Two-body quench dynamics of harmonically trapped interacting particles

We consider the quantum evolution of a pair of interacting atoms in a three dimensional isotropic trap where the interaction strength is quenched from one value to another. Using exact solutions of the static problem we are able to evaluate time-dependent observables such as the overlap between initial and final states and the expectation value of the separation between the two atoms. In the case where the interaction is quenched from the non-interacting regime to the strongly interacting regime, or vice versa, we are able to obtain analytic results. Examining the overlap between the initial and final states we show that when the interaction is quenched from the non-interacting to strongly interacting regimes the early time dependence dynamics are consistent with theoretical work in the single impurity many-body limit. When the system is quenched from the strongly to non-interacting regime we predict large oscillations in the separation between the two atoms, which arises from a logarithmic divergence due to the zero-range nature of the interaction potential.

cond-mat.quant-gas↗

arXiv2020

The Influence of s-wave interactions on focussing of atoms

The focusing of a rubidium Bose-Einstein condensate via an optical lattice potential is numerically investigated. The results are compared with a classical trajectory model which under-estimates the full width half maximum of the focused beam. Via the inclusion of the effects of interactions, in the Bose-Einstein condensate, into the classical trajectory model we show that it is possible to obtain reliable estimates for the full width half maximum of the focused beam when compared to numerical integration of the Gross-Pitaevskii equation. Finally, we investigate the optimal regimes for focusing and find that for a strongly interacting Bose-Einstein condensate focusing of order 20 nm may be possible.

quant-ph↗

arXiv2020

Optical focusing of Bose-Einstein condensates

We theoretically investigate the optical focusing of a rubidium Bose-Einstein condensate onto a planar surface. Our analysis uses a Gaussian variational method that includes the effects of two-body atom-atom interactions and three-body recombination losses. The essential factors such as the width, peak density and atom loss rate of the focused BEC profile on the surface are investigated and compared to Gross-Pitaevskii numerical simulations. We find a reasonable agreement in the results between our analytical approach and the numerical simulations. Our analysis predicts that condensates of $10^5$ atoms could be focused down to $\sim 10$nm widths, potentially allowing nanometer-scale atomic deposition with peak densities greater than $10^5$ atoms/$μ$m$^2$.

quant-ph↗

arXiv2020

Interplay between geometric and dynamic phases in a single spin system

We use a combination of microwave fields and free precession to drive the spin of a nitrogen-vacancy (NV) center in diamond on different trajectories on the Bloch sphere, and investigate the physical significance of the frame-dependent decomposition of the total phase into geometric and dynamic parts. The experiments are performed on a two-level subspace of the spin-1 ground state of the NV, where the Aharonov-Anandan geometric phase manifests itself as a global phase, and we use the third level of the NV ground state triplet to detect it. We show that while the geometric Aharonov-Anandan phase retains its connection to the solid angle swept out by the evolving spin, it is generally accompanied by a dynamic phase that suppresses the geometric dependence of the system dynamics. These results offer insights into the physical significance of frame-dependent geometric phases.

quant-ph↗

arXiv2019

Dynamics of the creation of a rotating Bose-Einstein condensate by two-photon Raman transition using Laguerre-Gaussian pulse

We examine the dynamics associated with the creation of a vortex in a Bose-Einstein condensate (BEC), from another nonrotating BEC using two-photon Raman transition with Gaussian (G) and Laguerre-Gaussian (LG) laser pulses. In particular, we consider BEC of Rb atoms at their hyperfine ground states confined in a quasi two dimensional harmonic trap. Optical dipole potentials created by G and LG laser pulses modify the harmonic trap in such a way that density profiles of the condensates during the Raman transition process depend on the sign of the generated vortex. We investigate the role played by the Raman coupling parameter manifested through dimensionless peak Rabi frequency and intercomponent interaction on the dynamics of the population transfer process and on the final population of the rotating condensate. During the Raman transition process, the two BECs tend to have larger overlap with each other for stronger intercomponent interaction strength.

cond-mat.quant-gas↗