arXiv subjects

Mark Sadgrove

Publications and source records attributed to Mark Sadgrove.

At least 19 recordsLinked to original sources

arXiv2026

Writing and readout of luminescent defects on an optical nanofiber using an electron beam

We demonstrate the writing and readout of groups of luminescent defects on an optical nanofiber (ONF), with readout performed through the fiber modes. In the write step, a focused electron beam incident on the fiber surface created luminescent defect centers in the fiber's silica in a region of diameter $\sim200$ nm localized about the beam center. In the readout step, the electron beam was scanned over the fiber and the induced cathodoluminescence from the defects was coupled into the ONF and detected. The regions with electron beam-induced luminescent defects exhibited a strongly enhanced optical signal, despite minimal observable change in the fiber surface, as judged by secondary electron imaging over the same region. Spectral measurements show that the enhanced luminescence stems from the creation of both oxygen deficiency centers (ODCs) and non-bridging oxygen hole centers (NBOHCs) by the electron beam.

physics.optics↗

arXiv2025

Chiral light from an emitter coupled to an achiral particle via the Purcell effect

We demonstrate that non-chiral nanoparticles can produce chiral light when point emitters are coupled to their surface plasmon modes (SPMs) under certain conditions. Chiral emission arises from asymmetrical plasmon mode propagation from the source combined with the spin-momentum locked nature of the SPMs. The Purcell regime of cavity quantum electrodynamics (QED) ensures that radiation from the coupled mode dominates over that from the emitter itself, giving rise to photons with a circularly polarized component -- i.e. chiral light. We experimentally demonstrate this effect using electron beam-induced cathode luminescence from a gold nanorod, coupling it evanescently to a nanofiber probe which also supports spin-momentum locked light. This converts the net spin of the emission into a net directionality of propagation in the fiber modes.

quant-ph↗

arXiv2025

Detecting entanglement between quantum emitters using directional emission

Recently, it was shown that quantum interference in a system containing a polarized and unpolarized emitter can allow directional emission of photons into a circulating cavity. Here, we ask whether high directionality of photon emission in this system implies a high degree of quantum correlation between the two emitters. We show that the answer is a qualified "yes", with photon emission directionality and emitter-emitter entanglement showing a monotonic relationship over a broad parameter range. The relationship only breaks down in the limit of perfect directionality. Furthermore, under reasonable assumptions for experimental parameters and stability, we show that the statistics of measured directionality allow a reliable estimate of the concurrence. This result implies that directionality of photon emission in the state preparation stage can be used to determine the entanglement between the emitters, with potential applications to more generic cases including quantum networks.

quant-ph↗

arXiv2025

Chirality-selective optical transport of nanoparticles in the evanescent field of a nanofiber

Optical nanofibers are waveguides known for their unique property to produce intense evanescent fields which have subwavelength transverse confinement easily extendable over thousands of wavelengths along the fiber axis. Moreover, circularly polarized fundamental modes of a nanofiber are chiral, that is, lacking mirror symmetry. Here, we use these two properties to demonstrate chirality-selective optical transport of a waterborne chiral material object - a chemically synthesized gold nanocube with twisted faces. Our experiments, supported by numerical simulations, show that right- and left-handed circularly polarized modes produce clearly distinct velocities of optically trapped nanocubes along the nanofiber axis, whereas non-chiral gold nanospheres of a similar size do not show any such dissymmetry. Furthermore, using a counterpropagating mode configuration, the non-chiral component of the optical force can be effectively zeroed out, yielding selective forward and backward transport of chiral nanocubes. In addition, the chiral optical force was found to be significant even for particle ensembles with natural variations in size and form, showing average behavior in agreement with numerical simulations. This is a clear implementation of optical separation of chiral enantiomers at the scale of 100 nm. Further development towards waveguide-assisted enantio-selective manipulation approaching the molecular scale can be envisaged.

physics.optics↗

arXiv2024

Single-photon generation from a neodymium ion in optical fiber at room temperature

The realization of single-photon generation is important for implementing various quantum information technologies. The use of rare-earth ions in an optical fiber is a promising single photon generation method due to its ability to operate at room temperature as well as the low cost involved. Neodymium ions are especially interesting because the ions are one of the most commercially affordable rare-earth materials in the current industry. The neodymium ion also has the advantage of having a rich energy level structure, which offers several possible wavelengths for emitted single photons from visible to near-telecommunication wavelengths. In this paper, we experimentally demonstrated single-photon generation using an isolated single neodymium ion in tapered silica fiber at room temperature. Our results have significant implications as a platform for low-cost wavelength-selectable single-photon sources and photonic quantum applications.

quant-ph↗

arXiv2023

Room-temperature addressing of single rare-earth atoms in optical fiber

Rare-earth (RE) atoms in solid-state materials are attractive components for photonic quantum information systems because of their coherence properties even in high-temperature environments. We have experimentally performed the single-site optical spectroscopy and optical addressing of a single RE atom in an amorphous silica optical fiber at room temperature. The single-site optical spectroscopy of the tapered RE-doped fiber shows nonresonant emission lines similar to those seen in the case of an unstructured fiber and the autocorrelation function of photons emitted from the fiber shows the antibunching effect due to the spatial isolation given by the tapered fiber structure. The ability to address single RE atoms at room temperature provides a very stable and cost-effective technical platform for the realization of a solid-state system for a large-scale quantum optical network and other quantum technologies based on a large number of spectral channels from visible to midinfrared wavelengths.

quant-ph↗

arXiv2023

Position-controlled trapping of nanoparticles and quantum dots on a fiber taper

We investigate numerically and experimentally the properties of a two color optical fiber taper trap, for which the evanescent field of the modes in the fiber taper give rise to a three-dimensional trapping potential. Experimentally, we use the technique to confine colloidal nanoparticles near the surface of an optical fiber taper, and show that the trapping position of the particles is adjustable by controlling the relative power of two modes in the fiber. We also demonstrate a proof of principle application by trapping quantum dots together with gold nanoparticles in a configuration where the trapping fields double as the excitation field for the quantum dots. This scheme will allow the positioning of quantum emitters in order to adjust coupling to resonators combined with the fiber taper.

quant-ph↗

arXiv2022

Plasmon-enhanced polarized single photon source directly coupled to an optical fiber

A bright source of fiber-coupled, polarized single photons is an essential component of any realistic quantum network based on today's existing fiber infrastructure. Here, we develop a Purcell enhanced, polarized source of single photons at room temperature by coupling single colloidal quantum dots to the localized surface plasmon-polariton modes of single gold nanorods, combined on the surface of an optical nanofiber. A maximum enhancement of photon emission of 62 times was measured corresponding to a degree of polarization of 86 $\%$, and a brightness enhancement of four times in the fiber mode. Evanescent coupling of photons to the nanofiber guided modes ensures automatic coupling to a single mode fiber. Our technique opens the way to realizing bright sources of polarized single photons connected to fiber networks using a simple composite technique.

quant-ph↗

arXiv2021

Interference-induced directional emission from an unpolarized two level emitter into a circulating cavity

Chiral coupling between quantum emitters and evanescent fields allows directional emission into nanophotonic devices and is now considered to be a vital ingredient for the realization of quantum networks. However, such coupling requires a well defined circular dipole moment for the emitter -- something difficult to achieve for solid state emitters at room temperature due to thermal population of available spin states. Here, we demonstrate that a two level emitter with a randomly polarized dipole moment can be made to emit directionally into a circulating cavity if a separate emitter is chirally coupled to the same cavity, for the case when both emitter-cavity couplings are strong but in the bad-cavity regime. Our analysis of this system first considers a transient scenario, which highlights the physical mechanism giving rise to the directional emission of the two level emitter into the cavity. An alternative setup involving a weak laser field continuously driving the system is also considered, where the directionality (our proposed figure of merit for this scheme) is shown to be significantly more robust against noise processes. The results presented here take the form of approximate analytical expressions backed by complete numerical simulations of the system.

quant-ph↗

arXiv2021

Probing local density of states near the diffraction limit using nanowaveguide coupled cathode luminescence

The photonic local density of states (PLDOS) determines the light matter interaction strength in nanophotonic devices. For standard dielectric devices, the PLDOS is fundamentally limited by diffraction, but its precise dependence on the size parameter $s$ of a device can be non-trivial. Here, we measure the PLDOS dependence on the size parameter in a waveguide using a new technique - nanowaveguide coupled cathode luminescence (CL). We observe that depending on the position within the waveguide cross-section, the effective diffraction limit of the PLDOS varies, and the PLDOS peak shape changes. Our results are of fundamental importance for optimizing coupling to nanophotonic devices, and also open new avenues for spectroscopy based on evanescently coupled CL.

quant-ph↗

arXiv2021

Optically induced sieve effect for nanoparticles near a nanofiber taper

We demonstrate size selective optical trapping and transport for nanoparticles near an optical nanofiber taper. Using a two-wavelength, counter-propagating mode configuration, we show that 100 nm diameter and 150 nm diameter gold nanospheres (GNSs) are trapped by the evanescent field in the taper region at different optical powers. Conversely, when one nanoparticle species is trapped the other may be transported, leading to a sieve-like effect. Our results show that sophisticated optical manipulation can be achieved in a passive configuration by taking advantage of mode behavior in nanophotonics devices.

quant-ph↗

arXiv2020

Optical transport of sub-micron lipid vesicles along an optical nanofibre

Enhanced manipulation and analysis of bio-particles using light confined in nano-scale dielectric structures has proceeded apace in the last several years. Small mode volumes, along with the lack of a need for bulky optical elements give advantages in sensitivity and scalability relative to conventional optical manipulation. However, manipulation of lipid vesicles (liposomes) remains difficult, particularly in the sub-micron diameter regime. Here we demonstrate the optical trapping and transport of sub-micron diameter liposomes along an optical nanofiber using the nanofiber mode's evanescent field. We find that nanofiber diameters below a nominal diffraction limit give optimal results. Our results pave the way for integrated optical transport and analysis of liposome-like bio-particles, as well as their coupling to nano-optical resonators.

physics.bio-ph↗

arXiv2020

Optical detection of nano-particle characteristics using coupling to a nano-waveguide

Recently, much research concerning the combination of nano-scale waveguides with nano-crystals and other nano-particles has been reported, because of possible applications in the field of quantum information and communication. The most useful and convenient method to verify the nature of such systems is optical detection. However, due to the diffraction limit, optical identification of characteristics such as particle type, particle position, etc is difficult or impossible. However, if such particles are placed on a waveguide, the coupling of scattered light to the waveguide guided modes can reveal the information about the particles. Here we consider how illumination with light of arbitrary polarization can reveal the difference between isotropic and non-isotropic nano-particles placed on the surface of an optical nanofiber. Specifically, we measure the polarization response function of gold nano-rods (GNRs) on an optical nanofiber surface and show that it is qualitatively different to that for gold nano-spheres (GNSs). This experimental technique provides a simple new tool for the optical characterization of hybrid nano-optical devices.

physics.optics↗

arXiv2017

Hamiltonian ratchets with ultra-cold atoms

Quantum-resonance ratchets have been realized over the last ten years for the production of directed currents of atoms. These non-dissipative systems are based on the interaction of a Bose-Einstein condensate with an optical standing wave potential to produce a current of atoms in momentum space. In this paper we provide a review of the important features of these ratchets with a particular emphasis on their optimization using more complex initial states. We also examine their stability close to resonance conditions of the kicking. Finally we discuss the way in which these ratchets may pave the way for applications in quantum (random) walks and matter-wave interferometry.

quant-ph↗

arXiv2016

Composite device for interfacing an array of atoms with a single nanophotonic cavity mode

We propose a method of trapping atoms in arrays near to the surface of a composite nanophotonic device with optimal coupling to a single cavity mode. The device, comprised of a nanofiber mounted on a grating, allows the formation of periodic optical trapping potentials near to the nanofiber surface along with a high cooperativity nanofiber cavity. We model the device analytically and find good agreement with numerical simulations. We numerically demonstrate that for an experimentally realistic device, an array of traps can be formed whose centers coincide with the antinodes of a single cavity mode, guaranteeing optimal coupling to the cavity. Additionally, we simulate a trap suitable for a single atom within 100 nm of the fiber surface, potentially allowing larger coupling to the nanofiber than found using typical guided mode trapping techniques.

quant-ph↗

arXiv2014

Cavity QED on a nanofiber using a composite photonic crystal cavity

We demonstrate cavity QED conditions in the Purcell regime for single quantum emitters on the surface of an optical nanofiber. The cavity is formed by combining an optical nanofiber and a nanofabricated grating to create a composite photonic crystal cavity. Using this technique, significant enhancement of the spontaneous emission rate into the nanofiber guided modes is observed for single quantum dots. Our results pave the way for enhanced on-fiber light-matter interfaces with clear applications to quantum networks.

physics.optics↗

arXiv2014

Control of spin current in a Bose gas by bang-bang pulses

We generate spin currents in an $^{87}$Rb spin-2 Bose-Einstein condensate by application of a magnetic field gradient. The spin current destroys the spin polarization, leading to a sudden onset of two-body collisions. In addition, the spin coherence, as measured by the fringe contrast using Ramsey interferometry, is reduced drastically but experiences a weak revival due to in-trap oscillations. The spin current can be controlled using periodic $π$ pulses (bang-bang control), producing longer spin coherence times. Our results show that spin coherence can be maintained even in the presence of spin currents, with applications to quantum sensing in noisy environments.

cond-mat.quant-gas↗

arXiv2013

Spin-Echo-Based Magnetometry with Spinor Bose-Einstein Condensates

We demonstrate detection of a weak alternate-current magnetic field by application of the spin echo technique to F = 2 Bose-Einstein condensates. A magnetic field sensitivity of 12 pT/Hz^1/2 is attained with the atom number of 5*10^3 at spatial resolution of 99 μm^2. Our observations indicate magnetic field fluctuations synchronous with the power supply line frequency. We show that this noise is greatly suppressed by application of a reverse phase magnetic field. Our technique is useful in order to create a stable magnetic field environment, which is an important requirement for atomic experiments which require a weak bias magnetic field.

physics.atom-ph↗