arXiv subjects

Richard J. Warburton

Publications and source records attributed to Richard J. Warburton.

At least 19 recordsLinked to original sources

arXiv2026

High-fidelity entangled photon pairs from a quantum-dot-based single-photon source

Entangled photon pairs are a ubiquitous resource in quantum technologies, used in quantum key distribution and quantum networking as well as fundamental tests of non-locality. For scalable quantum networks, pairs that are indistinguishable in all unentangled degrees of freedom are essential, as they enable high-fidelity entanglement swapping across network nodes. To date the most-studied sources of "swappable" entangled photon pairs have been based on spontaneous parametric down-conversion (SPDC) in non-linear crystals. However, the probabilistic nature and unavoidable trade-off between brightness and unwanted multi-photon emission limits their performance in lossy channels. Here, we demonstrate a high-fidelity source of "swappable" entangled photon pairs using a semiconductor quantum dot (QD) coupled to a tunable microcavity. By actively modulating the QD emission between orthogonal polarisation states, delaying one path in a low-loss Herriott cell, and recombining the two on a balanced beam splitter, we generate entangled photon pairs with a fidelity of $96.1\pm0.5$ %. We identify and mitigate fidelity-limiting factors, achieving a maximum fidelity of $98.1\pm0.5$ % through time-resolved post-selection. The scheme suppresses residual multi-photon events concentrated near the excitation pulse and has only a modest impact on the rate. Furthermore, the photons are mutually indistinguishable, enabling efficient entanglement swapping. Our results establish semiconductor QDs as a viable platform for quantum network-compatible swappable entangled photon pair generation, with feasible entanglement generation rates exceeding 0.5 Gpairs/s.

quant-ph↗

arXiv2026

High-quality single photons from cavity-enhanced biexciton-to-exciton transition

Resonant laser excitation of a two-level system with subsequent single-photon emission can be used to generate single photons with high indistinguishability or Hong-Ou-Mandel (HOM) visibility. However, spectral overlap between excitation laser and emitted photons generally poses significant challenges. Furthermore, emitter re-excitation intrinsically limits achievable single-photon purity. Established solutions mitigate these issues at significant cost to source efficiency and with increased source complexity. This motivates the use of few-level systems with spectral separation of excitation and emission pathways. One option is a three-level cascade. However, without targeted lifetime engineering of emitting states, the cascade naturally limits achievable photon indistinguishability. Here we study a semiconductor quantum dot with resonant and selective cavity-enhancement of biexciton-to-exciton transition. Following resonant two-photon excitation of the biexciton state, we collect the emitted single photon with the cavity. This approach circumvents emitter re-excitation and naturally introduces spectral separation of excitation laser and emitted single photon. Supported by first experimental results, we demonstrate theoretically that with selective Purcell enhancement, the observed quality quantifiers of single-photon emission (purity, equivalently $g^{(2)}(0)$, and HOM visibility $\mathcal{V}$, equivalently indistinguishability) are competitive with respect to high-quality deterministic quantum-dot single-photon sources. This is already achieved without systematic optimization or targeted system engineering, which firmly places the reported approach as a viable route to the next generation of highest-quality quantum-dot based deterministic single-photon sources.

quant-ph↗

arXiv2026

Four-Photon Interference with a High-Efficiency Quantum Dot Source

While two-photon Hong-Ou-Mandel interference visibility has become a standard metric for single-photon sources, many optical quantum technologies require the generation and manipulation of larger photonic states. To date, efficiency limitations have prevented scaling quantum dot-based interference to the coalescence of more than two photons at a single beamsplitter. We overcome this limitation by combining a state-of-the-art quantum dot source with deterministic demultiplexing, enabling the direct observation of quantum interference fringes arising from up to four photons. We measure high mean interference contrasts of $93.0 \pm 0.1~\%$ for two photons, and $84.1 \pm 1.0~\%$ for four photons, with the complex fringe structure fully reproduced by a theoretical model. These results reveal the existence of "deep fringes" whose minima are unaffected by distinguishable photons, rendering the maximum contrast of four-photon interference highly sensitive to multi-photon emission but robust against photon distinguishability. We predict that these phenomena will extend to interference of larger numbers of photons, with relevance across a range of potential optical quantum technologies. A Fisher information analysis demonstrates that interference fringes from our source can exhibit phase sensitivity beyond the standard quantum limit, illustrating potential applications in quantum metrology.

quant-ph↗

arXiv2025

In-situ control of hole-spin driving mechanisms

Hole-spin qubits enable fast, all-electrical spin manipulation through electric-dipole spin resonance (EDSR), arising from two microscopic mechanisms rooted in their intrinsically strong spin-orbit interaction. Depending on how the electric field acts on the quantum dot, the spin can be driven either by a modulation of its g-factor or by a displacement of the wavefunction. Here, we demonstrate in-situ control over the dominant EDSR driving mechanism of a hole-spin qubit in a silicon fin field-effect transistor by applying microwave signals to two different gate electrodes, thereby tuning the orientation of the local electric field. We measure the effective g-factor, its electrical tunability, and the Rabi frequency as functions of magnetic-field orientation. Their distinct angular dependencies, analyzed using a g-matrix formalism, allow us to identify the underlying driving processes and track their relative contributions for different drive configurations. By selecting the drive electrode, we can switch from a regime dominated by g-factor modulation to one with a strong contribution from wavefunction displacement. This in-situ tunability provides direct experimental access to both spin-driving mechanisms and offers a route toward optimized spin-qubit performance.

cond-mat.mes-hall↗

arXiv2025

Indistinguishable photons from a two-photon cascade

Decay of a four-level diamond scheme via a cascade is a potential source of entangled photon pairs. A solid-state implementation is the biexciton cascade in a semiconductor quantum dot. While high entanglement fidelities have been demonstrated, the two photons, XX and X, are temporally correlated, typically resulting in poor photon coherence. Here, we demonstrate a high two-photon interference visibility (a measure of the photon coherence) for both XX (V=94$\pm$2%) and X (V=82$\pm$6%) photons. This is achieved by Purcell-enhancing the biexciton transition in a low-noise device. We find that the photon coherence follows the well-known quantum optics result upon tuning the XX:X lifetime ratio over two orders of magnitude.

quant-ph↗

arXiv2025

A quantum-coherent photon--emitter interface in the original telecom band

Quantum dots stand out as the most advanced and versatile light-matter interface available today. Their ability to deliver high-quality, high-rate, and pure photons has set benchmarks that far surpass other emitters. Yet, a critical frontier has remained elusive: achieving these exceptional capabilities at telecom wavelengths, bridging the gap to fiber-optic infrastructure and scalable silicon photonics. Overcoming this challenge demands high quality quantum materials and devices which, despite extensive efforts, have not been realized yet. Here, we demonstrate waveguide-integrated quantum dots and realize a fully quantum-coherent photon-emitter interface operating in the original telecommunication band. The quality is assessed by recording transform-limited linewidths only 8 % broader than the inverse lifetime and bright 41.7 MHz emission rate under 80 MHz $π$-pulse excitation, unlocking the full potential of quantum dots for scalable quantum networks.

physics.optics↗

arXiv2025

Optical microcavity characterization via resonance spectra and modes

This paper describes how resonance spectra and mode profiles can be used to characterize and quantify the mode-shaping effects in open-access plano-concave optical microcavities. The presented semi-analytic theory is based on the application of perturbation theory to the roundtrip evolution of the optical field. It includes various mirror-shape and nonparaxial effects and extends the nonparaxial theory presented by van Exter et al. (2022, Phys. Rev. A 106, 013501) and verified by Koks et al. (2022, Phys. Rev. A 105, 063502) to the common case of an anisotropic Gaussian mirror. The presented measurements and analyses of resonance spectra and mode profiles demonstrate how the different mode-shaping effects can be individually distinguished and quantified. Spin-orbit coupling, which is one of the nonparaxial effects, is prominently visible in the intriguing polarization patterns of the resonant modes, while polarization tomography yields the shape-induced birefringence and associated polarization splitting of the fundamental modes.

physics.optics↗

arXiv2025

Homogeneous Free-Standing Nanostructures from Bulk Diamond over Millimeter Scales for Quantum Technologies

Quantum devices based on optically addressable spin qubits in diamond are promising platforms for quantum technologies such as quantum sensing and communication. Nano- and microstructuring of the diamond crystal is essential to enhance device performance, yet fabrication remains challenging and often involves trade-offs in surface quality, aspect ratio, device size, and uniformity. We tackle this hurdle with an approach producing millimeter-scale, thin (down to 70 nm) and highly parallel (< 0.35 nm/$\mathrm{μm}$}) membranes from single-crystal diamond. The membranes remain contamination-free and possess atomically smooth surfaces ($\mathrm{R_q}$ < 200 pm) as required by state-of-the-art quantum applications. We demonstrate the benefits and versatility of our method by fabricating large fields of free-standing and homogeneous photonic nano- and microstructures. Leveraging a refined photolithography-based strategy, our method offers enhanced scalability and produces robust structures suitable for direct use, while remaining compatible with heterogeneous integration through pick-and-place transfer techniques.

quant-ph↗

arXiv2024

Realisation of a Coherent and Efficient One-Dimensional Atom

A quantum emitter interacting with photons in a single optical-mode constitutes a one-dimensional atom. A coherent and efficiently coupled one-dimensional atom provides a large nonlinearity, enabling photonic quantum gates. Achieving a high coupling efficiency ($β$-factor) and low dephasing is challenging. Here, we use a semiconductor quantum dot in an open microcavity as an implementation of a one-dimensional atom. With a weak laser input, we achieve an extinction of $99.2\%$ in transmission and a concomitant bunching in the photon statistics of $g^{(2)}(0) = 587$, showcasing the reflection of the single-photon component and the transmission of the multi-photon components of the coherent input. The tunable nature of the microcavity allows $β$ to be adjusted and gives control over the photon statistics -- from strong bunching to anti-bunching -- and the phase of the transmitted photons. We obtain excellent agreement between experiment and theory by going beyond the single-mode Jaynes-Cummings model. Our results pave the way towards the creation of exotic photonic states and two-photon phase gates.

quant-ph↗

arXiv2024

All-electrical operation of a spin qubit coupled to a high-Q resonator

Building a practical quantum processor involves integrating millions of physical qubits along with the necessary components for individual qubit manipulation and readout. Arrays of gated silicon spins offer a promising route toward achieving this goal. Optimized radio frequency resonators with high internal quality factor are based on superconducting inductors and enable fast spin readout. All-electrical spin control and gate-dispersive readout remove the need for additional device components and simplify scaling. However, superconducting high-Q tank circuits are susceptible to crosstalk induced ringup from electrical qubit control pulses, which causes fluctuations of the quantum dot potential and is suspected to degrade qubit performance. Here, we report on the coherent and all-electrical control of a hole spin qubit at 1.5K, integrated into a silicon fin field-effect transistor and connected to a niobium nitride nanowire inductor gate-sensor. Our experiments show that qubit control pulses with their broad range of higher harmonics ring up the tank when the control pulse spectrum overlaps with the tank resonance. This can cause a reduction of the readout visibility if the tank ringing amplitude exceeds the excited state splitting of the quantum dot, lifting Pauli spin blockade and thus leading to state preparation and measurement errors. We demonstrate how to circumvent these effects by engineering control pulses around the tank resonances. Importantly, we find that the ringup does not limit the spin coherence time, indicating that efficient high-Q resonators in gate-sensing are compatible with all-electrical spin control.

cond-mat.mes-hall↗

arXiv2024

Cavity-assisted resonance fluorescence from a nitrogen-vacancy center in diamond

The nitrogen-vacancy center in diamond, owing to its optically addressable and long-lived electronic spin, is an attractive resource for the generation of remote entangled states. However, the center's low native fraction of coherent photon emission, $\sim$3\%, strongly reduces the achievable spin-photon entanglement rates. Here, we couple a nitrogen-vacancy center with a narrow extrinsically broadened linewidth (\unit[159]{MHz}), hosted in a micron-thin membrane, to the mode of an open optical microcavity. The resulting Purcell factor of $\sim$1.8 increases the fraction of zero-phonon line photons to above 44\%, leading to coherent photon emission rates exceeding four times the state of the art under non-resonant excitation. Bolstered by the enhancement provided by the cavity, we for the first time measure resonance fluorescence without any temporal filtering with $>$10 signal-to-laser background ratio. Our microcavity platform would increase spin-spin entanglement success probabilities by more than an order of magnitude compared to existing implementations. Selective enhancement of the center's zero-phonon transitions could furthermore unlock efficient application of quantum optics techniques such as wave-packet shaping or all-optical spin manipulation.

quant-ph↗

arXiv2023

A single-photon emitter coupled to a phononic-crystal resonator in the resolved-sideband regime

A promising route towards the heralded creation and annihilation of single-phonons is to couple a single-photon emitter to a mechanical resonator. The challenge lies in reaching the resolved-sideband regime with a large coupling rate and a high mechanical quality factor. We achieve all of this by coupling self-assembled InAs quantum dots to a small-mode-volume phononic-crystal resonator with mechanical frequency $Ω_\mathrm{m}/2π= 1.466~\mathrm{GHz}$ and quality factor $Q_\mathrm{m} = 2.1\times10^3$. Thanks to the high coupling rate of $g_\mathrm{ep}/2π= 2.9~\mathrm{MHz}$, and by exploiting a matching condition between the effective Rabi and mechanical frequencies, we are able to observe the interaction between the two systems. Our results represent a major step towards quantum control of the mechanical resonator via a single-photon emitter.

cond-mat.mes-hall↗

arXiv2023

A quantum dot coupled to a suspended-beam mechanical resonator: from the unresolved- to the resolved-sideband regime

We present experiments in which self-assembled InAs quantum dots are coupled to a thin, suspended-beam GaAs resonator. The quantum dots are driven resonantly and the resonance fluorescence is detected. The narrow quantum-dot linewidths, just a factor of three larger than the transform limit, result in a high sensitivity to the mechanical motion. We show that one quantum dot couples to eight mechanical modes spanning a frequency range from $30$ to $600~\mathrm{MHz}$: one quantum dot provides an extensive characterisation of the mechanical resonator. The coupling spans the unresolved-sideband to the resolved-sideband regimes. Finally, we present the first detection of thermally-driven phonon sidebands (at $4.2~\mathrm{K}$) in the resonance-fluoresence spectrum.

cond-mat.mes-hall↗

arXiv2023

Exchange energy of the ferromagnetic electronic ground-state in a monolayer semiconductor

Mobile electrons in the semiconductor monolayer-MoS$_2$ form a ferromagnetic state at low temperature. The Fermi sea consists of two circles, one at the $K$-point, the other at the $\tilde{K}$-point, both with the same spin. Here, we present an optical experiment on gated MoS$_2$ at low electron-density in which excitons are injected with known spin and valley quantum numbers. The resulting trions are identified using a model which accounts for the injection process, the formation of antisymmetrized trion states, electron-hole scattering from one valley to the other, and recombination. The results are consistent with a complete spin polarization. From the splittings between different trion states, we measure the exchange energy, $Σ$, the energy required to flip a single spin within the ferromagnetic state, as well as the intervalley Coulomb exchange energy, $J$. We determine $Σ=11.2\,$meV and $J=5\,$meV at $n=1.5 \times 10^{12}\,$cm$^{-2}$, and find that $J$ depends strongly on the electron density, $n$.

cond-mat.str-el↗

arXiv2023

Dual epitaxial telecom spin-photon interfaces with correlated long-lived coherence

Optically active solid-state spin qubits thrive as an appealing technology for quantum interconnect and quantum networking, owing to their atomic size, scalable creation, long-lived coherence, and ability to coherently interface with flying qubits. Trivalent erbium dopants in particular emerge as a compelling candidate with their telecom C band emission and shielded 4f intra-shell spin-optical transitions. However, prevailing top-down architecture for rare-earth qubits and devices has not allowed simultaneous long optical and spin coherence necessary for long-distance quantum networks. Here we demonstrate dual erbium telecom spin-photon interfaces in an epitaxial thin-film platform via wafer-scale bottom-up synthesis. Harnessing precise controls over the matrix purity, dopant placement, and symmetry unique to this platform, we simultaneously achieve millisecond erbium spin coherence time and $<$3 kilohertz optical dephasing rate in an inversion-symmetry protected site and realize both optical and microwave control in a fiber-integrated package for rapid scaling up. These results demonstrate a significant prospect for high-quality rare-earth qubits and quantum memories assembled using a bottom-up method and pave the way for the large-scale development of quantum light-matter interfaces for telecommunication quantum networks.

quant-ph↗

arXiv2023

Enhanced Electron Spin Coherence in a GaAs Quantum Emitter

A spin-photon interface should operate with both coherent photons and a coherent spin to enable cluster-state generation and entanglement distribution. In high-quality devices, self-assembled GaAs quantum dots are near-perfect emitters of on-demand coherent photons. However, the spin rapidly decoheres via the magnetic noise arising from the host nuclei. Here, we address this drawback by implementing an all-optical nuclear-spin cooling scheme on a GaAs quantum dot. The electron-spin coherence time increases 156-fold from $T_2^*$ = 3.9 ns to 0.608 $μ$s. The cooling scheme depends on a non-collinear term in the hyperfine interaction. The results show that such a term is present even though the strain is low and no external stress is applied. Our work highlights the potential of optically-active GaAs quantum dots as fast, highly coherent spin-photon interfaces.

quant-ph↗

arXiv2023

Phase driving hole spin qubits

The spin-orbit interaction in spin qubits enables spin-flip transitions, resulting in Rabi oscillations when an external microwave field is resonant with the qubit frequency. Here, we introduce an alternative driving mechanism of hole spin qubits, where a far-detuned oscillating field couples to the qubit phase. Phase driving at radio frequencies, orders of magnitude slower than the microwave qubit frequency, induces highly non-trivial spin dynamics, violating the Rabi resonance condition. By using a qubit integrated in a silicon fin field-effect transistor (Si FinFET), we demonstrate a controllable suppression of resonant Rabi oscillations, and their revivals at tunable sidebands. These sidebands enable alternative qubit control schemes using global fields and local far-detuned pulses, facilitating the design of dense large-scale qubit architectures with local qubit addressability. Phase driving also decouples Rabi oscillations from noise, an effect due to a gapped Floquet spectrum and can enable Floquet engineering high-fidelity gates in future quantum processors.

cond-mat.mes-hall↗

arXiv2023

Quantum Interference of Identical Photons from Remote GaAs Quantum Dots

Photonic quantum technology provides a viable route to quantum communication, quantum simulation, and quantum information processing. Recent progress has seen the realisation of boson sampling using 20 single-photons and quantum key distribution over hundreds of kilometres. Scaling the complexity requires architectures containing multiple photon-sources, photon-counters, and a large number of indistinguishable single photons. Semiconductor quantum dots are bright and fast sources of coherent single-photons. For applications, a significant roadblock is the poor quantum coherence upon interfering single photons created by independent quantum dots. Here, we demonstrate two-photon interference with near-unity visibility ($93.0\pm0.8$)\% using photons from two completely separate GaAs quantum dots. The experiment retains all the emission into the zero-phonon-line -- only the weak phonon-sideband is rejected -- and temporal post-selection is not employed. Exploiting the quantum interference, we demonstrate a photonic controlled-not circuit and an entanglement with fidelity ($85.0\pm 1.0$)\% between photons of different origins. The two-photon interference visibility is high enough that the entanglement fidelity is well above the classical threshold. The high mutual-coherence of the photons stems from high-quality materials, a diode-structure, and the relatively large quantum dot size. Our results establish a platform, GaAs QDs, for creating coherent single photons in a scalable way.

quant-ph↗