Session K74: Semiconductor Qubits: Novel Spin Qubit Materials and Technologies II

Deterministic Laser-Writing of Spin Defects in Nanophotonic Cavities

Robust engineering and characterization of spin-defect coupling to nanophotonic cavities remains a challenge in developing scalable quantum network nodes. Here we demonstrate direct laser writing of cavity-integrated spin defects using a nanosecond-pulsed above-bandgap laser. Photonic crystal cavities in 4H-silicon carbide serve as a nanoscope monitoring silicon monovacancy (V_Si^-) defect formation within the 100 nm³ cavity mode volume. We observe spin resonance, cavity-integrated photoluminescence, and excited-state lifetimes consistent with conventional defect formation methods, without need for post-irradiation thermal annealing. We further find an exponential reduction in excited state lifetime at fluences approaching the cavity amorphization threshold, and show single-shot laser annealing of intrinsic background defects at V_Si^- formation sites.   

Fabrication of Nanophotonic Structures in Silicon Carbide with Integrated Defects and an Efficient Taper-Fiber Interface

Defect centers in silicon carbide (SiC) have high potential for quantum information applications. Silicon vacancies (V_Si) in SiC show highly coherent spin-optical properties [1] up to temperatures of 20K [2], even after nanophotonic integration [3], [4]. It was also shown that triangular nanophotonics can efficiently guide the emission of these defects, while providing cavity designs that are resilient to fabrication errors [5].

 

To boost the overall emission of V_Si centers and increase the Debye-Waller factor (resonant emission), photonic crystal cavities (PCC) are highly desirable. Towards this goal, we show direct waveguide-to-fibre coupling [6], achieving near-deterministic efficiencies. We also model the observation of reduced efficiencies, which gives guidelines for improved tapered fiber generation. With this technology, we show an efficient room-temperature single-photon source based on a fiber-coupled V_Si center. Our latest steps towards low-temperature integration, as well as fabrication of PCCs will be discussed, too.

 

[1] R. Nagy et al., Nat. Commun. 10, 1954 (2019).

[2] P. Udvarhelyi et al., Phys. Rev. Appl. 13, 054017 (2020).

[3] C. Babin et al., Nat. Mater. 21, 67 (2022).

[4] D.M. Lukin et al., arXiv.2202.04845 (2022).

[5] S. Majety et al., J. Phys. Photonics 3, 034008 (2021).

[6] M. J. Burek et al., Phys. Rev. Appl. 8, 024026 (2017).   

Towards quantum networks based on the VSi defect in Silicon-Carbide

Large-scale quantum networks based on solid-state defects require both good spin and optical coherence times while simultaneously satisfying a high emission into the desired optical mode to generate entanglement^[1,2]. Recently the V_Si defect in silicon-carbide has gained significant interest because of both theoretical and experimental advancements that demonstrate promising results in this context, even when integrated into nano-photonic surfaces^[3,4]. Besides, silicon-carbide benefits from a large industrial backbone in terms of e.g., its wafer scale availability and research on surface termination, which makes it a promising material for future scalability.

Here we present our recent progress to use V_Si defects in SiC for future quantum networks. We discuss our work on nanophotonic structures to improve the collection efficiency and boost the emission of light into the desired optical mode for remote entanglement generation. In addition, we present our progress in understanding and utilizing multi-qubit registers based on spins in SiC for quantum networks and sensing applications.

[1] Pompili et al. Science 372, 6539, 359-264 (2021).

[2] Hermans et al. Nature 604, 663-668 (2022).

[3] Babin et al. Nature Materials 21, 67-73 (2022).

[4] Lukin et al. Nature Photonics 14, 330-334 (2020).

    

Controlling optical properties of the near-surface silicon vacancies in nanostructured silicon carbide via surface passivation

Silicon carbide (SiC) hosts a number of spin-active color centers (deep level defects), such as the negatively charged silicon vacancy, which are important for use in quantum applications. In their role as qubit-candidates in different applications, these defects are inevitably placed in a nanostructured host. This is mostly done for enhancing the signal from the defects. A recent work showed how finite size and surface effects modify not only the frequencies of the quantum emission from the color centers, but also adversely affect a defect’s photo-stability [PRX QUANTUM 3, 020325 (2022)].  Using density functional theory (DFT)-based calculations, we have explored chemical means of mitigating these detrimental effects of surface states in the as-created SiC nanostructures. We show that surface passivation with hydrogen and/or mixed hydrogen and hydroxyl groups can effectively remove surface states from SiC’s bandgap, thereby reducing their hybridization with the defect states of the near-surface defects. This can also effectively reduce/remove the observed blinking and charge-state conversion of these defects from the bright negatively charged to the neutral dark state.   

Magnetoelastic Control of Semiconductor Spins

Defect spins in semiconductors show great potential for quantum information processing and quantum sensing, offering long spin coherence times and single-spin sensitivities. However, current approaches to manipulating these spins can significantly perturb the local environment and often require high-power microwave sources, presenting a significant challenge for both sensing and scalable information processing applications. Building on previous demonstrations of acoustically-driven spin manipulation [1], we demonstrate efficient control of individual NV centers using magnetoelastic films coupled to Surface Acoustic Wave (SAW) devices. We explore experimental design considerations to mitigate deleterious effects of having a magnetic film in close proximity to defect spin sensors and characterize spatial heterogeneity in the films using a scanning-tip approach. 

[1] D. Labanowski, Vi. P. Bhallamudi, Q. Guo, C. M. Purser, B. A.. McCullian, P. C. Hammel, and S. Salahuddin. Voltage-driven, local, and efficient excitation of nitrogen-vacancy centers in diamond. Science Advances, 4(9), 2018   

Delta-Doped GeV- in Diamond

Owing to its strong optical transition and inversion symmetry, GeV^- centers are an alluring system for engineering many-body physics. However, realizing center-to-center interactions has been a challenge due to the dilute stochastic formation of color centers. In this work, we demonstrate the creation of GeV^- in diamond via in-situ depth localized doping (δ-doping) and discuss ways to overcome this interaction limitation. We report in-situ Germanium incorporation of 0.5 ppm and confinement to a sub-20nm layer. Additionally, we will describe the diamond growth, dopant incorporation, and present the optical characterization.

    

Indistinguishable Single Photon Emission from Single Erbium Ions

Individually addressed erbium ions in solid-state hosts are promising for application in long-distance quantum repeater networks because of their optical transition at 1.5 μm. However, their spin and optical coherence can be limited by noise from the solid-state environment. In this work we introduce a new host for single Er ions, CaWO4, that combines a non-polar site symmetry and low magnetic noise environment to significantly improve the optical and spin coherence properties. Using a high-Q, low mode-volume nanophotonic crystal cavity we isolate single Er ions and efficiently collect their emitted photons. We demonstrate indistinguishable photon emission, as well as significant improved spin coherence. This represents a significant step towards the development of quantum repeater networks based on individually addressed erbium ions.   

Automatic characterization of random nuclear spin baths in semiconductors

Spin-defects in semiconductors, such as nitrogen-vacancy centers in diamond or divacancies in silicon carbide, are promising candidates for qubits, due to their optical controllability and relatively long coherence times. In most semiconductors the interaction of the central defect spin of the electron with isotopic nuclear spins is an important cause of decoherence. At present there are no techniques to characterize the local random nuclear spin baths automatically and efficiently in materials containing spin-defects, since direct experimental characterization is too time-consuming and labor-intensive for many samples [1]. However, using spin Hamiltonians with parameters derived from first principle calculations, one can efficiently simulate dynamical decoupling experiments for a given bath spin configuration [2]. In this way, one can augment, with simulated data, shorter dynamical decoupling experiments. We present numerical and data-driven methods to extract hyperfine components from short dynamical decoupling experiments, paving the way for automatic high-throughput characterization of defects in semiconductors and for their eventual manipulation to build nuclear memories.

 

[1] Npj Quantum Inf. 7(1), 1-9 (2021).

[2] Adv. Theory Simul. 2100253 (2021).   

Atomic-Scale Visualization and Manipulation of Nitrogen-Vacancy Centers in Nanodiamonds

Nitrogen vacancy (NV) centers in diamond have emerged as a leading candidate for qubit applications due to their millisecond spin coherence times and optical addressability. While previous work has primarily focused on NVs in bulk diamond, a new direction involves integration of NV charge centers in nanometer-scale diamond particles, termed nanodiamonds. NV centers embedded in nanodiamonds are highly photostable, and the nanoparticles themselves are highly mobile, providing a novel platform for quantum information and imaging applications. Despite intense research efforts, however, an atomic scale picture of the electronic states in nanodiamond NV centers has remained elusive. In our work, we first show the existence and distribution of NV centers in individual nanodiamonds using tip-enhanced Raman spectroscopy and photoluminescence. We then use scanning tunneling microscopy and spectroscopy to visualize the local electronic density-of-states (DOS) of NV centers in nanodiamonds on an atomic scale. We further probe changes in the defects' atomic-scale DOS under 532 nm laser illumination, thus providing valuable insights for future quantum information applications of NV centers in nanodiamonds.   

Representation learning for identifying spin-spin interactions with reconstructive latent embeddings

We present an analysis model with nueral network for examining spin-spin interactions in diamond. With representation learning of dynamical decoupling signals induced from spin-spin interactions, two cases that could not been hitherto dealt with are addressed here; (1) overlapped signals of nuclear spins with similar periods. (2) split signals induced by nuclear-nuclear interaction. We train classification model with contrastive-center loss and regression model with reconstructive embedding learning especially identifying undistinguishable signals that cannot be evaluated by traditional regression approaches. Experimentally, we measure Carr-Purcell-Meiboom-Gill(CPMG) signal with the total evolution time of only less than 5 µs and with various numbers of unit π pulses controlling interacting time. Our method successfully recognizes the existence of nuclear-nuclear interaction and the undistinguishable overlapped signals up to 92% accuracy and estimates hyperfine interaction parameters up to 94% accuracy. We also distribute fully automated python modules for analyzing CPMG signals with various external magnetic field to obtain individual spin-spin interaction strengths.   

Magnetic evanescent-wave Johnson noise from BCS superconductors

There is evanescent-wave Johnson noise in the vicinity of metallic elements in qubit devices that contributes to qubit decoherence. This noise depends on the dielectric function of the metal, so it changes rather abruptly as the metal becomes superconducting.  We use the BCS form for the wave-vector- and frequency-dependent response function to compute the magnetic noise near the surface of a superconducting half space.  The calculations take the non-locality of the response fully into account.  We present T₁ for a spin qubit, finding a non-monotonic dependence of T₁ on temperature. This effect depends on the impurity density of the superconductor. We also present T₁ as a function of distance from the surface and as a function of qubit frequency. The results suggest that a spin qubit such as an NV center could be used as a probe of the properties of superconducting devices. Additionally, they describe the conditions when superconducting elements can either enhance or mitigate qubit decoherence.

    

Spin Acoustic Control and Metrology with Silicon Vacancies in Silicon Carbide

We demonstrate direct, acoustically mediated spin control of naturally occurring negatively charged silicon monovacancies (V− Si) in a high quality factor Lateral Overtone Bulk Acoustic Resonator fabricated out of high purity semi-insulating 4H-Silicon Carbide. We compare the frequency response of silicon monovacancies to a radio-frequency magnetic drive via optically-detected magnetic resonance and the resonator's own radio-frequency acoustic drive via optically-detected spin acoustic resonance and observe a narrowing of the spin transition to nearly the linewidth of the driving acoustic resonance. We show that acoustic driving can be used at room temperature to induce coherent population oscillations. Spin acoustic resonance is then leveraged to perform stress metrology of the lateral overtone bulk acoustic resonator, showing for the first time the stress distribution inside a bulk acoustic wave resonator. Our work can be applied to the characterization of high quality-factor micro-electro-mechanical systems and has the potential to be extended to a mechanically addressable quantum memory.   

Probing fractional quasiparticle mutual exchange statistics in quantum simulators

Recent advances in programmable quantum devices brought to the fore the possibility of using them to realise and investigate topological quantum spin liquid phases. This new exciting direction brings about important research questions on how to probe and determine the presence of such exotic, massively entangled phases. One of the most promising tools is investigating the behaviour of its topological excitations, and in particular their fractional statistics. In this work we put forward a generic route to achieve this, and we illustrate it in the specific case of a $mathbb{Z}_2$ topological spin liquids implemented with the aid of combinatorial gauge symmetry. We design a convenient architecture to study signatures of fractional statistics via quasiparticle interferometry, and we assess its robustness to diagonal and off-diagonal disorder, as well as to dephasing -- effects that are generally pervasive in noisy quantum programmable devices. A useful counterpart of our scheme is that it provides a clear test of the `quantumness' of these devices, since the signatures that we are looking for crucially hinge on quantum coherence and quantum interference effects in the system.