Session Q46: Defect Qubits III - Optical Properties, Charge Control, and Coherence

Suppression of the Optical Linewidth and Spin Decoherence in Diode-Embedded Spin Centers: Leveraging Advanced Optimization Algorithms

Reducing noise experienced by spin centers constitutes one of the main goals of solid-state-based quantum technologies. Embedding spin centers in p-n diodes under reverse bias voltage has been proven to be a suitable technique to reduce charge noise while controlling the wavelength emission [1,2]. Given the multiple combinations of manufacturing parameters that a diode can have (e.g.: size, doping densities, temperature, bias voltage), a question that has not been answered yet is what set of diode design parameters yields the lowest possible electric noise. In this work, we address this question by developing a scaled-gradient-descent-based algorithm that minimizes the optical linewidth of spin centers. By solving the diode’s Poisson equation, we calculate the noise level that arises from the non-depleted regions using the formalism developed in Candido and Flatté [2]. The noise minimization is subject to physical constraints such as dielectric breakdown, doping densities, temperature, diode’s length, and leakage current.

[1] C. P. Anderson, A. Bourassa, K. C. Miao, G. Wolfowicz, P. J. Mintun, A. L. Crook, H. Abe, J. Ul Hassan, N. T. Son, T. Ohshima, and D. D. Awschalom, Science 366, 1225 (2019).

[2] Denis R. Candido and Michael E. Flatté, PRX Quantum 2, 040310 (2021)   

First-principles investigation of near surface divacancies in 3C-SiC

The realization of quantum sensors using spin defects in semiconductors requires a thorough understanding of the physical properties of the defects in proximity of surfaces. Here we focus on silicon carbide (SiC), a promising material for quantum applications. We report a first-principles study of the divacancy (V_SiV_C) in 3C-SiC as a function of surface reconstruction and termination with -H, -OH, -F and oxygen groups. We show that a divacancy close to hydrogen-terminated (2×1) surfaces is a robust spin-defect with a triplet ground state, no surface states in the band gap and with small variations of many of its physical properties relative to the bulk, including the zero-phonon line and zero-field splitting. However, the Debye-Waller factor decreases in the vicinity of the surface and our calculations indicate it may be improved by strain-engineering. Overall our results show that the divacancy close to SiC surfaces is a promising spin defect for quantum applications, similar to its bulk counterpart.   

Defect-based quantum emitters in aluminum nitride-passivated silicon carbide

Silicon carbide (SiC) hosts several defect-based single photon emitters, making it particularly useful for applications in quantum technology. In order to enhance the signal coming from these quantum emitters, the SiC host is often nanostructured. This can result in the proximity of quantum emitters to the surfaces of SiC. In a recent work [1], we showed that a near-surface defect in SiC can lose its photostability due to surface effects.  In principle, this issue can be remedied by uniform passivation of the dangling bonds on the surface with a suitable adsorbate, such as hydrogen or the mixed hydrogen/hydroxyl groups. However, these passivation schemes may not be reliable in the long-term due to their limited chemical and/or thermal stability [2]. In this first principles study, we employ an AlN/SiC core shell nanowire to study how passivation with an AlN layer affects different properties of SiC defects. Interestingly, we find that our proof-of-principle defect, the negatively charged silicon vacancy, has very different properties when created at the interface versus when created in the SiC core.

 

References:

 

[1] T. Joshi and P. Dev, “Site-Dependent Properties of Quantum Emitters in Nanostructured Silicon Carbide,” PRX Quantum 3, 020325 (2022)

 

[2] M. J. Polking, A. M. Dibos, N. P. de Leon, and H. Park, “Improving Defect-Based Quantum Emitters in Silicon Carbide via Inorganic Passivation,” Adv. Mater. 30, 1704543 (2018).

 

    

Rotation of the center-of-mass of single fluorescent defects in silicon

The boom of silicon in semiconductor technologies was closely tied to the ability to control its density of lattice defects. After being regarded as detrimental to the crystal quality in the first half of the 20th century, point defects have become an essential tool to tune the electrical properties of this semiconductor, leading to the development of a flourishing silicon industry. At the turn of the 21^st century, progress in Si-fabrication and implantation processes has triggered a radical change by enabling the control of these defects at the single level. This paradigm shift has brought silicon into the quantum age, where individual dopants are nowadays used as robust electrical quantum bits to encode and process quantum information. Fluorescent defects recently isolated at single-defect scale in silicon could follow suit.

Among the many color centers isolated in silicon over the past three years, the G center has attracted particular attention because it combines telecom single-photon emission with a metastable spin triplet that could potentially be used as a qubit. As demonstrated in recent theoretical studies, this defect is said to present a remarkable microscopic configuration associated with a rotational dynamics. However, the rotation of the G center has never been demonstrated on the scale of a single defect. In this talk, we will investigate individual G centers in silicon through low-temperature photoluminescence measurements. We will reveal a fine structure in their photoluminescence spectra and multipolar emission that are compatible with a model of center-of-mass rotation of G centers during optical excitation. Using symmetry point group theory, we will model the rotational states of the center G under external perturbations to identify the dominant interaction producing the observed fine structures. These results highlight the rich physics of this quantum system and open the way to exploring its rotational quantum degree of freedom.   

Voltage-controlled switching of the charge state of Si-vacancy defects and N-vacancy defects in diamond

We report a voltage-controlled mechanism by which the photoluminescent (PL) emission from silicon-vacancy (SiV) defects and nitrogen-vacancy (NV) defects in diamond can be modulated. These voltage-induced changes in charge states are probed by their photoluminescence spectral analysis. In particular, we can selectively produce emission from the negatively charged state of the silicon-vacancy defect (i.e., SiV^-), which exhibits narrow (Γ = 4 nm) emission at 738 nm. This approach uses high voltage (2–5 kV) nanosecond pulses applied across top and bottom electrodes on a 0.5 mm thick diamond substrate. In the absence of high voltage pulses, we observe no emission at 738 nm. This feature increases monotonically with peak pulse voltage, pulse repetition rate (i.e., frequency), and incident laser intensity. We observe saturation of the PL intensity for pulse voltages above 3.2 kV and frequency above 100 Hz. The high-voltage pulses also enable manipulating the charge states of NV defects efficiently. Based on electrostatic simulations, we estimated the local electric field intensity near the tip of the Cu electrode to be 2.8 x 10⁶ V/cm at these voltages. However, as a function of laser power, we observe a linear dependence of PL intensity without saturation. These saturating and non-saturating behaviors provide important insight into the voltage-induced charging mechanisms and kinetics associated with this process.

 

Weng, et al., Applied Physics Letters, 119, 171101 (2021). https://doi.org/10.1063/5.0066537

 

Pambukhchyan, et al., Materials for Quantum Technology. Volume 3, page 035005 (2023). https://iopscience.iop.org/article/10.1088/2633-4356/acf750   

Scanning Tunneling Microscopy and Spectroscopy of Diamond with Nitrogen-Vacancy Centers

Spin qubits using nitrogen-vacancy (NV) centers in diamond are leading candidates for quantum information applications as a result of their millisecond spin coherence times, optical addressability, and room-temperature operation. While very extensive work has been undertaken in characterizing and applying NV centers, an atomic-scale picture of the defects' electronic states has remained elusive. Scanning tunneling microscopy (STM) would typically be an ideal technique for such atomic-scale visualization. However, thus far, STM has been unable to probe these defects due to the highly insulating nature of the diamond host crystal. In this work, we pursue a novel approach to visualizing NVs using STM. We image the atomic-scale topographic features of our samples, identify unique subsurface defects, and characterize their electronic states using scanning tunneling spectroscopy. Finally, we test the manipulability of these defects in our in-situ STM set-up using laser illumination and the STM tip.   

Near-unity charge state initialization of NV centers in diamond using photo-activation of phosphorus dopants

Nitrogen vacancy (NV) centers in diamond are widely explored as quantum sensors and qubits because of their long coherence time, optical spin readout, and optical spin initialization in ambient conditions. However, the NV center suffers from imperfect charge state initialization, leading to state preparation and measurement errors around 30%. This poor charge state initialization arises from ionization and recombination under green excitation, leading to continuous charge cycling. A natural approach to circumvent this problem is to avoid direct excitation of the NV center and instead utilize itinerant carrier capture to prepare the charge state. Here we demonstrate that selective ionization of implanted phosphorous (P) donors using near-infrared excitation can enhance NV center charge state initialization to near-unity fidelity, with no decay in the charge population detectable out to 1.5 seconds. This initialization is effective even for NV centers that are located over 10 μm away from the P bath, making the contribution of P electronic spins to NV center decoherence negligible. Our work shows that the large difference between electronic and magnetic interaction length scales in high purity diamond can be exploited to produce tailored materials for quantum technologies.   

Photoionization spectroscopy of the long-lived 1E singlet state of NV centers in diamond

The nitrogen-vacancy (NV) center in diamond is valued for its accessible quantum properties and for quantum sensing applications. The NV’s basic energy level diagram and optical cycle explain the NV’s key characteristics and are well-understood. For applications, there are significant efforts to improve optical and electrical readout by controlling ionization and recombination between NV^- and NV⁰ charge states. In this work, we use photoionization spectroscopy to determine the ionization energy of the long-lived ¹E singlet state, the final energy level in the optical cycle yet to be determined.  In our measurements, we modulate the populations of the optical cycle states by applying and removing a 100 mT magnetic field, while observing the resulting amplitude changes in the NV⁰ and NV ^- photoluminescence spectra.  The extensive measurements cover a region of parameter space spanning 488 nm to 560 nm in excitation wavelength, 1.5 µW to 40 µW in excitation power, and 1.6 K to 295 K in temperature.  Ionization from ¹E indicated by field-induced increases in NV⁰ photoluminescence is observed for excitation wavelengths 532 nm and below but not 550 nm and above.  From this data, we conclude that the ionization threshold from the singlet is between 2.25 eV and 2.33 eV, in agreement with theoretical predictions based on singlet-triplet transition rates. Subtle narrowing of the NV⁰ zero-phonon line with the applied field is consistent with a reduction in charged defects.  At the same time, a strong, zero-field, spin-mixing phenomenon between 10 K and 100 K is revealed.   

Ab-initio theory of nuclear spin flip processes within NV center of diamond

SDS (zero-field), (SAI) hyperfine and (IQI) quadrupolar tensors of NV (nitrogen vacancy) are both extensively measured and theoretically modeled before already[1,2]. All of these tensors depict the interactions between electronic (S) and nuclear (I) spin. ¹⁴N nuclei spins are usually treated devoid from relaxation processes during timeframes of optical cycles. However, optical pumping into the ³E excited state offers additional ΔI=±1,±2 flipping channels via orbital coupling of the quadrupolar (Q) tensor. In conjunction with experimental work under resonant optical excitation at cryogenic conditions we theoretically model the possible spin-flip processes acting within the ³E excited state. In summary, in the present talk I will show that the strength of such processes can be predicted directly from first principles supporting the experimental observations [3].

[1] M. W. Doherty et al. Physics Reports 528.1 (2013): 1-45.

[2] J. R. Maze, et al. New Journal of Physics 13.2 (2011): 025025.

[3] R. Monge, T. Delord, G. Thiering, A. Gali, C. A. Meriles, “Resonant versus non-resonant spin readout of a nitrogen-vacancy center in diamond under cryogenic conditions”, submitted. https://cmeriles.ccny.cuny.edu/?page_id=523   

Surface spin hopping dynamics on diamond surfaces

Unpaired surface electronic spins (dark spins) on a diamond surface, arising from dangling bonds, are recognized as a major source of magnetic noise, and they limit the practical utility of NV-based quantum sensors. To either suppress these dark spins or harness them as a quantum resource, a comprehensive understanding of their dynamics is crucial. Recent experimental observations have indicated that surface dark spins can change their positions between measurements, with approximately 80% of them undergoing hopping [1].

Consequently, any static model for these surface spins is inadequate for accurately describing their dynamics. Utilizing experimentally derived density data for dark spins on a diamond surface, we present a reliable model based on multisite spin-hopping that effectively accounts for both coherent (tunneling) and incoherent (hopping) dynamics. Our study shows how the dynamics of dark spins affects the decoherence of near-surface NV center spin qubits. We also uncover how the stretched exponent of the coherence function, which characterizes the noise of the qubit environment, depends on the inter-spin distances on the surface and the depth of the NV center.

Our results provide a way to accurately describe surface spin noise and underscore the potential of spin qubits for characterizing both the static and dynamical properties of interacting surface spin baths.   

Decoupling magnetic dipolar interactions between electron spins in diamond

Synthetic diamond produced by high-pressure, high-temperature (HPHT) techniques is known to contain inhomogeneous distributions of substitutional nitrogen (P1) defects. Electron irradiation of these samples creates nitrogen-vacancy (NV) centers with a range of local P1 concentrations which determine their coherence times. Extending the coherence times of ensembles of P1 or NV spins requires decoupling both local Zeeman disorder and the magnetic dipolar interactions between spins. While the CPMG pulse sequence can simultaneously decouple disorder and dipolar interactions between dissimilar spins, it does not decouple interactions between spins of the same species. Decoupling such interactions is becoming important as systems of interacting P1 and NV centers are increasingly used for many-body physics and ensemble quantum sensing. Here we explore the performance of a series of dipolar decoupling sequences such as epsilon-CPMG and WAHUHA on both weakly and strongly interacting spins using inductively detected bulk electron spin resonance experiments. We find that for the weakly dipolar coupled NV centers, epsilon-CPMG shows enhancement of coherence for slight over and under rotations, while WAHUHA has little effect. For the strongly dipolar coupled P1 centers, epsilon-CPMG shows a small enhancement of coherence, while WAHUHA shows a dramatic increase in coherence.   

Oral:Optimizing Dynamical Decoupling Sequence Using Real-Time Noise Sensing.

Nitrogen Vacancy (NV) Centers are a promising candidate for the implementation of quantum technologies.  This is partly due to the high coherence times of the nuclear spin, even at room temperature, that can be extended with dynamical decoupling (DD). However, in an experimental setting the noise can fluctuate and to accommodate for that we can use the electron spin in NV center as a spectator qubit. This choice is motivated by the contrasting gyromagnetic ratio of the electrons corresponding to the spectator qubit, with the very low gyromagnetic ratio of nuclei corresponding to the memory qubit. The dynamics of the spectator qubits are therefore much faster than the memory qubits. By using Coherent Population Trapping (CPT), a Λ-type system trapped in the dark-state can be used to sense the noise in real-time. The noise causes a non-zero population of the excited state, leading to photon emission. The time series of the photon emissions has been used to sense the noise in real-time^[1]. We use this to obtain a probability distribution of the characteristic time of the noise. Consequently, we show that the optimal time between two DD pulses (τ) can be determined analytically. By continuously updating τ, we can actively tailor the pulse sequence in order to have a longer coherence time.   

Optical Spin Readout of Nuclear Spins Beyond the NV Electron T1

We investigate optical readout of nitrogen nuclear spin states coupled to dephased electron spins by optical re-pumping. The nitrogen nuclear spin intrinsic to each diamond nitrogen-vacancy (NV) center has been demonstrated as a basis for rotation sensing. The sensitivity is partially determined by the coherence time of the nuclear spins, which is limited by the NV center electron spin T1. To improve sensing protocols, decoupling electron spins from nuclear spins will allow nuclear spin coherence to persist past the electron T1. This introduces a challenge for future sensors as current protocols use the conditional rotation of electron spins to optically readout the nuclear spins, which cannot work if the electron spins have depolarized. To address this, we investigate the efficiency of repumping thermalized NV center electron spins without disturbing nuclear spins. We measure at fields near 500 G, where spin flip-flops between the nuclear and electron spins in the NV excited state are a significant source of nuclear spin dephasing. We find that while the spin flip-flops harm the nuclear spin fidelity, this sensitivity loss can be compensated by optical contrast enhancement between nuclear spin states caused by the same flip-flops.