Temperature has less impact on this hBN qubit’s stability

Mechanically isolated quantum emitters in hexagonal boron nitride maintain narrow optical linewidths even at room temperature, a characteristic that sets them apart from other solid-state candidates for coherent optical control. Researchers are now focusing on addressing persistent spectral instabilities that hinder their potential; these emitters frequently exhibit fluctuations and intermittency on timescales ranging from nanoseconds to seconds. The work demonstrates detailed examination of a bright, single hBN emitter within a donor-acceptor pair framework to understand its spectral stability, a step toward realizing robust spin-photon interfaces.

hBN Defect Origins: Donor-Acceptor Pair Framework

High-resolution spectroscopy revealed two zero-phonon lines originating from a single hexagonal boron nitride (hBN) emitter, demonstrating distinct spectral diffusion dynamics despite arising from the same defect. These closely spaced transitions, observed during investigation of a mechanically isolated emitter, suggest separate recombination pathways with differing sensitivities to local electrostatic fluctuations. Researchers utilized a high numerical aperture (NA = 0.9) objective within a cryostat to efficiently excite and collect emissions from the hBN defect using a confocal setup, enabling detailed examination of its spectral properties.

The observed behavior supports a donor-acceptor pair (DAP) framework for understanding the emitter’s characteristics; within this model, emission energy is acutely dependent on the separation between the donor and acceptor, as well as the surrounding electrostatic environment. This sensitivity naturally predisposes DAP transitions to spectral diffusion, a phenomenon where the emission wavelength fluctuates over time due to environmental factors.

Multiple zero-phonon lines, each responding differently to charge fluctuations, can arise from various recombination pathways within a single defect complex, a nuance the team explored through careful analysis. The study’s findings indicate that the dominant transition exhibits comparatively weak temperature dependence and moderate spectral diffusion, while a nondominant transition is strongly influenced by thermally activated fluctuations. Magnetic-field-dependent photoluminescence, optical detection of magnetic resonance (ODMR), and time-resolved pump-probe measurements further illuminated the emitter’s spin dynamics.

Combining ODMR measurements with magnetostatic simulations, the team estimated the magnetic field at the emitter location to be approximately 40±10mT, assuming a magnet-sample separation of 6±1mm. A photoluminescence spectrum measured at 77K showed a zero-phonon line at 585nm with linearly polarized emission, confirming the emitter’s brightness; a weaker emission line at 579nm, exhibiting a different polarization dipole, was attributed to a neighboring emitter and excluded from further analysis.

The DAP-like character of this emitter could prove advantageous for future coupled-emitter devices, as the associated electric dipole moment may facilitate interactions over larger distances, potentially reducing the need for precise, near-atomic defect placement.

Mechanically Isolated Emitter: Brightness and Polarization

A resonant saturation count rate exceeding expectations confirms the exceptional optical brightness of a mechanically isolated quantum emitter in hexagonal boron nitride, positioning the defect as a strong candidate for integration into quantum photonic circuits. This high photon flux, combined with a nanosecond excited-state lifetime and moderate Debye-Waller factor, distinguishes this emitter from previously reported hBN defects.

The researchers determined this secondary emission originates from a neighboring emitter, not the defect under investigation. Further spectral analysis revealed an energy gap of 2±0. 5THz between the zero-phonon line and the first acoustic phonon mode, indicating reduced coupling between the electronic orbital states and low-energy phonon modes. The presence of optical phonon sidebands at 631.6 and 645.0nm, corresponding to phonon energies of 157 and 197meV respectively, aligns with observations from other mechanically isolated emitters in hexagonal boron nitride.

Variations in the local crystal field, potentially caused by strain or electrostatic changes, can modify orbital level splittings and state mixing, ultimately influencing spin-dependent relaxation pathways that govern optical spin polarization and readout. The disappearance of the ODMR signal following thermal cycling suggests this environmental sensitivity is a significant factor in maintaining stable spin control. This detailed understanding of the emitter’s spectral and spin properties is essential for realizing its potential in advanced quantum technologies.

Spectral Instability: Zero-Phonon Line Dynamics

The sensitivity of emission energy to local electrostatic fluctuations directly impacts the stability of recombination pathways within hexagonal boron nitride (hBN) emitters. Detailed analysis of these pathways reveals multiple zero-phonon lines (ZPLs), each exhibiting differing responses to environmental charge fluctuations, a phenomenon observed through power-dependent measurements of switching rates. At 77K, resonant excitation alone increases the off-rate, the speed at which an emitter ceases to glow, indicating that driving the emitter accelerates spectral diffusion, the random drift of its emission frequency.

Introducing a 5μW blue laser alongside resonant excitation then boosts this off-rate by approximately a factor of two, suggesting that additional photons contribute to the instability. Measurements at 8K demonstrate a weak temperature dependence of the primary noise source affecting the dominant ZPL transition; the off-rate decreased from approximately 85kHz at 77K to 63kHz at 8K with a resonant power of 20μW.

This suggests that the mechanisms causing spectral instability are not solely reliant on thermally activated charge dynamics, which remain largely frozen at this cryogenic temperature. The addition of blue illumination at 8K has minimal impact on the off-rate, further supporting the conclusion that thermally driven processes are not the primary cause of spectral diffusion at low temperatures. These findings are crucial because Rabi oscillations, a key technique for coherent control of quantum states, have been demonstrated with these hBN emitters up to 20K, establishing a limited operating range for current experiments.

The nondominant ZPL transition displays distinct behavior, with power-dependent switching rates differing from the dominant line. At 77K, the off-rate of this weaker emission increases with resonant excitation, similar to the dominant line, but the response to added blue illumination is also present. This suggests that both recombination pathways are susceptible to external influences, but to varying degrees.

Analysis of the off-rates at 8K reveals a nearly linear power dependence with blue illumination, indicating that even at low temperatures, the nondominant pathway responds to external stimuli. The timescale of these instabilities ranges from nanoseconds to seconds, presenting a significant challenge to maintaining stable quantum signals and precise control over the emitter’s spin.

Photoluminescence Setup: Cryogenic Spectroscopy & Filtering

High-numerical aperture objectives, boasting an NA of 0.9, were integrated into the cryostat to maximize both excitation and emission efficiency from the hexagonal boron nitride (hBN) defect, enabling detailed spectroscopic analysis at cryogenic temperatures. This setup facilitated the acquisition of photoluminescence spectra using a 532nm laser for off-resonant excitation, with a 550nm long-pass filter removing scattered light before signal collection by a spectrometer. Complementary photoluminescence excitation (PLE) measurements employed a tunable dye laser for resonant excitation, and a tunable long-pass filter isolated the zero-phonon line (ZPL) signal from the phonon sideband.

Analysis of the emitter’s spectral characteristics revealed two closely spaced ZPL transitions originating from the same defect, each exhibiting unique spectral diffusion dynamics. The observed spectral gap between the ZPL and the first acoustic phonon sideband measured 2 ± 0.5 THz, a value extracted from Gaussian fits applied to the peaks in the photoluminescence spectrum recorded at 77K.

Characteristic phonon sidebands at 631.6nm further confirmed the mechanical isolation of the hBN emitter. Data processing involved careful background subtraction to isolate the emitter’s fluorescence, excluding contributions with approximately linear dependence on excitation power. This refined signal then underwent saturation modeling, yielding a saturation power and a saturation count rate. The high saturation count rate was further analyzed considering the detection efficiency of the setup, the Debye-Waller factor, and the quantum efficiency of the emitter itself.

Resonant excitation of each ZPL transition produced identical phonon sideband profiles, which also aligned with those observed under off-resonant excitation, confirming a shared origin for these spectral features. Second-order autocorrelation measurements, conducted under off-resonant green excitation, revealed a value of, indicating single-emitter behavior. Time-resolved PL spectroscopy, displaying a sequence of spectra recorded under 532nm excitation, provided additional insight into the emitter’s dynamic properties.

Measurements of transient contrast at 50° demonstrated enhanced initialization fidelity, supporting a reduced spin-mixing rate at that field orientation. This higher contrast, coupled with reduced steady-state photoluminescence intensity, indicated increased population transfer into a metastable shelving state.

Magnetic Field Calibration: ODMR and Magnetostatics

Magnetostatic simulations, combined with optical detection magnetic resonance (ODMR) measurements from nitrogen-vacancy (NV) nanocrystals, allowed researchers to estimate the magnetic field experienced by the hexagonal boron nitride (hBN) emitter at approximately 40±10mT. This calibration relied on an estimated emitter-sample separation of 6±1mm, a critical parameter for accurately correlating the ODMR signal from the NV centers with the field at the hBN defect. The team aligned the magnet to ensure the field was primarily within the plane of the sample, accounting for a potential ±12° deviation in field direction based on the simulations.

Pump-probe recovery measurements revealed magnetic-field-dependent spin mixing, exhibiting a minimum in signal near 50° and a maximum near 140°. Under off-resonant excitation at 77K, the recovery dynamics at 50° yielded a fluorescence contrast of approximately 18%, while 140° produced a contrast of approximately 16%.

Removing the magnetic field accelerated recovery, with measurements showing a contrast reduction to roughly 11%, indicating that the observed relaxation dynamics are sensitive to Zeeman splitting. This sensitivity suggests that the lifting of degeneracy in spin sublevels, induced by the magnetic field, modifies spin-mixing pathways and suppresses relaxation channels. The extracted $T_1$ value, representing an optically conditioned population recovery time, further supports the influence of spin relaxation under specific initialization and readout conditions.

These findings align with a recent study on hBN, which demonstrated that ODMR contrast dependence on excitation wavelength stems from varying coupling strengths between optically excited and inactive states, highlighting the role of excitation-selective spin dynamics. Interestingly, off-resonant excitation at zero magnetic field generated a higher fluorescence contrast than resonant excitation.

This discrepancy suggests that distinct mechanisms govern contrast generation depending on the excitation scheme. The team observed a resonance when performing ODMR measurements at 77K in the presence of the external magnetic field. “The magnetic-field dependence of both the recovery time and fluorescence contrast indicates spin-dependent relaxation dynamics,” the researchers noted, emphasizing the intricate interplay between magnetic field orientation and the emitter’s spin state. The ability to precisely calibrate and control the magnetic field environment is important for manipulating and reading out the quantum state of these hBN emitters.

Millisecond Relaxation: Spin Dynamics via Pump-Probe

Millisecond-scale relaxation dynamics have been observed in a mechanically isolated single emitter within hexagonal boron nitride, revealing a sensitivity to excitation conditions and magnetic field orientation. Pump-probe recovery measurements performed under resonant excitation demonstrated a contrast reaching approximately 30%, a figure of merit directly influencing spin-readout fidelity and magnetic-field sensitivity for potential defect-based quantum sensors.

The team prepared hBN samples by dissolving commercially available powder in ethanol and spin-coating the solution onto a tapered coplanar waveguide, then annealing the sample under vacuum at 800°C for 1h. The sample was cooled from 800°C to room temperature over a period of 2h.

This fabrication process is important for mechanically isolating the single emitter, enabling the observation of these long-lived spin dynamics. The observation of millisecond-scale relaxation, coupled with excitation-dependent spin selectivity, positions these hBN emitters as promising candidates for quantum sensing applications requiring precise control over spin coherence.

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