Resonant Excitation and Purcell Enhancement of Coherent Nitrogen-Vacancy Centers Coupled to a Fabry-Perot Microcavity
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-02-19 |
| Journal | Physical Review Applied |
| Authors | M Ruf, M.J. Weaver, S. B. van Dam, Hanson R, M Ruf |
| Institutions | QuTech, Delft University of Technology |
| Citations | 66 |
| Analysis | Full AI Review Included |
Resonant Excitation and Purcell Enhancement of Coherent Nitrogen-Vacancy Centers
Section titled âResonant Excitation and Purcell Enhancement of Coherent Nitrogen-Vacancy CentersâExecutive Summary
Section titled âExecutive SummaryâThis research demonstrates a critical advancement toward scalable quantum networks by achieving resonant optical control and Purcell enhancement of single, coherent Nitrogen-Vacancy (NV) centers coupled to a fiber-based Fabry-PĂ©rot micro-cavity operating at 4 K.
- Resonant Addressing Achieved: The system successfully overcomes previous limitations related to poor NV center optical coherence (GHz linewidths) by demonstrating frequency-selective resonant excitation of individual NV centers within the cavity.
- Purcell Enhancement Verified: A Purcell enhancement factor (FPZPL) of up to 4 was measured under both off-resonant and resonant excitation protocols, confirming enhanced emission into the Zero-Phonon Line (ZPL).
- High Coherence Demonstrated: Spectral diffusion limited linewidths of 190 ± 9 MHz were measured, comparable to bulk diamond samples used in previous entanglement experiments.
- Current Performance Benchmark: The probability of detecting a ZPL photon per excitation pulse is currently 9.3 x 10-5, which, when corrected for low initialization/excitation probability, is comparable to state-of-the-art Solid Immersion Lens (SIL) systems (up to 2 x 10-3).
- Future Scalability Projection: A detailed loss model predicts that implementing three achievable improvements (spin pumping, vibration reduction, and higher finesse fiber) will increase the ZPL photon detection probability to approximately 10%.
- System Architecture: The setup utilizes a closed-cycle cryostat (4 K) containing a diamond membrane (~5.8 ”m thick) bonded to a flat mirror, forming a cavity with a laser-ablated fiber mirror.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Operating Temperature | 4 | K | Closed-cycle cryostat operation. |
| NV Center Lifetime (Off-Resonant) | 11.8 ± 0.2 | ns | Consistent with bulk diamond. |
| NV Center Lifetime (Resonant, Enhanced) | 9.77 ± 0.08 | ns | Measured during low vibration periods. |
| Measured Purcell Factor (FPZPL) | Up to 4 | Dimensionless | Enhancement of ZPL emission into the cavity mode. |
| Theoretical Max Purcell Factor (FPZPL) | â 7 | Dimensionless | Estimated based on cavity parameters. |
| NV Center Linewidth (Centered) | 190 ± 9 | MHz | Spectral diffusion limited linewidth. |
| Cavity Finesse (Design) | 6200 | Dimensionless | Target value based on mirror coatings. |
| Cavity Finesse (Operating) | 1000 to 2500 | Dimensionless | Limited by clipping loss in the fiber mirror. |
| Diamond Membrane Thickness | ~5.8 | ”m | Final etched thickness in the cavity region. |
| Air Gap (Typical) | 7.3 | ”m | Measured air gap between diamond and fiber. |
| Cavity RMS Vibrations (Ïvib) | 0.1 to 0.2 | nm | Root mean squared fluctuation in cavity length. |
| ZPL Detection Probability (Current) | 9.3 x 10-5 | Photons/Pulse | Measured absolute count rate under resonant excitation. |
| ZPL Detection Probability (Projected) | ~10 | % | Predicted maximum efficiency with future upgrades. |
| Coupling Regime | Weak Coupling | N/A | Îł < g < Îș (NV decay rate < coupling rate < cavity decay rate). |
| NV Center ZPL Wavelength | ~637 | nm | Zero-Phonon Line transition frequency. |
Key Methodologies
Section titled âKey MethodologiesâThe experiment relies on a highly stabilized, cryogenic fiber-cavity system combined with advanced pulsed optical protocols to address and characterize single NV centers.
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Diamond Membrane Fabrication:
- Electron irradiated and annealed bulk diamond was etched down to a final thickness of ~5.8 ”m.
- The membrane was bonded to the flat, super-polished cavity mirror. This process preserves the optical coherence of the near-surface NV centers.
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Cryogenic Cavity Setup:
- An open, fiber-based Fabry-PĂ©rot micro-cavity was assembled inside a closed-cycle cryostat, operating at 4 K.
- The cavity length was actively tuned in situ using a piezo positioning stage.
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Frequency Stabilization Protocol:
- A ~637 nm laser was frequency stabilized to a wavemeter (Step 1).
- The cavity length was actively stabilized to this laser frequency (Step 2) to compensate for slow thermal and mechanical drifts.
- Measurement blocks (Step 3) were interleaved between stabilization rounds to maintain spectral alignment.
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NV Center Characterization (PLE Scans):
- Photoluminescence Excitation (PLE) scans were performed using short green pulses (initialization into NV-, ms=0 state) interleaved with red measurement pulses (resonant excitation).
- Fluorescence was collected in the Phonon-Sideband (PSB) path to detect the spin-conserving optical transitions (Ex and Ey).
- Spectral diffusion was mitigated by fitting individual PLE traces and shifting them to a common center frequency, reducing the effective linewidth from 224 MHz to 190 MHz.
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Purcell Enhancement Measurement:
- Off-Resonant: Pulsed green excitation (~532 nm) was used while sweeping the cavity detuning. Purcell enhancement was extracted from the simultaneous decrease in NV center lifetime (Ï) and increase in ZPL fluorescence counts.
- Resonant: Pulsed red resonant excitation was used while sweeping the cavity detuning. Single-emitter confirmation was achieved via g(2) measurements (g(2)(0) < 0.5). Purcell enhancement was extracted via joint fitting of the lifetime and PSB count curves.
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ZPL Photon Collection:
- ZPL photons were separated from the bright resonant excitation pulse using cross-polarization detection and time-bin filtering.
- The ZPL fluorescence decay was recorded simultaneously with the PSB fluorescence to benchmark the system performance.
Commercial Applications
Section titled âCommercial ApplicationsâThe successful integration of coherent, resonantly addressable NV centers with a high-efficiency optical interface is foundational for several emerging quantum technologies.
- Quantum Networks and Quantum Internet:
- The primary application is enabling high-fidelity, high-rate entanglement generation between distant quantum nodes. The projected 10% ZPL collection efficiency represents a potential 100x speedup for single-click entanglement protocols compared to current non-cavity systems.
- Distributed Quantum Computing:
- High-speed, low-loss photonic links are essential for connecting stationary qubits (like NV centers) into a distributed quantum processor architecture.
- Quantum Enhanced Sensing:
- NV centers are highly sensitive quantum sensors (magnetometry, thermometry). Integrating them into a high-finesse cavity enhances the photon collection rate, improving the signal-to-noise ratio and thus the sensitivity and speed of quantum sensing protocols.
- Solid-State Quantum Emitters:
- The methodology developed for overcoming spectral diffusion and achieving resonant control in a cryogenic, fiber-coupled system is transferable to other solid-state emitters (e.g., SiV, GeV, rare earth ions) that require high-coherence optical interfaces.
- Cryogenic Optical Systems:
- The robust design and stabilization protocols for the fiber-based Fabry-PĂ©rot micro-cavity operating within a closed-cycle cryostat (4 K) are valuable for developing commercial cryogenic quantum hardware, eliminating the need for liquid helium infrastructure.
View Original Abstract
<p>The nitrogen-vacancy (N-V) center in diamond has been established as a prime building block for quantum networks. However, scaling beyond a few network nodes is currently limited by low spin-photon entanglement rates, resulting from the N-V centerâs low probability of coherent photon emission and collection. Integration into a cavity can boost both values via the Purcell effect, but poor optical coherence of near-surface N-V centers has so far prevented their resonant optical control, as would be required for entanglement generation. Here, we overcome this challenge, and demonstrate resonant addressing of individual, fiber-cavity-coupled N-V centers, and collection of their Purcell-enhanced coherent photon emission. Utilizing off-resonant and resonant addressing protocols, we extract an enhancement of the zero-phonon line emission by a factor of up to 4, consistent with a detailed theoretical model. This model predicts that the probability of coherent photon detection per optical excitation can be increased to 10% for realistic parameters - an improvement over state-of-the art solid immersion lens collection systems by 2 orders of magnitude. The resonant operation of an improved optical interface for single coherent quantum emitters in a closed-cycle cryogenic system at TâŒ4 K is an important result towards extensive quantum networks with long coherence.</p>