Towards a spin-ensemble quantum memory for superconducting qubits

At a Glance

Metadata	Details
Publication Date	2016-07-27
Journal	Comptes Rendus Physique
Authors	Cécile Grèzes, Yuimaru Kubo, Brian Julsgaard, T. Umeda, Junichi Isoya
Institutions	Université Paris-Sud, Centre National de la Recherche Scientifique
Citations	43
Analysis	Full AI Review Included

Technical Documentation and Analysis: Towards a Spin-Ensemble Quantum Memory

This documentation analyzes the key material science and engineering requirements outlined in the research paper “Towards a spin-ensemble quantum memory for superconducting qubits,” and correlates them directly with the advanced capabilities offered by 6CCVD.

Executive Summary

The paper successfully demonstrates the fundamental building blocks for a multi-mode hybrid quantum memory, combining long-coherence electron spin ensembles (NV centers in diamond) with superconducting qubits (cQED architecture).

Hybrid Architecture: The system uses a superconducting microwave resonator as a quantum bus to mediate coherent quantum state transfer between a transmon qubit and an ensemble of NV centers in MPCVD diamond.
Strong Coupling Verified: Researchers achieved collective strong coupling ($g_{ens}/2\pi = 11$ MHz) between the NV spin ensemble and the resonator, a critical prerequisite for high-fidelity writing of quantum states.
Material Purity Criticality: Initial experiments were limited by high concentrations of residual P1 centers (nitrogen defects) in the HPHT diamond, leading to short spin coherence times (T₂ $\approx$ 8 µs).
Coherence Improvement: Improved results utilized isotopically enriched (¹²C) and lower P1 concentration diamond, extending the coherence time to T₂ $\approx$ 84 µs, enabling single-photon level storage.
On-Demand Retrieval: Retrieval of stored microwave fields was demonstrated using a Two-Pulse Echo (2PE) refocusing sequence, confirming the feasibility of the read step protocol.
Material Processing: Successful implementation relies on precise NV creation (via irradiation and subsequent high-temperature annealing: 800-1000 °C) in high-quality diamond substrates.
Future Requirements: Further optimization requires homogeneous microwave field coupling and ultra-high purity diamond (low P1, high ¹²C enrichment) to reach spin coherence times exceeding milliseconds.

Technical Specifications

Key physical parameters and experimental results extracted from the study:

Parameter	Value	Unit	Context
Material Base	HPHT/CVD Diamond	N/A	Host material for NV centers.
NV Concentration (Target)	1 - 10	ppm	Required concentration for efficient strong coupling.
Initial P1 Concentration	$\approx$150	ppm	Limiting factor for T₂ in initial HPHT sample.
Improved P1 Concentration	$\approx$16	ppm	Used in later experiment (Section 6) for improved T₂.
Zero-Field Splitting (D/2π)	2.88	GHz	NV center electronic ground state triplet.
Operating Frequency ($\omega_{r}/2\pi$)	2.88 - 2.95	GHz	Microwave resonator frequency range.
Magnetic Field (B_NV)	$\approx$1	mT	Used to lift degeneracy and tune spin transition frequency.
Strong Coupling Strength ($g_{ens}/2\pi$)	11	MHz	Collective coupling achieved between NV ensemble and resonator.
Resonator Damping Rate ($\kappa$)	$9 \times 10^{6}$	s^-1	Used for strong coupling condition $g_{ens}$ $\gg$ $\kappa$.
Spin Ensemble Linewidth ($\Gamma/2\pi$)	3	MHz	Due primarily to inhomogeneous broadening.
Spin Coherence Time (T₂)	8	µs	Typical T₂ for high-P1 HPHT diamond used in initial tests.
Improved T₂	84	µs	Achieved using low-P1, isotopically enriched (¹²C) diamond.
Memory Storage Time	100	µs	Used for single-photon level retrieval demonstration.
Echo Retrieval Efficiency (E)	$3 \times 10^{-3}$	N/A	Best observed efficiency at single-photon level (after 100 µs storage).
Operating Temperature	$\approx$40	mK	Required for superconducting circuit operation.

Key Methodologies

The experiment utilized a complex combination of advanced material preparation, cQED fabrication, and pulsed electron spin resonance techniques adapted for cryogenic temperatures.

Material Growth and Preparation:
- Diamond crystals were grown via High Pressure High Temperature (HPHT) or Chemical Vapor Deposition (CVD) methods, with specific control over initial Nitrogen (P1) concentrations.
- NV Creation: Samples were subjected to particle irradiation (protons or electrons) to create vacancies in the lattice.
- Annealing: Subsequent high-temperature annealing ($800$ °C to $1000$ °C) was performed for several hours to mobilize vacancies, allowing them to bind with nitrogen atoms to form the required NV centers.
Hybrid Circuit Fabrication:
- Superconducting circuits (coplanar waveguide resonator and transmon qubit) were fabricated on silicon substrates.
- Integration: The prepared diamond crystal containing the NV ensemble was physically glued onto the center of the coplanar waveguide resonator using vacuum grease to maximize the mode filling factor.
- Metalization: Resonator tuning elements (SQUIDs) were integrated to enable dynamic control of frequency ($\omega_{r}$) and quality factor (Q or $\kappa$).
Quantum Memory Protocol Implementation (Write/Read):
- Cryogenic Operation: Experiments conducted at millikelvin temperatures ($T \approx 40$ mK).
- Write Step (Transfer): An arbitrary qubit state was transferred to the resonator using an adiabatic SWAP (aSWAP) gate, followed by dynamic tuning of the resonator frequency ($\omega_{B}$) into resonance with the NV spin transition ($\omega_{s}$) for a duration $\tau$.
- Active Reset: Between successive sequences, the spin memory was actively reset by applying 532 nm green laser pulses (1.5 mW, 1s duration) through an optical fiber glued to the diamond, repumping NVs to the $m_{s}=0$ ground state.
- Read Step (Retrieval): The stored information was retrieved using a two-pulse (Hahn) echo sequence (2PE), where a $\pi$-pulse acts as a time-reversal trigger, causing the emission of the stored microwave field (echo) at a specific time $2\tau - t_{i}$.

6CCVD Solutions & Capabilities

This research highlights the absolute necessity of ultra-high purity, custom-engineered diamond substrates—precisely the niche where 6CCVD excels. Replicating or advancing this quantum memory research requires materials science expertise beyond standard commercial offerings.

Applicable Materials

To achieve the long coherence times (T₂ $\gg$ 100 µs) necessary for an operational quantum memory, researchers must minimize paramagnetic impurities (P1 centers) and nuclear spin bath noise (¹³C).

Application Requirement	Recommended 6CCVD Material	Key 6CCVD Capability
Ultra-High Coherence	Optical Grade SCD (Single Crystal Diamond)	SCD wafers with extremely low P1 concentration (< 5 ppb).
Spin Bath Minimization	Isotopically Enriched SCD	CVD growth specialized in ¹²C enrichment (> 99.99%) for maximal T₂.
Integrated cQED Circuitry	Custom Thin SCD Plates	SCD thicknesses down to 0.1 µm for minimized thermal mass and high filling factor in resonators.
High Density NV Creation	Custom Nitrogen-Doped Precursors	Highly controlled initial N concentration during growth phase for post-processing optimization (1-10 ppm target).

Customization Potential

The experimental setup requires integrating the diamond physically and electromagnetically with the cQED circuit. 6CCVD offers the customization necessary for seamless device integration:

Precision Sizing and Shaping: The diamond plates must be precisely sized for coupling to the planar microwave resonator. 6CCVD provides custom laser cutting and shaping services to match specific resonator geometries, minimizing passive loss and maximizing the mode filling factor.
Surface Preparation: Successful coupling requires extremely low surface roughness to avoid losses. 6CCVD guarantees ultra-smooth polishing (Ra < 1 nm for SCD), crucial for minimizing defects and ensuring close proximity to the superconducting circuit.
Metalization Services: While the diamond itself is placed on the resonator, future integration (e.g., direct electrical readout, on-chip strain tuning) may require local contacts. 6CCVD offers in-house metalization capabilities (Au, Pt, Ti, W, etc.) compatible with quantum device standards.

Engineering Support

The creation of optimal quantum grade diamond requires tight control over growth, irradiation, and annealing parameters to achieve the specific NV concentration (1-10 ppm) while maintaining minimal P1 concentration. The annealing temperature window ($800$ °C to $1000$ °C) is critical.

6CCVD’s in-house PhD team specializes in MPCVD diamond engineering and post-processing protocols. We provide expert consultation for clients working on spin-based quantum memory projects, assisting with:

Optimizing precursor gas mixture ratios to achieve required N-doping targets (P1).
Consulting on post-growth irradiation and annealing recipes (e.g., temperature profile and duration) to maximize NV yield and optimize charge state stability.
Selecting the appropriate surface orientation and dimension for integration with novel cQED architectures.

Call to Action

To unlock millisecond-level coherence times required for scalable quantum memory, researchers need material purity that only high-quality, isotopically enriched MPCVD Single Crystal Diamond can provide.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

This article reviews efforts to build a new type of quantum device, which combines an ensemble of electronic spins with long coherence times, and a small-scale superconducting quantum processor. The goal is to store over long times arbitrary qubit states in orthogonal collective modes of the spin-ensemble, and to retrieve them on-demand. We first present the protocol devised for such a multi-mode quantum memory. We then describe a series of experimental results using NV (as in nitrogen vacancy) center spins in diamond, which demonstrate its main building blocks: the transfer of arbitrary quantum states from a qubit into the spin ensemble, and the multi-mode retrieval of classical microwave pulses down to the single-photon level with a Hahn-echo like sequence. A reset of the spin memory is implemented in-between two successive sequences using optical repumping of the spins.

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.crhy.2016.07.006

References

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2013 - Quantum memory for microwave photons in an inhomogeneously broadened spin ensemble [Crossref]
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