Improving the lifetime of the nitrogen-vacancy-center ensemble coupled with a superconducting flux qubit by applying magnetic fields

At a Glance

Metadata	Details
Publication Date	2015-04-23
Journal	Physical Review A
Authors	Yuichiro Matsuzaki, Xiaobo Zhu, Kosuke Kakuyanagi, Hiraku Toida, Takaaki Shimo-Oka
Institutions	Chinese Academy of Sciences, National Institute of Informatics
Citations	26
Analysis	Full AI Review Included

Hybrid Quantum Systems Documentation: Enhancing NV Center Coherence via Magnetic Field Application

This technical documentation analyzes the integration of Single Crystal Diamond (SCD) NV⁻ ensembles with superconducting flux qubits (FQ) to enhance coherence time for quantum memory applications, focusing on the findings detailed in the paper “Improving the lifetime of the NV center ensemble coupled with a superconducting flux qubit by applying magnetic fields.”

Executive Summary

The following points summarize the key technical achievements and implications for quantum device fabrication using diamond NV centers:

Hybrid Quantum Architecture: The research successfully demonstrated a functional hybrid system coupling a superconducting flux qubit (acting as the processor) with an ensemble of Nitrogen-Vacancy (NV⁻) centers in diamond (acting as quantum memory).
VRO Lifetime Improvement: The application of a small, in-plane external magnetic field (2.6 mT) resulted in a nearly twofold improvement in the lifetime of Vacuum Rabi Oscillations (VRO), extending the clear oscillation duration from less than 100 ns to approximately 170 ns.
Mechanism of Coherence Enhancement: Theoretical modeling confirmed that the external magnetic field functions by significantly suppressing the inhomogeneous broadening in the NV ensemble, specifically the decoherence caused by internal lattice strain.
Material Requirements: The experiment utilized a diamond crystal with a high NV⁻ density (5 x 10¹⁷ cm^-3) created via ion implantation, requiring high-purity, electronic-grade Single Crystal Diamond (SCD).
Strain Mitigation: This work introduces a critical method—applying a transversal magnetic field—that is complementary to conventional techniques (like reducing P1 center concentration) for suppressing inhomogeneous broadening in NV ensembles.
Application Relevance: The demonstrated coherence improvement is a vital step toward realizing long-lived quantum memory components necessary for scalable quantum computation and advanced quantum metrology.

Technical Specifications

Parameter	Value	Unit	Context
Applied External Magnetic Field (B_ex)	2.6	mT	Applied along the [100] crystalline axis
Operating Temperature	< 50	mK	Measurements performed in a dilution refrigerator
NV^- Ensemble Density	5 x 10¹⁷	cm^-3	Achieved via ion implantation and annealing
FQ-Diamond Coupling Distance	< 1	µm	Gap distance critical for magnetic coupling
VRO Lifetime (Unmagnetized)	< 100	ns	Oscillation quickly suppressed by decoherence
VRO Lifetime (2.6 mT B-field)	~170	ns	Lifetime nearly doubled due to strain suppression
NV Zero Field Splitting (D/2π)	2.878	GHz	Natural electronic spin property of NV
Inhomogeneous Strain Broadening (FWHM)	4.4	MHz	Decoherence source suppressed by B-field
Simulated Collective Coupling Strength (√(N)g/2π)	13	MHz	Based on an ensemble size N=1200
FQ Decay Rate (Simulated Γ_e/2π)	0.3	MHz	Characterized via T₁ measurement

Key Methodologies

The experimental approach focused on integrating and controlling two disparate quantum systems in an ultra-low-temperature environment:

System Preparation: A gap-tunable superconducting flux qubit (FQ), featuring four Josephson junctions, was fabricated. A diamond crystal containing the NV⁻ ensemble was physically bonded on top of the FQ.
NV Ensemble Creation: The NV⁻ centers were generated within the diamond crystal via ion implantation followed by high-temperature annealing conducted in vacuum. The resulting density was optimized at approximately 5 x 10¹⁷ cm^-3.
Proximity Control: The diamond was mounted to achieve a precise coupling gap of less than 1 µm between the FQ control loop and the diamond surface, optimizing magnetic coupling.
Cryogenic Operation: All Vacuum Rabi Oscillation (VRO) measurements were conducted in a highly stable, ultra-low-temperature environment using a dilution refrigerator, maintaining temperatures below 50 mK.
Quantum State Manipulation (VRO Sequence):
- The FQ was initially excited using a microwave pulse while intentionally detuned (decoupled) from the NV ensemble.
- A controllable magnetic flux was applied to tune the FQ into resonance with the NV⁻ centers.
- The transfer of quantum state probability (VRO) was measured over time via a SQUID coupled to the FQ.
Strain Mitigation via Field Application: An external magnetic field of 2.6 mT was applied parallel to the surface along the diamond’s [100] crystalline axis to test the hypothesis that transversal fields suppress strain-induced decoherence.

6CCVD Solutions & Capabilities

6CCVD is an expert provider of MPCVD diamond solutions optimized for quantum memory, sensing, and processor applications. The replication and advancement of this hybrid FQ-NV system are highly dependent on ultra-pure, low-strain diamond substrates and precise fabrication capabilities.

6CCVD Solution Category	Relevance to Research Requirements	6CCVD Specific Offering & Advantage
Applicable Material	Requires high-purity material for effective NV creation and minimal background noise (P1 centers).	Electronic/Quantum Grade Single Crystal Diamond (SCD): Provides the lowest nitrogen incorporation (< 1 ppb P1 concentration possible) and highest structural perfection necessary to sustain the long coherence times targeted in this research.
Strain Management	The core finding relies on mitigating strain (4.4 MHz FWHM broadening). Starting material must have minimal intrinsic strain.	Ultra-Low Strain MPCVD Growth: Our proprietary CVD recipes are optimized to reduce lattice defects and bulk strain inherent to the growth process, resulting in superior quality substrates for high-fidelity quantum applications.
Surface Finish & Integration	Critical requirement for a precise coupling gap of < 1 µm between the diamond surface and the FQ.	Precision Polishing: We guarantee surface roughness Ra < 1 nm (SCD). This ultra-flatness is essential for minimizing surface-induced strain and ensuring perfect intimate contact necessary for high-fidelity hybrid coupling.
Dimensions & Thickness Control	Need for custom geometry for bonding and thin layers for coupling distance optimization.	Custom Dimensions & Thickness: We provide SCD/PCD wafers up to 125mm in size, with thickness control ranging from 0.1 µm to 500 µm, allowing researchers to specify the optimal substrate depth for near-surface NV creation.
Metalization Services	Hybrid systems often require stable superconducting circuits or bonding layers (e.g., Ti/Pt/Au for FQ integration).	In-House Custom Metalization: 6CCVD offers complete deposition services for common quantum fabrication stacks including Au, Pt, Pd, Ti, W, and Cu, simplifying the integration of diamond chips with complex superconducting circuitry.
Engineering Support	Assistance in designing diamond specs for specific ion implantation recipes (e.g., [100] axis growth) and subsequent annealing to optimize NV yield and coherence.	Expert Consultation: 6CCVD’s in-house PhD team can assist with material selection, orientation specification, and post-processing considerations for similar Quantum Memory/Hybridization projects aiming to achieve maximum NV center coherence.

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

View Original Abstract

One of the promising systems to realize quantum computation is a hybrid system where a superconducting flux qubit plays a role of a quantum processor and the NV center ensemble is used as a quantum memory. We have theoretically and experimentally studied the effect of magnetic fields on this hybrid system, and found that the lifetime of the vacuum Rabi oscillation is improved by applying a few mT magnetic field to the NV center ensemble. Here, we construct a theoretical model to reproduce the vacuum Rabi oscillations with/without magnetic fields applied to the NV centers, and we determine the reason why magnetic fields can affect the coherent properties of the NV center ensemble. From our theoretical analysis, we quantitatively show that the magnetic fields actually suppress the inhomogeneous broadening from the strain in the NV centers.

Tech Support

Original Source

DOI: https://doi.org/10.1103/physreva.91.042329