Robust Quantum-Network Memory Using Decoherence-Protected Subspaces of Nuclear Spins

At a Glance

Metadata	Details
Publication Date	2016-06-22
Journal	Physical Review X
Authors	Andreas Reiserer, Norbert Kalb, Machiel S. Blok, Koen J. M. van Bemmelen, Tim H. Taminiau
Institutions	QuTech, Delft University of Technology
Citations	106
Analysis	Full AI Review Included

Robust Diamond Quantum Memory: Technical Analysis and 6CCVD Solutions

This document analyzes the research detailing a robust quantum network memory based on Nitrogen-Vacancy (NV) centers in diamond and outlines how 6CCVD’s advanced MPCVD diamond materials and customization capabilities can support and extend this critical quantum technology.

Executive Summary

Quantum Network Node Demonstration: Experimental realization of a prototype quantum network node consisting of a single NV electronic spin hyperfine-coupled to five individually controlled ¹³C nuclear spin qubits.
Decoherence Mechanism Identified: The primary limitation to quantum state storage fidelity during remote entanglement protocols was identified as dephasing induced by the stochastic trajectory of the electronic spin reset.
Decoherence Protection Achieved: Quantum states were successfully encoded into a Decoherence-Protected Subspace (DPS) utilizing two nuclear spins.
Enhanced Robustness: Encoding in the DPS enabled the maintenance of quantum coherence for over 1000 repetitions of the remote entangling protocol, significantly exceeding single-qubit performance.
Material Requirement: The experiment relies on high-quality, low-strain Single Crystal Diamond (SCD) to host stable NV centers and provide a clean nuclear spin environment.
Future Scaling: The results pave the way for practical quantum repeaters and remote entanglement purification using NV center quantum network nodes.

Technical Specifications

The following hard data points were extracted from the experimental results, focusing on the performance of the ¹³C nuclear spin qubits and the operational environment.

Parameter	Value	Unit	Context
Operating Temperature	4	K	Helium bath cryostat
Applied Magnetic Field (B)	40	mT	Along NV symmetry axis
Controlled Nuclear Qubits	5	N/A	Individual ¹³C spins (I = 1/2)
Maximum Repetitions (DPS)	> 1000	N/A	Before Bloch vector length drops to 1/e
Electronic Spin Initialization Fidelity	> 0.99	N/A	Using spin-selective optical transitions
Electronic Spin Readout Fidelity	~ 0.94	N/A	Single shot average fidelity
Fastest Reset Timescale (E_1,2 config)	48(1)	ns	Fast component of double-exponential decay
Slowest Reset Timescale (Singlet State)	~ 440	ns	Metastable singlet state decay constant
Hyperfine Coupling Range (A_\|\|)	-48.6 to 23.7	kHz	Parallel component for 5 ¹³C spins
Dephasing Time Range (T₂*)	12 to 55	ms	Measured for individual ¹³C spins
Initialization/Readout Fidelity Range (F_i,r)	0.89(2) to 0.97(2)	N/A	Combined fidelity for 5 ¹³C spins

Key Methodologies

The experiment successfully integrated advanced diamond material science with precise quantum control techniques:

Material Preparation: Experiments were performed on a diamond device containing NV centers and natural abundance ¹³C nuclear spins, cooled to 4 K under a 40 mT magnetic field.
Electronic Spin Control: The NV electronic spin was initialized and read out using spin-selective optical transitions and controlled using microwave (MW) pulses.
Nuclear Spin Control: High-fidelity individual control over five ¹³C nuclear spin qubits was achieved using tailored MW pulse sequences on the electronic spin.
Entanglement Protocol: The Barrett-Kok inter-node entangling sequence was employed, requiring repeated electronic spin reset steps after failed attempts.
Spin Reset Optimization: The electronic spin reset time was optimized by applying repump laser fields on resonance with specific transitions (A_1,2 or E_1,2), minimizing the time the electronic spin spent in an unknown state.
Dynamical Decoupling: A dynamical decoupling sequence was implemented within the entanglement protocol, with timing optimized (τ ≈ 0.44 µs) to match the slow timescale of the initialization process (metastable singlet state decay).
Decoherence Protection: Quantum states were encoded into a two-spin Decoherence-Protected Subspace (DPS) using basis states where the net parallel hyperfine coupling strength was minimized (Δω ~ 2π × (A_||,i - A_||,j)).

6CCVD Solutions & Capabilities

The realization of robust quantum network nodes requires diamond materials with exceptional purity, precise isotopic control, and integrated device features. 6CCVD is uniquely positioned to supply the next generation of MPCVD diamond required to scale this research.

Applicable Materials for Quantum Network Scaling

Research Requirement	6CCVD Material Solution	Technical Rationale
Ultra-Low Strain Host	Optical Grade Single Crystal Diamond (SCD)	Essential for minimizing inhomogeneous broadening of the NV optical transition and maximizing electronic spin coherence (T₂), crucial for high-fidelity remote entanglement.
Nuclear Spin Bath Engineering	Isotopically Purified ¹²C SCD (> 99.99%)	Reducing the natural abundance of ¹³C minimizes background nuclear spin noise, leading to significantly longer intrinsic nuclear spin coherence times (T₂).
Deterministic Qubit Placement	Controlled ¹³C Doping/Implantation	Allows for the precise placement of ¹³C qubits at desired distances from the NV center, enabling the engineering of specific hyperfine coupling strengths (A_\|\|, A_⊥) necessary for optimized DPS encoding.
High-Power Optical Interface	Boron-Doped Diamond (BDD) Electrodes	BDD films can be integrated for on-chip electrical control or charge state stabilization, complementing the optical reset protocols.

Customization Potential for Device Integration

The complexity of the NV quantum network node requires integration of microwave control lines and precise material dimensions. 6CCVD offers comprehensive customization services:

Custom Dimensions: We supply high-quality SCD plates and wafers up to 125mm (PCD) and substrates up to 10mm thick, accommodating large-scale device fabrication and integration into cryogenic systems.
Advanced Metalization: The experiment relies heavily on MW pulses. 6CCVD provides in-house metalization (Au, Pt, Pd, Ti, W, Cu) services for depositing high-conductivity striplines and antennas directly onto the diamond surface, ensuring optimal MW field delivery and minimal pulse errors.
Precision Fabrication: We offer precision laser cutting and etching services to create custom geometries, alignment features, and microstructures necessary for integrating optical cavities (as suggested for future improvements in the paper) and fiber coupling.
Polishing Excellence: Our SCD wafers feature ultra-smooth polishing (Ra < 1 nm), critical for minimizing optical loss and maximizing the efficiency of the spin-photon entanglement interface.

Engineering Support

6CCVD’s in-house team of PhD material scientists and quantum engineers specializes in optimizing diamond properties for solid-state quantum applications. We can assist researchers in selecting the ideal material specifications—including isotopic purity, NV concentration, and surface preparation—required to replicate or extend this research into scalable quantum networking and quantum memory projects.

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

View Original Abstract

<p>The realization of a network of quantum registers is an outstanding challenge in quantum science and technology. We experimentally investigate a network node that consists of a single nitrogen-vacancy center electronic spin hyperfine coupled to nearby nuclear spins. We demonstrate individual control and readout of five nuclear spin qubits within one node. We then characterize the storage of quantum superpositions in individual nuclear spins under repeated application of a probabilistic optical internode entangling protocol. We find that the storage fidelity is limited by dephasing during the electronic spin reset after failed attempts. By encoding quantum states into a decoherence-protected subspace of two nuclear spins, we show that quantum coherence can be maintained for over 1000 repetitions of the remote entangling protocol. These results and insights pave the way towards remote entanglement purification and the realization of a quantum repeater using nitrogen-vacancy center quantum-network nodes.</p>

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevx.6.021040