Room-temperature storage of quantum entanglement using decoherence-free subspace in a solid-state spin system

At a Glance

Metadata	Details
Publication Date	2017-10-31
Journal	Physical review. B./Physical review. B
Authors	Fei Wang, Yufei Huang, Zhuo Zhang, Chong Zu, Pan‐Yu Hou
Institutions	University of Michigan, Tsinghua University
Citations	15
Analysis	Full AI Review Included

Technical Documentation & Analysis: Room-Temperature Quantum Entanglement Storage in Diamond

Executive Summary

This research demonstrates a significant advance in solid-state quantum memory by achieving robust, room-temperature storage of quantum entanglement using Nitrogen-Vacancy (NV) centers in diamond.

Core Achievement: Experimental demonstration of quantum entanglement storage between two $^{13}$C nuclear spins coupled to a single NV electronic spin at room temperature.
Decoherence Mitigation: Successful implementation of the Decoherence-Free Subspace (DFS) passive error control strategy to protect entangled states from general collective noise (dephasing and relaxation).
Coherence Time Improvement: The entangled state protected by the DFS exhibited a memory time ($T_{est}$) of approximately 2.2 ms, limited by the electronic spin $T_{1}$ relaxation time.
Performance Metric: This DFS protection resulted in an order of magnitude increase in coherence time compared to unprotected entangled states (360 µs).
High-Fidelity Gates: Achieved high-fidelity quantum control, including conditional X gates with fidelities up to F ≈ 0.988, crucial for scalable quantum information processing.
Material Requirement: The work underscores the critical need for high-purity, low-defect diamond material to maximize electronic spin coherence and enable practical quantum memory applications.

Technical Specifications

Parameter	Value	Unit	Context
Qubit System	NV center + two $^{13}$C nuclear spins	N/A	Solid-state spin system in diamond
Operating Environment	Room Temperature	°C	Demonstrated robust operation without cryogenics
External Magnetic Field ($B_{z}$)	480	Gauss	Applied along the NV symmetry axis
DFS Entanglement Memory Time ($T_{est}$)	≈ 2.2	ms	Coherence time under general collective noise (Singlet state $
Non-DFS Entanglement Memory Time ($T_{est}$)	≈ 360	µs	Coherence time under general collective noise (Triplet state $
Electronic Spin Relaxation Time ($T_{1}$)	≈ 2.5	ms	Limiting factor for DFS memory time
Conditional X Gate Fidelity (Spin 1)	≈ 0.988	N/A	Extracted from slow decay of oscillations
Nuclear Spin 1 Parallel Hyperfine ($A_{		1}$)	-77.02(3)
Nuclear Spin 2 Parallel Hyperfine ($A_{		2}$)	71.03(3)
Magnetic Field Fluctuation (Simulated)	0.15	G	Causes entanglement fidelity drop from 1 to 0.92

Key Methodologies

The experiment relied on precise control and calibration of the three-qubit system (one electronic spin, two nuclear spins) within a diamond lattice.

Qubit Platform: Utilizing the NV electronic spin (spin-1 system) as a quantum bus (handle) to coherently control and entangle two weakly coupled $^{13}$C nuclear spins.
Initialization and Readout: Optical initialization and readout of the NV electronic spin using a 350 ns green laser pulse. Nuclear spin state is read out by swapping its polarization back onto the electronic spin.
Coherent Control: Microwave and radio frequency (rf) fields were used to manipulate the spins. Pulse sequences, including Carr-Purcell-Meiboom-Gill (CPMG) and XY8, were applied to decouple the electronic spin from the spin bath and perform conditional gate operations.
High-Precision Calibration: Nuclear Spin Optical Detected Magnetic Resonance (ODMR) was employed to calibrate the parallel ($A_{||}$) and transverse ($A_{\perp}$) hyperfine interaction parameters with high resolution (down to 0.05 kHz standard deviation).
Entanglement Preparation: Entanglement was generated by first creating an electron-nuclear entangled state, followed by a swap operation and conditional rotations to produce the target entangled states ($|S\rangle$ and $|T\rangle$) within the nuclear spin DFS.
Noise Modeling: General collective noise (including dephasing and relaxation) was experimentally realized by injecting a noisy radio-frequency field centered at the nuclear spin Larmor frequency (10 kHz bandwidth) into the system.

6CCVD Solutions & Capabilities

This research highlights the critical role of high-quality, customized diamond materials for advancing solid-state quantum computing. 6CCVD is uniquely positioned to supply the necessary Single Crystal Diamond (SCD) substrates and engineering services required to replicate and scale this work.

Research Requirement	6CCVD Solution & Capability	Technical Advantage
Ultra-High Purity Diamond	Single Crystal Diamond (SCD) - Electronic Grade	Our MPCVD SCD features ultra-low nitrogen (< 1 ppb) and defect concentrations, essential for maximizing the NV electronic spin coherence time ($T_{1}$ and $T_{2}$). Longer $T_{1}$ directly translates to longer DFS memory times, currently limited to 2.5 ms.
Isotopic Control for Noise Reduction	Custom Isotopic SCD (e.g., < 0.1% $^{13}$C)	While the experiment utilized natural abundance $^{13}$C, future scaling requires reducing the background nuclear spin bath noise. 6CCVD offers SCD with precise isotopic purification to extend NV $T_{2}$ coherence and mitigate nuclear spin crosstalk errors, as suggested by the authors. We can also supply SCD with controlled $^{13}$C concentrations for specific qubit coupling requirements.
Large-Area Scalability	Custom Dimensions (Plates/Wafers up to 125 mm)	For transitioning from single-NV experiments to integrated quantum circuits, 6CCVD provides large-area Polycrystalline Diamond (PCD) and SCD plates, enabling wafer-scale fabrication and integration of quantum devices.
Optical and RF Integration	Precision Polishing (Ra < 1 nm for SCD)	We guarantee surface roughness (Ra < 1 nm for SCD) necessary for high-quality optical access (green laser initialization/readout) and seamless integration with on-chip microwave/RF delivery structures (e.g., coplanar coils and waveguide transmission lines).
On-Chip Control Structures	Custom Metalization Services (Ti, Pt, Au, W, Cu)	6CCVD offers in-house metal deposition capabilities to fabricate the necessary RF/microwave antennas directly onto the diamond surface, ensuring optimal coupling and high Rabi frequencies required for high-fidelity gate operations.
Cryogenic Operation Support	Thick Substrates (up to 10 mm)	The paper suggests significant fidelity improvement under cryogenic conditions. Our robust SCD substrates (up to 10 mm thick) maintain superior thermal properties and structural stability required for low-temperature quantum experiments.

Engineering Support

6CCVD’s in-house PhD team of material scientists and quantum engineers provides expert consultation on material selection, defect engineering (NV creation), and surface preparation tailored specifically for solid-state quantum memory and NV-based quantum computing projects. We ensure your diamond material meets the stringent requirements for high-fidelity, long-coherence quantum operations.

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

View Original Abstract

We experimentally demonstrate room-temperature storage of quantum\nentanglement using two nuclear spins weakly coupled to the electronic spin\ncarried by a single nitrogen-vacancy center in diamond. We realize universal\nquantum gate control over the three-qubit spin system and produce entangled\nstates encoded within the decoherence-free subspace of the two nuclear spins.\nBy injecting arbitrary collective noise, we demonstrate that the\ndecoherence-free entangled state has coherence time longer than that of other\nentangled states by an order of magnitude in our experiment.\n

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Original Source

DOI: https://doi.org/10.1103/physrevb.96.134314