Coherence Time Extension by Large-Scale Optical Spin Polarization in a Rare-Earth Doped Crystal

At a Glance

Metadata	Details
Publication Date	2020-09-16
Journal	Physical Review X
Authors	Sacha Welinski, Alexey Tiranov, Moritz Businger, Alban Ferrier, Mikael Afzelius
Institutions	Université Paris Sciences et Lettres, Sorbonne Université
Citations	12
Analysis	Full AI Review Included

Technical Documentation & Analysis: Coherence Time Extension via Diffusion Enhanced Optical Pumping (DEOP)

Reference: Welinski et al., “Coherence Time Extension by Large Scale Optical Spin Polarization in a Rare-Earth Doped Crystal,” arXiv:1910.07907v1 (2019).

Executive Summary

This research demonstrates a powerful technique, Diffusion Enhanced Optical Pumping (DEOP), to significantly extend optical coherence lifetimes ($T_{2,o}$) in paramagnetic solid-state systems, a critical requirement for scalable quantum technologies.

Core Achievement: Achieved large-scale spin polarization ($> 90%$) in a $^{171}\text{Yb}^{3+}:\text{Y}{2}\text{SiO}{5}$ (YSO) crystal at 2 K and zero magnetic field by optically pumping only a small fraction ($\approx 0.5%$) of ions.
Coherence Enhancement: The resulting decrease in spin-spin interactions extended the optical coherence lifetime ($T_{2,o}$) from $278 \text{ µs}$ (thermal equilibrium) to $782 \pm 30 \text{ µs}$.
Record Performance: The $782 \text{ µs}$ coherence time is the longest reported optical $T_{2}$ for any paramagnetic solid-state system operating at zero or very-low magnetic fields.
Methodology: DEOP relies on optical pumping coupled with spin diffusion via flip-flop interactions to polarize the entire spin ensemble.
Quantum Relevance: This method is directly applicable to other concentrated, optically active spin ensembles, including color centers in MPCVD diamond, paving the way for high-density quantum memories, processors, and optical-microwave transducers interfacing with superconducting circuits.
6CCVD Value Proposition: 6CCVD provides the high-purity, low-strain Single Crystal Diamond (SCD) substrates and advanced metalization required to implement and scale DEOP-based quantum devices, such as NV$^{-}$ or SiV$^{-}$ centers in diamond.

Technical Specifications

Parameter	Value	Unit	Context
Host Material	$\text{Y}{2}\text{SiO}{5}$ (YSO)	Crystal	Single crystal, Czochralski grown
Dopant Concentration	10	ppm	$^{171}\text{Yb}^{3+}$ (94% isotopic purity)
Operating Temperature	2	K	Liquid helium bath cryostat
Magnetic Field	Zero / Very-low	T	Optimized for magnetic insensitive transitions
Optical Transition	978.854	nm	Vacuum wavelength ($\text{^2F}{7/2} \rightarrow \text{^2F}{5/2}$)
Laser Linewidth	$\approx 1$	MHz	Used for optical pumping (OP)
Spin Polarization Achieved	$> 90$	%	Achieved after 20 s of OP
Optical $T_{2,o}$ (Thermal Eq.)	$278 \pm 20$	µs	Coherence lifetime without DEOP
Optical $T_{2,o}$ (DEOP Enhanced)	$782 \pm 30$	µs	Coherence lifetime after 10 s DEOP
Homogeneous Linewidth ($\Gamma_{h,o}$)	$407 \pm 15$	Hz	Narrowest reported at zero magnetic field for RE (excluding $\text{Eu}^{3+}$)
$T_{2}$ Enhancement Factor	2.5	Fold	Increase achieved via DEOP

Key Methodologies

The Diffusion Enhanced Optical Pumping (DEOP) technique was implemented using precise optical and cryogenic control:

Sample Preparation: A single crystal of $\text{Y}{2}\text{SiO}{5}$ was doped with 10 ppm of isotopically purified $^{171}\text{Yb}^{3+}$ (94% purity) via the Czochralski technique.
Cryogenic Environment: The sample was placed in a liquid helium bath cryostat and maintained at 2 K to minimize spin-lattice relaxation (SLR) rates.
Optical Pumping (OP): A narrow linewidth ($\approx 1 \text{ MHz}$) tunable single mode diode laser (Toptica DL 100) was used to excite the $\text{^2F}{7/2} \rightarrow \text{^2F}{5/2}$ transition at 978.854 nm.
DEOP Implementation: The laser was fixed at a specific frequency for 10s of seconds (up to 20 s) to optically pump a small fraction ($\approx 0.5%$) of ions, initiating the spin diffusion process.
Population Measurement: Absorption spectra were recorded after OP to determine the normalized ground state spin level populations ($k_{ig}$), confirming polarization levels up to $96 \pm 1%$.
Coherence Measurement: Optical coherence lifetimes ($T_{2,o}$) were measured using a standard Hahn photon echo sequence ($\pi/2 - \tau - \pi - \tau - \text{echo}$) to quantify the reduction in spin-spin interactions.

6CCVD Solutions & Capabilities

The demonstrated DEOP mechanism is highly relevant for engineering robust, scalable quantum systems. While this paper focuses on rare-earth doped crystals, the authors explicitly note the applicability of DEOP to color centers in diamond, which is 6CCVD’s core expertise.

6CCVD is uniquely positioned to supply the advanced diamond materials and fabrication services necessary to replicate and extend this breakthrough research into practical quantum devices.

Applicable Materials for DEOP Implementation

To leverage the DEOP mechanism for quantum computing, memory, and sensing applications based on diamond color centers (e.g., NV$^{-}$, SiV$^{-}$), 6CCVD recommends the following engineered materials:

6CCVD Material Grade	Specification & Relevance	Customization Potential
Optical Grade SCD	Ultra-high purity, low-strain Single Crystal Diamond. Essential for maximizing the intrinsic $T_{2}$ of color centers (like NV or SiV) by minimizing background spin noise (the “spin bath”).	Custom nitrogen or silicon doping levels for precise color center creation.
High-Purity PCD	Polycrystalline Diamond plates up to 125mm in diameter. Ideal for scaling up quantum sensor arrays or integrating large-area microwave circuits.	Custom dimensions up to 125mm diameter, with polishing down to $\text{Ra} < 5 \text{ nm}$ for optimal optical coupling.
Boron-Doped Diamond (BDD)	Highly conductive BDD films. Necessary for integrating diamond quantum systems with superconducting qubits and resonators (as referenced in the paper’s discussion of zero-field operation).	Precise control over Boron doping concentration to tune conductivity and minimize microwave loss.

Customization Potential for Advanced Quantum Architectures

The integration of quantum materials often requires complex geometries and specialized interfaces. 6CCVD offers full in-house engineering capabilities to meet these demands:

Custom Dimensions and Thickness: We provide SCD and PCD plates with custom dimensions up to 125mm, and precise thickness control (SCD/PCD from $0.1 \text{ µm}$ to $500 \text{ µm}$) for integration into nanophotonic cavities or microwave circuits.
Ultra-Low Roughness Polishing: Achieving high-fidelity optical coupling requires exceptional surface quality. 6CCVD guarantees $\text{Ra} < 1 \text{ nm}$ for SCD and $\text{Ra} < 5 \text{ nm}$ for inch-size PCD, critical for minimizing scattering losses.
Integrated Metalization Services: The paper discusses interfacing with superconducting qubits and resonators. We offer internal metalization capabilities, including Ti, Pt, Au, Pd, W, and Cu, allowing researchers to define custom contact pads, waveguides, or superconducting circuits directly onto the diamond substrate.

Engineering Support

6CCVD’s in-house PhD team specializes in the material science of MPCVD diamond for quantum applications. We can assist researchers in:

Spin Bath Engineering: Applying the principles of DEOP to diamond systems by optimizing isotopic purity (e.g., $^{12}\text{C}$ enrichment) and controlling defect concentrations to minimize the spin bath noise and maximize coherence times.
Material Selection: Consulting on the optimal diamond grade (SCD vs. PCD) and doping strategy required for specific quantum memory or optical-microwave transducer projects.
Global Logistics: Ensuring reliable global shipping (DDU default, DDP available) for sensitive, high-value quantum materials.

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

View Original Abstract

Optically addressable spins are actively investigated in quantum\ncommunication, processing and sensing. Optical and spin coherence lifetimes,\nwhich determine quantum operation fidelity and storage time, are often limited\nby spin-spin interactions, which can be decreased by polarizing spins in their\nlower energy state using large magnetic fields and/or mK range temperatures.\nHere, we show that optical pumping of a small fraction of ions with a fixed\nfrequency laser, coupled with spin-spin interactions and spin diffusion, leads\nto substantial spin polarization in a paramagnetic rare earth doped crystal,\n$^{171}$Yb$^{3+}$:YSO. Indeed, up to more than 90 % spin polarizations have\nbeen achieved at 2 K and zero magnetic field. Using this spin polarization\nmechanism, we furthermore demonstrate an increase in optical coherence lifetime\nfrom 0.3 ms to 0.8 ms, due to a strong decrease in spin-spin interactions. This\neffect opens the way to new schemes for obtaining long optical and spin\ncoherence lifetimes in various solid-state systems such as ensembles of rare\nearth ions or color centers in diamond, which is of interest for a broad range\nof quantum technologies.\n

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevx.10.031060