Extending qubit coherence by adaptive quantum environment learning
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-03-01 |
| Journal | New Journal of Physics |
| Authors | Eleanor Scerri, Erik M. Gauger, Cristian Bonato, Eleanor Scerri, Erik M. Gauger |
| Institutions | Heriot-Watt University |
| Citations | 14 |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: Extending Qubit Coherence in MPCVD Diamond
Section titled âTechnical Documentation & Analysis: Extending Qubit Coherence in MPCVD DiamondâThis document analyzes the research paper âExtending qubit coherence by adaptive quantum environment learningâ and maps its material requirements and technical achievements directly to the advanced capabilities of 6CCVDâs MPCVD diamond products.
Executive Summary
Section titled âExecutive SummaryâThe research demonstrates a significant breakthrough in extending solid-state qubit coherence using adaptive quantum environment learning applied to Nitrogen-Vacancy (NV) centers in diamond.
- Core Achievement: Implementation of an adaptive Bayesian protocol to learn and narrow the probability distribution of the surrounding nuclear spin bath ($^{13}$C).
- Coherence Enhancement: The protocol deterministically increased the qubit coherence time ($T_2$) by a factor of 10, from approximately $10 \mu\text{s}$ (non-adaptive) to $\sim 100 \mu\text{s}$ (adaptive).
- Material System: The study focuses on the electronic spin of an NV center in diamond coupled to a dilute bath of $^{13}$C nuclear spins.
- Methodology: Real-time adaptation of Ramsey measurement parameters (sensing time $\tau$ and detection angle $\phi$) based on previous measurement outcomes.
- Repeatability: The scheme demonstrated repeatable spin bath refocusing, allowing extended coherence to be maintained indefinitely through intermittent narrowing sequences.
- Material Requirement: Success hinges on using high-quality, isotopically controlled diamond (low $^{13}$C concentration) to minimize intrinsic decoherence sources.
Technical Specifications
Section titled âTechnical SpecificationsâThe following hard data points were extracted from the simulation and experimental context described in the paper:
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Qubit System | NV Center Electronic Spin | N/A | Solid-state quantum technology platform |
| Spin Bath Composition | $^{13}$C Nuclear Spins | N/A | Dilute environment (Natural abundance $\sim 1.1%$) |
| Required $^{13}$C Concentration | As low as 0.01% | % | For isotopically modified diamond samples |
| Initial Coherence Time ($T_2$) | $\sim 10$ | $\mu\text{s}$ | Non-adaptive Ramsey sequence performance |
| Final Coherence Time ($T_2$) | $\sim 100$ | $\mu\text{s}$ | Achieved via adaptive Bayesian protocol |
| Narrowing Factor (N.F.) Enhancement | $\ge 10$ | Factor | Average improvement over 100 simulated spin baths |
| Applied Magnetic Field ($B_z$) | 250 | $\text{mT}$ | External field aligned along the NV axis |
| Minimum Sensing Time ($\tau_0$) | 1 | $\mu\text{s}$ | Shortest measurement time used in the protocol |
| Repetition Parameters (G, F) | $G=3, F=2$ | N/A | Used for the $M_k = G + kF$ Ramsey sequence repetitions |
| Operating Temperature | Cryogenic | K | Assumed for high-fidelity initialization and readout |
Key Methodologies
Section titled âKey MethodologiesâThe adaptive protocol relies on real-time feedback and Bayesian estimation to optimize the measurement sequence.
- System Model: Simulation of an NV electronic spin coupled to a bath of up to 10 $^{13}$C nuclear spins, requiring full quantum simulation due to electron-nuclear correlations.
- Initial State: Nuclear spin bath is assumed to be in a thermal (completely mixed) state, resulting in a broad, uniform probability distribution $P(A_z)$ for the hyperfine field.
- Ramsey Measurement Sequence: Interference experiments are performed to detect the phase acquired by the central spin, providing partial information on the hyperfine field projection ($A_z$).
- Bayesian Update: After each measurement outcome ($\mu_m \in {0, 1}$), the classical probability distribution $P(A_z)$ representing knowledge of the bath is updated using Bayesâ theorem.
- Adaptive Parameter Selection:
- Sensing Time ($\tau$): The optimal sensing time $\tau_{k_{opt}}$ is chosen to be as close as possible to the current estimated coherence time $T_2^*$, derived from the Holevo variance $V_H(A_z)$.
- Detection Angle ($\phi$): The rotation angle $\phi_{opt}$ is locally optimized to minimize the Holevo variance in Fourier space, steering the distribution towards a narrow, uni-modal state.
- Refocusing Scheme: The adaptive narrowing sequence is applied intermittently (e.g., 1ms narrowing followed by 8ms free evolution) to counteract spin bath diffusion and maintain extended coherence time.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThe success of this quantum environment learning technique is fundamentally dependent on the quality and isotopic purity of the diamond material. 6CCVD is uniquely positioned to supply the necessary Single Crystal Diamond (SCD) substrates to replicate and advance this research.
| Research Requirement | 6CCVD Material Solution | 6CCVD Technical Capability & Sales Advantage |
|---|---|---|
| Ultra-Low $^{13}$C Concentration (Required to minimize background decoherence and isolate the dilute spin bath) | Isotopically Engineered SCD | We specialize in MPCVD growth of SCD with controlled isotopic purity, offering wafers with $^{13}$C concentrations significantly below natural abundance (e.g., < 100 ppm), essential for achieving long intrinsic $T_2$ times. |
| High-Fidelity Qubit Platform (Requires low defect density and high surface quality for optical control) | Optical Grade Single Crystal Diamond (SCD) | SCD plates up to 500 $\mu\text{m}$ thick, featuring ultra-low strain and defect density, ensuring stable NV center formation and optimal performance at cryogenic temperatures. |
| Precision Surface Finish (Critical for high-fidelity optical initialization and readout) | Ultra-Low Roughness Polishing | Internal polishing capability guarantees surface roughness $R_a < 1\text{nm}$ on SCD wafers, minimizing scattering losses and surface-related decoherence. |
| Custom Device Integration (Need for specific geometries and electrical contacts for control pulses) | Custom Dimensions & Metalization | We provide custom dimensions and laser cutting services for plates/wafers up to 125mm. Our internal metalization capability supports deposition of Au, Pt, Ti, and W contacts required for high-speed programmable electronics and qubit control. |
| Replication and Extension of Hamiltonian Learning (Complex protocols require deep material understanding) | Expert Engineering Support | 6CCVDâs in-house PhD team can assist researchers in selecting the optimal SCD growth parameters (e.g., nitrogen concentration, isotopic purity) to tailor the material for similar Quantum Sensing and Coherence Extension projects. |
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
Abstract Decoherence, resulting from unwanted interaction between a qubit and its environment, poses a serious challenge towards the development of quantum technologies. Recently, researchers have started analysing how real-time Hamiltonian learning approaches, based on estimating the qubit state faster than the environmental fluctuations, can be used to counteract decoherence. In this work, we investigate how the back-action of the quantum measurements used in the learning process can be harnessed to extend qubit coherence. We propose an adaptive protocol that, by learning the qubit environment, narrows down the distribution of possible environment states. While the outcomes of quantum measurements are random, we show that real-time adaptation of measurement settings (based on previous outcomes) allows a deterministic decrease of the width of the bath distribution, and hence an increase of the qubit coherence. We numerically simulate the performance of the protocol for the electronic spin of a nitrogen-vacancy centre in diamond subject to a dilute bath of 13 C nuclear spin, finding a considerable improvement over the performance of non-adaptive strategies.
Tech Support
Section titled âTech SupportâOriginal Source
Section titled âOriginal SourceâReferences
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