A Hahn-Ramsey scheme for dynamical decoupling of single solid-state qubits

At a Glance

Metadata	Details
Publication Date	2022-11-29
Journal	Frontiers in Photonics
Authors	Nikola Sadzak, Alexander Carmele, Claudia Widmann, Christoph E. Nebel, Andreas Knorr
Institutions	Technische Universität Berlin, Humboldt-Universität zu Berlin
Citations	1
Analysis	Full AI Review Included

Executive Summary

This research introduces and experimentally validates a novel Hahn-Ramsey dynamical decoupling (DD) scheme designed to enhance the coherence and visibility of solid-state qubits, specifically Nitrogen-Vacancy (NV) centers in diamond.

Core Achievement: Demonstrated a Hahn-Ramsey sequence using variable detuning (Δ) of radiofrequency (RF) pulses to modulate the filter function, achieving superior noise suppression.
Coherence Improvement: The electron spin coherence time (T₂) was extended from (1.9 ± 0.1) µs (standard Ramsey) to (3.1 ± 0.1) µs (Hahn-Ramsey) in a low-noise environment.
Noise Filtering: The scheme provides strong suppression of low-frequency magnetic noise, approaching the performance limit of the standard Hahn-Echo sequence.
DC Magnetometry: When the detuning (Δ) is comparable to the Rabi frequency (Ω_Rabi), the sequence generates a detuning-dependent component (cos(Δτ)) that is sensitive to small DC magnetic fields, offering improved sensitivity for magnetometry.
Robustness: The method is robust in noisy environments and provides performance improvements (up to ~1.5x sensitivity factor) compared to classic Ramsey interferometry, particularly when low-power RF pulses are utilized.

Technical Specifications

Parameter	Value	Unit	Context
Qubit System	Single NV Center	-	Solid-state qubit in [111] CVD-grown diamond.
Initial Ramsey T₂	1.9 ± 0.1	µs	Coherence time measured without DD.
Hahn-Ramsey T_2,HR	3.1 ± 0.1	µs	Coherence time achieved with DD scheme (low noise).
Static Magnetic Field (B₀)	~500	G	Applied parallel to NV axis for nuclear spin hyperpolarization.
Optical Initialization	532	nm	Diode laser wavelength used for polarization and readout.
Objective Numerical Aperture (NA)	1.4	-	Oil immersion objective used for collection.
RF Pulse Delivery Medium	50	µm	Thickness of copper wire/stripline used for RF delivery.
Nominal Detuning (Δ)	4	MHz	Used in the first demonstration (Δ < Ω_Rabi condition).
Artificial Noise Correlation Time (τ_c)	0.5	µs	Used in controlled noise experiments (Poissonian statistics).
Artificial Noise Bandwidth	2	MHz	-3 dB bandwidth of added noise.
Artificial Noise Strength (Γ)	~200	kHz	Jump intensity variance of added noise.
Sensitivity Scaling (η_HR)	∝ 1 / (3πγ√T_2,HR)	-	Derived sensitivity for DC magnetometry (maximized at θ ≈ 0.2π).

Key Methodologies

The experimental demonstration utilized a delta-doped diamond plate containing ¹⁵NV centers, manipulated and measured using a confocal microscopy setup integrated with RF control.

Material and Setup: A type [111] CVD-grown diamond plate was used. RF pulses were delivered via a 50 µm thick copper stripline placed on a microwave waveguide.
Initialization and Readout: NV centers were optically initialized and read out using a pulsed 532 nm laser focused through an NA = 1.4 oil immersion objective. Fluorescence was collected and measured by single photon detectors.
Spin Preparation: A static magnetic field of approximately 500 G was applied parallel to the NV axis to achieve nuclear spin hyperpolarization (80%-90%), creating an effective two-level spin-1/2 system (m_s = 0, -1).
Ramsey Sequence: The baseline coherence was established using the standard Ramsey sequence (π/2 pulse, free evolution τ, π/2 pulse).
Hahn-Ramsey Sequence: The DD sequence consisted of an initial and final π/2 pulse with detuning +Δ, separated by a free precession time τ, and a central refocusing π pulse with opposite detuning -Δ.
Detuning Control: Experiments were performed under two conditions: low detuning (Δ < Ω_Rabi) to maximize T₂ extension, and high detuning (Δ ≈ Ω_Rabi or Δ > Ω_Rabi) to maximize DC magnetic field sensitivity.
Noise Characterization: In the second phase, controlled magnetic noise (simulating classical Poissonian noise) was added externally via a solenoid driven by a function generator to test the sequence’s filtering capabilities against known spectral densities.

Commercial Applications

The enhanced coherence and sensing capabilities provided by the Hahn-Ramsey DD scheme are critical for advancing solid-state quantum technologies.

Quantum Sensing and Metrology:
- DC Magnetometry: Highly sensitive detection of static magnetic fields, crucial for geological surveys, medical imaging (MRI), and fundamental physics research.
- Nanoscale Sensing: Probing magnetic fields generated by single molecules or magnetic domain particles (electron spin relaxometry).
Quantum Information Processing (QIP):
- Quantum Memory: Utilizing NV centers as robust quantum memories with extended T₂ coherence times, essential for long-distance quantum communication and quantum computing architectures.
- Qubit Control: Providing robust control sequences that maintain coherence in complex, noisy solid-state environments.
Quantum Synchronization:
- Developing highly stable quantum clocks and synchronization systems by leveraging the long coherence times of electron spin states in diamond.
Integrated Quantum Devices:
- The scheme’s effectiveness in low-power RF regimes supports the development of compact, portable, and integrated quantum sensors and processors where power constraints are significant.

View Original Abstract

Spin systems in solid state materials are promising qubit candidates for quantum information in particular as quantum memories or for quantum sensing. A major prerequisite here is the coherence of spin phase oscillations. In this work, we show a control sequence which, by applying RF pulses of variable detuning, allows to increase the visibility of spin phase oscillations. We experimentally demonstrate the scheme on single NV centers in diamond and analytically describe how the NV electron spin phase oscillations behave in the presence of classical noise models. We hereby introduce detuning as the enabling factor that modulates the filter function of the sequence, in order to achieve a visibility of the Ramsey fringes comparable to or longer than the Hahn-echo T 2 time and an improved sensitivity to DC magnetic fields in various experimental settings.

Tech Support

Original Source

DOI: https://doi.org/10.3389/fphot.2022.932944

References

2012 - Ultrasensitive magnetic field detection using a single artificial atom [Crossref]
2020 - Sensitivity optimization for nv-diamond magnetometry [Crossref]
2011 - Dynamical decoupling sequence construction as a filter-design problem [Crossref]
2019 - Non-markovian features in semiconductor quantum optics: Quantifying the role of phonons in experiment and theory [Crossref]
2010 - Universal dynamical decoupling of a single solid-state spin from a spin bath [Crossref]
2017 - Quantum sensing [Crossref]
2009 - Demonstration of two-qubit algorithms with a superconducting quantum processor [Crossref]
2007 - Quantum register based on individual electronic and nuclear spin qubits in diamond [Crossref]
2013 - Detection of a few metallo-protein molecules using color centers in nanodiamonds [Crossref]
2014 - Room-temperature detection of a single 19nm super-paramagnetic nanoparticle with an imaging magnetometer [Crossref]