[Paper Review] Coherent spin dynamics of hyperfine-coupled vanadium impurities in silicon carbide

Joop Hendriks, Carmem M. Gilardoni+3 more|arXiv (Cornell University)|Oct 18, 2022

Diamond and Carbon-based Materials Research4 citations

TL;DR

This study investigates coherent spin dynamics of hyperfine-coupled vanadium defects in 4H-SiC, demonstrating dephasing times up to 7.2 μs and Hahn spin-echo coherence lifetimes exceeding 26 μs. Despite strong coupling to neighboring 29Si nuclear spins, the central vanadium spin maintains long coherence, identifying its potential as a robust quantum register in a scalable platform.

ABSTRACT

Progress with quantum technology has for a large part been realized with the nitrogen-vacancy centre in diamond. Part of its properties, however, are nonideal and this drives research into other spin-active crystal defects. Several of these come with much stronger energy scales for spin-orbit and hyperfine coupling, but how this affects their spin coherence is little explored. Vanadium in silicon carbide is such a system, with technological interest for its optical emission at a telecom wavelength and compatibility with semiconductor industry. Here we show coherent spin dynamics of an ensemble of vanadium defects around a clock-transition, studied while isolated from, or coupled to neighbouring nuclear spins. We find spin dephasing times up to 7.2 $μ$s, and via spin-echo studies coherence lifetimes that go well beyond tens of microseconds. We demonstrate operation points where strong coupling to neighbouring nuclear spins does not compromise the coherence of the central vanadium spin, which identifies how these can be applied as a coherent spin register. Our findings are relevant for understanding a wide class of defects with similar energy scales and crystal symmetries, that are currently explored in diamond, silicon carbide, and hexagonal boron nitride.

Motivation & Objective

• To investigate coherent spin dynamics of vanadium defects in 4H-SiC under varying nuclear spin environments.
• To determine how hyperfine coupling with neighboring 29Si nuclear spins affects spin dephasing and coherence times.
• To evaluate the effectiveness of spin-echo techniques in preserving coherence in the presence of strong nuclear spin interactions.
• To identify operation points where strong hyperfine coupling does not degrade central spin coherence, enabling use as a quantum register.
• To assess the viability of vanadium defects in SiC as a scalable, room-temperature-compatible platform for quantum technologies.

Proposed method

• Optically detected magnetic resonance (ODMR) was used to probe spin states of vanadium defects at 2 K, with tunable optical probing wavelengths to selectively address subensembles.
• Ramsey interference measurements were performed to extract dephasing times (T2*) by applying two microwave pulses with variable delay.
• Hahn spin-echo sequences were applied to measure coherence times (T2) by refocusing dephasing due to inhomogeneous broadening and hyperfine interactions.
• Microwave driving at 25 dBm power was used to induce Rabi oscillations and Ramsey fringes, with signal modulation analyzed to extract coherence properties.
• Subensembles were distinguished by optical wavelength: 1278.76 nm probed defects with nearby 29Si nuclear spins (DT I/II), while 1278.86 nm targeted those without (DT 0).
• Simulations of ODMR spectra were used to validate experimental data, fitting with five Gaussian line shapes to account for hyperfine splitting and inhomogeneous broadening.

Experimental results

Research questions

• RQ1 What is the impact of hyperfine coupling to neighboring 29Si nuclear spins on the dephasing time (T2*) of vanadium defects in 4H-SiC?
• RQ2 Can Hahn spin-echo sequences preserve coherence in vanadium defects despite strong hyperfine coupling, and what is the resulting T2 lifetime?
• RQ3 How does optical probing wavelength selectively access subensembles with different nuclear spin configurations (DT 0, DT I, DT II) in the vanadium defect ensemble?
• RQ4 Are there operation points—such as clock transitions—where strong hyperfine coupling does not compromise central spin coherence?
• RQ5 To what extent do dynamical decoupling sequences remain effective in systems with anisotropic coupling to magnetic fields, as in this defect system?

Key findings

• The longest observed dephasing time (T2*) for vanadium defects in 4H-SiC was 7.2 μs, measured at a magnetic field of 30 mT near a clock transition.
• Hahn spin-echo measurements showed no decay in echo amplitude up to a total free precession time of 26 μs, indicating that the coherence time (T2) is at least an order of magnitude longer than T2*.
• Optical probing at 1278.76 nm revealed sidepeaks in ODMR spectra due to hyperfine coupling with one or two neighboring 29Si nuclear spins, confirming the presence of DT I and DT II subensembles.
• Probing at 1278.86 nm suppressed the sidepeak signals, confirming selective access to the DT 0 subensemble with no nearby 29Si spins.
• The absence of echo decay over 26 μs demonstrates that strong hyperfine coupling does not necessarily degrade coherence, enabling robust quantum operation.
• The results show that vanadium defects in SiC can function as a coherent spin register even when coupled to environmental nuclear spins, due to favorable symmetry and energy scale matching.