Ramsey Interferometry Quantifies Spectator-Crosstalk in Silicon Carbide S=3/2 Qudits

Researchers are tackling the challenge of crosstalk in quantum information processing using silicon carbide, a material poised to revolutionise scalable quantum technologies. Jun-Jae Choi, Seung-Jae Hwang, and Seoyoung Paik, all from the Department of Physics and Photon Science at Gwangju Institute of Science and Technology, alongside Juhwan Kim et al., have demonstrated a novel method for understanding and mitigating unwanted interactions within a silicon vacancy qudit , a promising building block for quantum computers. Their work, utilising broadband Ramsey interferometry, reveals how seemingly harmless microwave pulses can inadvertently drive unintended transitions between energy levels, creating ‘spectator’ crosstalk. This detailed analysis not only maps these interactions with unprecedented precision, but also provides a practical framework for either suppressing this crosstalk or, crucially, harnessing it for improved quantum control and calibration , paving the way for more robust and reliable quantum devices.

The research team achieved this by employing broadband Ramsey interferometry, a sensitive technique for probing phase evolution in quantum systems, to map the complex interplay of energy levels and coherent pathways within the SiV centre. Compact amplitudes, determined by the initial state preparation and microwave pulse parameters, were assigned to each line, providing a clear understanding of the contribution from different transitions. Crucially, numerical time-domain propagation, using experimental sampling parameters, accurately reproduced the observed detuning map, with measured peak positions coinciding with the analytic predictions without any frequency fitting.

This level of agreement validates the analytical model and confirms the accurate identification of spectator transitions and their impact on the system’s behaviour. The approach provides clear direction for in-situ pulse calibration, enhancing the precision of quantum gate operations, and for phase-sensitive state and process estimation, improving the fidelity of quantum measurements. This work opens possibilities for utilising these spectator lines as additional constraints, refining control and characterisation of the SiV qudit. The study highlights how short, detuned microwave pulses can coherently drive these non-addressed level pairs, creating crosstalk that can limit performance. By meticulously characterising these off-resonant pathways, the team has not only identified the source of these errors but also provided a pathway to mitigate or harness them, paving the way for more robust and efficient quantum devices. This detailed understanding of multilevel dynamics is crucial for realising the full potential of the SiV centre as a building block for future quantum networks and repeaters.

Spectator Crosstalk in Silicon Vacancy Qudits reveals new

Scientists investigated color centers in 4H-SiC, identifying them as promising building blocks for scalable quantum technologies due to their wafer-scale maturity, long spin coherence, and chip-level photonics. The study focused on the silicon vacancy, possessing a unique S=3/2 ground state, a native qudit, but acknowledged the potential for crosstalk arising from spectator levels driven by off-resonant pulses. Researchers employed broadband Ramsey interferometry to reveal and quantify this spectator-induced crosstalk, a crucial step towards precise qudit control. Experimentally, the team utilized a 28μm thick 4H-SiC layer grown by chemical vapor deposition on an n-type a-plane 4H-SiC wafer, achieving an isotope purity of ~99.85% for 28Si and ~99.98% for 12C, verified by secondary ion mass spectrometry.

Prior to silicon vacancy creation, the layer underwent annealing at 1150°C for one hour to minimise surface defects, followed by electron irradiation with 2 MeV electrons at a fluence of 1×1012cm-2, delivered at a flux of 4.8×1010cm-2s-1 at room temperature, creating a low density of approximately one V1 center per 1 μm2. Subsequent annealing at 400°C for 30 minutes removed interstitial-related defects, and 200 keV proton irradiation with a fluence of 1×1015cm-2, through a 60μm aluminum mask, generated a bright ensemble reference for magnetic field alignment. The experiments were conducted within a cryogenic confocal microscope, cooled to ~4-5 K using a closed-cycle cryostat (Montana Instruments s200). A three-axis low-temperature nanopositioning stage facilitated sample scanning, while a Zeiss EC Epiplan-Neofluar 100×, NA 0.75 air objective focused the excitation beam.

A 785nm laser (Thorlabs L785H1) depolarized ground state spin-sublevels, and an 861.4nm external-cavity diode laser (TOPTICA DL pro) resonated with the zero-phonon line (ZPL), stabilized by a wavemeter (HighFinesse WS-8-10) referenced to a frequency-stabilized He-Ne laser (632.991405nm, ±5MHz). Acousto-optic modulators enabled pulsed operation for Ramsey sequences, and the combined beams were directed via a dichroic mirror (Thorlabs DMLP805). Collected fluorescence, filtered by an 875nm long-pass filter (Thorlabs FELH0875), was detected using a superconducting nanowire single-photon detector (Single Quantum Eos R12). Microwave control was provided by a vector signal generator (Rohde & Schwarz SMBV100A), and an FPGA-based control unit (National Instrument) generated timing triggers for lasers, microwave pulses, and photon event counting. A static magnetic field was applied using an external permanent magnet positioned outside the cryostat.

Spectator-crosstalk enables six-branch qudit control with high fidelity

Analytically, researchers mapped each spectral line to a pairwise energy difference between qudit levels within the rotating-frame Hamiltonian, assigning weights determined by the prepared state and microwave pulse parameters. This mapping predicted a deterministic six-branch structure, providing a clear understanding of the system’s behaviour. The team measured peak positions coinciding with the analytic branch lines without any frequency fitting, validating the theoretical model and demonstrating exceptional precision in the experimental setup. Numerical time-domain propagation, using experimental sampling parameters, accurately reproduced the detuning map, further solidifying the connection between theory and observation.

Researchers focused on the negatively charged silicon vacancy, notable for its spin-3/2 ground state, which realises a natural four-level qudit or ‘ququart’. This unique structure supports higher information density and reduced circuit depth in certain quantum architectures, alongside hardware-efficient quantum error correction and multi-transition sensing. However, closely spaced transitions render the system susceptible to off-resonant driving, potentially causing coherent leakage and crosstalk, unintended excitation mediated by residual couplings. Using optically detected magnetic resonance (ODMR), the study implemented broadband Ramsey spectroscopy to probe these multilevel dynamics, resolving crosstalk and coherent spectator excitation among the sublevels.

Specifically, the VV1 center, exhibiting a ground-state zero-field splitting of approximately 4.5MHz, was used as a testbed, bringing neighbouring transitions within the bandwidth of short π/2 pulses. Measurements confirmed that a resonant drive seeds coherent amplitudes in spectator transitions, accumulating multiple relative phases during free precession and ultimately manifesting as detuning-dispersive, multi-frequency Ramsey spectra. This breakthrough delivers a pathway towards enhanced control and characterisation of quantum systems in silicon carbide.

Six-component structure unlocks qudit control in silicon vacancy

This research reveals that Ramsey responses comprise coherences across four spin sublevels, identified through both experimental measurements and analytical mapping. This framework offers guidance for suppressing unwanted crosstalk or, conversely, exploiting spectator lines for enhanced state and process estimation. The authors acknowledge that short, finite-bandwidth microwave pulses inevitably seed coherences in off-resonant sublevel pairs, leading to intrinsic crosstalk, a fundamental aspect of the measurement rather than a minor imperfection. Future research directions include systematic exploration of pulse parameter regimes to minimize or shape crosstalk, and investigations into the impact of spectator transitions on dynamical decoupling sequences, potentially leading to improved quantum control and metrology.