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Finding Clock Transitions in Point Defects using High-Throughput Data

Alice Eriksson

Published Jan 1, 2026• 0 citations
OpenAlex

Abstract

Quantum computing has the potential to meet the increasing demand for computational power by solving some unique issues more efficiently than classical computers. To realize the full capabilities of quantum computing, a reliable physical implementation of quantum bits is essential. Point defects in semiconductors may offer a promising solution for this implementation. However, point defects, like most qubit implementations, would benefit from increased coherence times. For certain point defects, this improvement comes from clock transitions, where the transition frequency becomes locally independent of the magnetic field. This thesis consists of a thorough analysis of clock transitions in point defects. First, the clock transition of the bismuth vacancy (BiV) in CaO is reproduced. Then, an automatic workflow for finding clock transitions and converging computational parameters are built as an extension to the ADAQ (Automatic defect analysis andqualification) software. 10243 clock transitions were found in a set of 3704 defects, 143 of these are due to zero-field splitting, and the rest due to hyperfine interactions. Clock transitions that arise only from zero-field splitting have a clear correlation between separation and the 2nd derivative. Clock transitions that emerge from hyperfine interactions have no such clear trend, meaning that they need to be studied in more detail for individual defects.

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arXiv:2302.07523•Nature Communications•Jun 6, 2024•21 citations

Cluster correlation expansion for studying decoherence of clock transitions in spin baths

Geng-Li Zhang, Wen-Long Ma, Ren‐Bao Liu

The clock transitions (CTs) or optimal working points (OWPs) of central qubits have long coherence times because their frequency fluctuations vanish in the linear order of external field noise (such as Overhauser fields from nuclear spin baths). Therefore, CTs (or OWPs) are useful for quantum technologies. Also, the quadratic dependence of frequencies on noises makes the CT decoherence an interesting physics problem. Thus we are motivated to study the decoherence of CTs. We consider noise from spin baths, which is one of the most relevant mechanisms of qubit decoherence. Various quantum many-body methods have been developed to study the decoherence of a central spin in spin baths. In particular, the cluster-correlation expansion (CCE) systematically accounts for the many-body correlations that cause the central spin decoherence. However, the CCE cannot be straightforwardly applied to CTs in spin baths, for the expansion may fail to converge due to the effective long-range interactions resulting from the quadratic term of the noise (e.g., the second-order interaction mediated by hyperfine interactions for a nuclear spin bath). In this work, we develop a modified CCE method to tackle this class of decoherence problems. By diagonalizing the central spin Hamiltonian for each bath eigenstate of the hyperfine interaction, we find that the effects of long-range interactions are absorbed as fluctuations of central spin eigenenergies in the form of single-spin correlations. We apply the method to two important systems, namely, nitrogen vacancy center electron spins in near zero magnetic field and the singlet-triplet transition of two electrons in a double quantum dot. The numerical simulation shows that the modified CCE converges rapidly for the CTs. This modified cluster correlation expansion provides a nonperturbative approach to treating the central spin mediated long-range interactions in central spin decoherence problems.

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Yihuang Xiong, Yizhi Zhu, Shay McBride +2 more

Point defects in semiconductors are becoming central to quantum technologies. They can be used as spin qubits interfacing with photons, which are fundamental for building quantum networks. Currently, the most prominent quantum defect in diamond is the nitrogen-vacancy (NV) center. However, it suffers from spectral diffusion that negatively impacts optical coherence and is due to the coupling of the emission energy with uncontrolled electric fields. The group IV vacancy complexes on the other hand have shown to be significantly more spectrally-stable as they are centrosymmetric and thus immune to the linear Stark shift. They however suffer from several issues ranging from low operation temperature to low optical efficiency due to dark states and difficulty in stabilizing the right defect charge state. Here we search for alternative to the group IV vacancy complex in diamond by systematically evaluating all possible vacancy complex using high-throughput first-principles computational screening. We identify the defects that combine centrosymmetry, emission in the visible range, as well as favorable and achievable electronic structure promoting higher operation temperature and defect levels well within the band gap. We find Zn$V^{-2}$ to be especially appealing.

arXiv:2504.11598•Apr 15, 2025•0 citations
Quantum computing has the potential to meet the increasing demand for computational power by solving some unique issues more efficiently than classical computers. To realize the full capabilities of quantum computing, a reliable physical implementation of quantum bits is essential. Point defects in semiconductors may offer a promising solution for this implementation. However, point defects, like most qubit implementations, would benefit from increased coherence times. For certain point defects, this improvement comes from clock transitions, where the transition frequency becomes locally independent of the magnetic field. This thesis consists of a thorough analysis of clock transitions in point defects. First, the clock transition of the bismuth vacancy (BiV) in CaO is reproduced. Then, an automatic workflow for finding clock transitions and converging computational parameters are built as an extension to the ADAQ (Automatic defect analysis andqualification) software. 10243 clock transitions were found in a set of 3704 defects, 143 of these are due to zero-field splitting, and the rest due to hyperfine interactions. Clock transitions that arise only from zero-field splitting have a clear correlation between separation and the 2nd derivative. Clock transitions that emerge from hyperfine interactions have no such clear trend, meaning that they need to be studied in more detail for individual defects.
Abstract Virtually noiseless due to the scarcity of spinful nuclei in the lattice, simple oxides hold promise as hosts of solid-state spin qubits. However, no suitable spin defect has yet been found in these systems. Using high-throughput first-principles calculations, we predict spin defects in calcium oxide with electronic properties remarkably similar to those of the NV center in diamond. These defects are charged complexes where a dopant atom — Sb, Bi, or I — occupies the volume vacated by adjacent cation and anion vacancies. The predicted zero phonon line shows that the Bi complex emits in the telecommunication range, and the computed many-body energy levels suggest a viable optical cycle required for qubit initialization. Notably, the high-spin nucleus of each dopant strongly couples to the electron spin, leading to many controllable quantum levels and the emergence of atomic clock-like transitions that are well protected from environmental noise. Specifically, the Hanh-echo coherence time increases beyond seconds at the clock-like transition in the defect with 209Bi. Our results pave the way to designing quantum states with long coherence times in simple oxides, making them attractive platforms for quantum technologies.
The clock transitions (CTs) or optimal working points (OWPs) of central qubits have long coherence times because their frequency fluctuations vanish in the linear order of external field noise (such as Overhauser fields from nuclear spin baths). Therefore, CTs (or OWPs) are useful for quantum technologies. Also, the quadratic dependence of frequencies on noises makes the CT decoherence an interesting physics problem. Thus we are motivated to study the decoherence of CTs. We consider noise from spin baths, which is one of the most relevant mechanisms of qubit decoherence. Various quantum many-body methods have been developed to study the decoherence of a central spin in spin baths. In particular, the cluster-correlation expansion (CCE) systematically accounts for the many-body correlations that cause the central spin decoherence. However, the CCE cannot be straightforwardly applied to CTs in spin baths, for the expansion may fail to converge due to the effective long-range interactions resulting from the quadratic term of the noise (e.g., the second-order interaction mediated by hyperfine interactions for a nuclear spin bath). In this work, we develop a modified CCE method to tackle this class of decoherence problems. By diagonalizing the central spin Hamiltonian for each bath eigenstate of the hyperfine interaction, we find that the effects of long-range interactions are absorbed as fluctuations of central spin eigenenergies in the form of single-spin correlations. We apply the method to two important systems, namely, nitrogen vacancy center electron spins in near zero magnetic field and the singlet-triplet transition of two electrons in a double quantum dot. The numerical simulation shows that the modified CCE converges rapidly for the CTs. This modified cluster correlation expansion provides a nonperturbative approach to treating the central spin mediated long-range interactions in central spin decoherence problems.
This thesis describes studies on atomic point defects in semiconductors for applications in quantum information. In particular, its focus is on defects whose energy levels reside deep within the bandgap certain semiconductors because these systems can behave like artificial trapped atoms in the solid state, possessing optical addressability, long coherence times, and the ability to be manipulated at microwave frequencies. The most prominent defect in this field of study is the diamond nitrogen-vacancy center, whose functionality is well-known to persist to ambient temperature. The first part of this thesis investigates the persistence of this functionality to temperatures upwards of 600 K. Importantly, these investigations also demonstrate that the diamond nitrogen-vacancy center can be a sensitive thermometer (~10 mK/Hz^(1/2)) over a wide range of temperatures with nanoscale spatial resolution. The latter part of the thesis describes experimental measurements of divacancy defects in silicon carbide, a potential alternative materials host to diamond with established wafer-scale growth and microfabrication capabilities. Single divacancy spins are isolated and coherently controlled for the first time in the 4H polytype of silicon carbide, and the divacancy's coherence is shown to be a remarkably long T2 = 1.2 ms at T = 20 K. The last chapter extends these results by isolating single divacancies in the 3C polytype, experimentally demonstrating a qualitative and quantitative model of their spin and orbital dynamics, and revealing the divacancy's excited state level structure in both the 3C and 4H polytypes using resonant laser techniques. These excited state transitions are shown to posses a structure identical to the diamond nitrogen-vacancy center and a subset are shown to have favorable cycling properties. The location of a clear spin-photon interface has important implications for high-fidelity quantum state readout and photon-based entanglement schemes that use divacancy spins.
A quantum computer will potentially solve far-reaching problems which are currently intractable on any classical computer. Many technological obstacles have prevented the realization of a quantum computer, the main obstacle being decoherence, which is the loss of quantum information. Decoherence arises from the undesired interaction between qubits and their environment. Isolated qubits have better coherence but are more difficult to control. Superconducting qubits are a promising platform since their macroscopic size allows for easy control and coupling to other qubits. While the coherence of superconducting qubits has substantially improved over the past two decades, further improvements in coherence are required.We have repeatedly and reliably increased the coherence times of superconducting qubits. Currently decoherence in these devices is dominated by coupling to material defects. These defects are present in the dielectrics used to fabricate these devices or introduced during fabrication. Using simpler resonators as a testbed, we individually isolate, characterize, and then improve each step of the more complicated fabrication of superconducting qubits. We increased the quality factor of resonators by a factor of four by first identifying the surfaces and interfaces as a major source of loss and then by optimizing the substrate preparation. Furthermore, we measure and subsequently mitigate additional defect loss, which is dependent on the position of ground plane holes used to limit the loss from magnetic vortices. Implementing these improvements led to an increase of our qubit coherence times by more than an order of magnitude.The progress made in coherence while maintaining a high degree of connectivity and controllability has been directly used in more complex circuits. One such device is a fully connected three qubit ring with both tunable qubit frequencies and adjustable qubit-qubit couplings. The considerable level of control allows us to generate the quantum dynamics resulting from a collision between Na and He atoms by encoding the problem into a time-dependent multi-qubit interaction. We compute a scattering cross section by measuring the probability of exciting the Na atom over a wide range of collision parameters. The scattering cross-section shows that there is a velocity where the excitation probability is maximized, in agreement with scattering experiments.
During the last ten years, basic quantum operations on a single electronic spins in the nitrogen-vacancy defect center in diamond have been a rich field of study. Quantum operations coherently manipulating the spin of the nitrogen-vacancy center have become promising candidates for easy to manipulate qubits. However in order to implement these qubits in quantum computers robust high-fidelity control is needed. Building on an existing confocal microscopy setup, pulsed and quadrature-amplitude modulated microwave signals were used to realize coherent spin manipulation measurements. Especially designed Smooth Optimal Control pulses (robust against detuning and different control amplitudes) were used to develop measurement schemes resulting in the aforementioned increase of fidelity. The first part of the thesis describes the implementation of these pulses into the existing experimental setup and the calibration work necessary prior to measurements, due to imperfections of devices in the microwave chain. The second part is dedicated to the analysis of the pulses applied to a single spin by comparing the experimental results to theoretical simulations. This was done not only for one resonant spin with accurate control amplitude, but also for a range of different detunings and amplitudes. A more thorough analysis of the pulses was then achieved by using state/process tomography, resulting in a complete description of the process induced by these optimized pulses. Finally, after verifying the effect of the pulses, measurement schemes for sensing alternating magnetic fields were implemented, resulting in measurements with improved sensitivity for magnetic fields.
Point defects in semiconductors are becoming central to quantum technologies. They can be used as spin qubits interfacing with photons, which are fundamental for building quantum networks. Currently, the most prominent quantum defect in diamond is the nitrogen-vacancy (NV) center. However, it suffers from spectral diffusion that negatively impacts optical coherence and is due to the coupling of the emission energy with uncontrolled electric fields. The group IV vacancy complexes on the other hand have shown to be significantly more spectrally-stable as they are centrosymmetric and thus immune to the linear Stark shift. They however suffer from several issues ranging from low operation temperature to low optical efficiency due to dark states and difficulty in stabilizing the right defect charge state. Here we search for alternative to the group IV vacancy complex in diamond by systematically evaluating all possible vacancy complex using high-throughput first-principles computational screening. We identify the defects that combine centrosymmetry, emission in the visible range, as well as favorable and achievable electronic structure promoting higher operation temperature and defect levels well within the band gap. We find ZnV−2V^{-2}V−2 to be especially appealing.