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arXiv:2305.05049 · Physical Review Research

Phonon-induced decoherence in color-center qubits

Prajit Dhara, Saikat Guha

Published Jan 16, 2024• 8 citations
arXivHTMLPDF

Abstract

Electron spin states of solid-state defects such as nitrogen- and silicon-vacancy in diamond are a leading quantum-memory candidate for quantum communications and computing. Via open-quantum-systems modeling of spin-phonon coupling—the major contributor of decoherence—at a given temperature, we derive the time dynamics of the density operator of an electron-spin qubit. We use our model to corroborate experimentally measured decoherence rates. We further derive the temporal decay of distillable entanglement in spin-spin entangled states heralded via photonic Bell-state measurements. Extensions of our model to include other decoherence mechanisms, e.g., undesired hyperfine couplings to the neighboring nuclear-spin environment, will pave the way to a rigorous predictive model for engineering artificial-atom qubits with desirable properties. Published by the American Physical Society 2024

DOI
10.1103/physrevresearch.6.013055

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Electron spin states of solid-state defects such as nitrogen- and silicon-vacancy in diamond are a leading quantum-memory candidate for quantum communications and computing. Via open-quantum-systems modeling of spin-phonon coupling—the major contributor of decoherence—at a given temperature, we derive the time dynamics of the density operator of an electron-spin qubit. We use our model to corroborate experimentally measured decoherence rates. We further derive the temporal decay of distillable entanglement in spin-spin entangled states heralded via photonic Bell-state measurements. Extensions of our model to include other decoherence mechanisms, e.g., undesired hyperfine couplings to the neighboring nuclear-spin environment, will pave the way to a rigorous predictive model for engineering artificial-atom qubits with desirable properties. Published by the American Physical Society 2024
Quantum decoherence induced by defects remains a major limitation for solid-state quantum technologies, yet predicting decoherence in realistic materials remains computationally challenging. Complex defect populations are often approximated as homogeneous spin baths, obscuring the role of defect-specific electronic structure and spin dynamics. Here, we develop a predictive framework for decoherence in diamond by combining first-principles electronic-structure calculations, quantum many-body spin-bath simulations, and experimental validation. The framework incorporates defect-resolved spin Hamiltonians and heterogeneous spin baths containing multiple paramagnetic defect species. Using diamond nitrogen-vacancy ensembles as a model platform, we investigate mixed nitrogen-, vacancy-, and hydrogen-related defect environments. We show that decoherence depends not only on defect density but also on defect identity and bath composition, whose distinct electronic structures, hyperfine interactions, and spin dynamics produce different coherence behavior. Heterogeneous defect populations can either suppress or enhance decoherence, producing trends unexplained by homogeneous-bath models. Magnetic-field-dependent Hahn-echo measurements on samples with different defect concentrations validate the framework. The calculations reproduce the observed coherence times and stretched-exponential decay behavior across a broad magnetic-field range and identify vacancy-related defects as critical contributors beyond the conventionally assumed P1 spin bath. By linking atomistic defect properties to quantum coherence, our framework provides a predictive route for identifying hidden defect environments and optimizing decoherence in defect-based quantum materials.
The negatively charged silicon-vacancy (SiV^{-}) color center in diamond has recently emerged as a promising system for quantum photonics. Its symmetry-protected optical transitions enable the creation of indistinguishable emitter arrays and deterministic coupling to nanophotonic devices. Despite this, the longest coherence time associated with its electronic spin achieved to date (∼250 ns) has been limited by coupling to acoustic phonons. We demonstrate coherent control and suppression of phonon-induced dephasing of the SiV^{-} electronic spin coherence by 5 orders of magnitude by operating at temperatures below 500 mK. By aligning the magnetic field along the SiV^{-} symmetry axis, we demonstrate spin-conserving optical transitions and single-shot readout of the SiV^{-} spin with 89% fidelity. Coherent control of the SiV^{-} spin with microwave fields is used to demonstrate a spin coherence time T_{2} of 13 ms and a spin relaxation time T_{1} exceeding 1 s at 100 mK. These results establish the SiV^{-} as a promising solid-state candidate for the realization of quantum networks.
Group-IV color centers in diamond (SiV, GeV, SnV) have emerged as leading solid-state spin-photon interfaces for quantum information processing applications. However, these qubits require cryogenic temperatures to achieve high fidelity operation due to interactions with the thermal phonon bath. In this work, we: (i) derive a detailed model of the decoherence from first-order acoustic phonon processes acting on the spin-orbit fine structure of these color centers; (ii) demonstrate agreement of the model's predicted coherence times with previous measurements; (iii) identify regimes to suppress phonon-mediated decoherence by changing magnetic-field and strain bias to allow higher temperature operation. This methodology enables prediction of decoherence processes in other color centers and solid-state qubit systems coupled to a thermal bath via a parasitic two-level system. By experiment-anchored decoherence models, we facilitate optimizing qubit coherence for specific applications and devices.
Qubit coherence is an essential figure of merit for quantum information processing applications such as quantum computing, or quantum repeaters. Understanding the coherence properties of the underlying physical qubits that facilitate such applications is therefore are often limited by coupling to lattice phonons, which in turn constrains operation temperature. Here we study phonon induced electronic spin decoherence in group-IV vacancy centers in diamond. We begin by modeling the spin-phonon interaction and then employ the widely used Born-Markov approximation, highlighting its inconsistencies in this setting and its deviations from experimental observations. To close the gap between theoretical predictions and experimental results, we relax certain approximations, investigate their contributions to the predicted coherence times, and identify the dominant sources of discrepancy. We further demonstrate that experimentally measured electronic spin coherence dynamics are consistently captured within a non-Markovian framework, and we show how the magnetic field orientation influences the qubit coherence time.
Realization of physical systems for quantum information processing fundamentally rests on the ability to generate entanglement among many quantum bits. To this end, solid state quantum systems are often considered a scalable approach due to the relative ease of generating of a large number of individual qubits. However, realizing strong coherent interactions that dominate decoherence processes in the solid state environment is a formidable challenge that has continued to motivate the investigation of new physical systems to store quantum information and new mechanisms to achieve controllable interactions. This thesis takes steps in these directions with diamond color centers, which can store optically accessible quantum information in their electronic spin. We study color center spins as acoustic two level systems, and consider whether their interaction with single phonons in diamond can be engineered to be coherent. Through spectroscopy of color centers in diamond nanomechanical devices and follow-up theoretical work, we identify the negatively charged silicon vacancy (SiV) color center as a promising candidate for phonon-mediated quantum information processing. In the process, an electromechanical device platform capable of tuning the strain environment of color centers is developed. It is used to mitigate thermal decoherence of the SiV spin and demonstrate photon-mediated quantum interference between two SiV centers on a diamond chip, highlighting the utility of nanomechanical devices for photonic quantum networks. Finally, we present high quality factor, wavelength scale acoustic resonators in diamond using phononic crystals towards a coherent interface between SiV centers and single phonons.
Abstract Spin qubits with long dephasing times are an essential requirement for the development of new quantum technologies and have many potential applications ranging from quantum information processing to quantum memories and quantum networking. Here we report a theoretical study and the calculation of the spin dephasing time of defect color centers for the negatively charged nitrogen vacancy center in diamond. We employ ab initio density functional theory to compute the electronic structure, and extract the dephasing time using a cumulant expansion approach. We find that phonon-induced dephasing is a limiting factor for T_2 at low temperatures, in agreement with recent experiments that use dynamical decoupling techniques. This approach can be generalized to other spin defects in semiconductors, molecular systems, and other band gapped materials.