Recent progress of quantum control in solid-state single-spin systems
At a Glance
Section titled āAt a Glanceā| Metadata | Details |
|---|---|
| Publication Date | 2022-01-01 |
| Journal | Acta Physica Sinica |
| Authors | Tingwei Li, Xing Rong, Jiangfeng Du |
| Analysis | Full AI Review Included |
Executive Summary
Section titled āExecutive Summaryā- Robust Qubit Platform: The Nitrogen-Vacancy (NV) center in diamond is confirmed as a leading solid-state spin qubit system (S=1), uniquely capable of high-fidelity quantum control at room temperature.
- Ultra-High Gate Fidelity: Achieved record-high average fidelity for single-qubit gates (0.999952) using the BBlinC pulse sequence, successfully suppressing both environmental quasi-static noise (Ī“0) and control field noise (Ī“1).
- Fault-Tolerance Threshold Met: The demonstrated single-qubit fidelity exceeds the necessary threshold for fault-tolerant quantum computation (FTQC). Two-qubit CNOT gate fidelity reached 0.9920.
- Universal Control Methodology: Quantum state initialization and readout are performed optically (532 nm excitation, 637-750 nm fluorescence), while universal control is achieved using precise microwave (MW) and radio frequency (RF) pulses.
- Time Optimization: Implemented Time-Optimal Control (TOC) based on geodesic line methods, significantly reducing the execution time of complex two-qubit gates, thereby minimizing decoherence effects.
- Programmable Quantum Processor: Demonstrated a universal programmable quantum logic circuit using the NV electron spin and an adjacent nuclear spin, enabling flexible execution of various quantum algorithms (e.g., Grover search).
- Non-Hermitian Physics: Successfully utilized the NV system to experimentally simulate and observe the dynamics of Parity-Time (PT) symmetric Hamiltonians, including the critical PT-symmetry breaking transition.
Technical Specifications
Section titled āTechnical Specificationsā| Parameter | Value | Unit | Context |
|---|---|---|---|
| Qubit System | Nitrogen-Vacancy (NV) Center | N/A | Electron spin (S=1) in diamond |
| Ground State Zero-Field Splitting (D) | 2870 | MHz | Splitting between |
| Electron Gyromagnetic Ratio (Ī³e) | 2.8 | MHz/G | Used for Zeeman splitting calculations |
| Optical Excitation Wavelength | 500-600 | nm | Used for spin initialization and excitation |
| Fluorescence Readout Wavelength | 637-750 | nm | Spin-dependent radiative decay |
| Longitudinal Relaxation Time (T1) | Milliseconds | ms | Coherence time at room temperature |
| Dephasing Time (T2*) | 1.68(3) | Āµs | Measured via Free Induction Decay (FID) |
| Coherence Time (T2) | Hundreds of Āµs to ms | N/A | Measured via Hann echo sequence |
| Single-Qubit Gate Fidelity (BBlinC) | 0.999952(6) | N/A | Highest fidelity achieved, surpassing FTQC threshold |
| Two-Qubit Gate Fidelity (CNOT) | 0.9920(1) | N/A | Achieved using optimized gradient ascent pulse sequences |
| Time-Optimal C-U Gate Duration | 446.1 | ns | Minimum time for two-qubit controlled-U operation |
| Spin Polarization Efficiency | >95 | % | Achieved at room temperature using optical pumping |
Key Methodologies
Section titled āKey Methodologiesā-
Optical Spin Initialization and Readout:
- Electron spins are polarized to the |ms = 0> state using continuous-wave laser excitation (532 nm).
- Spin state readout is achieved by measuring the spin-dependent fluorescence intensity (637-750 nm), leveraging the difference in non-radiative decay paths (ISC) between |ms = 0> and |ms = Ā±1>.
-
Universal Quantum Control via MW/RF:
- Single-qubit operations on the electron spin are implemented using resonant microwave (MW) pulses tuned to the |ms = 0> ā |ms = -1> transition.
- Nuclear spins (e.g., 14N, 13C) are used as auxiliary qubits and controlled using radio frequency (RF) pulses.
- Two-qubit gates (e.g., CNOT, controlled-U) are realized by exploiting the intrinsic hyperfine coupling between the electron and nuclear spins.
-
Advanced Dynamic Decoupling (DD) Sequences:
- SUPCODE (Soft Uniaxial Positive Control): Pulse sequences (3-piece, 5-piece) designed to suppress quasi-static environmental magnetic noise (Ī“0) up to high orders (e.g., 6th order for 5-piece).
- BB1 (Broadband Number 1): Sequences used primarily to suppress control field amplitude noise (Ī“1).
- BBlinC: A nested sequence combining BB1 and CORPSE pulses, optimized to simultaneously suppress both Ī“0 and Ī“1 noise, achieving the highest single-qubit fidelity.
-
Randomized Benchmarking (RB):
- High-fidelity gates are characterized using the RB protocol to accurately measure the average gate fidelity (Fa), isolating the intrinsic gate error (Īµ) from state preparation and measurement errors.
-
Time-Optimal Control (TOC) Implementation:
- The quantum control problem is solved using geodesic line methods (analogous to Zermeloās navigation problem) to find the shortest time path (minimum T) for a target unitary operation (UF), subject to limited control Hamiltonian strength.
-
Non-Hermitian Quantum Simulation:
- The non-Hermitian PT-symmetric Hamiltonian (HPT) is mapped onto a larger, two-qubit Hermitian system (Hs,a(t)) involving the electron spin (system) and nuclear spin (ancilla).
- The dynamics of HPT are observed by performing selective MW pulses and post-selecting the measurement results on the ancilla qubit.
Commercial Applications
Section titled āCommercial Applicationsā- Fault-Tolerant Quantum Computing: The demonstrated high-fidelity single-qubit control (F > 0.9999) is critical for building scalable quantum processors based on solid-state spins, enabling error correction protocols.
- Nanoscale Quantum Sensing: NV centers function as highly sensitive, nanoscale probes for measuring physical quantities with high precision, applicable in:
- Biomedical Imaging: High-resolution magnetic field sensing for mapping neural activity or detecting single molecules in biological systems.
- Materials Characterization: Localized measurement of magnetic and electric fields in novel electronic materials and devices.
- Programmable Quantum Hardware: The development of a universal programmable quantum logic circuit allows for rapid prototyping and testing of quantum algorithms, accelerating software development for future quantum computers.
- Fundamental Physics and Simulation: The ability to precisely control and simulate exotic Hamiltonians (like PT-symmetric systems) opens new avenues for studying non-Hermitian physics, potentially leading to enhanced sensor sensitivity near exceptional points.
- Industrial Quality Control: High-sensitivity magnetometers based on NV centers can be used for non-destructive testing and quality control of microelectronic components and magnetic storage media.
View Original Abstract
In the field of quantum physics, quantum control is essential. Precise and efficient quantum control is a prerequisite for the experimental research using quantum systems, and it is also the basis for applications such as in quantum computing and quantum sensing. As a solid-state spin system, the nitrogen-vacancy (NV) center in diamond has a long coherence time at room temperature. It can be initialized and read out by optical methods, and can achieve universal quantum control through the microwave field and radio frequency fields. It is an excellent experimental platform for studying quantum physics. In this review, we introduce the recent results of quantum control in NV center and discuss the following parts: 1) the physical properties of the NV center and the realization method of quantum control, 2) the decoherence mechanism of the NV center spin qubit, and 3) the application of single-spin quantum control and relevant research progress.