Quantum error correction of spin quantum memories in diamond under a zero magnetic field
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2022-04-27 |
| Journal | Communications Physics |
| Authors | Takaya Nakazato, Raustin Reyes, Nobuaki Imaike, Kazuyasu Matsuda, Kazuya Tsurumoto |
| Institutions | Yokohama National University |
| Citations | 17 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research demonstrates the implementation of quantum error correction (QEC) on a solid-state spin memory operating under a completely zero magnetic field (B=0), a critical requirement for integrating spin qubits with superconducting quantum circuits.
- Core Achievement: Successful demonstration of three-qubit QEC (bit-flip and phase-flip codes) using a diamond Nitrogen-Vacancy (NV) center at B=0.
- Qubit System: The quantum memory is the 14N nuclear spin, protected by two surrounding 13C nuclear spins acting as ancilla qubits.
- Zero-Field Advantage: B=0 operation eliminates magnetic field penetration issues that destabilize superconducting qubits (e.g., flux qubits), enabling scalable integration and flexible quantum computer layout.
- Key Operational Gate: QEC relies on the holonomic Controlled-Phase (C-Z) gate, implemented using the geometric phase of the electron spin, which maintains high fidelity even without an external magnetic field.
- Performance: Achieved average state fidelities of 75.4% (bit-flip QEC) and 74.6% (phase-flip QEC), comparable to previous high-field demonstrations.
- Error Resilience: The QEC scheme successfully maintains fidelity when the intentional error probability (p) is greater than 0.15, overcoming operational and environmental errors.
- Future Impact: Opens a path toward distributed quantum computation and a quantum internet utilizing memory-based quantum interfaces and repeaters compatible with B=0 superconducting architectures.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Operating Magnetic Field (B) | 0 | Gauss | Residual magnetic field canceled by 3D coil system. |
| Operating Temperature | 5 | K | Used to prolong electron spin coherence time. |
| NV Center Material | High-purity Type IIa | CVD Diamond | Single, naturally occurring NV center (Element Six). |
| Zero-Field Splitting (D0) | 2.877 | MHz | NV electron spin ground state. |
| 14N Hyperfine Splitting | 2.2 | MHz | Coupling between electron and nitrogen nuclear spin. |
| 13C1 Hyperfine Splitting | 1.14 | MHz | Coupling for the first carbon ancilla qubit. |
| 13C2 Hyperfine Splitting | 0.33 | MHz | Coupling for the second carbon ancilla qubit. |
| GHZ Entanglement Fidelity | 0.78 (78) | N/A | Classical correlation measured for the three-qubit GHZ state. |
| Bit-Flip QEC Fidelity (Average) | 75.4 | % | State fidelity of the 14N nuclear spin after QEC. |
| Phase-Flip QEC Fidelity (Average) | 74.6 | % | State fidelity of the 14N nuclear spin after QEC. |
| QEC Crossover Point (p) | 0.15 | N/A | QEC fidelity exceeds non-QEC fidelity when error probability (p) is > 0.15. |
| Undetected Carbon Coupling | 0.06 to 0.12 | MHz | Range of hyperfine coupling causing ~10% fidelity degradation. |
Key Methodologies
Section titled âKey Methodologiesâ- Material Preparation: Used a single, naturally occurring NV center embedded in a high-purity Type IIa chemical-vapor deposition (CVD) grown diamond with <100> crystal orientation.
- Environmental Control: The diamond sample was cooled to 5 K to maximize the electron spin coherence time (T2).
- Zero-Field Cancellation: A three-dimensional coil system was used to cancel the residual magnetic field (including the geomagnetic field). The B=0 condition was verified by maximizing the spin-echo coherence time.
- Spin Control Hardware: Two orthogonal copper wires were attached to the sample surface to deliver microwaves (MW) with arbitrary polarization for electron spin manipulation (e.g., geometric qubit operations). Radiofrequency (RF) pulses were used for nuclear spin manipulation.
- Optical Initialization and Readout:
- Nonresonant Excitation: A green laser (515 nm) was used for charge and electron spin initialization.
- Resonant Excitation: Two red lasers (637 nm) were used for further electron spin initialization (A1 state) and spin measurement (Ey state) via optically detected magnetic resonance (ODMR).
- Quantum Gate Implementation: The holonomic C-Z gate, essential for encoding and decoding, was implemented using the GRAPE (gradient ascent pulse engineering)-optimized waveform to achieve high fidelity, particularly against weakly coupled environmental carbons.
- Nuclear Spin Initialization: 13C spins were initialized via probabilistic projection (measurement-based initialization, >99% fidelity). 14N spin was deterministically initialized (approx. 95% fidelity).
Commercial Applications
Section titled âCommercial ApplicationsâThe demonstration of fault-tolerant quantum memory operating at zero magnetic field directly addresses major engineering challenges in scaling quantum systems.
- Distributed Quantum Computing:
- Enables the construction of large-scale, distributed quantum computers by connecting remote quantum nodes.
- Provides memory-based quantum interfaces and quantum repeaters for long-haul communication networks.
- Hybrid Quantum Systems:
- Crucial for integrating spin-based quantum memories (like NV centers) with superconducting qubits (e.g., transmon or flux qubits), which are highly sensitive to magnetic fields and require B=0 operation for stability.
- Quantum Network Infrastructure:
- The NV centerâs long coherence time and excellent optical accessibility make it an ideal candidate for quantum repeaters, serving as the memory element in a quantum internet.
- Fault-Tolerant Qubit Design:
- The demonstrated three-qubit QEC code is a fundamental building block for larger, more robust stabilizer codes (like Shorâs nine-qubit code or surface codes), necessary for practical, fault-tolerant quantum computation.