Geometric entanglement of a photon and spin qubits in diamond
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-12-15 |
| Journal | Communications Physics |
| Authors | Yuhei Sekiguchi, Yuki Yasui, Kazuya Tsurumoto, Yuta Koga, Raustin Reyes |
| Institutions | Yokohama National University |
| Citations | 17 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research demonstrates the generation of geometric entanglement between a photon and an electron spin qubit utilizing a diamond Nitrogen-Vacancy (NV) center, paving the way for robust quantum networking infrastructure.
- Core Achievement: Successful generation of entanglement between a geometric photon qubit (polarization) and a geometric spin qubit (NV electron spin) via spontaneous emission.
- Noise Resilience: The system operates under a zero magnetic field, defining the qubit in a degenerate subspace. This geometric approach offers intrinsic noise resilience compared to conventional dynamic qubits.
- High Fidelity: An entanglement state fidelity of 86.8% was experimentally achieved, limited primarily by NV-axis misalignment (6%) and spin phase rotation (2%).
- Mode-Matching Tolerance: The demonstrated entangled emission, combined with entangled absorption, generates purely geometric entanglement insensitive to errors in time, frequency, and spatial mode matching.
- Quantum Network Advancement: This work establishes a protocol for building a noise-resilient quantum repeater network or quantum internet capable of interconnecting heterogeneous quantum systems.
- Mechanism: Entanglement is generated through the spin-orbit entanglement inherent in the NV center structure during the relaxation of the excited |A2> state.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Entanglement State Fidelity | 86.8 | % | Measured Bell state fidelity (limited by systematic errors). |
| Operating Temperature | 5 | K | Required for coherent control of electron orbitals. |
| Diamond Type | High-purity type IIa | - | CVD-grown material hosting the NV center. |
| Crystal Orientation | <100> | - | Substrate orientation used in the experiment. |
| Magnetic Field | Zero | - | Residual field canceled by 3D coil system for geometric operation. |
| Microwave Frequency (Spin) | 2.88 | GHz | Corresponds to the zero-field splitting (D) of the NV center. |
| Microwave Rabi Frequency | 2.5 | MHz | Maximum Rabi frequency used for GRAPE-optimized pulses. |
| Initialization Laser | 515 | nm | Green laser used for charge and spin initialization. |
| ZPL Emission Wavelength | 637 | nm | Zero-Phonon Line emission used for the photon qubit. |
| NV Axis Tilt Angle | 53.0 | ° | Estimated tilt relative to the surface normal (source of 6% fidelity error). |
| Spin Measurement Probability | 0.15 | - | Probability of detecting the spin state. |
| Photon Detection Probability | 3 x 10-6 | - | Probability of detecting the ZPL photon. |
| Spin Phase Rotation Error | 2 | % | Attributed to residual magnetic field and hyperfine interaction. |
Key Methodologies
Section titled âKey MethodologiesâThe experiment relies on precise control of a single NV center in diamond under cryogenic and zero-magnetic field conditions, utilizing polarized microwave and optical fields.
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Material Preparation and Environment Control:
- A single, naturally occurring NV center in a high-purity type IIa CVD diamond (<100> orientation) was used.
- The diamond was cooled to 5 K using a cryostat to maintain electron orbital coherence.
- A zero magnetic field environment was achieved by canceling the residual magnetic field (including geomagnetic field) using a three-dimensional coil system, optimized by maximizing the spin-echo coherence time.
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Geometric Spin Qubit Manipulation:
- The geometric spin qubit is defined in the degenerate |ms = ±1>s subspace of the spin-triplet ground state.
- Microwaves were applied via two orthogonal copper wires attached to the sample surface.
- Arbitrarily polarized microwave Ï-pulses were generated using GRAPE (Gradient Ascent Pulse Engineering) algorithms to achieve high-fidelity manipulation (97% average fidelity) within a 2.5 MHz Rabi frequency limit.
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Entanglement Generation Sequence (Spontaneous Emission):
- The NV center was initialized using a nonresonant green laser (515 nm).
- The electron orbital was excited to the |A2> state using a resonant red laser (637 nm).
- Spontaneous emission from |A2> to the ground state generated the entangled photon (ZPL emission) and spin state.
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Conditional Measurement:
- The ZPL photon polarization was measured using a variable wave plate (VWP) and a polarizer, coupled to an Avalanche Photodiode (APD).
- The spin state was measured conditioned on the detection of the polarized ZPL photon.
- The spin measurement utilized a microwave pulse to rotate the qubit state to the |0>s ancillary state, followed by spin-dependent excitation and detection of the Phonon Sideband (PSB) emission.
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Data Acquisition:
- An FPGA (100 MHz clock) controlled the experimental sequences and processed the APD signals in real-time to perform conditional spin measurements based on ZPL photon detection.
Commercial Applications
Section titled âCommercial ApplicationsâThe development of robust, geometric entanglement generation is critical for next-generation quantum technologies, particularly those requiring long-distance communication and interfacing.
- Quantum Networking and Internet:
- Enabling the construction of long-distance quantum communication links (quantum repeaters) that are inherently tolerant to environmental fluctuations and manufacturing imperfections (time/frequency/space mode mismatching).
- Quantum Transduction:
- Developing high-efficiency quantum transducers that interface disparate quantum systems (e.g., linking superconducting qubits or trapped ion qubits, which operate via microwave photons, to optical photons for long-haul transmission).
- Noise-Resilient Quantum Memory:
- Utilizing the geometric spin qubit in diamond as a robust, solid-state quantum memory element that maintains coherence under ambient noise conditions due to its zero-field operation.
- Quantum Computation Resources:
- Generating complex multiphoton entangled resources necessary for advanced measurement-based quantum computation protocols.
- Advanced Quantum Sensing:
- Providing a stable, high-fidelity platform for quantum metrology applications where environmental insensitivity is paramount.
View Original Abstract
Abstract Geometric nature, which appears in photon polarization, also appears in spin polarization under a zero magnetic field. These two polarized quanta, one travelling in vacuum and the other staying in matter, behave the same as geometric quantum bits or qubits, which are promising for noise resilience compared to the commonly used dynamic qubits. Here we show that geometric photon and spin qubits are entangled upon spontaneous emission with the help of the spin â orbit entanglement inherent in a nitrogen-vacancy center in diamond. The geometric spin qubit is defined in a degenerate subsystem of spin triplet electrons and manipulated with a polarized microwave. An experiment shows an entanglement state fidelity of 86.8%. The demonstrated entangled emission, combined with previously demonstrated entangled absorption, generates purely geometric entanglement between remote matters in a process that is insensitive of time, frequency, and space mode matching, which paves the way for building a noise-resilient quantum repeater network or a quantum internet.