Scalable spin–photon entanglement by time-to-polarization conversion
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2020-01-28 |
| Journal | npj Quantum Information |
| Authors | Rui Vasconcelos, Sarah Reisenbauer, C. L. Salter, Georg Wachter, Daniel Wirtitsch |
| Institutions | Vienna Center for Quantum Science and Technology, University of Vienna |
| Citations | 40 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This analysis focuses on a novel, scalable protocol for generating spin-photon entanglement using Time-to-Polarization Conversion (TPC), significantly relaxing the physical constraints traditionally placed on quantum emitters.
- Core Innovation: The TPC protocol converts entanglement from the robust, but difficult-to-use, time basis into the polarization basis using a single, unbalanced, asymmetric polarizing interferometer.
- Relaxed Emitter Requirements: The scheme requires only a single optical transition, eliminating the need for energy-degenerate transitions and complex tuning required by traditional Cluster-State Machine Gun (CSMG) protocols.
- Experimental Demonstration: The fundamental unit of the TPC protocol was successfully demonstrated using a Nitrogen-Vacancy (NV) center in diamond, a system typically challenging for CSMG due to complex excited-state manifolds.
- High Fidelity: The resulting spin-photon entanglement achieved a lower bound fidelity of F > 64.7 ± 1.3% with respect to the ideal Bell state, significantly exceeding the classical limit (F < 50%) by over 11 standard deviations.
- Scalability: The TPC scheme is inherently scalable, allowing for the iteration of the entanglement generation step to produce long strings of entangled photons (cluster states) necessary for measurement-based quantum computation (MBQC).
- Performance Enhancement: This classically assisted protocol (using the interferometer as a classical counterpart) broadens the range of suitable solid-state emitters (e.g., quantum dots, single atoms) and enables performance improvements for existing platforms.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Emitter Material | Artificial, single-crystal diamond | N/A | Natural isotopic abundance, {1, 1, 1} surface orientation. |
| Emitter Type | Nitrogen-Vacancy (NV) Center | N/A | Used as the matter qubit (electron spin subspace { |
| Operating Temperature | ~4.5 | K | Maintained via closed-cycle cryostat. |
| Resonant Laser Wavelength | ~637.2 | nm | Used for the working transition ( |
| Transition Detuning | 0.87 | GHz | Detuning of the working transition from other transitions (ensuring negligible cross-excitation). |
| Qubit Initialization Fidelity | 97.9 ± 1.6 | % | Fidelity of preparing the electron spin in the |
| Nuclear Polarization (mI = -1) | 83.8 ± 1.9 | % | Polarization within the mI = -1 manifold (used to reduce dephasing). |
| TPC Interferometer Type | Mach-Zehnder (Unbalanced) | N/A | Fiber-based, polarization-maintaining. |
| Propagation Delay (TPC) | 262 | ns | Time difference between the short and long arms, matched to the pulse separation. |
| ZPL Efficiency (Pulse to Click) | ~2 x 10-5 | N/A | Low efficiency due to high decay (~97%) into the Phonon-Side Band (PSB). |
| Spin Readout Probability (ms=0) | 16.7 ± 0.1 | % | Average probability of detecting a spin-readout click when prepared in ms = 0. |
| Entangling Event Rate | 25 | events/hour | Measured coincidence events in the quadrature ports (D, A, R, L). |
| Entanglement Fidelity (Lower Bound) | F > 64.7 ± 1.3 | % | Fidelity relative to the ideal Bell state (after background subtraction). |
| Raw Entanglement Fidelity | F > 56.0 ± 0.9 | % | Fidelity including background light. |
| Correlation Czz | 83.7 ± 1.6 | % | Measured correlation in the σz ⊗ σz basis. |
| Correlation Cxx | 40.7 ± 2.9 | % | Measured correlation in the σx ⊗ σx basis. |
Key Methodologies
Section titled “Key Methodologies”The experiment utilized a custom-fabricated NV center in diamond coupled to a fiber-based unbalanced interferometer for TPC.
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Sample Preparation and Enhancement:
- An artificial, single-crystal diamond with a {1, 1, 1} surface was used, hosting a pre-allocated NV center.
- A solid-immersion microlens was fabricated over the NV center using focused-ion-beam milling to improve photon collection efficiency.
- The surface was coated with 110 nm of SiO2 to minimize Fresnel reflection and laser backscatter.
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Spin Initialization (Matter Qubit Preparation):
- The NV charge state was initialized to NV- using a green laser pulse.
- Electron and nuclear spins were initialized iteratively using resonant optical pumping and nuclear spin selective microwave pulses to prepare the state in ms = -1, mI = 0.
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Entanglement Generation (TPC Protocol):
- A microwave Ry(π/2) pulse created an equal spin superposition (|−1> - |0>)/√2.
- First Excitation: A resonant optical π-pulse (a1) generated a photon conditional on the spin state, resulting in an H-polarized photon in the long arm of the interferometer.
- Spin Inversion: During the 262 ns storage time, a microwave π-pulse rotated the spin to the orthogonal state.
- Second Excitation: A second optical π-pulse (a2) generated a V-polarized photon in the short arm of the interferometer.
- Conversion: By matching the time delay between the two excitation pulses to the 262 ns propagation delay, the two wavepackets overlapped at the output, erasing the time/path information and converting the time-bin entanglement into polarization entanglement.
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Tomography and Readout:
- Photonic Readout: The interferometer output was monitored using four quadrature detectors (D, A, R, L) to project the photon onto equatorial (|Φ>) or polar (|H>/|V>) bases.
- Spin Readout: The electron spin state was rotated using a Ry(θ)-pulse, followed by a final 5 µs laser pulse, with readout performed by detecting fluorescence from the far off-resonant Phonon-Side Band (PSB).
Commercial Applications
Section titled “Commercial Applications”The TPC protocol, demonstrated using NV centers in diamond, is critical for advancing quantum technologies by providing a robust and scalable method for generating resource states.
| Industry/Sector | Application | Technical Relevance |
|---|---|---|
| Quantum Computing (LOQC) | Generation of photonic cluster states and resource states for Measurement-Based Quantum Computation (MBQC). | TPC enables scalable generation of entangled photon strings using emitters with simplified level schemes. |
| Quantum Communication | Development of all-photonic quantum repeaters and long-distance quantum networks. | The protocol is robust and adaptable, allowing integration with various solid-state emitters (NV centers, quantum dots). |
| Quantum Sensing & Metrology | High-precision sensing based on entangled states. | NV centers are known for excellent coherence properties, which are leveraged by the TPC scheme for high-fidelity entanglement. |
| Solid-State Emitter Technology | Broadening the applicability of quantum dots and single molecules as quantum light sources. | By requiring only a single optical transition, the TPC protocol makes previously unsuitable emitters viable for entanglement generation. |
| Diamond Technology | Integration of quantum devices into robust, solid-state platforms. | Utilizes the stable, high-coherence properties of NV centers embedded in artificial diamond for cryogenic operation. |
View Original Abstract
Abstract The realization of quantum networks and quantum computers relies on the scalable generation of entanglement, for which spin-photon interfaces are strong candidates. Current proposals to produce entangled-photon states with such platforms place stringent requirements on the physical properties of the photon emitters, limiting the range and performance of suitable physical systems. We propose a scalable protocol, which significantly reduces the constraints on the emitter. We use only a single optical transition and an asymmetric polarizing interferometer. This device converts the entanglement from the experimentally robust time basis via a path degree of freedom into a polarization basis, where quantum logic operations can be performed. The fundamental unit of the proposed protocol is realized experimentally in this work, using a nitrogen-vacancy center in diamond. This classically assisted protocol greatly widens the set of physical systems suited for scalable entangled-photon generation and enables performance enhancement of existing platforms.