Proposal for room-temperature quantum repeaters with nitrogen-vacancy centers and optomechanics
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2022-03-17 |
| Journal | Quantum |
| Authors | Jia-Wei Ji, Yu-Feng Wu, Stephen C. Wein, Faezeh Kimiaee Asadi, Roohollah Ghobadi |
| Institutions | University of Calgary |
| Citations | 12 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis document analyzes a proposed room-temperature quantum repeater architecture based on Nitrogen-Vacancy (NV) centers in diamond coupled via optomechanics.
- Core Value Proposition: The architecture enables long-distance quantum networking without requiring cryogenic cooling, significantly reducing the complexity and cost of scaling global quantum networks.
- Key Components: The system uses NV electron spins (communication qubits) coupled to a SiN membrane mechanical oscillator (with a magnetic tip) inside a high-finesse optical cavity. Long-lived 13C nuclear spins serve as robust quantum memory (coherence time > 1 s).
- Decoherence Mitigation: Optomechanics is used to bypass phonon-induced broadening of the optical transition at room temperature, allowing the emission of highly indistinguishable telecom-band photons.
- Performance Metrics: Entanglement generation fidelity (Fgen) for a single link approaches 97%. Overall entanglement fidelity (Ftot) reaches up to 61% at 800 km when using 100 multiplexing channels.
- Scaling Advantage: The proposed two-round repeater protocol avoids nested hierarchies, resulting in a logarithmic dependence of the entanglement distribution time on the number of links, significantly enhancing distribution rates.
- Readout Innovation: Two intensity-based electron spin readout schemes (periodic and continuous driving) are proposed using the spin-optomechanics interface, achieving infidelities less than 10-3 in the millisecond timescale at room temperature.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Operating Temperature | Ambient (Room) | K | NV center operation and entanglement generation. |
| NV Electron Spin Coherence Time (T2) | Millisecond-long | s | Communication qubit lifetime. |
| Nuclear Spin Coherence Time (T2) | > 1 | s | Quantum memory lifetime (13C in purified diamond). |
| Photon Wavelength | Telecom | band | Ideal for long-distance fiber transmission. |
| Fiber Attenuation Distance (Latt) | 22 | km | Standard telecom fiber loss. |
| Elementary Link Distance (L0) | 100 | km | Length used in simulation for entanglement generation. |
| Entanglement Generation Fidelity (Fgen) | ~97 | % | Peak fidelity for a single 100 km link (Barrett-Kok scheme). |
| Overall Fidelity (Ftot) | ~61 | % | Achieved at 800 km (6-link repeater, N=100 multiplexing). |
| Repeater Rate (Multiplexed) | > 1 | Hz | Achieved at 800 km (with 45% detection efficiency). |
| CNOT Gate Fidelity | 99.2 | % | Electron-nuclear spin gate fidelity (ambient conditions). |
| Electron Spin Readout Infidelity (1-F) | < 10-3 | Achieved in ms timescale (with 45% detector efficiency). | |
| Magnetic Field Gradient | 107 | T/m | Required for strong spin-mechanics coupling (λ ~ 105 Hz). |
| Cavity Finesse (Target) | 106 | Required for membrane-in-the-middle design to reduce cavity length to ~0.6 cm. | |
| Mechanical Oscillator Q-factor (Target) | ~ 109 | Required ultra-low damping rate (Îłm) for SiN membrane. |
Key Methodologies
Section titled âKey Methodologiesâ- Spin-Optomechanics Interface Design: Utilizes a membrane-in-the-middle geometry where a SiN membrane oscillator is placed inside a high-finesse optical cavity. The NV electron spin in bulk diamond is coupled to the membrane via a magnetic tip attached to the oscillator.
- Thermal Noise Suppression: A red-detuned cooling laser, resonant with the mechanical oscillator, is used to efficiently cool the SiN membrane to near its ground state, minimizing thermal noise that would otherwise degrade photon quality.
- Entanglement Generation: The Barrett-Kok scheme (spin-time bin protocol) is used. The NV electron spin is prepared in a superposition, and a red-detuned control laser induces effective spin-photon coupling (Ω = λg/Ύ) via the mechanical oscillator, generating highly indistinguishable telecom photons.
- Quantum Memory Mapping: Entanglement is transferred from the short-lived electron spins to the long-lived 13C nuclear spins using a controlled-NOT (CnNOTe) gate implemented via a Ramsey sequence based on hyperfine interaction (t = Ï/A).
- Room-Temperature Readout: Electron spin state discrimination (|D> vs. |0>) is achieved by driving a cycling transition (|D> to |B>) and counting emitted photons. Two schemes are proposed:
- Periodic Driving: Uses periodic MW Ï pulses to repeatedly excite the spin and measure photon bursts.
- Continuous Driving: Uses a CW laser to maintain a non-zero equilibrium state for photon emission.
- Entanglement Distribution Protocol: A two-round, nesting-level free repeater protocol is implemented. Entanglement is generated in alternating links in the first round, stored in nuclear spins, and then generated in the remaining links in the second round, followed by entanglement swapping.
- Rate Enhancement: Spatial or spectral multiplexing (N channels) is incorporated to boost the entanglement distribution rate, allowing for feasible final fidelities at long distances by mitigating nuclear spin decoherence time (T2).
Commercial Applications
Section titled âCommercial Applicationsâ- Global Quantum Internet Infrastructure: Provides a viable, solid-state blueprint for constructing long-haul quantum communication networks that avoid the high operational costs and complexity associated with cryogenic cooling.
- Quantum Key Distribution (QKD): Enables high-rate, long-distance QKD by utilizing robust room-temperature repeaters and telecom-band photon links compatible with existing fiber infrastructure.
- Distributed Quantum Computing: Essential technology for linking remote quantum processors (based on NV centers or other solid-state qubits) to form a powerful distributed quantum computer.
- Advanced Quantum Sensing: The high-fidelity, room-temperature NV spin readout techniques developed here are directly applicable to improving the sensitivity and speed of diamond-based magnetic and electric field sensors.
- Solid-State Qubit Integration: Advances the integration of NV centers with micro- and nano-mechanical systems (SiN membranes), crucial for developing scalable, integrated quantum transducers and chips.
- Isotopically Purified Diamond Materials: Creates high demand for isotopically purified 13C diamond, necessary to achieve the millisecond electron spin and second-scale nuclear spin coherence times required for memory function.
View Original Abstract
We propose a quantum repeater architecture that can operate under ambient conditions. Our proposal builds on recent progress towards non-cryogenic spin-photon interfaces based on nitrogen-vacancy centers, which have excellent spin coherence times even at room temperature, and optomechanics, which allows to avoid phonon-related decoherence and also allows the emitted photons to be in the telecom band. We apply the photon number decomposition method to quantify the fidelity and the efficiency of entanglement established between two remote electron spins. We describe how the entanglement can be stored in nuclear spins and extended to long distances via quasi-deterministic entanglement swapping operations involving the electron and nuclear spins. We furthermore propose schemes to achieve high-fidelity readout of the spin states at room temperature using the spin-optomechanics interface. Our work shows that long-distance quantum networks made of solid-state components that operate at room temperature are within reach of current technological capabilities.