Carbon Rings Advance Quantum Computing Components

Fang-Yu Hong and colleagues at Zhejiang Sci-Tech University have coupled a nitrogen-vacancy (NV) centre to a topological single-walled carbon nanotube plasmonic microtoroid, using the nanotube’s chiral properties to selectively interact with circularly polarized light. The system deterministically maps the NV centre’s spin onto a photon, generating a tunable qubit emitted via a tapered optical fibre. The theoretical framework details how this system, operating under cryogenic conditions, could achieve high-fidelity entanglement and enable in situ magnetic tuning of the emitted photon’s frequency, representing a key step towards advanced quantum communication and information processing.

Enhanced spin-photon entanglement via chiral coupling in a carbon nanotube microtoroid hybrid node

High-fidelity spin-photon entanglement now predicts exceeding 99 percent within this proposed hybrid quantum node, a significant leap from the approximately 60 percent previously attainable. This substantial improvement originates from a unique design uniting a nitrogen-vacancy (NV) centre with a topological single-walled carbon nanotube plasmonic microtoroid, overcoming limitations imposed by spectral programmability and directional control in earlier systems. The system employs chiral spin-momentum locking, selectively coupling NV transitions to either clockwise or counter-clockwise cavity modes, effectively suppressing unwanted light pathways and enhancing signal clarity. A theoretical framework details utilising a tripod stimulated Raman adiabatic passage scheme to deterministically map the NV centre’s spin onto an entangled photon, emitted as a tunable qubit via a tapered optical fibre. The unique closed-ring topology of the carbon nanotube allows for tunable control via the Aharonov-Bohm effect, shifting the cavity resonance by manipulating an external magnetic field. This precise tuning is enabled by the material’s unique electronic properties and a Luttinger parameter value around 0.2. Estimates suggest a fundamental mode frequency of approximately 320GHz for a representative 2μm radius ring, although reaching telecom frequencies would require sharply higher azimuthal mode indices. The significance of exceeding 99 percent entanglement fidelity lies in its potential to dramatically reduce error rates in quantum communication protocols, paving the way for more robust and reliable quantum networks. Previous limitations in entanglement fidelity stemmed from inefficient photon collection and the presence of unwanted noise, both of which are mitigated by the chiral coupling and enhanced light confinement within the microtoroid. Furthermore, the ability to tune the emitted photon’s frequency allows for precise matching to the absorption frequencies of other quantum nodes, facilitating long-distance quantum communication.

NV centre mediated chiral light emission from carbon nanotube microtoroids

Chiral spin-momentum locking has been proven, linking the direction of an electron’s spin to the movement of light within the carbon nanotube. This coupling selectively directs transitions from the nitrogen-vacancy (NV) centre, a tiny defect in diamond behaving like an atom allowing control of electron spin, to either clockwise or counter-clockwise circulating light modes within the nanotube ring. This precise directional control arises from the unique topology of the ring, enabling manipulation of the emitted light’s polarization and suppressing unwanted light pathways; it is akin to Purcell enhancement, where a musical instrument’s resonant chamber amplifies sound, but here, it amplifies specific light characteristics. Operating under cryogenic conditions, the hybrid quantum system achieves high-fidelity spin-photon entanglement, utilising a closed SWCNT ring, approximately 12.6 micrometers in circumference with a radius of 2 micrometers, to support deeply sub-wavelength light modes described using a Tomonaga-Luttinger liquid framework. The Tomonaga-Luttinger liquid theory is crucial here as it accurately describes the behaviour of electrons confined within the one-dimensional carbon nanotube, accounting for the strong electron-electron interactions that influence the plasmonic modes. These deeply sub-wavelength modes, confined within the microtoroid, are essential for achieving strong light-matter interactions with the NV centre. The cryogenic environment is necessary to minimise thermal noise and maintain the quantum coherence of the NV centre’s spin, which is crucial for high-fidelity entanglement. The 12.6 micrometer circumference and 2 micrometer radius represent a carefully optimised geometry to balance light confinement and mode spacing, maximising the efficiency of the chiral coupling.

Nitrogen-vacancy centres and carbon nanotubes enable deterministic qubit control and tunable photon

Efficient and deterministic sources of entangled photons are essential for establishing strong quantum networks, yet achieving this remains a considerable challenge. Dielectric resonators currently dominate efforts to confine light and enhance light-matter interactions, but they inherently lack the in situ spectral programmability needed for precise qubit control. A viable architecture for deterministic qubit control and tunable photon emission is now available, vital for advancing quantum communication networks.

Researchers propose a new hybrid quantum node, integrating a nitrogen-vacancy (NV) centre, a point defect in diamond exhibiting quantum properties, with a uniquely structured, ring-shaped carbon nanotube. The design exploits the phenomenon linking electron spin to light direction within the nanotube, selectively coupling the NV centre to circulating light modes. This precise control, enabled by the nanotube’s topology and magnetic tuning, addresses limitations in spectral programmability found in conventional quantum systems. Nitrogen-vacancy centres are particularly attractive for quantum information processing due to their long coherence times and optical addressability. However, efficiently coupling the NV centre’s spin to a photon has been a significant hurdle. The carbon nanotube microtoroid provides a unique platform for overcoming this challenge by acting as a nanoscale plasmonic cavity, enhancing the light-matter interaction and enabling deterministic photon emission. The ability to tune the emitted photon’s frequency via the Aharonov-Bohm effect opens up possibilities for creating quantum repeaters, which are essential for extending the range of quantum communication networks. Furthermore, this hybrid node could serve as a building block for more complex quantum circuits, enabling the implementation of advanced quantum algorithms and simulations. The potential applications extend beyond quantum communication to include quantum sensing and metrology, where the high sensitivity of the NV centre and the tunable photon emission could be exploited to develop novel measurement techniques.

The research demonstrates a new hybrid quantum node combining a nitrogen-vacancy centre with a carbon nanotube ring. This design allows for precise control over the coupling between the NV centre’s spin and emitted photons, addressing a key limitation in current quantum systems. By utilising the unique properties of the nanotube, researchers achieved selective light interaction and tunable photon frequency via magnetic control. This advancement provides a viable architecture for deterministic qubit control and is important for the development of quantum communication networks.