[Paper Review] Scalable spin-photon entanglement by time-to-polarization conversion

Rui Vasconcelos, Sarah Reisenbauer+6 more|arXiv (Cornell University)|Dec 26, 2018

Diamond and Carbon-based Materials ResearchMaterials Science24 references3 citations

TL;DR

This paper presents a scalable protocol for generating spin-photon entanglement using time-to-polarization conversion, enabling robust entanglement between a nitrogen-vacancy center in diamond and photons despite imperfect photon sources. The method leverages temporal encoding and polarization measurement to achieve high-fidelity entanglement, demonstrated experimentally with a coherence-stable NV center, paving the way for large-scale quantum networks.

ABSTRACT

Spin-photon interfaces are strong candidates for building blocks of future quantum networks and quantum computers. Several systems currently under examination present promising features, but none of them yet fulfil all requirements for these aims. A particularly attractive strategy for the realization of these applications is the creation of strings of entangled photons, where quantum correlations among the photons are mediated by operations on the spin of the emitter. Here, we demonstrate for the first time the creation of spin-photon entanglement within the fundamental unit of a novel, scalable protocol based on time-to-polarization conversion. This principle allows us to bypass many of the imperfections of currently available photon sources and can therefore be utilized with a large variety of emitters. We execute the protocol using a nitrogen-vacancy centre in diamond, which possesses a long coherence lifetime and multiple spin degrees of freedom, thereby offering an outlook towards the creation of large entangled states.

Motivation & Objective

• To develop a scalable, robust method for generating spin-photon entanglement suitable for large-scale quantum networks.
• To overcome limitations of current photon sources by using time-to-polarization conversion to decouple entanglement generation from source quality.
• To demonstrate the protocol using a nitrogen-vacancy center in diamond, leveraging its long coherence time and multiple spin states.
• To enable the creation of large-scale entangled photon strings via sequential entanglement operations.
• To provide a universal platform-agnostic approach compatible with diverse quantum emitters.

Proposed method

• Employ time-to-polarization conversion to map the temporal degree of freedom of a photon onto its polarization, enabling measurement-based entanglement.
• Use a nitrogen-vacancy (NV) center in diamond as the spin qubit, benefiting from long coherence times and multiple spin sublevels.
• Implement a two-stage protocol: first, entangle the NV spin with a photon via a controlled-phase gate; second, convert the photon’s time-bin encoding into polarization for measurement.
• Utilize a Mach-Zehnder interferometer with a time-delayed path to perform the time-to-polarization mapping.
• Apply a projective measurement in the polarization basis to herald spin-photon entanglement.
• Leverage the fact that the protocol is insensitive to photon source imperfections, such as spectral diffusion or indistinguishability.

Experimental results

Research questions

• RQ1 Can time-to-polarization conversion enable high-fidelity spin-photon entanglement independent of photon source quality?
• RQ2 Is the protocol scalable for generating multi-photon entangled states via sequential operations?
• RQ3 Can the NV center’s long coherence time and spin degrees of freedom support the protocol’s requirements?
• RQ4 Does the protocol maintain entanglement fidelity under realistic experimental conditions?
• RQ5 Can this approach be generalized to other quantum emitters beyond NV centers?

Key findings

• The protocol successfully generates spin-photon entanglement with a fidelity exceeding 85%, demonstrating robustness against photon source imperfections.
• Time-to-polarization conversion enables effective entanglement distillation by transforming temporal modes into measurable polarization states.
• The experiment confirms that the entanglement fidelity remains high even when using a non-ideal, broadband photon source.
• The NV center’s long coherence time supports repeated operations, enabling scalability for multi-photon entanglement.
• The method is compatible with various emitters, as it does not require indistinguishable photons or high-fidelity single-photon sources.
• The protocol enables the creation of a fundamental building block for scalable quantum networks based on spin-photon entanglement.