Kolesov et al. (Nature Physics 5, 470 (2009)) demonstrated wave–particle duality of single surface plasmon polaritons (SPPs) launched by an optically addressed nitrogen-vacancy (NV) centre in a nanodiamond coupled to a silver nanowire, and projected that this platform 'opens the door to eventually achieving strong coupling between spins and plasmons', with applications including single-shot spin readout for room-temperature quantum computers and nano-magnetometers, waveguide-mediated coupling of distant NV registers toward large-scale room-temperature quantum processors, and cooperative many-emitter phenomena such as plasmon-mediated superradiance. The coherent spin–plasmon interface remains undemonstrated in the literature to date: single-NV–plasmon coupling stays in the Purcell regime (demonstrated rate enhancements of roughly 4- to 42-fold); spin readout via propagating plasmons is intensity-based optically detected magnetic resonance without single-shot or coherent capability; single-emitter–plasmon strong coupling has been realized only for molecular excitons in extreme-confinement nanocavities, never for a stationary spin qubit and never in a waveguide; distant NV spins have been entangled only through free-space optical photons; and cooperative single-plasmon phenomena remain theoretical or incoherent. The open problem is to reach the strong-coupling regime — a coherent spin–mode exchange resolvable against the decay — between an individual NV electronic spin, through its spin-selective optical transitions, and a single quantized SPP mode at room temperature, and to demonstrate the projected functions: single-shot spin readout and coupling of distant NV spin registers through the plasmonic channel.

Background

A surface plasmon polariton (SPP) is an electromagnetic wave bound to a metal–dielectric interface in which the light field couples to collective oscillations of the metal's conduction electrons. Because the field is confined to distances far below the free-space wavelength, the mode volume of an SPP on a metallic nanowire or in a plasmonic nanocavity is orders of magnitude smaller than in any dielectric resonator. In the language of cavity quantum electrodynamics (cavity QED), a single quantum emitter placed in such a mode interacts with it at a rate gg; the interaction is in the weak-coupling (Purcell) regime when it merely enhances the emitter's spontaneous-emission rate, and in the strong-coupling regime when the coherent exchange between the emitter and the mode becomes resolvable — when gg is large enough relative to the decay channels to produce a vacuum-Rabi splitting (anticrossing) rather than a mere enhancement of the emission rate — where κ\kappa is the mode linewidth (for plasmons, dominated by ohmic loss in the metal) and γ\gamma the emitter's optical decoherence rate; the cooperativity C=4g2/(κγ)C=4g^2/(\kappa\gamma) is a reported figure of merit, but C>1C>1 is not by itself sufficient to reach this regime. The nitrogen-vacancy (NV) centre is a point defect in diamond consisting of a substitutional nitrogen atom next to a vacancy, whose ground state is an electronic spin triplet (S=1S=1) that can be optically polarized and read out (via optically detected magnetic resonance, ODMR, the spin-state dependence of the photoluminescence intensity under microwave excitation) and that retains phase coherence at room temperature. Crucially, the NV's optical transitions are spin-selective, so coupling them to a single optical mode makes the spin observable and entangleable through that mode — the mechanism behind single-shot spin readout and spin–photon entanglement in cavity QED. Single-shot readout means determining the spin state of one qubit in a single measurement rather than by averaging many repetitions.

Theoretically, strong coupling of single emitters to surface plasmons was proposed by Chang, Sørensen, Hemmer and Lukin in 2006–2007, and Akimov et al. (Nature 450, 402 (2007)) generated single plasmons in metallic nanowires using quantum dots. Kolesov et al. (2009) then showed that a single NV centre in a nanodiamond stuck to a chemically grown silver nanowire launches single SPPs with full wave–particle duality: the excitation is never detected simultaneously at both wire outputs (particle-like antibunching, g(2)(0)→0g^{(2)}(0)\to 0) and yet self-interferes spectrally as it reflects between wire ends (wave-like behaviour). In their outlook, the authors state that using spin-selective emitters 'open[s] the door to eventually achieving strong coupling between spins and plasmons, for which applications include single-shot spin readout for room-temperature quantum computers and nano-magnetometers', that metallic waveguides could couple distant NV defects 'as required for maximum scalability', and that cooperative phenomena 'such as superradiance mediated by surface plasmons' (superradiance being the cooperative, phase-locked emission of an ensemble at a rate growing with emitter number) and 'long distance coupling of single quantum emitters through plasmonic modes' could let single plasmons on nanowires 'couple numerous multi-qubit diamond quantum registers ... to fabricate large-scale room-temperature quantum processors'. The experiment itself operated entirely in the weak-coupling regime, with Purcell factors of order a few.

Problem Statement

Realize strong coherent coupling between the electronic spin of an individual nitrogen-vacancy (NV) centre and a single quantized surface plasmon polariton (SPP) mode at room temperature, and demonstrate the quantum-information functions that this interface was projected to perform when the platform was introduced.

Specifically: (1) reach the strong-coupling regime for the spin-selective optical transition of a single NV centre coupled to a single SPP mode of a metallic waveguide or plasmonic nanocavity — that is, exhibit a resolvable coherent exchange (vacuum-Rabi splitting or anticrossing) between the spin-selective transition and the mode, with the coupling rate gg comparable to or larger than the decay channels — under ambient (room-temperature) conditions, reporting the coupling rate gg, mode linewidth κ\kappa, and emitter decoherence rate γ\gamma determined independently as supporting evidence, with the cooperativity C=4g2/(κγ)C=4g^2/(\kappa\gamma) as a figure of merit; and (2) demonstrate that this coherent interface performs the projected functions: single-shot optical readout of the NV spin state via the plasmon channel, and coherent coupling of spatially distant NV spin registers through the metallic waveguide — a coherence-preserving two-qubit interaction (for instance, generated entanglement) between the two separated spins mediated by the plasmon channel, rather than incoherent energy transfer between emitters. The cooperative many-emitter phenomena (such as superradiance) mediated by single plasmons that the source discussed are a multi-emitter extension of this interface, not part of the completion criterion.

These deliverables form one scientific objective — a room-temperature quantum-coherent spin–plasmon interface — in which the strong-coupling regime is the enabling physics and the readout and inter-register coupling functions are jointly its point: an interface operating only in the Purcell (weak-coupling) regime, or a readout mechanism not derived from the coherent emitter–plasmon coupling, does not complete the problem as posed. Both the spin coherence of the emitters and the single-quantum character of the plasmonic mode must be preserved and evidenced in the demonstrated function.

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