A photon exists in all places at once. Light is both a particle and a wave, and the photon is governed by quantum waves that determine where it might be at any time. But its position is only a probability, and until it is measured it can only be guessed. Until that time, it is everywhere and nowhere particular, like the electrons in the carbon atoms that made rough wood under Will Parry's fingertips.
Years ago, he had carved their names into the arm of the bench. Will is no stranger to physics or to a plurality of universes, and perhaps in some other universe the same carving has appeared for Lyra to see. Perhaps not. She might have done one of her own. They could be identical, for all Will knows. The roughness is comforting, solid and real, even though he understands that none of it is solid or real. He is intimately connected to this bench, its atoms shared with an identical (or near-enough) bench in another world altogether, where Lyra sits beside-not-beside him. If he wishes hard enough, the atoms of this bench become indistinguishable from the atoms of that bench, give or take a carving of their names, so it is as if he is truly beside her.
Will sometimes suspects he won't be able to keep his promise of being truly happy until he has died, which is morbid.
Still, sitting in the sunlight, he can imagine that the photons which touch his face are simultaneously touching hers. He has no way to measure them, so they exist in all places in all universes, connected by imaginary quantum strings. It makes everything a little more bearable, for a little while.