We use heralded single photons to perform an antibunching experiment in which the clicks at the detectors are spacelike separated events. The idea of such experiment dates back to the 5th Solvay conference, when it was proposed by Einstein as an expression of his concerns about quantum theory.
In this chapter, we address proposals (by Einstein and by Bohr, Kramers and Slater) according to which quantum events are influenced by ‘guiding fields’ in 3-space. These ideas led to a predicted violation of energy-momentum conservation for single events, in contradiction with experiment. The contradiction was resolved only by the introduction of guiding fields in configuration space. All this took place before the fifth Solvay conference, but nevertheless forms an important background to some of the discussions that took place there. Einstein's early attempts to formulate a dynamical theory of light quanta Since the publication of his light-quantum hypothesis in 1905, Einstein had been engaged in a solitary struggle to construct a detailed theory of light quanta, and to understand the relationship between the quanta on the one hand and the electromagnetic field on the other. Einstein's efforts in this direction were never published. We know of them indirectly: they are mentioned in letters, and they are alluded to in Einstein's 1909 lecture in Salzburg. Einstein's published papers on light quanta continued for the most part in the same vein as his 1905 paper: using the theory of fluctuations to make deductions about the nature of radiation, without giving details of a substantial theory. Einstein was essentially alone in his dualistic view of light, in which localised energy fragments coexisted with extended waves, until the work of de Broglie in 1923 – which extended the dualism to all particles, and made considerable progress towards a real theory (see Chapter 2).
In the famous Solvay 1927 conference, Einstein discussed a gedanken experiment involving a single photon diffracted at an aperture and impinging on a screen. He devised the example to support De Broglie's hypothesis of the pilot wave, and his own ideas on the incompleteness of the description of physical reality provided by Quantum Mechanics. Partial realizations of Einstein's example have been performed, but the complete experiment has not been attempted (for good practical reasons) yet. Here I describe how to do it with accessible means. The setup will make possible to test Hellwig and Kraus postulate of covariant reduction of the quantum state.