Perhaps one of the most famous experiments in physics has been giving us the wrong message all these years. For nearly two centuries, the double-slit experiment has been providing a direct visual confirmation of the wave properties of light through its interference pattern of dark and light fringes. However, new research conducted under the lead of Gerhard Rempe from the Max Planck Institute of Quantum Optics challenges conventional understandings and suggests that such interference patterns might be modeled without invoking wave properties whatsoever.
The particle-based model of the team is founded on the principles of bright and dark photon modes. The bright modes are those which are detectable by a detector. This refers to the ability to detect the photon. On the other hand, the dark modes are those which are not detectable by a normal detector. This implies the photon is present but cannot be detected using a normal detector. The two modes give the regions where the interference pattern takes place. The regions of constructive interference represent the bright modes. The dark modes are located in regions of destructive interference.
Such a notion finds its inspiration from Dicke’s bright and dark states of atoms, pioneered in the 1950s, but now generalized for optical modes. In the double-slit experiment, photons are in a quantum superposition of bright and dark states. One is familiar with maxima and minima in an interference pattern being statistical distributions of locations where photons interact or do not interact with the measuring device. According to Rempe, “maxima and minima result from entangled bright (that couple) and dark (that do not couple) particle states.”
The implications of these ideas are profound. From classical electromagnetic theory, based on Maxwell’s equations, it follows that where there is total destructive interference, where the average value of the electric field is zero, light cannot, by definition, interact with matter. But quantum optics permits photon states to exist within these ‘voids’, which correspond to places where light is not expected to be, challenging these notions about these places being empty. Indeed, photons occupying dark states might be able to interact under specific conditions, like where a measurement changes a dark state to a bright state without transferring momentum.
Single-photon detection has become accessible with this new interpretation thanks to recent technological advancements. Current technologies in quantum imaging with undetected photons have already harnessed the use of paired entangled photons to image a sample without having to detect the photon used for imaging. These technologies rely on the use of the interference pattern of a secondary photon for data on an object, even in high levels of noise. These can be used for the search for photons in dark states through phase modulation or coherence.
The set of tools for experimental proof is broadening and intensifying. It is possible to set up quantum optics experiments based on cavity quantum electrodynamics that can put atoms into states of selective interaction and blindness to bright and dark states, respectively. One can manipulate interactions between light and atoms at the single-photon level to demonstrate or falsify ideas about destructive interference of photons and to create photon detectors sensitive to dark states by harvesting optical information from regions of classical zero intensity.
The reinterpretation is also consistent with frontier work on wavefunctions of photons. The wavefunctions of photons in momentum space are described in terms of transverse vectorial wavefunctions with regard to their helicity, energy, and spin. In coordinate space, definitions with Riemann-Silberstein vectors establish a direct relationship between the quantum state of photons and the field components of the electromagnetic fields. In this theoretical framework, bright and dark modes are different only in terms of their interaction with quantum degrees of freedom of a detector and not in terms of their field amplitudes.
Applications would go well beyond fundamental tests. Quantum communication networks might utilize bright/dark-state encoding in order to enhance channel capacity or optimize security, given that dark-states are unaffected by conventional interception. In spectroscopy, detectors adapted for conversion of dark to bright states might allow for analysis of spectral signatures that are inaccessible using classical techniques. Even the search for gravitational waves would be aided inasmuch as the sensitivity of a interferometer might be improved by considering populations of unseen photons in regions of destructive interference.
But in this particle model, it does not dismiss wave-particle duality at all. Rather, it interprets interference in terms of superposition and entanglement of modes of detection. In either case, it’s a story of a gateway model of relevance for physics students, a transition model between classical field theories and quantum states. For science popularizers, it’s a warning that even a most celebrated experiment could still hold secrets if looked upon through a different perspective one which looks behind, where dark photons dwell.
