Photon Bunching in a Rotating Reference Frame

Although quantum physics is well understood in inertial reference frames (flat spacetime), a current challenge is the search for experimental evidence of nontrivial or unexpected behavior of quantum systems in noninertial frames. Here, we present a novel test of quantum mechanics in a noninertial reference frame: we consider Hong-Ou-Mandel (HOM) interference on a rotating platform and study the effect of uniform rotation on the distinguishability of the photons. Both theory and experiments show that the rotational motion induces a relative delay in the photon arrival times at the exit beam splitter and that this delay is observed as a shift in the position of the HOM dip. This experiment can be extended to a full general relativistic test of quantum physics using satellites in Earth’s orbit and indicates a new route toward the use of photonic technologies for investigating quantum mechanics at the interface with relativity.

Figure 1

Rotating interferometer schematic layouts: “Sagnac” indicates the classical interferometer formed from two counter-propagating beams that then overlap at a small angle on a camera that measures the resulting interference fringes. Platform rotation is detected as a lateral shift in the fringes. “HOM” indicates the quantum Hong-Ou-Mandel interferometer that can be implemented on the same platform as the classical Sagnac interferometer by introducing a down-conversion crystal in the plane of the camera. The two down-converted photons enter the interferometer and propagate in opposite directions, interfere at the beam splitter (BS), and are coincidence counted at the two BS output ports with single-photon avalanche detectors, SPADs (A) and (B). Rotation is detected by measuring a variation in the coincidence counts.

Figure 2

Classical experiment. (a) Experimental layout. (b) Results showing measured Sagnac phase shifts (circles) and fitted line (dashed).

Figure 3

Quantum experiment. (a) Experimental layout. (b) HOM interference dip measured by scanning the delay stage with no rotation of the experiment. The vertical dashed line marks the delay corresponding to the point of maximum steepness of the HOM dip. The shift can then be measured by fixing the stage at this delay and observing the changes in coincidence counts when the interferometer is in rotation. (c) Results showing measured HOM interference shift (circles) and fitted line (dashed).