Quantum Interference between Light Sources Separated by 150 Million Kilometers

Abstract

We report an experiment to test quantum interference, entanglement and nonlocality using two dissimilar photon sources, the Sun and a semiconductor quantum dot on the Earth, which are separated by ~150 million kilometers. By making the otherwise vastly distinct photons indistinguishable in all degrees of freedom, we observe time-resolved two-photon quantum interference with a raw visibility of 0.796(17), well above the 0.5 classical limit, providing the first evidence of quantum nature of thermal light. Further, using the photons with no common history, we demonstrate post-selected two-photon entanglement with a state fidelity of 0.826(24), and a violation of Bell's inequality by 2.20(6). The experiment can be further extended to a larger scale using photons from distant stars, and open a new route to quantum optics experiments at an astronomical scale.Can any two photons in the Universe, no matter how distantly and independently they originate from, show quantum interference and entanglement? According to quantum theory, when two quantum-mechanically indistinguishable single photons impinge upon a 50/50 beam splitter, they bunch together out of the same output port due to bosonic statistics. The classical picture of electromagnetic fields failed in understanding the interference of two photons from independent sources with a visibility better than 50% 1-4 , which can be explained by quantum interference of the probability amplitudes of the twophoton events 5 . This effect, also known as Hong-Ou-Mandel (HOM) two-photon interference 6 , poses a strong conceptual challenge to the celebrated statement by Dirac that "Each photon then interferes only with itself. Interference between different photons never occurs" 7 .

Imaging Spatiotemporal Hong-Ou-Mandel Interference of Biphoton States of Extremely High Schmidt Number

“…Most of these experiments and protocols used the coherence time properties of the biphoton state, and the measurements are performed by means of bucket detectors and coincidence counters gated in time. HOM interference is obtained if the two photons involved are indistinguishable, whatever their origin, meaning that extremely dissimilar light sources [19] can be used if the corresponding modes are thoroughly tailored. On the other hand, genuine multimode HOM interference implies entanglement, as quoted by Lee et al [20], and SPDC remains the simplest way to produce entangled photon pairs of high dimensionality.…”

Deterministic control of photonic de Broglie waves using coherence optics

“…Here, it should be noted that perfect correlation between two lights (E 3 /E 4 or E 7 /E 8 ) is achieved by path indistinguishability in MZI, and proved by either anticorrelation [1][2][3][4][5][6] or Bell inequality violation [25][26][27][28] . Thus, the specific phase relation with ϕ n between two superposed coherent lights in MZI of Fig.…”

Measuring the time–frequency properties of photon pairs: A short review

“…All measurements described so far implement inter- [111] ferometric and spectrographic techniques directly inherited from the classical world. One might seek to apply reconstruction methods exploiting quantum features in Hong-Ou-Mandel (HOM) interferometry [112], as this is a commonplace technique when working with few photons [113,114]. In a HOM experiment, two photons arrive at the surface of a symmetric beamsplitter with a relative delay τ , and coincidences are measured between the two output modes.…”