Quantum Interference Across an Astronomical Distance

In 1987, a landmark experiment demonstrated a striking quantum optical effect: When two identical photons simultaneously enter a beam splitter, quantum interference forces both of them to bunch together and always exit from the same beam splitter port. Subsequent variations of the experiment have tested a variety of settings to show that photons from distant sources can show quantum interference. Now, a team of researchers has harnessed sunlight to demonstrate quantum interference between photons generated 150 million km apart, setting the stage for quantum optics experiments on astronomical scales.