In this paper, we have primarily considered how distributed quantum entanglement can potentially improve optical interferometry. For radio frequencies, interferometry can be performed robustly today even between telescopes spread across the planet. Optical frequencies are much higher, so fewer photons arrive per second, making interferometry much more difficult. In telescope design, the arriving light is usually treated classically, but when the number of photons arriving is small, the quantum state of the light may become important. Thus, the eld of quantum information is well-suited to provide advances. Quantum repeaters have until now been under development primarily for use in quantum communications, so interferometry o ers a very interesting new venue for the application of quantum information techniques.
As we have shown, in the long run, quantum repeaters can completely lift the upper limit on distance over which it is possible to do interferometry. In order to do so, a number of technical hurdles need to be overcome, either by improved technology or by theoretical innovations. We need high-rate bandwidthmatched single-photon sources, and high efficiency photodetectors with very fast time resolution. We also need robust quantum repeater protocols, including entanglement distillation. One additional requirement we have is that the light arrives in the telescope in certain modes, and the quantum repeater modes must adapt to that. Therefore, the repeater protocols need to work at the desired optical frequencies or we need to implement technologies to shift the frequencies of either the arriving light or the entangled photons.
Quantum information technology may o er even more signi cant applications to help improve astronomical observations. For instance, it may be advantageous to coherently store arriving photons using a quantum memory and then perform the quantum Fourier transform, rather than measuring, waiting and performing the classical Fourier transform. The quantum Fourier transform works reasonably well even with a small number of photons, whereas if we measure rst, we need enough photons to get a reliable measurement of each phase.