Quantum coherence of photons to cosmological distances

Viability of quantum communication across interstellar distances

“…This is a remarkable result, since in principle one has to assume there is very little chance to observe such a quantum communication signal arriving from interstellar distance. Nevertheless this paper along with our previous ones [17,18] have demonstrated that such a signal can propagate through large interstellar distances without decoherence. As such, this is a viable signal for astronomy.…”

“…which is much longer than the observable Universe. A more thorough calculation arriving to the same conclusion (lack of interaction of X-rays with CMB radiation) is also shown in [18]. The interstellar medium is dominated by electrons, protons and CMB photons.…”

“…A first attempt at this question was reported in [17], where the mean free paths of photons at galactic scales were computed, pointing out that if these are large enough the quantum coherent state of the photon will be sustained. A complementary analysis considering cosmological distances was presented in [18], where the conclusions from [17] were reinforced, i.e., X-ray photons are arguably the best alternative for establishing quantum communication at such scales, with the additional point that the transparency window extends up to z ∼ 100 [53].…”

“…In this paper, we explore some of the potential factors that could prevent us from detecting quantum signals (or establishing quantum communication channels) at interstellar scales. In previous works, we have established that factors like the expansion of the Universe or the interaction with CMB radiation and other particles at galactic scales do not destroy the quantum coherence of a photon state [17,18]. Here, we will pay attention to the potential role of gravity as a decoherence factor for photons.…”

Introduction to gravitational redshift of quantum photons propagating in curved spacetime

“…Nevertheless, recent developments in the field of quantum information [36], and relativistic quantum information in particular [37], have fuelled novel approaches to the study of physics at the overlap of general relativity and quantum mechanics without the need for a complete Theory of Everything. Many new models and experimental proposals are now being put forward to deepen our understanding of this key area of physics: these range from collapse models [38] and gravitationally induced decoherence of a quantum state [39,40] to testing the quantum nature of gravity with tabletop experiments [41] or to modelling spacetime as a quantum channel for propagating quantum systems [42,43,44,45,46,47], from stochastic gravity [48] and semiclassical approaches to self gravitation of quantum systems [49] to gravitational quantum time dilation [50,51] and to developing setups geared at detecting the quantumness of gravity [52,53,54] or testing gravitationally-induced effects on the interferometric visibility [55,56].…”