Astronomy has always been in need of an unequivocally defined and well measurable time, ideally available to all Observatories in the world. This need is well reflected in history, through the definitions of Sidereal Time, Universal Time, Ephemeris Time, and finally the adoption of the International Atomic Time.
The opening of new observational windows on ground and Space (encompassing the whole electromagnetic spectrum, and extended to Neutrino Astronomy and Gravitational Wave Astronomy) has simply reinforced this need.
In the near future, Quantum Astronomy, as expounded in the Introduction, will raise the need to determine the arrival time of photons (briefly, to time-tag, or time-stamp, the photons) with an accuracy of 1 ns or better (the goal being1 ps), namely from 10^9 s to 10^12 s, in a continuous manner for the entire duration of the observations, be it seconds, minutes, even hours.

Goals of the analysis

We present here some considerations about the possibilities to measure and distribute to several telescopes this very accurate time. Two different scenarios will be considered for the generic astronomical needs:

a) single telescope operation
b) Very Long Baseline Optical Intensity Interferometry

The proper definition of these scenarios, and the full justification of the advantages expected from GNSS over other possibilities, will be the major output of our analysis in the course of the first part of this study.
It will be shown that only a Space system like EGNOS/GNSS presents the capability to link in an economic, efficient and certified manner the many Observatories on the ground and/or in Space. Furthermore, we propose to analyze in the course of this study the need for future developments, like:

• optical clocks,
• space facilities distributing to ground observers sets of photons with predetermined well controlled statistical properties (thermal source, laser, entangled photons),
• clock synchronization by means of novel quantum devices such as entangled photons.

Single telescope operation

The time information comes from a clock inside the Observatory, or remotely via fiber, or air, or satellite.
Quite often though the need arises to compare the instant of an event recorded by different Observatories, and here is when the availability of a well defined and certified time can bring to novel scientific results.
The example can be made of one of the most important astrophysical events of the recent past, namely the explosion of the SuperNova 1987A in the Large Magellanic Cloud. The lack of this certified time information prevented the proper synchronization of gravitational waves and neutrino detectors with optical and radio telescopes on the ground and in Space, caused the loss of information on the geometry of the event inside the star (it must be remembered that 1 ns in vacuum corresponds to a path of 30 cm), and finally was at the root of bitter discussions among scientists.
This case might reproduce in the future, when gravitational wave detectors like LISA of the European Space Agency, neutrino detectors in Antarctica, telescopes in Chile or Europe etc. might witness another similar event.

Regarding Quantum Astronomy, an example of the single telescope operational mode was discussed in detail in our paper about QUANTEYE (the quantum eye of the future extremely large telescope of ESO, Barbieri et al., 2006). There (see Fig. 1), we adopted a 'brute force' solution to satisfy the time tagging needs, namely equipping the detector (a distributed array of SPAD detectors, each capable to time stamp the incoming photon to better than 50 picoseconds) with a GPS receiver to provide the UTC start/stop signals, and a H-Maser clock.
However, this brute force solution is certainly not warranted on operational and economic grounds, because at any large telescope it is essentially impossible to operate for more than few hours for a very limited number of nights, so that the cost of buying and maintaining a H-Maser clock is unjustified. This problem could be circumvented by a proper utilization of the EGNOS- GNSS signals, with suitable error correction routines. We plan to test this alternative strategy by using as soon as possible the Asiago and Vega telescopes at our disposal, equipping them with a SPAD detector.

Fig. 1 - The 'brute force' solution for QUANTEYE timing. The GPS (or EGNOS/GNSS) provides the UTC start/stop signals, while a local device (here a H-Maser) insures the time stability during the observations. The first 'plus' added by the EGNOS/GNSS availability is a certified rigorous UTC. We plan to demonstrated inside this work that clever strategies and proper algorithms can afford to dispense with the local H-Maser

Very Long Baseline Optical Intensity Interferometry

In their pioneering work, Hanbury Brown and Twiss demonstrated the feasibility of optical Intensity Interferometry, using two detectors on two distant movable telescopes.
To be clear, Intensity HBT Interferometry, does not rely on measuring the phase of the optical wave, it is a property of the quantum field that can be exploited by high precision time tagging of the incoming photons. As such, HBT interferometry is largely independent on atmospheric effects, such as seeing or intensity scintillation.

In a modern realization of the HBTI, the two telescopes could well be very distant from each other, in the limit one of them could be on the ground and the other on a low Earth orbit telescope, provided there is the possibility to be reliably synchronized to a common central clock, as routinely done in VLBI operations thanks to the local availability of H-Maser clocks.

We plan to explore, again using our Asiago and Vega telescopes and the EGNOS/GNSS time signals, the feasibility of this modern version of HBTI, because it would bring an exceptionally valuable and entirely new way of doing optical astronomy. For instance, two telescopes 100 km apart could resolve the surface of the nearby stars to the point of actually seeing the analogous of the solar spots (see Fig. 2).

Fig. 2 - The resolution of the Very Long Baseline Optical Intensity Interferometry