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Astronomy and space

Physicists propose space mission to test quantum gravity

28 May 2018 Michael Banks
International Space Station
Fundamental probe: Could the International Space Station be the basis for a first test of quantum gravity? (Courtesy: NASA)

A global team of researchers is planning to use the International Space Station (ISS) to test the fundamental nature of quantum mechanics. The Space QUEST proposal aims to investigate, for the first time, whether gravity can affect a quantum state of light over large distances by firing entangled pairs of photons from a ground station to the ISS. If the European Space Agency (ESA) gives the proposal the green light, the experiment could begin by the early 2020s.

The work is based on a theory, developed by Timothy Ralph from the University of Queensland and colleagues, based on the notion that quantum states should behave differently than classical counterparts under the influence of gravity. “We have tested quantum mechanics and general relativity separately to incredible precision, but they are so fundamentally different that it is hard to reconcile the two in a theory of quantum gravity,” says Siddarth Joshi from the University of Bristol who is part of the team. “This theory is one of a few that are actually testable with current technology.”

This is a measure of how excited and interested the community is in this proposal

Siddarth Joshi

The experiment, which is described in the New Journal of Physics, involves creating entangled photons at a ground station and sending them to the ISS where they are detected. The ground station will first send photons that are not part of an entangled state before quickly sending up photons that are. If there is an effect due to gravity on the entangled pair, they should arrive at a random time. Ralph’s theory predicts that there will be a change in the arrival time of approximately a few percent of the photons, with Siddarth adding that the experiment should be sensitive such changes.

Going through the phases 

The new experiment has its roots in a proposal sent to ESA in 2008 to use the ISS to test quantum com­munication. The idea was to use the ISS to generate entangled pairs that would then be sent to two ground sta­tions across the globe. Studying the effects of gravity on these entangled states was meant to be a side pro­ject to that mission. But when the proposal went through initial study rounds, reviewers felt that this “sec­ondary objective” was much more compelling, which resulted in the collaboration revising the scope of the mission.

To reduce the new mission’s cost, the complexity of the experiment will be in the ground station. It is planned that the ISS would host four single-photon detectors at varying degrees of polarization – vertical, horizontal, +45° and –45°. The ground station will be used to generate the single photons using a faint pulsed source. Siddarth says that while one ground station would be enough, more would be better, and while the location for the first ground station has not been agreed, it will likely be at La Palma or Tenerife.

ESA proposals usually undergo a Phase 0 study to show that a mission is feasible followed by Phase A and then B, which involve a cost evalu­ation and prototype development, respectively. Phase 0 was completed recently, but ESA then decided to join Phase A and B together to accel­erate the process. “I think this is a measure of how excited and inter­ested the community is in this pro­posal,” says Siddarth, who expects the Phase AB study to be complete by the end of the year.

Siddarth says that if the experi­ment is successful and it verifies Ralph’s theory, it would be a “mas­sive” result that would mean quan­tum mechanics is even stranger than we realize. However, he says that even a negative result would also be useful as it would place limits on the effect of such gravitational effects on quantum systems, which could be used to rule out some theories.

More tests to come

Siddarth and colleagues’ experiment is not the only one searching for quantum gravity. Another proposal from Sougato Bose and colleagues from University College London, for example, involves entangling a mass with a second identical mass via the gravitational field. To do this, the two masses would first be prepared using two adjacent, identical inter­ferometers. If gravitational fields are truly quantum in nature, the gravitational attraction between the two masses would become entangled once they have left their respective interferometers.

Siddarth says that such set-ups are much more complex and there­fore harder to accurately determine if there is any effect due to gravity and that they cannot be used to test theories similar to those devised by Ralph and colleagues. Indeed, Bose himself admits that the ISS proposal is “one of the simplest experiments one can do” to test quantum fields interacting with gravity adding that most of the technology is ready.

“Certain degradation of the cor­relations between entangled photons is predicted due to general relativity, which will be a very novel effect to check,” says Bose. He is not clear, however, what conclusions about quantum gravity could be drawn from the results. “While experi­ments exploring such regimes may indirectly shed some light on quan­tum gravity, it is perhaps not a priori that clear whether that will be the case or precisely how it will illumi­nate that path,” he adds.

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