View from the conference centre, Erice
I'm lucky enough to be attending a COST Action workshop on quantum foundations currently taking place in Erice, Italy, with lots of FQXi folk in attendance. (Thank you to the organisers, FQXi's Angelo Bassi, Catalina Oana Curceanu and Detlef Duerr and the meeting's sponsors for inviting me.) I'll keep you posted on the highlights throughout the week (and beyond, with a few podcast specials in the works).
Much of the first day was taken up with discussions of matter-wave interferometry. Quantum mechanics tells us that the boundary between waves and particles is murky at small scales. But just how far does this ambiguity stretch? Markus Arndt and Jonas Rodewald of the University of Vienna, and James Bateman, of Southampton University, opened the meeting by talking about ways to test whether ever-larger particles display quantum properties such as superposition, the ability to be in two places at the same time, for instance. (Bateman works with FQXi member Hendrik Ulbricht and you can read more about Ubricht's Southampton tests of the quantum limits in an article written by reporter Sophie Hebden.)
Bateman's talk particularly caught my attention when he mentioned plans to put such quantum experiments in space to try to detect dark matter. Although we have good evidence for the existence of dark matter -- which is invisible but exerts a gravitational pull on other matter -- from astronomical observations, physicists still do not know what it is and have devised numerous clever experiments to try to detect it directly and help identify it.
The inspiration to link dark matter to quantum optics came from an intriguing suggestion in 2013 by C. Jess Reidel that dark matter could be causing decoherence in matter wave experiments (knocking fragile quantum objects and causing them to lose their nifty quantum properties). As Reidel said in the abstract to a paper in Phys. Rev. D: "The apparent dark matter wind we experience as the Sun travels through the Milky聽Way ensures interferometers and related devices are directional detectors, and so are able to provide unmistakable evidence that decoherence has Galactic origins."
It was a provocative notion and Bateman and co decided to investigate further. "This was an interesting idea, but essentially none of the details had been worked out, so we started talking to theoretical particle physicists," Bateman told me. Those theoretical physicists were initially dismissive, but together with the experimental team, they came up with a candidate dark matter particle that would have evaded detection by dedicated collider experiments and would not have shown up in high energy collider experiments, but might show itself in a matter-wave experiment.
It turned out that this candidate could not actually be the decoherence mechanism in any existing matter-wave experiments, says Bateman. The hypothesised particle has a mass of about 0.02 per cent of the electron. "It is very low mass, which means its de Broglie wavelength" -- the wavelength associated with quantum particles -- "is large compared with the nuclei in normal matter," Bateman explains. This means that it coherently interacts strongly with normal matter -- so much so that "it couldn't penetrate Earth's atmosphere, let alone the glass and metal vacuum chambers of, for example, Markus Arndt's interferometers," says Bateman.
It's not too much of a disappointment, though. A dark matter decoherence mechanism, despite sounding cool, isn't that different from other decoherence mechanisms, in which the environment interacts with a quantum system, destroying its fragile properties. So, notes Bateman, even if it had worked in theory, it wouldn't have magically solved the quantum measurement problem in a new way.
But this still left open the exciting possibility that a quantum optics experiment carried out beyond the atmosphere could be sensitive enough to pick up signs of this particle. The team outlined such an experiment in a paper published in Scientific Reports, involving a suspended nanoparticle. The way in which the nanoparticle's position changes will tell them something about dark matter.
Scientific Reports
The idea is to place the experiment in a spacecraft located at Lagrange point 2 (a stable point in our solar system beyond the Earth). The nanoparticle's position can be precisely tracked by firing laser light at the particle and collecting it with a lens. Light that has been scattered by the particle will interfere with the rest of the laser light, changing the intensity of light picked up by detectors, in a way that can be precisely monitored. (Image from Scientific Reports 5, article number: 8058, courtesy of James Bateman.)
The team plans to send the experiment into space as part of the MAQRO (microscopic quantum resonators) consortium. You can read more about space-based quantum tests in a Q&A that reporter Colin Stuart carried out with David Rideout and in an article that I wrote for Nature about the quantum space race between China and Europe.