To say the quantum world is unintuitive is a staggering understatement. Particles end up in more than one place at a time, and the instances interact with each other. Decisions made after a photon has traversed an obstacle course determine the path it took through it. Entangled quantum objects can be in separate galaxies, yet measuring one will instantly set the fate of the second. Obviously, things like this don't take place in the world of our common experience.

So where's the boundary that separates the quantum world from ours? While the experiments above were first demonstrated with individual particles, researchers have revisited some of them with ever-larger objects, showing that entire molecules will act just like an electron does. Now, the limit's been pushed back even further, as a collection of papers describes the entanglement of objects that consist of thousands of atoms.

A quantum cloud

Three of the papers grace the pages of Science, and all rely on a similar material: clouds of ultra-cold atoms, up to 20,000 of them. These rely on yet another quantum quirk: if two particles become physically indistinguishable, they start to behave like a single system of entangled particles. (As one of the more lucid teams of researchers write, "The entanglement generation relies on the fundamental particle-exchange symmetry in ensembles of identical particles.") Conveniently, if you pick the right atoms to make a cloud, they'll naturally form a Bose-Einstein condensate, in which all of them adopt the same state.

That makes it relatively easy for the atoms to become entangled with each other. But, since they normally remain indistinguishable, there's been no way to do anything useful with that. The atoms of the cloud may be entangled, but we can really only do experiments with the cloud rather than its individual entangled atoms.

The solution to this turned out to be remarkably simple. The researchers simply shut off the system that's holding the atoms in the cloud together in a bunch. Freed of this restraint, the atoms naturally started to drift, causing the cloud to expand. As it expanded, the researchers could simply experiment with different parts of it—in this case, measuring the spin of the atoms.

For two of the papers, the researchers performed measurements that showed the two halves of the cloud remained entangled as it expanded. In the third paper, they used measurements of one half to determine the properties of the second with a precision greater than is possible under Heisenberg's uncertainty principle (a technique called "quantum steering").

While dividing a cloud in half doesn't give us many entangled objects to work with, one team showed that it's possible to make the divisions arbitrarily shaped, while a second team performed five measurements on a single expanding cloud.

The nice thing about this is that cold atomic clouds are extremely well understood physical systems—we're pretty good at generating and manipulating them. So this could be a good system for testing quantum behaviors. It's going to be very challenging to use this approach to generate qubits for quantum computing, though, because any direct intervention to split up the cloud into parts could easily ruin the entanglement.

Good vibes

Two other papers, appearing in Nature, also look at a big quantum system. In this case, it's a pair of quantum oscillators—think mirrors stuck on springs. When a photon hits one of the mirrors, there's a chance that it will give up some of its energy to set a mirror vibrating. If the mirrors are kept sufficiently cold, preventing environmental noise from getting in the way, the photon can transfer some of its energy to the mirror, setting it vibrating with a single quantum of energy.

That allows them to be entangled. Both research groups used a setup where two vibrating mirrors were set up opposite each other with a path for photons to travel between them. In one case, the entanglement was done by sending a photon into the device so that it bounced off one of the mirrors. But the researchers couldn't tell which mirror it hit. This left both mirrors in a physically indistinguishable superposition of vibrating and not vibrating. And, since they were indistinguishable, the mirrors behaved like a single quantum system—in other words, they were entangled.

There are a couple of notable features of this system. To begin with, the mirrors are, in quantum terms, massive, consisting of an estimated 10¹² atoms. That dwarfs the thousands of atoms used in the other sets of experiments.

The second is that the distance between the mirrors could be arbitrarily large. In one case, they were placed on a single chip, with an optical channel between them. In the second, however, they were on separate chips. (This was out of necessity. The researchers couldn't manufacture the devices well enough to guarantee that the properties of any two would match well enough to work. So they simply made a bunch on two separate chips and tested until they found a match.) In the experiment, the two chips were 20cm apart, but the researchers say they could have placed them up to 70m apart without any significant changes to their setup.

The hardware was also built to operate at telecommunications wavelengths and should work out to 75 kilometers at 95 percent of the efficiency it did in the lab. And that gets to the possible practical benefits here: easy entanglement of two distant devices provides a great opportunity for things like quantum key distribution.

But the striking thing about all of this is the physical size of the systems being entangled. They're still microscopic, but they're still considerably bigger than the things we typically think of as belonging in the quantum world. It doesn't tell us where the line between the two breaks down, but it's certainly looking like that boundary will depend on the specific features of a given system, rather than simply the number of atoms involved.

Science, 2017. Papers linked via this DOI: 10.1126/science.aat4590  (About DOIs).
Nature, 2017. DOI: 10.1038/s41586-018-0038-x, s41586-018-0036-z  (About DOIs).