To say that the quantum world is not intuitive is a staggering understatement. Particles end up in more than one place at a time and the bodies work together. Decisions made after a photon has gone through an obstacle course determines the path it has traveled. Entangled quantum objects may be in separate galaxies, but measuring one will immediately determine the fate of the second. Obviously, things like this don’t happen in the world of our common experience.
So where is the boundary that separates the quantum world from ours? While the above experiments were first demonstrated with individual particles, researchers have revisited some of them with increasingly larger objects, showing that whole molecules will behave just like an electron. Now the limit has been pushed even further, as a collection of papers describes the entanglement of objects made up of thousands of atoms.
A quantum cloud
Three of the newspapers adorn the pages of Science, and they all rely on a similar material: clouds of ultra-cold atoms, up to 20,000. These depend on another quantum quirk: when two particles become physically indistinguishable, they start to behave as a single system of entangled particles. (As one of the more lucid teams of researchers writes, “The generation of entanglement relies on the fundamental symmetry of the particle exchange in ensembles of identical particles.”) Choosing the right atoms to create a cloud, they form itself naturally. a Bose-Einstein condensate, in which they all assume the same state.
As a result, the atoms can become entangled relatively easily. But since they are normally indistinguishable from each other, there has been no way to do anything useful with them. The atoms of the cloud may be entangled, but we can really only do experiments with the cloud rather than with its individual entangled atoms.
The solution to this turned out to be remarkably simple. The researchers simply turn off the system that holds the atoms in the cloud together in a bundle. Freed from this constraint, the atoms began to drift naturally, causing the cloud to expand. As it expanded, the researchers could easily experiment with different parts of it — in this case, measuring the spin of the atoms.
For two of the papers, the researchers took measurements that showed that the two halves of the cloud remained entwined as it expanded. In the third paper, they used measurements of one half to determine the properties of the second with an accuracy greater than the Heisenberg uncertainty principle (a technique called “quantum steering”).
While dividing a cloud in half doesn’t leave us with many entangled objects to work with, one team showed that it is possible to randomize the distributions, while a second team took five measurements on a single expanding cloud.
The cool 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 to test quantum behavior. However, using this approach to generate qubits for quantum computing will be challenging, as any direct intervention to split the cloud into parts can easily ruin the entanglement.
Two other newspapers, which appear in Nature, also look at a large quantum system. In this case, it’s a pair of quantum oscillators – think mirrors glued to springs. When a photon hits one of the mirrors, it’s likely to give up some of its energy to make a mirror vibrate. If the mirrors are kept cold enough to prevent ambient noise from getting in the way, the photon can transfer some of its energy to the mirror, causing it to vibrate with a single amount of energy.
As a result, they can become entangled. Both research groups used an arrangement where two vibrating mirrors were arranged 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 say which mirror it hit. This left both mirrors in a physically indistinguishable superposition of vibrating and non-vibrating. And because they were indistinguishable, the mirrors behaved like a single quantum system — in other words, they were entwined.
There are a few notable features of this system. For starters, the mirrors are, in quantum terms, huge, consisting of an estimated 1012 atoms. That reduces the thousands of atoms used in the other series of experiments.
The second is that the distance between the mirrors can 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 fabricate the devices well enough to guarantee that the properties of each two would match well enough to work. So they just bunched up two separate chips and tested until they found a match. ) In the experiment, the two chips were 20 cm apart, but the researchers say they could have placed them up to 70 meters apart without significant changes in their arrangement.
The hardware is also built to work on telecommunications wavelengths and should work up to 75 kilometers at 95 percent of the efficiency it did in the lab. And that adds to the potential practical benefits here: easy entanglement of two distant devices presents a great opportunity for things like quantum key distribution.
But the striking thing about all this is the physical size of the systems that become entangled. They’re still microscopic, but they’re still significantly bigger than the things we usually think belong in the quantum world. It doesn’t tell us where the line breaks between the two, but it certainly looks like that boundary will depend on the specifics of a particular system, rather than just the number of atoms involved.
Science2017. Papers linked through this DOI: 10.1126/science.aat4590 (About DOIs).
Nature2017. DOI: 10.1038/s41586-018-0038-x, s41586-018-0036-z (About DOIs).