I know I have a reputation for liking lasers. But I’m a bit of a philanderer—I have a secret and unrequited love for Bose-Einstein condensates (BECs). BECs are to physicists what lasers were in 1970: an amazing new tool that we are only now coming to grips with.

BECs are not formless clouds of atoms. They are carefully sculpted into pretty shapes that aren’t just beautiful; a BEC’s shape, in a sense, defines its quantum properties. New shapes mean new properties and maybe new applications. A recipe created by a group of theoreticians for creating BEC donuts should have physicists licking their chops.

Bose-Einstein condensate: What is it good for?

Absolutely everything, and you don’t need to say that again.

Essentially, a BEC is a bunch of atoms that are very, very cold (less than a microKelvin). At this temperature, quantum mechanics takes over: the individual atoms lose their identity, and the entire bunch becomes, almost literally, one. The BEC is a single quantum object that is a few millimeters in diameter and is very nearly unmoving. You can test all sorts of rules of quantum mechanics and measure the result just by taking a picture of the BEC. The result is generally encoded in the position or momentum of the atoms that make up the BEC.

The shape of a BEC is generally created by lasers. Instead of delving into the details, let’s put it like this: the wavelength and intensity profile of a laser beam creates a pot that holds the BEC.

You can even create a whole lot of tiny pots: the BEC will divide itself among them. When you take a picture of the BEC, you will find an atom or two in every pot. The shape of the BEC is defined by the 3D arrangement of the pots.

Atoms can (and do) jump out of one pot and into a neighboring one by a process called quantum tunneling. It is this process of moving between locations that keeps the BEC together. The BEC is not just a cloud of atoms that is a single quantum object; that object is a wave. When we impose a shape on the BEC, it has to rearrange itself so that the wave fits in the shape.

To make this idea concrete, imagine that we create a ring of pots, with the BEC spread evenly over them. The wave will also spread around the ring, and it has a single value at any point in space. No matter where in the ring we start it, the wave will travel around the ring and meet itself.

This creates a potential problem, since the wave could have two values at the point where it meets itself. And the wave continues to travel and comes back a third time, so now we could have three values at the same point in space. Nature doesn’t like that situation, so the wave-like nature of the BEC takes on a state that prevents it from occurring. No matter where you start and how many times you send the BEC’s wave around the ring, it always returns to you with the same value.

The shape has determined the state of the BEC.

The shape I’m in

This is where the new research comes into play. A two-dimensional shape, like a ring, only offers the BEC one degree of freedom. The only thing that matters is the length of the perimeter of the ring. A three-dimensional shape, like a donut, offers an extra degree of freedom.

The researchers showed that the BEC only takes on particular states that relate to spiraling around the donut. The rule is the same: the wave, when it meets itself, no matter what path it has taken around the donut, must have the same value.

Summarizing that result is selling the researchers short though. The important part of the research is in the details of creating the donut. Essentially, the researchers were able to tune the intensity and wavelength of laser beams to create a set of pots that have very high walls along the edges. None of the BEC can make it into the hole of the donut, nor can the BEC get into the dough of the donut. Instead, the entire BEC is confined to a set of pots that line the surface of the donut—a BEC glaze, if you will.

The way in which the donut was made is quite flexible. It should be relatively simple to create more complicated shapes, such as three-dimensionally interlinked donuts.

What to do with BEC donuts?

While all of this is impressive from a physics perspective, it may also have practical implications: using donuts as a building block for quantum logic gates.

But Chris, I hear you say, we’ve already got quantum logic gates—you never stop bleating on about them. True, but the donut logic gate is different. Because the shape of the BEC defines its state, as long as the shape is kept fixed, the state cannot change. This form of protection means that information stored in its quantum state should last. Operations on a computer based on arrangements of donuts should be more reliable.

That last point is really important, because quantum computing needs error correction. For every computational quantum bit, you need seven or more quantum bits to ensure that you have no errors. But, if errors can be significantly reduced or even eliminated via the protection offered by the shape, then we only need the computational qubits.

And, then, yes, a BEC in every home. I think I may need a lie-down.

Physical Review Letters, 2018, DOI: 10.1103/PhysRevLett.121.133002 (About DOIs)