One of light’s most redeeming features is that it can be guided in optical fibers with very little change to its properties. In terms of quantum information processing and sensing, this is important: you stick a bit of information into an optical fiber, and you will get that bit out the other end with very high probability.

Matter is not so nice to work with, but the sensitivity that makes matter difficult also makes it promising for sensing applications and quantum computation. Researchers have recently demonstrated that matter can be guided down an optical fiber without losing its quantum state, opening up the possibility of combining the best properties of light and matter.

A train track of quantum states

I think the atomic clock best demonstrates the benefits of moving quantum states around. In an atomic clock, we shoot atoms upward in a vacuum to measure the passage of time. The end result is measurement accuracy down to about 10 attoseconds (10-18s).

But if we could shuffle atoms around in a more controlled way, we could do more. There are many sensing applications, like magnetic field sensors or vibration sensors, that might gain a lot from using the quantum states of matter as a probe. Not only that, atomic clocks might benefit from moving from fountains to atoms being moved around a track.

The essential idea is that you should be able to set the quantum state of a cloud of atoms (all cooled to within a whisker of absolute zero). The atom cloud should be transported to a new location. Any changes in the quantum state should be due to something we want to sense, not the guiding process.

This is exactly what the researchers have demonstrated.

Hollow fibers, filled with matter

Matter can be trapped and guided by light. If we shine a tightly focused laser beam on a cloud of cold atoms, the atoms will experience a force due to the shape of the light field. The force pushes them toward the bright center of the laser beam. The atomic cloud can be waved around by shifting the point of focus of the laser beam, but the movement and finesse of the operation is very limited.

Enter the hollow fiber, a type of optical fiber that guides light in an open structure. This is important, because the open core of the fiber provides the space for atoms to travel along the fiber. However, it is the shape of light intensity within the hollow core that pushes the atoms away from the wall and back to the center. You can think of this as a two-step process: the optical fiber guides the light down the hollow core, while the light guides the atoms down the fiber’s path.

Guiding and preserving

Guiding matter down a fiber is not entirely new. And even though the light keeps the atoms away from the sides, the quantum state is still modified by the presence of the glass. Glass has electrons, those electrons respond to the presence of the light, and, in turn, the atoms respond to the electrons. The end result is that over time, each atom’s state is modified, and that modification depends on its path down the fiber.

The researchers showed that by using a careful sequence of microwave pulses, the influence of the wall could be partially removed. Essentially, the first pulse sets the state to change with time. The atoms traveling down the fiber start getting all messed up. Then, at the halfway point (in time not distance), a second pulse reverses the direction of change with time (think of it as inverting the state). As the atoms continue to interact with the wall, the mess created in the first period of time is reversed on average. After the atoms have reached their destination and hung about for a while, the first pulse is undone, returning the atoms to their original state. Once all that is completed, the researchers measure the number of atoms that are in the expected state.

What does this mean? It means that the quantum state of the atoms is still preserved after the atoms are dropped into the fiber, transported 11cm down the fiber, and then left hanging in place for 100ms.

That is a very impressive achievement. To put it in perspective, most matter transport experiments move clouds of atoms a few millimeters at most—now we have something a factor of 100 better. Even more important for sensing applications, the further you can transport a quantum state, the better. Essentially, distance and sensitivity are related (this is why gravitational wave observatories are several kilometers in size). Now we are on the verge of having matter guides that can gain sensitivity through distance as well.

The best is yet to come

We are also nowhere near the end of improving the preservation of the quantum state. The researchers estimate that they can improve the coherence time (the main property that they measure) by at least a factor of ten with relatively straightforward improvements.

However, that is a side issue for researchers. At the moment, this researcher’s measurements have focused on the states of the individual atoms. But these cold atoms form a Bose Einstein condensate, meaning that they are also a matter wave. It is not yet known if the wave-like characteristics of the condensate is preserved. That will be the next step. In the meantime, bring on the hollow fiber quantum state sensors.

Physical Review Letters, 2019, DOI: 10.1103/PhysRevLett.122.163901 (About DOIs)