Bose Einstein condensation (BEC) was one of the highlights of late 20th century physics. Scientists showed that if you cooled atoms enough, they would all get together in the same quantum state. At this point, it becomes meaningless to speak of "this" atom or "that" atom, since they are all absolutely identical and indistinguishable and can be manipulated collectively. Because they all start in the same place—quantum mechanically speaking—they all end up in the same place.

This robustness seems like it would be really useful, but there's a catch: creating a BEC requires an optical table, an enormous vacuum system, and a generally large and complicated setup. The vacuum system is unavoidable, but researchers have been working hard on miniaturizing the rest of the setup. Traps (basically wires on chips) can now hold atoms in large areas, a consequence of the low frequency currents that run through the wires and create a trapping magnetic field. To create traps that confine atoms within a few nanometers, researchers have now created traps using plasmonic fields.

How to trap an atom

Traps for neutral atoms come in three different flavors. One is called an optical molasses trap, where the Doppler shift is used to turn the atom's translational motion into light, which prevents atoms from flying away from the trap. Atoms absorb very specific frequencies of light. If an atom is moving towards the light field, the Doppler shift ensures it will see the frequency as slightly higher than a motionless atom will. If I choose my frequency carefully, moving atoms will absorb light while stationary atoms will not.

On absorbing the light, the atom gets a small kick, slowing its motion along the direction of the light beam. Furthermore, eventually it will radiate the energy, giving it a second kick in a random direction. If I use three pairs of laser beams all facing a cloud of atoms, then no matter what direction the atoms are moving, they always get driven back to the place where the laser beams meet.

The big drawback of this trap is that it puts atoms into an excited state. From there they have many options, only one of which involves emitting the absorbed photon—atoms that don't choose this path escape the trap. Furthermore, the excited state destroys a BEC (remember, all the atoms need to be in the same state to form a BEC), so the trap must be switched off at some point in the preparation process and cannot be used to manipulate the BEC.

One alternative is a magnetic trap. Most atoms—atoms that have an uneven number of electrons— are small magnets. With a good magnetic kicking, you can encourage a cloud of atoms to orient so that their magnetic fields are all pointed in the same direction. Once in the same state, you can trap them by introducing a magnetic field with the opposite orientation. This is as easy as running a current through a wire, making magnetic traps excellent for holding atoms in place on small scales. Create a glass chip with a wire running along it, run some current through it, and, provided you have your atoms cold enough, they will hang around the vicinity of the wire.

Even through the frequency of the current in this trap is low, the trap dimensions can be very small. Basically, you can run huge currents through the wire, and, because the atoms sit right next to it, the field gradient is also really huge, creating a very deep hole for the atoms to fall into. The atoms don't have much energy anyway, so they stay right at the bottom.

The downside? Magnetic traps only work for atoms oriented in the right way. If a collision causes an atom to spin-flip, it will be accelerated out of the trap. A consequence of this is that researchers must either choose to use states that don't involve spin-flips (and accept that they will slowly lose atoms from their BEC) or turn the trap off before starting to play.

A second type of optical trap is called an optical dipole trap. This trap makes use of the fact that laser beams don't have an even intensity profile—they are brightest in the center and fade toward the outside. This can be used to create a force to slow atoms.

Take an atom sitting just to the right of the center of the laser beam. As photons pass through the electron cloud of this atom they are deflected—those on the left-hand side are deflected to the right, while those on the right are deflected to the left. But, because the atom is off-center, there are more photons being deflected on one side. As a result, the atom feels a small force that drives it back to the center. Once there, the photons are still deflected, but the deflection is symmetric, holding the atom in place along the axis of the laser beam.

The optical dipole trap is much less invasive, quantum mechanically speaking—it can still excite atoms out of the desired state and destroy the BEC, but the probability is low. The consequence of this non-invasiveness is that the trap tends to be rather large and weak. So there is a huge amount of interest in creating dipole traps that are small enough to hold just a few atoms, or even a single one.

Plasmonics

Researchers from Germany have now made a small dipole trap. They placed a series of different gold structures on a transparent substrate. Some structures were simple pads, while others were more complicated, striped structures designed to either focus the plasmon field or to create moving traps. When illuminated by a laser from the backside, a plasmon can be created that either oscillates back and forth on a pad or travels along a structure. Either way, the plasmon creates a strong field in the vicinity of the metal. The trap is created by a balance between the fields from the displaced electrons in the gold (which attracts) and the repulsive force of the light field.

To get atoms into the trap, the researchers cheated a little bit, using a prepared BEC that was gently pushed into the trapping region. And, in fact, the BEC was much larger than any of the traps, so most of the atoms simply bounced off the surface and were lost. But those that were trapped ended up sitting just a few nanometers from the gold surface. The lateral size of the trap was much larger, because this depends on the width of the laser beam and the type of metal structure that the plasmon is travelling through.

Although the researchers' traps were no smaller than the magnetic ones, they think that by using the right plasmonic structures, the trap dimensions could be reduced to as little as 100nm. This is smaller than is possible with other on-chip methods, and it has the benefit of being a dipole trap.

But there is more to this than tiny traps, as the structures that create the plasmon allow a degree of control over the BEC. The researchers demonstrated this with a diffraction grating experiment. One of the structures was a series of closely spaced gold wires. Plasmons travel down the wires, and as they travel they jump from wire to wire, interfering with each other. As they do so, they carry their individual BECs with them, which interfere with each other as well. As a result, certain parts of the cloud end up with a momentum kick compared to the rest. So, atoms end up traveling in different directions depending on how and when the plasmons cross.

The actions of the light field on the BEC depend on the structure of the light field, which, in this case, depends on the metallic structures that are illuminated. One can imagine trapping a single condensate on a pad and moving the condensate along the pad by simply moving the laser beam. You could split the condensate by dividing the pad in two and running each condensate through its own series of operations by passing it over different structures. If you recombined the two condensates to interfere, the resulting pattern could be used to determine the outcome of the operations. These are the basic sorts of manipulations required for quantum information processing.

The results presented here are not too startling: it would have been more surprising if it hadn't worked. Nevertheless, this is one of those critical steps that is required to keep things moving from fundamental physics to eventual applications.

Nature Photonics, 2011, DOI: 10.1038/NPHOTON.2011.159