During the last decades, quantum physicists have been working towards reaching their dream: to be able to control single atoms, molecules or other tiny particles governed by the laws of quantum physics so accurately that it would be possible to build new types of computers and other machines with them. However, many control techniques that are easy in the classical world of big things that we experience every day become extremely difficult when one tries to apply them to the very small. Imagine, for instance, a stationary marble at the bottom of a salad bowl. To make the marble oscillate back and forth in the bowl, all one has to do is to briefly shake the bowl fast enough to one side and back. The marble, owing to its inertia, will then roll up the bowl and down again, then up and down on the other side, and so forth. Joseba Alonso and Florian Leupold, postdocs in the group of ETH professor Jonathan Home, and their colleagues have now demonstrated a technique that achieves the equivalent of the oscillating marble with single atoms trapped in electric fields. Their results set a record for oscillating quantum states of massive particles and may prove to be useful for speeding up the operations of a quantum computer.

In the quantum realm, an electrically charged atom, also called an ion, oscillating in a "salad bowl" created by electric fields can only take on certain well-defined or quantized motional states that are arranged in the order of increasing size and energy, like rungs on a ladder. The lowest rung on that ladder corresponds to the ion standing still (or almost, as quantum particles always have a tiny so-called zero-point motion). Up to now, a typical experiment aimed at getting an ion to move back and forth would shine laser light of a particular frequency onto the atom, or apply an oscillating electric field at the oscillation frequencies. These would then coax the ion slowly into higher excited states. In that way, oscillatory states corresponding to about a hundred rungs could be created.

Solving the switching problem

Alonso and his collaborators chose a different, at first sight much less delicate approach known as "bang-bang". By suddenly applying an additional electric field that shifted the position of the electric salad bowl to one side, the ion could be made to oscillate violently, just like the marble in the above example. In doing so, the ion effectively jumped 10,000 rungs up the ladder of quantum levels. “This bang-bang technique is very efficient at creating highly excited quantum states, but so far it had been hampered by technical problems”, explains Alonso. In particular, switching the electric fields in the vacuum apparatus that contains the trapped ion was a challenge.

ETH professor Home's team solved this problem by placing a digital electric switch inside the apparatus, very close to the ion. This allowed them to move the electric trapping fields in a few billionths of a second and hence they could move the bowl much faster than the ion can respond – which is characterized by the time it takes the ion to complete one oscillation cycle inside the bowl.

Accelerating ion transport

In order to prove that after the first “bang” of the procedure the ion was, indeed, in a well-defined quantum state, the researchers performed a test by applying a second bang after one oscillation period. In this way the ion should be caught exactly at its starting point and be at rest again. The ion passed that test, showing that coherence was preserved during the procedure, and that the new experimental control technique is precise enough for exploration of quantum effects.

The method now demonstrated by the physicists at ETH could, for example, speed up the transport of ions in a quantum computer, which currently represents a bottleneck in the overall speed of such a computer. Moreover, the virtually infinite number of quantum states available in an oscillator might even represent an alternative to the standard approach to quantum computation, in which only two internal energy states (quantum bits) are used for performing calculations which cannot be performed on today’s classical computers.

Tests for interaction with light

At a more fundamental level, these huge oscillating states are also excellent probes for how extended quantum states interact with light. "There are theoretical calculations for this", explains Alonso, "but they had never been tested with states that are much larger than the wavelength of the lasers, as ours are. With this new technique we are now able to verify the calculations with high accuracy”.

Finally, being able to go from small to very large motional states of an ion will also help researchers to explore what happens at the so-called quantum divide, the boundary between the quantum and the classical worlds.