Hitting a specific point on a screen with a laser pointer during a presentation isn't easy - even the tiniest nervous shaking of the hand becomes one big scrawl at a distance. Now imagine having to do that with several laser pointers at once. That is exactly the problem faced by physicists who try to build quantum computers using individual trapped atoms. They, too, need to aim laser beams - hundreds or even thousands of them in the same apparatus - precisely over several metres such as to hit regions only a few micrometres in size that contain the atoms. Any unwanted vibration will severely disturb the operation of the quantum computer.
At ETH in Zurich, Jonathan Home and his co-workers at the Institute for Quantum Electronics have now demonstrated a new method that allows them to deliver multiple laser beams precisely to the right locations from within a chip in such a stable manner that even the most delicate quantum operations on the atoms can be carried out.
Aiming for the quantum computer
To build quantum computers has been an ambitious goal of physicists for more than thirty years. Electrically charged atoms - ions - trapped in electric fields have turned out to be ideal candidates for the quantum bits or qubits, which quantum computers use for their calculations. So far, mini computers containing around a dozen qubits could be realized in this way. "However, if you want to build quantum computers with several thousand qubits, which will probably be necessary for practically relevant applications, current implementations present some major hurdles," says Karan Mehta, a postdoc in Home's laboratory and first author of the study recently published in the scientific journal "Nature". Essentially, the problem is how to send laser beams over several metres from the laser into a vacuum apparatus and eventually hit the bull's eye inside a cryostat, in which the ion traps are cooled down to just a few degrees above absolute zero in order to minimize thermal disturbances.
Optical setup as an obstacle
"Already in current small-scale systems, conventional optics are a significant source of noise and errors - and that gets much harder to manage when trying to scale up", Mehta explains. The more qubits one adds, the more complex the optics for the laser beams becomes which is needed for controlling the qubits. "This is where our approach comes in", adds Chi Zhang, a PhD student in Home's group: "By integrating tiny waveguides into the chips that contain the electrodes for trapping the ions, we can send the light directly to those ions. In this way, vibrations of the cryostat or other parts of the apparatus produce far less disturbance."
The researchers commissioned a commercial foundry to produce chips which contain both gold electrodes for the ion traps and, in a deeper layer, waveguides for laser light. At one end of the chips, optical fibres feed the light into the waveguides, which are only 100 nanometres thick, effectively forming optical wiring within the chips. Each of those waveguides leads to a specific point on the chip, where the light is eventually deflected towards the trapped ions on the surface.