New ion trap for larger quantum computers
Researchers in Switzerland have developed a new technique to scale ion trap quantum processors for larger quantum computers.
Rather than using an oscillating magnetic field to trap the ions, researchers at ETH Zurich developed a way to use a static field that supports larger ion traps.
The energy states of electrons in an atom follow the laws of quantum mechanics: they are not continuously distributed but restricted to certain well-defined, quantised values. The quantised states are the basis for quantum bits (qubits).
Strong trapping can be achieved by ionising the atoms, which means giving them an electric charge. However, a fundamental law of electromagnetism states that electric fields that are constant in time cannot trap a single charged particle. By adding an oscillating electromagnetic field, on the other hand, one obtains a stable ion trap, also known as a Paul trap.
This has allowed ion traps containing around 30 qubits but this doesn’t scale as the oscillating field causes heating.
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Instead the team at ETH led by Jonathan Home has now demonstrated that ion traps suitable for use in quantum computers can also be built using static magnetic fields with an additional magnetic field, called Penning traps, This supports both arbitrary transport and the necessary operations for the future quantum supercomputers.
“Traditionally, Penning traps are used when one wants to trap very many ions for precision experiments, but without having to control them individually”, says researcher Shreyans Jain. “By contrast, in the smaller quantum computers based on ions, Paul traps are used.”
However Penning traps require extremely strong magnets, which are very expensive and rather bulky, and are symmetric, which is a problem with chip-scale structures.
Putting the experiment inside a large magnet makes it difficult to guide the laser beams necessary for controlling the qubits into the trap, while strong magnetic fields increase the spacing between the energy states of the qubits. This, in turn, makes the control laser systems much more complex: instead of a simple diode laser, several phase-locked lasers are needed.
Instead Home and his collaborators built a Penning trap with a superconducting magnet and a microfabricated chip with several electrodes, which was produced at the Physikalisch-Technische Bundesanstalt in Braunschweig.
The magnet used delivers a field of 3 Tesla with a system of cryogenically cooled mirrors, the researchers managed to channel the necessary laser light through the magnet to the ions.
A single trapped ion, which can stay in the trap for several days, could now be moved arbitrarily on the chip by controlling the different electrodes. This was not previously possible with ions using the previous approach based on oscillating fields, say the researchers.
This also allows many more traps to be packed onto a single chip. “Once they are charged up, we can even completely isolate the electrodes from the outside world and thus investigate how strongly the ions are disturbed by external influences”, says Tobias Sägesser, who was involved in the experiment as a PhD student.
The researchers also demonstrated that the qubit energy states of the trapped ion could also be controlled while maintaining quantum mechanical superpositions. Coherent control worked both with the electronic (internal) states of the ion and the (external) quantised oscillation states as well as for coupling the internal and external quantum states. This is a prerequisite for creating entangled states, which are important for quantum computers.
As a next step, Home wants to trap two ions in neighbouring Penning traps on the same chip and thus demonstrate that quantum operations with several qubits can also be performed. This would be the definitive proof that quantum computers can be realized using ions in Penning traps.
There are other applications, for example to probe electric, magnetic or microwave fields near surfaces. This opens up the possibility to use these systems as atomic sensors of surface properties.
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