Quantum Hardware

AQTION - Addressing Optics for a Quantum Computer

Realization of a scalable quantum computer

The goal of the “Advanced Quantum Computing with Trapped Ions”  (AQTION) project – part of the EU Quantum Flagship Initiative – is to realize a scalable quantum computer. This computer will be assembled at the University of Innsbruck (UIBK), which leads the AQTION consortium.

Researchers at Fraunhofer IOF are developing a laser-optical setup within this initiative that enables the manipulation of ions in an ion trap for quantum computers.

 

Measuring ions

Within the AQTION quantum computer, the quantum bits (Qbits) are represented by Ca+-ions, which are confined within a Paul trap. In order to prepare the quantum states and execute calculations at the quantum gate, different laser wavelengths are used. This includes to illuminate single ions with a laser spot of a wavelength of 729 nm. The result of the calculations is "read out" from the status of the ions that are arranged in a linear chain within the trap. The status of the ion at the time of the measurement becomes apparent by checking whether a fluorescence signal is emitted or not.

Laboratory setup of addressing optics.
© Fraunhofer IOF
Laboratory setup of addressing optics.

Correct addressing of ions

Reliable addressing of the ions that are spaced by 3 microns approximately in the center of the trap requires diffractionlimited spots, on one hand, and on the other a means of tracking the spots along the trap axis with sub-micron accuracy. To this end, a particular optomechanical unit was developed where in a special solid-state-joint configuration piezo-actuators induce a linear movement of micro-prisms. The optomechanical unit transforms the fixed array of input-fibers in a dynamically adjustable arrangement of sources. The additional optics ensure the demagnification of the source distances at the input down to the ion distances, as well as the appropriate spot sizes in the plane of the trap. In addition to a special objective, which is corrected for the wavelengths of the addressing beam and the fluorescence detection, gradient index and achromatic lenses are used.

 

System miniaturization

For the overall optical setup, a multiple folding of the beam path is required to ensure the potential for integration. To have the optimum focal length of the parabolic mirror – an essential component for the device principle (patent pending) – this mirror is now under construction by ultra-precision machining at Fraunhofer IOF.

Detailed view of addressing optics.
© Fraunhofer IOF
Detailed view of addressing optics.