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.

Entangled photon pair source for quantum communication

EPS – Entangled photon pair source.
© Fraunhofer IOF
EPS – Entangled photon pair source.
View inside the periodically poled  KTP crystal, which is elementary for the creation of stable and bright polarization entangled photon pairs.
© Fraunhofer IOF
View inside the periodically poled KTP crystal, which is elementary for the creation of stable and bright polarization entangled photon pairs.

Quantum Key Distribution (QKD) is the distribution of keys to sender and receiver for encrypting and decrypting data. The key information is carried by photons that are quantum mechanically entangled. This means that a pair of entangled photons, where one photon is going to the sender and the other to the receiver, cannot be eavesdropped by a third party without destroying the entanglement state, which can be observed by sender and receiver. QKD is thus believed to be the future of encrypted communication; it provides keys that are safe from being hacked by means of the laws of physics.

Due to the required long coherence length of entangled photons that can only currently be realized by free-space optics, satellite optical links are preferred to distribute keys over large distances to different customers. At Fraunhofer IOF, the prototype of a space-suitable Entangled Photon Source (EPS) has been developed for such satellite links within the framework of the European Space Agency ARTES telecom program.



The EPS is based on a hybrid setup, where a non-linear, periodically poled KTP crystal is double-side pumped using up to 8 mW pumping power at 405 nm in a sagnac-interferometer scheme. The pumping power triggers spontaneous down conversion (SPDC) in the crystal that generates polarization entangled photons within sender and receiver channel at rates up to 300.000 pairs per second and at visibilities between 96 - 99 %. At higher pumping power, it is expected that up to 1 mio. pairs/s can be achieved, which will be the basis for efficient key transmission at highly attenuated optical links.

© Fraunhofer IOF
Pair count rate at 35 °C homogeneous crystal temperature.
© Fraunhofer IOF
Pair count rate at 35 °C homogeneous crystal temperature.

Space suitability

For the space-suitable design of the EPS, a compact, precision mechanics, thermo-mechanically stable platform has been selected, on which the optical setup of the source was integrated via effective and deterministic assembly algorithms, achieving high accuracies. Temperature leveling of the ppKTP crystal within this platform was realized at temperature homogeneity of 0.1 K along the 30 mm long optical axis of the crystal. To fixate the sensitive alignment state of the EPS with respect to long-term stability under space conditions, specific laser based soldering and optics glueing technologies have been used. The source was positively evaluated for its quantum optical parameters within and after typical test cycles for space assembly for thermal and mechanical loads as well as thermal-vacuum.


As a result of the development activities and results, it is planned to further optimize the EPS and implement it in an upcoming satellite mission for the demonstration of QKD links.


Authors: Erik Beckert, Oliver deVries, Christoph Damm

The operating principle of the entangled photon source for quantum communication

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