SC5 / OPTICS FOR QUANTUM TECHNOLOGY
The seemingly exotic laws of the quantum universe are one of the most important scientific achievements of the last century. We can use them to understand the properties and behavior of matter on the nanoscale. Progress in science and technology allow for increasingly illustrating quantum physical effects on a macroscopic scale like it is formidably proved the Nobel prizes for physics in 1997 , 2001 , 2012  und 2016 . These works herald a new era of quantum technology , where technological applications make use of quantum physical states in our daily life.
This is valid, particularly for quantum computers [6, 7] and quantum communication. They will revolutionize our existing security infrastructure of digital web, which builds the base of our digital society and global economy. It is founded on the fact that classical computers cannot factorize large numbers in prime numbers.
Quantum computers will be – amongst many other applications – solve this problem efficiently, make conventional security architecture obsolete  and therefore revolutionize fundamentally. Simultaneously quantum communication allows for encryption of messages in a way that every attempt of eavesdropping becomes impossible. Already in April 2004 in Vienna, Austria, the first wire transfer was encrypted with quantum technology  and 2007 quantum cryptography was used to protect the networks during the election of the Swiss National Council .
The core of the network security are systems for exchanging digital keys. The quantum key distribution is intrinsically secure against eavesdropping. Quantum secure classical encryption algorithm are using these keys to transport the data on securely. Therefore, quantum optical systems are promising approach. In a later stage even all information can be transferred via quantum optical technology; a classical encryption would become obsolete.
The optical quantum key distribution opens up a new epoch of cyber security. At first, this technology will be utilized for highly confidential information transfer, e.g., in banking, intelligence, military, politics or health. Costs are prognosticated in the order of a few cents per kbit  and enables the technology for a full data transfer, when highest confidential level and cyber security is required.
The applications of quantum science span even further beyond information technology. Based on quantum sensing completely new scientific realms and areas will grow up, e.g., the gravitational imaging and consequently the quantum navigation . Further potential applications are earth observation, measurement of magnetic fields as well as quantum remote imaging .
The mentioned scenarios show that due to future developments in quantum technology multiple research areas will arise, where scientists will have the need for fundamental optical technologies and components. For this purpose, the Fraunhofer techreport »Quantum technologies« specifies micro- and nanofabrication as well as high power and highly coherent laser sources . Particularly, this is valid for applications of secure communication and sensing: according to the Quantum Technology Roadmap Report  improved sensors for multiple applications and security threads are predicted as technological and economical drivers. The research and long-term manifestation the infrastructure of NPL builds a profound base, which enables the scientific community to realize quantum optical systems. Already at this state, NPL is linked with four of the ten most powerful European Institutes in quantum technology and therefore has an established base for the science case »Optics for quantum technology«.
SCENARIO: QUANTUM KEY DISTRIBUTION
A technological challenge of the quantum key distribution is the need for direct line of sight between sender and receiver because quantum signal cannot be amplified in optical fiber networks [15, 16]. Therefore, quantum communication can only be realized via satellites and demands therefore an establishment of respective infrastructure .
The development of satellite-based quantum communication is still in an early stage. Currently, a distance of 143 km between the Canary Islands La Palma and Tenerife was bridged . The communication via a low-earth orbit was demonstrated as well. Therefore, a retroreflector of 3 cm diameter and terrestrial telescope with a diameter of 1.7 m² as ground station was used . In converse, this means a compact ground station requires powerful quantum emitters, adaptive optics and mirrors up to 1 m in diameter are needed for the orbital relay station. Such a value chain is currently not available.
Furthermore, an intercontinental communication would require reliable satellite-to-satellite interlinks so that in future further demands for optical system technology arise.
ENABLER: MULTIFUNCTIONAL INTEGRATION OF SOURCES AND OPTICS
The realization of such an infrastructure and the manifestation of quantum sensing systems satellites as well as (transportable) ground stations as integrated systems have to be developed. They need optical components with extreme demands:
- Low noise, monolithically integrated high intensity lasers build the base for quantum emitters since robust high power sources for entangled photons almost doesn't exist. Further development works have to concentrate on the simplification and integration of the optical systems (Fig. 1).
- Large freeform mirrors on satellites and ground stations are demanded for efficient communication and to achieve a diffraction-limited beam quality spanning the 1000 km range. For the application on satellites the light weight construction plays an important role. The reduction of the number of components due to nanostructuring is a further development step (Fig. 2).
- Microstructured monolithical mirrors offer an ideal platform for highly sensitive quantum sensing (Fig. 3).
- Adaptive Optics  are required for tracking the communication partner and for correction of atmospheric distortions (Fig. 4).
The nature of quantum photonics sets high requirements on the optical components. Hence, quantum optical systems will profit from the increased capability of optical components concerning reduced scattering, high reflectivity and wave front accuracy and from the integration of optical functionalities due to active correction mechanism, freeforms as well as micro- and nanostructures. Also for sources of entangled photons an increased merging of optical functionalities in form of monolithical components, e.g., with fiber lasers, freeform elements or multi surface optics is aspired. Such a wide spanning area of challenges can only be addressed via a broad and holistic technology platform like NPL.
Optical components for quantum photonics need substrate with highest form accuracy and lowest roughness. To enable a large variety of applications, particularly freeforms for large geometrical sizes are extremely important. The optical components need to be smoothed by first-class polishing techniques. The functionalization of the surface by coating and nanostructuring expands the functionality spectrum and reduces amount of components.
Core technological challenges are:
- Freeform manufacturing as well as polishing on curved surfaces with lowest roughness and highest form accuracy.
- Procedures for ultraprecise coating, e.g., atomic layer deposition especially also for freeforms and nanostructures.
- Application of optical nanostructures.
- High-resolution characterization methods (Scanning electron microscopy, Atomic-Force-Microscopy, scattered light analysis), particularly on large substrates and freeforms.
For the development and establishment of the required technologies the NPL has the necessary knowledge, which is given due to to the conduction of numerous projects for quantum optical solutions in the fields of optical coatings, nanostructuring or surface characterization and especially for light source development. The main machines and technologies for the envisaged elaboration of the technology chain will be developed in accordance with applicants and industrial partners.
The described demands result in the following development demands of the core technologies:
IMPORTANT PROJECTS OF THE NPL-PARTNERS WITH RELATION TO THE SCIENCE CASE
- Entangled Photon Source (EPS) for SpaceEPS at European Space Agency (ESA) in corporation with Institut für Quantenoptik und Quanteninformation Vienna (IQOQI).
- Ultra Stable High Power Photon Sources for PowerQuant at Max-Planck-Fraunhofer-Kooperationsprojekt in corporation with Max-Planck-Institut für Quantenoptik (MPI MPQ).
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 C. for Physics of the Royal Swedish Academy of Sciences. Measuring and manipulating individual quantum systems. Scientific Background on the Nobel Prize in Physics (2012).
 C. for Physics of the Royal Swedish Academy of Sciences. Topological phase transitions and topological phases of matter. Advanced information on the Nobel Prize in Physics (2016).
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- fundamental research
- information technology
- quantum cryptography
- quantum communication
- quantum sensing
- highly brillant light sources
- leight weight mirrors
- nanostructured freefrom optics
- active und adaptive optics
- integrated light sources