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As the 21^st century is entering its third decade, we are witnessing a second quantum revolution, powered by such quantum-mechanical phenomena as superposition, quantum entanglement, and interference. Superposition suggests that a quantum particle, isolated from its environment, can exist in any combination of quantum states at the same time: different energy levels, spin, polarization, or even positions in space. Compared to the conventional bit, the basic building block of our digital world, and its only two discrete states – 0 or 1 – the quantum bit or qubit has an infinite number of possible identities – and this range of possible states for every qubit was recognized as the potential basis for the computers and algorithms of the future (quantum computing and quantum simulation) almost half a century ago.

When two qubits are entangled, we can read the state of one qubit to know the state of its twin – immediately, without delay, and crucially, even if the two qubits are physically separated from each other. This phenomenon, termed quantum teleportation, forms the basis for secure, bug-proof quantum communication and quantum-based imaging technologies (ghost imaging).

Not unlike waves, qubits can also interfere with each other and even with themselves. When sub-waves overlap, the measurable amplitude of the wave is determined by the quantum-mechanical phase captured by the sub-waves. It reacts to even the tiniest outside forces, such as electrical or magnetic fields, Earth’s acceleration at the specific time and place, or simply the passing of time. Quantum sensors can use this to outperform conventional sensors, as their measuring data does not have to be calibrated, but simply understood in relation to several known and absolute constants of nature.

Fraunhofer IZM has been harnessing innovative techniques for photonic system integration and miniaturization to tackle the challenges posed by quantum technologies (QT) and seize their enormous potential to overcome the inherent limitations of current technology. The ambition is to make the leap from highly complicated, sensitive, and power-hungry custom laboratory systems to devices that are easy and cheap to produce and reliable for use in the real world. Progress has been particularly fast in quantum communication, where qubits are carried by individual photons. Today’s telecommunications industry already has the infrastructure, processes, and technical means to manage and transfer these photons. Quantum sensing and computing, on the other hand, depend on interactions between photons and atoms, ions, molecules, or other atom-level systems like nitrogen-vacancy centres. Researchers are working on ways to overcome the challenges in managing this fine interplay between the carriers, containers, and emitters of information on this level:

• Isolating environmental interference, e.g. with cryogenics or ultra-high vacuum;
• Maximizing the cross section, e.g. by placing or capturing specific quantum emitters with extremely closely controlled light intensity;
• Miniaturizing and scaling up the production of systems.

Building on many years of experience with glass fibers and thin display glass in opto-electronic systems, Fraunhofer IZM is actively pursuing new integrated photonic systems using glass substrates. The choice of glass beats standard semiconductor technology in several key aspects for quantum technology: i) its transparency allow the near-IR wavelengths used in QT, ii) optical waveguides embedded in glass benefit from far fewer losses than their counterparts in silicon, iii) compared to silicon nitride, there are no destructive effects with alkali atoms, iv) glass can be polished to an extremely smooth surface to reduce any remaining light scattering effects.

Several solutions are being pursued in the development of all-glass quantum chips:

• Hermetic connection of glasses for the scalable production of passive micro-UHV chambers
• Glass metallization and structuring of vias and conductor paths to integrate electric functionalities (DC and HF)
• Integration of optical waveguides inside the glass walls of UHV chambers
• Design and production of “optical chips” on glass (passive waveguide structures like splitters, beam combiners, resonators)
• Adapted processing for custom refractive indices of optical waveguides for:
  • Glass-integrated waveguides for vacuum feedthroughs;
  • Creation of strong evanescent fields in the immediate vicinity of quantum emitters (e.g. nitrogen-vacancy centres);
  • Capturing of neutral atoms in the evanescent field;
  • Decoupling of guided optical beams into free or formed beams;
• Assembly of microoptics in UHV chambers
• Integration of light sources and their electronic controls on glass substrates and coupling of light into waveguides
• Highly efficient coupling of fibre optics with innovative connection technologies