Challenge no. 1:

Perfectly isolate the quantum system

The basic principle of any kind of quantum system is to isolate it from its environment such that at least some of the quantum particle’s properties are not influenced by its surroundings at all. Therefore, sending such systems over long distances – i.e., outside of the controlled environment of a laboratory – is quite challenging. However, if one uses **photons** (light particles) to transport quantum information, one benefits from intrinsically very weak interactions with their environment on one hand and from their moving at the speed of light on the other hand. QUAPITAL, as the “P” in the acronym suggests, therefore focuses on photons as carriers of quantum information.

Challenge no. 2:

Produce entangled photons at a reasonable rate

The state-of-the-art way to produce photons sharing the quantum property of **entanglement** is by focusing a strong laser beam into a tailor-made crystal. The crystal’s lattice properties make high-energy laser photons split up into two entangled photons of lower energy. These photons are now, by making use of a sophisticated arrangement of beam-splitters and mirrors, separated and coupled into glass fibres. The consortium members of QUAPITAL are internationally renowned experts on such photon sources, holding a world record on the **“brightest” photon source**, i.e. the one producing highest number of pairs per second. The source in use for QUAPITAL will create about a billion entangled pairs per second.

Challenge no. 3:

Distribute photons to communication partners for them to measure the photon's polarization

The photons are produced at telecom wavelength (1550 nm), which is optimal for bridging long distances via readily deployed glass fibres used for high-speed internet. At this wavelength, absorption losses in fibres are extremely low, with only 2% of photons being lost over one kilometre. Since these photons are entangled, they cannot be considered as individual systems, no matter how far they are apart. In other words: When one measures e.g. the polarisation of one of the photons, one knows that a polarisation measurement of its partner photon will yield a correlated outcome. This strange “**spooky action at a distance**”, how Albert Einstein called it, enables two communication partners (traditionally called **Alice** and **Bob**) to create an unconditionally secure means of communication. Alice and Bob are each connected to the source of entangled photons via their own glass fibre, therefore getting one photon out of an entangled pair each.

Challenge no. 4:

Detect eavasdropping through error rate

With the use of a measurement device consisting of polarisation-separating mirrors and highly efficient nanowire photodetectors, they each **measure their photons’ polarisation** states. Although Alice (Bob) only performs the measurement on her (his) photon alone, she (he) simultaneously knows Bob’s (Alice’s) outcome. The crucial point is now that if some malicious eavesdropper **Eve** wants to listen in on these photons, she automatically introduces errors. This is mainly because a quantum state cannot be copied perfectly; so if she intercepts a photon and reads it out, she would have to send a perfect copy to Bob in order not for him to notice, which is forbidden by the law of physics.

Finally: Communication partners share a secret and completely random key

Now that both Alice and Bob have a sequence of measurement outcomes, they can assign values of 0 and 1 to them (e.g. horizontal polarization = 0, vertical polarization = 1). This string or **key** is secret and – another quantum property – completely random; however Alice and Bob can be sure that they have the same one. Alice now adds this **keystring of 0 and 1** to her message of the same length, making it total gibberish for anybody but Bob. She can now send this encrypted message via any classical channel or even print it out and nail it on the town hall’s door. Only Bob, when performing the same operation as Alice on the message using his key, will be able to read it.