“The best I can do with a conventional bit is to measure it, because then I know the information,” explains Stephan Ritter. “In quantum communication, in contrast, I change the content by the measurement in an irreversible way”. Since a measurement changes the quantum state of quantum bits, handling them becomes a delicate issue. This is made more difficult by the fact that they are much more sensitive to external disturbances than conventional bits.
Although the fragile nature of quantum information can also be used for some quantum communication applications – quantum cryptography, for example, uses this characteristic of quantum bits so that nobody can intercept information without it being noticed – it is this sensitivity that is the great experimental challenge for the physicists in Garching. The researchers have mastered the task over several years in a number of steps. They learned to trap atoms with laser beams between two almost perfect mirrors and to hold them there for several minutes. They also used laser pulses to convince the atoms to emit a photon precisely when they wanted them to. They found a possibility to store quantum information in an individual atom by accurately reversing the process in which the individual photons are being produced. And they can also recall the information from the atom.
Now the physicists have combined all of these abilities and produced the first nodes of a quantum network using two atoms in optical resonators, which they placed into neighbouring laboratories and linked up with a 60-metre fibre-optic cable. They also had to cope with the fact that not every step has a 100% success rate. This is because not every attempt results in a photon of the transmitter atom arriving at the receiver atom. But if it does – and the researchers have been able to prove this – the information in the receiver atom is also in agreement with the information originally present in the transmitting atom.
The atoms serve as stationary storage devices and the physicists store the information therein in an internal state which is determined by different quantum properties. With the help of an ingenious method, they transfer the information from the internal state of the transmitting atom to the direction of oscillation of the emitted photon, which has the form of a small wave packet. At the receiver atom, the information is then again transferred to the internal atomic state in a way which the researchers can accurately control.
The Garching-based researchers even went one step further: they entangled the two nodal points of the rudimentary quantum network with each other – this entanglement creation also being a very peculiar quantum process. This involved the physicists again transmitting a photon from one atom to the other, but this time one which is entangled with the transmitting atom. To this end, they carefully selected the process in which the photon is created. When the process is reversed, i.e when the receiver atom absorbs the photon, the entanglement is transferred to the second atom. They thus prepare the two atoms in such a way that both are in a superposition state and their internal states are mutually dependent. This means: the two atoms form a single quantum object and the manipulation on one of the two atoms has an inevitable effect on the other atom as well. This is precisely what the physicists in Garching have observed in a further experiment.
Entangled systems, in the form of atoms, for example, are also useful for quantum communication, sometimes even essential. When transmitting quantum bits over large distances, for example, the purpose of the entanglement is to facilitate the efficient transmission of quantum states despite the losses which inevitably occur in every transmission. The physicists in Garching are currently able to protect the entangled atoms against external disturbances which destroy the entanglement for 100 microseconds. This would already be long enough to carry out computations with the entangled atoms, for example.
A quantum network with more than two nodes would be required for applications in quantum communication, and also for quantum simulations. “Our approach to creating a quantum network is promising, mainly because it provides a clear perspective for scalability”, says Gerhard Rempe. He and his colleagues now plan to work on this issue, and they are also planning further steps. The physicists want to make the transmission of the quantum information more robust against external disturbances. Moreover, the researchers have a plan to determine whether the atoms are entangled without measuring the entangled state itself, and thus destroying it in the process. “This will enable us to take the next steps towards quantum communication over large distances and one day maybe even to a quantum Internet,” says Stephan Ritter.