Atoms in quantum dialogue

Quantum bits can now be transmitted between two atoms in a controlled way and reversibly stored in the atoms

It is possible that the progression from analogue to digital data processing has not been the last leap forward in information technology. Throughout the world, physicists are investigating the possibilities for processing quantum information. No one knows yet whether it will change our daily routine as much as digital information processing, as it is extremely fragile and its quantum properties disappear easily. However, it offers opportunities that are principally not available to conventional digital information. Physicists in the Quantum Dynamics Division headed by Gerhard Rempe at the Max Planck Institute of Quantum Optics in Garching are now the first to have transmitted quantum information in a controlled and reversible way.

One could say: the researchers let the individual atoms talk to each other in the language of individual quantum bits, which are transmitted by individual photons. The physicists in Garching succeeded in achieving something that is taken for granted in a conversation and also works in conventional data processing, but has not yet been possible at all in quantum communication. “Our approach is characterised by the fact that the system of atom and resonator can serve as transmitter and receiver. We can also store the information in the atoms and we can exchange it between them in a controlled and reversible way,” says Stephan Ritter, who was instrumental in facilitating the quantum dialogue. This means that any atom can speak the quantum language and transmit the information contained in it - without losing the sensitive quantum character of the information.

Physicists are also studying other systems for exchanging quantum information, such as ensembles of atoms or single atoms and molecules without a mirror. The special strong point of individual atoms in resonators is that they allow all operations, i.e. the sending, receiving and storage of quantum information, in equal measure.

New prospects for processing quantum information

One new development of fundamental interest is the fact that the Garching-based researchers can exchange quantum information in a controlled and reversible way - physicists use the term coherent. “We have created a breakthrough with our experiments in order to learn more about the fundamental properties of the quantum world,” says Gerhard Rempe, who, as a Director at the Max Planck Institute of Quantum Optics, headed the research. Since the researchers accurately control the exchange of quantum information and thus the quantum mechanical interactions between their networked atoms, they hope in the future to also be able to simulate quantum processes that physicists do not fully understand as yet. These include superconductivity - the flow of current with zero resistance - at relatively high temperatures. “A larger quantum network of the future could be particularly suitable for such quantum simulations because it’s particularly versatile,” says Stephan Ritter.

The atomic dialogue also opens up new prospects for the exchange and the processing of quantum information. Unlike conventional bits, quantum bits have the characteristic that the state which they are in is not determined as long as they are not measured. They are in a state which physicists call superposition state. This has a property that is hardly conceivable in our everyday world, which is governed by classical physics, because the result of a measurement on the superposition state is not determined right from the start, it is the measurement that makes the decision. The measurement changes the state in this sense.

The computing characteristics of quantum bits are based on this indecisiveness of the superposition state. A quantum bit therefore does not represent a zero or one, like a conventional bit, but has the possibility to be in both states at the same time. This characteristic can be used for ingenious and powerful computational methods, which are denied a conventional computer.

Quantum bits are very sensitive to external disturbances

“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 nodal points of the quantum network are entangled

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.

The quantum network is to be expanded

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.

PH