Coupling Photons to Rydberg Atoms

Steffen Schmidt-Eberle and colleague Thomas Stolz working in their lab at Max Planck Institute of Quantum Optics to gain fundamental insights for future quantum technologies. (Source: L. Husel, MPQ)

Since the early years of the century, scientists have been seeking ways to exploit the properties of Rydberg atoms for photon-based proces­sing of quantum information. “We have now taken an important step towards this goal,” says Steffen Schmidt-Eberle, a PhD student in the Max Planck Insti­tute of Quantum Optics at the Garching Research Campus near Munich. Under the supervision of Stephan Dürr and Gerhard Rempe, the Director of the Division of Quantum Dynamics, he has been working with a team of researchers on the develop­ment of a photon-photon quantum gate since 2014.

Rydberg atoms interact strongly with each other, and are therefore ideal for proces­sing quantum infor­mation. The idea behind the project is to map this well-known strong inter­action between Rydberg atoms on light, and use it as a means of imple­menting a logical operation between two photons. Such a controlled procedure is a basic prerequisite for the correct processing of quantum infor­mation in an optical quantum network. “The ability to control light and its properties is playing an increa­singly important role in the modern world. It provides the basis for what are now routine appli­cations in fields ranging from medicine to materials processing and high-speed data trans­mission. This explains why one major goal of research in this area is to find ways of mani­pulating the behavior of single photons with similar effi­ciency and versa­tility,” says Schmidt-Eberle.

Photons enable information to be disse­minated very rapidly and with high bandwidths. That is why optical techno­logies have become the standard for trans­mission of data across the Internet. Future quantum-based tech­nologies will most probably make use of these particular properties of photons to transmit quantum states and thus quantum infor­mation between the different nodes of quantum networks.

Initial attempts to harness light quanta not just for the trans­mission but for the processing of information failed, because photons do not normally interact with one another. The research group led by Dürr and Rempe has now solved this problem by demons­trating a practical method for coupling photons to Rydberg atoms, in which an electron is excited into a high-energy state. By taking advantage of the strong inter­action between such atoms, they were able for the first time to create an effective inter­action between photons and use it to realize a photon-photon quantum gate. In a future quantum computer or network, such photon-based quantum logic gates could play a role similar to that of the CPU in a classical computer.

The experi­mental approach success­fully used to create the all-optical logic gate consists of the following steps. First, the researchers cool a cloud of atoms to a tempera­ture of 1 microkelvin. A first photon is stored in this medium as a Rydberg excitation. They then send a second photon into the cloud. Finally, they retrieve the first photon and measure the polari­zation of both photons when they emerge from the cloud. Thanks to its inter­action with the Rydberg exci­tation, the influence of the first photon sent into the cloud on the second turns out to be so strong that the polari­zation of the second, depending on the polari­zation of the first photon, is rotated by 90 degrees from its input direction. “The very fact that one can achieve such a large effect using a single photon is fascinating,” says Schmidt-Eberle.

“One of the major diffi­culties associated with this experiment – making the inter­action strong enough that the rotation angle of the polarization was not too small – was resolved two years ago. The second challenge was to ensure that the rotation is not contingent on whether or not the first photon is irradiated, but depends on its polari­zation,” explains Stephan Dürr. In order to solve that problem, the team had to identify appro­priate atomic states to which the photons could be coupled, and then to actually implement the coupling interaction techni­cally.

“The search for ways to generate an effective inter­action between photons by means of Rydberg atoms is a young and dynamic area of research, which has made it possible to demonstrate optical non-li­nearity at the level of single photons. The reali­zation of an optical quantum gate based on Rydberg inter­actions is a critical milestone in the development of the field, which demonstrates that the properties of a photon can be maximally altered by the influence of a second single photon,” Dürr adds.

Gerhard Rempe provides the following assessment of the team’s achieve­ment: “Our experi­ments provide the first proof that it is in fact possible to process quantum infor­mation between photons with the aid of Rydberg systems. That in itself is a major step towards the reali­zation of a highly efficient photon-photon quantum gate.” (Source: MPQ)

Reference: T. Tiarks et al.: A photon–photon quantum gate based on Rydberg interactions, Nat. Phys., online 29. Oktober 2018; DOI: 10.1038/s41567-018-0313-7

Link: Quantum Dynamics (G. Rempe), Max-Planck-Institute of Quantum Optics, Garching, Germany

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