Twisted Light in Free Space

Screw of light on the 143 km long way between the canary islands of La Palma and Tenerife (Source: Univ. Vienna)

Screw of light on the 143 km long way between the canary islands of La Palma and Tenerife. (Source: Univ. Vienna)

The research team around Anton Zeilinger has succeeded in breaking two novel records while experi­menting with twisted particles of light. In one experiment, the scientists could show that the twist of light itself is maintained over a free-space propa­gation of 143 kilometers, which could revo­lutionize future data trans­mission. In the second study, the researchers teamed up with an Austra­lian research group to twist individual light quanta stronger than ever before while even preserving quantum ent­anglement with a second particle. Similarly, these results are not only of funda­mental interest but also give a hint towards the enormous information capacity a single particle of light may offer in future appli­cations.

Time and again, properties of the light surprise the research world. For example, light can be brought into a corkscrew-like form in order to produce “screws of light”, as Anton Zeilinger, quantum physicist at the University of Vienna, describes. The amazing fact is that one can in principle impose any number of windings on each indi­vidual photon. The larger the number of windings, the larger the quantum number with which the photon is described. The Viennese scientists results of the Vienna Center for Quantum Science and Techno­logy (VCQ) at the University of Vienna and the Institute of Quantum Optics and Quantum Infor­mation Vienna (IQOQI Vienna) at the Austrian Academy of Sciences have now made use of this feature, breaking previous records on the trans­mission distance and the magnitude of the quantum number.

In principle, twisted light can carry an arbi­trary large amount of information per photon. This is in contrast to the polari­zation of light, which is limited to one bit per photon. For example, data rates of up to 100 terabits per second have already been achieved under laboratory conditions. The trans­mission under realistic conditions, however, is still in its infancy. In addition to trans­mission over short distances in special fiber optics, transmission of such light beams over free space, required for instance for satellite communi­cation, was limited to three kilometers so far; achieved by the same Viennese team two years ago.

Now, the research team around Anton Zei­linger and Mario Krenn show that information encoded in twisted light can still be recon­structed even after more than 100 kilometers. The experiment has been conducted between the canary islands of La Palma and Tenerife, which is 143 kilometer away. “The message ´Hello World!´ has been encoded onto a green laser with an optical hologram, and recon­structed with an artificial neural network on the other island”, explains Krenn. Having shown that these light properties are in principle maintained over long distances, they now have to be combined with modern communi­cation techno­logies – a task which already several groups around the world are starting to address.

Together with the research group of Ping Koy Lam in Canberra, Australia, the Viennese group of Anton Zeilinger also inves­tigated how strongly single photons can be twisted into the screw-like structure without losing distinct quantum features. In other words, does quantum physics still hold in the limit of large quantum numbers or is classical physics and everyday experience taking over again? For this purpose, the researchers took advantage of a novel technique developed by their colleagues in Australia. There, they have established a technique to fabri­cate spiral phase mirrors to twist photons in an unpre­cedented strong manner and thus increase the quantum numbers to huge values. The mirrors, custom-made for the experiment in Vienna, allow the generation of screw-like photons with quantum numbers of more than 10,000, which is a hundred times larger than in previous experiments.

At first, the Viennese researchers generated entangled photon pairs. After completion of this initial step, the researchers then twisted one of the photons with the Australian mirrors without destroying the ent­anglement, thus demon­strating that quantum physics even holds if 5-digit quantum numbers are entangled. Although driven by founda­tional questions, future appli­cations can already be anti­cipated. “The enormous complexity of the light’s structure is fascinating and can be seen as an intuitive indication about how much infor­mation should fit on a single photon”, explains Robert Fickler, currently working as a post­doctoral fellow at the University of Ottawa, Canada. Hence, in both studies the researchers set up novel records to investigate founda­tional questions as well as pave the way to possible future techno­logies. (Source: U Vienna)

References: M. Krenn et al.: Twisted Light Transmission over 143 kilometers, Proc. Nat. Ac. Sc., online 15 November 2016; DOI: 10.1073/pnas.1612023113 • R. Fickler et al.: Quantum entanglement of angular momentum states with quantum numbers up to 10010, Proc. Nat. Ac. Sc., online 15 November 2016; DOI: 10.1073/pnas.1616889113

Links: Inst. for Quantum Optics and Quantum Information, Austrian Academy of Sciences, Vienna, Austria • Centre for Quantum Computation and Communication Technology, Research School of Physics & Engineering, Australian National University, Canberra, Australia

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