Can light go through matter

News portal - Ruhr University Bochum

In a quantum computer, information could be stored in certain structures of matter, known as quantum dots. In order to be able to transport the information over certain distances, for example through fiber optic cables, it has to be transferred from matter to light. In the journal “Nature”, researchers describe such a light-matter interface. The results were published online on October 21, 2019. Teams from the University of Basel, the Ruhr University Bochum and the Université de Lyon collaborated on the work.

Quantum bits from light and matter

Quantum dots can be realized in semiconductors, for example by locking an electron in a very limited area. The team around Dr. Arne Ludwig and Prof. Dr. Andreas Wieck from the Bochum Chair for Solid State Physics specializes in the production of these structures. The information units in such a system are called quantum bits or qubits for short. “In addition to matter qubits, we also generated flying qubits in the form of photons in our experiment,” says Arne Ludwig. The scientists coupled light and matter in such a way that information can be transferred from matter to light and back to matter.

For this purpose, they enclosed the quantum dot in a tiny cavity, the walls of which reflect light extremely efficiently. At the University of Basel, the researchers led by Dr. Daniel Najer and Prof. Dr. Richard Warburton approached matter in the quantum dot by irradiating a photon with a certain wavelength. This is absorbed by the quantum dot; so the photon disappears, but the matter is now in a higher energetic state. The state of excitation can then be transferred from the matter to a photon in the cavity. The energy is transferred from matter to the photon. The reverse is also possible: If a photon hits the quantum dot, the photon can be absorbed and the quantum dot itself can return to the excited state of matter.

The cavity is defined by mirrors

"However, this does not happen every time a photon with the correct wavelength passes the quantum dot," explains Arne Ludwig. The researchers increased the likelihood of this through the cavity that surrounds the quantum dot. It traps the photons and acts as a resonator. “The walls of the cavity can be imagined as mirrors that throw the photon back and forth over and over again,” compares Ludwig. "In doing so, it repeatedly passes the quantum dot, which it absorbs at some point."

The special thing about it: normal mirrors manage to throw a photon back and forth about 100 times. The semiconductor mirror that the former Bochum doctoral students Sascha Valentin and Dr. Rüdiger Schott created 100,000 reflections for the present work. The Bochum and Basel researchers can adjust the cavity so that it lets photons oscillate with exactly the wavelength that is needed for communication with the quantum dot.

Superimposed states of light and matter

“Since the light remains trapped in the cavity for a very long time, something very interesting from a quantum mechanical point of view happens,” Arne Ludwig describes. “Put simply, the photon is repeatedly absorbed and emitted by matter. To put it more realistically, however, we have to speak of a superposition of the states: The photon is there and not there at the same time. So we see a fusion between the states of light and matter. "

The researchers were able to measure the probability of the presence of the photon; it oscillated at a certain frequency. The experimentally determined frequency corresponded exactly to that which results from calculations from the theory of quantum electrodynamics (QED). The measurements thus provided evidence that QED makes precise predictions.

Schrödinger's Cat

The states "photon there" and "photon gone" - or, to put it another way, the states light and matter - did not simply alternate in the system; "It is only when we measure that we realize that the system has decided to be either light or matter," explains Arne Ludwig. "It's exactly like Schrödinger's cat from the somewhat absurd thought experiment: Only when you look into the box with the cat do you know whether it is dead or alive - before that it is both."

Application potential in quantum computers

"We succeeded in carrying out the experiments with photons at optical frequencies, not with photons in the microwave range as we did in the past," says Daniel Najer. "This means that we can transmit quantum information over long distances using photons and could potentially perform arithmetic operations much faster."

The work was financially supported by the Swiss National Science Foundation within the framework of projects with the numbers 200020 156637 and PP00P2 179109, within the framework of the National Center of Competence Research QSIT and the Eppic project (747866). On the German side, the project was supported by the Federal Ministry of Education and Research (Q.Link.X 16KIS0867) and the German Research Foundation (LU2051 / 1-1).

Daniel Najer et al .: A gated quantum dot strongly coupled to an optical microcavity, in: Nature, 2019, DOI: 10.1038 / s41586-019-1709-y

Dr. Arne Ludwig
Chair of Solid State Physics
Faculty of Physics and Astronomy
Ruhr-University Bochum
Tel .: 0234 32 25864
Email: [email protected]

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