No longer just a pipe dream: the spin of an artificial atom is entangled with the color of the light particle in a semi-conductor system. Illustration: Quantum Photonics Group / ETH Zurich    Back in his day, Albert Einstein dubbed it “spooky action at a distance”: the entanglement of two particles with the aid of quantum physics. And for laymen at least, how two atoms or light particles can be interlinked across great distances is not exactly comprehensible today, either. It is hard to imagine why one of these particles changes its quantum physical state if you manipulate and measure that of its entangled counterpart several kilometers away.

However, quantum physicists have already made this action at a distance a reality decades ago. Atac Imamoglu, a professor from the Institute of Quantum Electronics, and his team have now taken a major step forward in this research field and become the first to pull off a new kind of entanglement: between a stationary “artificial atom” and a mobile light particle (photon) in a semiconductor system.

Transferring information over long distances
This achievement is especially remarkable given the potential future applications.

“Entangling particles at a large distance is a major goal of quantum information science,” says Imamoglu.

This is essential if we eventually want to revolutionize telecommunications and information technology with the aid of quantum physics. One objective, for instance, is to implement secure communication.

“Based on our system, in the future it will be possible to entangle distant stationary particles, for instance,” says Imamoglu.

Because the physical and quantum-physical properties of all these particles such as the wavelengths—in other words, the color of the light particles—or the electron spin of the artificial atoms can be manipulated, information can be stored in them. And because photons are mobile, they might be able to transport information to a remote stationary system via an optic fiber cable and alter its quantum physical states in future.

Easy to enlarge
But we are not there yet. In the experimental stage, the ETH-Zurich researchers’ system consists of an electron spin and a photon. Therefore, it thus needs to be enlarged to include more particles first. According to Weibo Gao, a postdoc in Imamoglu’s group and first author of the paper published in the journal Nature, this should be relatively straightforward in the system they use.

The ETH-Zurich researchers achieved the entanglement in a semiconductor system. This kind of semi-conductor is comparable with a computer chip, only it is not made of silicon, but rather the compounds gallium arsenide and indium gallium arsenide, which are just the building blocks for quantum-optical experiments. Some years ago, scientists already managed to entangle a stationary and a mobile particle in a different system, a so-called ion trap.

“However, the present ion-trap experiments fill entire labs and are mainly interesting for basic research,” says Emre Togan, another scientist involved in the research project.

If a portable, powerful device for quantum communication needs to be built one day, semiconductor systems would be advantageous.

Observation of entanglement between a quantum dot spin and a single photon

Source: ETH Zurich