A bridge to the quantum world: taking a step towards a ‘quantum internet’

Entanglement created using mechanical oscillation — described by researchers as ‘a prototype quantum link’ — is a step towards a ‘quantum internet.’

Physicists from the Institute of Science and Technology Austria (IST Austria) developed a method of using a mechanical oscillator to produce to create entangled radiation. This method could prove to be extremely useful in the development of mechanisms to connect quantum computers.

This is an illustration of a prototype of what may, in the future, serve as a link to connect quantum computers. ( IST Austria/Philip Krantz, Krantz NanoArt)

Entanglement is a phenomenon typical of the quantum world, not present in the so-called classical world the so-called “classical world” we see around us on a day-to-day basis.

When two particles are entangled, the characteristics of one particle can be created by the observation of the state of its entangled partner. This means that even if the particles are separated by a distance equivalent to the universe itself, one particle instantly adopts a state that reflects the state of its distant partner.

Usually, when these states are measured, the entanglement between the two particles is destroyed.

Despite being restricted to the quantum world and seeming to be highly counter-intuitive — especially when considering the universal speed limit of c, the speed of light — the phenomenon of entanglement is actively used in quantum cryptography where it is said to lead to unbreakable codes.

Particles, though, aren’t the only thing that can be entangled. Radiation can also be entangled. And it is entangled radiation that is at the heart of the work of Shabir Barzanjeh — a postdoc in the group of Professor Fink at IST Austria and first author of the study published in the latest edition of the journal Nature.

Barzanjeh explains: “Imagine a box with two exits. If the exits are entangled, one can characterize the radiation coming out of one exit by looking at the other.”

Whilst entangled radiation is nothing new, this study represents the first time that it has been created with the use of a mechanical object.

Quantum mechanical — literally

The team created a silicon beam with a length of 30 micrometres and composed of about a trillion — 10¹² — atoms to use in their experiment. Though invisible to the human eye, the beam is relatively large for a quantum object.

Banzanjeh says: “For me, this experiment was interesting on a fundamental level. The question was: can one use such a large system to produce non-classical radiation? Now we know that the answer is: yes.”

The device also has practical value, with mechanical oscillators possibly serving as a link between the extremely sensitive quantum computers and optical fibres, connecting them inside data centres.

Banzanjeh states: “What we have built is a prototype for a quantum link.”

A quantum internet would be cool by necessity

In superconducting quantum computers, the electronics only work at extremely low temperatures which are only a few thousandths of a degree above ‘absolute zero’ (-273.15 °C).

Connections between quantum computers would be extremely vulnerable to changes in temperature

This is because such quantum computers operate on the basis of microwave photons which are extremely sensitive to noise and losses. If the temperature in a quantum computer rises, all the information is destroyed. This means that transferring information from one quantum computer to another would require it crossing an environment too hot for it to survive. Thus making such links virtually impossible, currently at least.

This means that to build connections similar to those used in classical computers networks — connected via optical fibres as optical radiation is very robust to disturbance — one would have to build a link that can convert the quantum computer’s microwave photons to optical information carriers. Or possibly, a device that generates entangled microwave-optical fields as a resource for quantum teleportation.

A link such as this could serve as a bridge between the room temperature optical and the cryogenic quantum world. The device developed by Barzanjeh and his team represents a step in that direction.

He says: “The oscillator that we have built has brought us one step closer to a quantum internet.”

As the researcher points out, this may not be the only application of the device, however.

Barzanjeh continues: “Our system could also be used to improve the performance of gravitational wave detectors.

Johannes Fink, one of the paper’s co-authors adds: “It turns out that observing such steady-state entangled fields implies that the mechanical oscillator producing it has to be a quantum object.

“This holds for any type of mediator and without the need to measuring it directly, so in the future, our measurement principle could help to verify or falsify the potentially quantum nature of other hard to interrogate systems like living organisms or the gravitational field.”