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Entanglement of Separate Nanomechanical Devices Heralds Quantum Internet

Creating a deep quantum link between nanofabricated resonators on silicon chips is a step toward spy-proof communication, physicists say.

The strange laws of quantum mechanics make it possible to send information from one part of the universe to another with perfect privacy. Eavesdroppers cannot spy on this communication, even in principle. So governments, the military, banks, and others are eagerly awaiting improvements in this kind of technology.

Indeed, a basic version is already available. Current quantum communication systems rely on direct optical-fiber connections from one place to another. But because fibers absorb light, this limits the distance quantum information can be sent to a couple of hundred kilometers.

Sending quantum information further afield requires a quantum Internet—a network of quantum routers linked by fibers. These routers must receive quantum information, store it, and then send it on through the network.

That’s difficult, because quantum information is famously fragile—it decoheres at the drop of a hat and leaks into the environment. So physicists would dearly love to have a robust device that can receive and store quantum states.

Today, Ralf Riedinger at the University of Vienna in Austria and a few pals say they have developed just such a device. Their nanomachine is capable of receiving quantum information sent down ordinary fiber-optic cables and storing it.

The new device consists of a pair of nanofabricated silicon resonators—tiny silicon beams that vibrate like a guitar string. These beams are a few micrometers in scale, a size chosen to ensure that they resonate at a precise frequency in the optical telecommunications range—in this case 5.1 gigahertz (equivalent to a wavelength of 1,553.8 nanometers).

In the experiment, the researchers cool the resonators to close to absolute zero so that they do not vibrate—in other words, they are in their quantum ground state.

Then they connect the two resonators with a fiber-optic cable that they fill with photons at the resonance frequency. This generates quantum phonons, or units of vibration, in each bar. In other words, the radiation pressure causes the beams to vibrate. And because this is a quantum process, the beams and photons become entangled.

That sounds straightforward, but it is hard to do, because the beams have to be identical to vibrate at precisely the same frequency. To find identical beams, Riedinger and co fabricate around 500 of them on a silicon chip using electron-beam lithography and plasma reactive-ion etching.

They then split the chip in two and measure the resonance frequency of all the bars on each chip to find pairs that are identical. “We find a total of 5 pairs fulfilling this requirement within 234 devices tested per chip,” they say.

In practice, the resonant frequencies can differ by a few megahertz, which the team can compensate for by manipulating the optical pulses in the fiber.

The team have put their proof-of-principle device through its paces with impressive results. They placed both chips in a fridge, connected by 70 meters of optical fiber over a distance of 20 centimeters.

They then entangled the two nanoresonators and measured the telltale quantum signatures.

There is nothing to prevent the setup from being significantly extended. “We do not see any additional restrictions to extend this to several kilometers and beyond,” say Riedinger and co.

The factor that limits the entanglement distance is the time over which the quantum state can be stored this way, because that determines how far an entangled photon can travel. Riedinger and co limited the coherence time of the phonons in their experiment to reduce the length of the measurements that were required.

But these resonators could easily reach state-of-the-art performance in this respect. “State of the art lifetimes for these engineered mechanical elements typically range between 1 µs and 1 s, which would allow for entanglement distribution on a regional or even continental level,” say the team.

All this is important because these nanoresonators could obviously act as quantum routers. “The system presented here is directly scalable to more devices and could be integrated into a real quantum network,” they say.

And because the system can be modified to transfer information into microwave frequencies, it could also link up with quantum computers that work at these frequencies.

With that kind of thinking, Riedinger and co have an ambitious conclusion: “Combining our results with optomechanical devices capable of transferring quantum information from the optical to the microwave domain could provide a backbone for a future quantum internet using superconducting quantum computers.”

The question is how soon.

Ref: : Remote Quantum Entanglement Between Two Micromechanical Oscillators

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