This is a quantum memory. Image: ICFO
Take an ordinary computer, and gift it with the remarkable powers of the subatomic world. That is to say, the world of the insanely small, where matter does astounding things, like existing in two places at once. Then you would have yourself a quantum computer—no small feat to create. Quantum computers can solve certain algorithms much faster than classical computers. But having a quantum computer by itself is not enough. Try to send a signal from this quantum computer to another, on the other side of the world. This is a pretty important function, as evidenced by the existence of the internet and everything it facilitates. So, how hard could this be? Surprisingly, as of now, quantum computers cannot make long distance calls; reliably sending quantum signals over long distances has not been achieved.
A workaround is to send signals over many short trips, storing them in memory in between. Now, researchers from the Institute of Photonic Sciences (ICFO) in Spain have shown how to drastically improve this method of using quantum “memories”. They reliably pass signals that last for roughly 1000 times longer than previous experiments have demonstrated. These signals are encoded by particles that have been “entangled”, taking advantage of a phenomenon that is central to quantum mechanics. The new work is published in Nature. “While entanglement between two such memories has already been demonstrated, it was... with a technique not directly extendable to long distance communication,” the authors write.Signals between quantum computers, most often conceived of as individual photons, or particles of light, have a problem. They tend to fade out, they are volatile, for entirely quantum-style reasons. Unlike standard electrical signals, it is much harder to amplify them without mucking them up. A major goal is therefore to figure out how to send reliable signals. One key method, considered in the study, is to chop the communication medium (say, fiber optic cable) into smaller parts. Rather than sending a photon across the world, you could send it from house to house, say, or town to town, before it has time to degrade. At each waypoint, the photon would be stored, and then have the next pipeline retrieve it. In other words, the idea is to equip each little repeating piece of cable with little memory storage units at both ends, which are synchronized with the ends of the next cables.
Classical computers also store and retrieve signals at many different segments of a network. However, quantum computers have a harder time. The signal is generally composed out of photons that are said to be entangled with other photons. You can think of entangled particles as being linked together, but acting oppositely, like a set of two coins that somehow always land on opposite sides. Even if the two coins are brought to the different ends of the universe, as long as entanglement is maintained, they will always share this “opposite” correlation.If you retrieve an entangled photon at one end of the cable, say, measuring a “heads” reading on a flipped coin, then it therefore tells you something about the photon at the other end: that it has landed tails. Quantum computers use this phenomenon to pass information.However, due to another phenomenon called decoherence, the act of measuring the photon destroys the entanglement. Once you measure heads or tails, at one end of the segment, the entanglement is lost. You can no longer entangle that coin with the next one, thereby keeping the original entanglement flowing on. The trick for quantum signalling is therefore to pass photons down each repeating pipe without losing entanglement. However, the trick is also to be sure that the photons are reliably entangled, without measuring them, which would destroy the entanglement. Somehow, you must know the coins will fall to different sides, without ever looking.
In the researchers’ experiment, they investigate a setup containing a length of fiber optic cable with a piece of quantum “memory” at each end. Like the memory in a classical computer, quantum memory is supposed to reliably store information so that it can later be retrieved. The researchers use a particular type of quantum memory that is actually made out of a crystal. When hit by a photon, the crystal vibrates in a distinctive way, different depending on whether the photon is heads versus tails. In this way, the crystal encodes the entanglement information. (In reality, photons do not have two sides like a coin, but more nuanced properties like polarization or spin, which are nevertheless encoded in an analogous way.)They show with their setup that the photons in each piece of quantum memory maintain entanglement, even after a relatively long time has passed. This means their setup could make for robust nodes in a repeating network. The maintenance of entanglement is indicated by the release of a separate photon, called the “heralding” photon, which is only emitted when the two particles are entangled.Because their “heralding” photons are generated at standard telecom frequencies, they believe their system can be translated to field-deployed systems and used in real networks between quantum computers, or in other words, a quantum internet. As co-author Hugues de Riedmatten remarked in a press release: “The next steps are to bring the experiment outside of the lab, to try and link different nodes together and distribute entanglement over much larger distances.”