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Today’s Internet may be a playground for hackers. From insecure communication links to inadequately guarded data within the cloud, vulnerabilities are everywhere. But if quantum physicists have their way, such weaknesses will soon go the way of the dodo. they need to create quantum networks sporting full-blown quantumness, where information is made , stored and moved around in ways in which mirror the bizarre behavior of the quantum world — consider the metaphorical cats which will be both dead and alive or particles which will exert “spooky action at a distance.” free of many limitations of “classical” networks, these systems could provide A level of privacy, security and computational clout that’s impossible to realize with today’s Internet.
Although a totally realized quantum network remains a far-off vision, recent breakthroughs in transmitting, storing and manipulating quantum information have convinced some physicists that an easy proof of principle is imminent.
From defects in diamonds and crystals that help photons change color, to drones that function spooky network nodes, researchers are employing a smorgasbord of exotic materials and techniques during this quantum quest. the primary stage, many say, would be a quantum network using standard glass fiber to attach a minimum of three small quantum devices about 50 to 100 kilometers apart.
Such a network could also be inbuilt subsequent five years, consistent with Ben Lanyon of the Institute for Quantum Optics and Quantum Information in Innsbruck, Austria. Lanyon’s team is a component of Europe’s Quantum Internet Alliance, coordinated by Stephanie Wehner of the Delft University of Technology within the Netherlands, which is tasked with creating a quantum network. Europe is competing with similar national efforts in China — which in 2016 launched Micius, a quantum satellite — also as within the U.S. Last December the U.S. government enacted the National Quantum Initiative Act, which can lavishly fund variety of research hubs dedicated to quantum technologies, including quantum computers and networks. “The main feature of a quantum network is that you simply are sending quantum information rather than classical information,” says Delft University’s Ronald Hanson. Classical information deals in bits that have values of either 0 or 1. Quantum information, however, uses quantum bits, or qubits, which may be during a superposition of both 0 and 1 at an equivalent time. Qubits are often encoded, for instance , within the polarization states of a photon or within the spin states of electrons and atomic nuclei.
QUANTUM NETWORKING
In what Hanson calls the “low hanging fruit of quantum networks,” qubits are already getting used for creating secret keys — random strings of 0s and 1s — which will then be wont to encode classical information, an application called quantum key distribution (QKD).
QKD involves one party, say Alice, sending qubits to Bob, who measures the qubits (Alice and Bob first appeared during a 1978 paper on public key cryptography, and have now become placeholders for nodes during a quantum network). just for certain sorts of measurements will Bob get an equivalent value that Alice encoded within the qubits. Alice and Bob can compare notes over a public channel to work out what those measurements are, without actually sharing the qubit values. they will then use those private values to make a secret shared key to encrypt classical messages. Crucially, if an intruder were to intercept the qubits, Alice and Bob could detect the intrusion, discard the qubits and begin over — theoretically continuing until nobody is eavesdropping on the quantum channel.
In July 2018 Alberto Boaron of the University of Geneva and his colleagues reported distributing secret keys using QKD over a record distance of quite 400 kilometers of glass fiber , at 6.5 kilobits per second. In contrast, commercially available systems, like the one sold by the Geneva-based company ID Quantique, provide QKD over 50 kilometers of fiber.
ALICE AND BOB GET SPOOKY
Ideally quantum networks will do quite QKD. subsequent step would be to transfer quantum states directly between nodes. Whereas qubits encoded employing a photon’s polarization are often sent over optical fibers (as is completed with QKD), using such qubits to transfer large amounts of quantum information is problematic. Photons can get scattered or absorbed along the way or may simply fail to register during a detector, making for an unreliable channel . Fortunately, there’s a more robust thanks to exchange quantum information — via the utilization of another property of quantum systems, called entanglement.
When two particles or quantum systems interact, they will get entangled. Once entangled, both systems are described by one quantum state, so measuring the state of 1 system instantly influences the state of the opposite , albeit they’re kilometers apart. Einstein called entanglement “spooky action at a distance,” and it’s a useful resource for quantum networks. Imagine two network nodes, Alice and Bob, each made from some isolated little bit of matter (the most blatant and reliable substrate for encoding and storing quantum states). Such “matter nodes” can become entangled with one another via a process that involves the exchange of entangled photons.
Using entangled matter nodes, Alice can exploit her share of the entanglement to send a whole qubit to Bob, without actually transmitting a physical qubit, making the transfer foolproof and secure. The key here is that when entanglement is established between the nodes, the protocol to transfer qubits from Alice to Bob is strong and deterministic.
But to try to to this across long distances, one first must distribute the entanglement — usually via standard fiber-optic networks. In January 2019, Lanyon’s team in Innsbruck reported setting the record for creating entanglement between matter and lightweight over 50 kilometers of glass fiber .
For matter, Lanyon’s team used a so-called trapped ion — one factor IV confined to an optical cavity using electromagnetic fields. When manipulated with lasers, the ion finishes up encoding a qubit as a superposition of two energy states, while also emitting a photon, with a qubit encoded in its polarization states. The qubits within the ion and therefore the photon are entangled. The task: to send this photon through an glass fiber while preserving the entanglement.
Unfortunately, the trapped ion emits a photon at a wavelength of 854 nanometers (nm), which doesn’t last long inside an glass fiber . Thus, Lanyon’s team sent the emitted photon into something called a nonlinear crystal being pumped with a strong laser. the whole interaction converts the incoming photon into another of “telecom” wavelength, one compatible for optical fibers.
The Innsbruck team then injected this photon into a 50-kilometer-long section of glass fiber . Once it reached the opposite end, they tested the ion and therefore the photon to ascertain if they were still entangled. They were.
SWAPPING ENTANGLEMENTS
Lanyon’s team now wants to entangle two trapped ion nodes that are 100 kilometers apart. Each node would transmit an entangled photon through 50 kilometers of glass fiber to a station within the middle. There the photons would be measured in such how that they lose entanglement with their respective ions, causing the ions themselves to urge entangled with one another . As a consequence, the 2 nodes, 100 kilometers apart, will each form a quantum link via a pair of entangled qubits. the whole process is named entanglement swapping. Though relatively inefficient for now, Lanyon calls the setup “a good start” for developing better, faster swapping systems.
Meanwhile Hanson’s team at Delft has demonstrated the way to entangle a special sort of matter node with a telecom-wavelength photon. The researchers used a defect in diamond called a nitrogen-vacancy (NV) center. The defect arises when a nitrogen atom replaces a atom within the gem’s crystalline structure, leaving a vacancy within the space lattice adjacent to the nitrogen atom. The team used lasers to control the spin of 1 “free” electron within the diamond NV center, placing the electron during a superposition of spin states, thus encoding one qubit. the method also leads to the emission of a photon. The photon is during a superposition of being emitted in one among two consecutive time slots. “The photon is usually there, but during a superposition of being emitted early or late,” Hanson says. The qubit stored within the electron’s spin and therefore the qubit stored within the photon’s presence or absence within the time slots are now entangled.
In 2015 the Delft team placed two spatially separated matter nodes made from diamond NV centers about 1.3 kilometers apart, linked by glass fiber . The group then transmitted an entangled photon from each node to some extent roughly midway on the trail between these two nodes. There the team swapped the entanglement, causing the 2 NV centers to become entangled. But even as with Lanyon’s experiment, the photons emitted by the Delft team’s apparatus have a wavelength of 637 nm. Such photons are terrible travelers when injected into optical fibers, diminishing in intensity by an order of magnitude for each kilometer they travel. “It makes it impossible to travel beyond a couple of kilometers,” Hanson says.
So, in May 2019, the Delft team reported a remedy almost like that developed by the Innsbruck team, also using nonlinear crystals and lasers to convert the photon to telecom wavelengths. during this approach, the qubits encoded by the NV center and telecom-wavelength photon remained entangled, setting the stage for entanglement swapping between two diamond NV center nodes.
Although they need not yet transmitted a diamond-entangled telecom-wavelength photon via any significant length of glass fiber , Hanson is confident that they will do so then entangle diamond NV centers 30 kilometers apart using entanglement swapping. “We are now building two of those nodes,” he says. “We’ll use optical fiber that’s already within the ground to entangle these two NV centers.” The team’s next goal is to entangle nodes using the preexisting fiber infrastructure between three cities within the Netherlands, where distances are amenable to such state-of-the-art experiments.
MIX AND MATCH: THE CHALLENGE AHEAD
The Innsbruck and Delft teams each worked with just one sort of matter for storing and entangling qubits. But real-life quantum networks may use differing types of materials in each node, counting on the precise task at hand — for instance , quantum computation or quantum sensing. And quantum nodes, besides manipulating qubits, can also need to store them for brief periods, in so-called quantum memories.
“It’s still not clear what is going on to be the proper platform and therefore the right protocol,” says Marcelli Grimau Puigibert of the University of Basel in Switzerland. “It’s always good to be ready to connect different hybrid systems.”
To this end, Puigibert, working with Wolfgang Tittel’s team at the University of Calgary, recently showed the way to entangle qubits stored in two differing types of materials. They started with a source that emits a pair of entangled photons, one at a wavelength of 794 nm and therefore the other at 1,535 nm. The 794-nm photon interacts with a lithium-niobate crystal doped with thulium, in order that the photon’s state becomes stored within the crystal. The 1,535-nm photon goes into an erbium-doped fiber, which also stores the quantum state.
Both memories were designed to reemit photons at a specific time. The team analyzed those reemitted photons and showed that they remained entangled. This, in turn, implies that the quantum memories were also entangled just before emitting those photons, thus preserving entanglement over time.
The photon wavelengths were also designed to cross-connect different transmission systems: optical fibers on one end (1,535 nm) and satellite communications on the opposite (794 nm). The latter is vital because if quantum networks are to travel intercontinental, entanglement will got to be distributed via satellites. In 2017 a team led by Jian-Wei Pan of the University of Science and Technology of China in Hefei used Micius, China’s quantum satellite, to distribute entanglement between ground stations on the Tibetan Plateau and southwest China.
Satellites, however, seem destined to remain an expensive, niche option of last resort for quantum networks. The next best choice may be relatively inexpensive drones. In May 2019, Shi-Ning Zhu of Nanjing University and his colleagues reported that they had used a 35-kilogram drone to send entangled photons to two quantum nodes 200 meters apart on the ground. The experiment used a classical communication link between the nodes to confirm that the photons they received were indeed entangled. The experiment succeeded in significantly varying conditions, working in sunlight and in darkness and even on rainy nights. If such drones can be scaled up and installed on high-altitude unmanned aerial vehicles, the distance between the nodes on the ground can extend to about 300 kilometers, the authors write.
Challenges remain in the march toward a fully functioning quantum network. Reliable quantum memories are one. Another important missing piece is the ability to extend the reach of a quantum link to arbitrarily long distances, using so-called quantum repeaters. Quantum states cannot be simply copied and regurgitated, as is done with classical information. Quantum nodes will need sophisticated quantum logic gates to ensure that entanglement is preserved in face of losses from interaction with the environment. “It’s definitely one of the next big challenges,” Lanyon says.
Nevertheless, the basic elements are falling into place for building a quantum network that connects at least three cities — and, perhaps, eventually the world. “We now have platforms with which we can start to explore true quantum networks for the first time,” Hanson says. More sophisticated networks beckon. “There’s no guarantee. There’s only promise there [of] the cool stuff we’ll be able to do if we succeed.”