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Quantum technology (QT) applies quantum mechanical properties such as quantum entanglement, quantum superposition, and No-cloning theorem to quantum systems such as atoms, ions, electrons, photons, or molecules. Quantum bit is the basic unit of quantum information. Whereas in a classical system, a bit is either in one state or the another. However, quantum qubits can exist in large number of states simultaneously, property called Superposition. Quantum entanglement is a phenomenon where entangled particles can stay connected in the sense that the actions performed on one of the particles affects the other no matter what’s the distance between them. The no-cloning theorem tells us that quantum information (qubit) cannot be copied.
Quantum communication refers to a quantum information exchange that uses photons as quantum information carriers over optical fibre or free-space channels. Today, quantum data transfer rates remain quite low, and so communicating entire messages is not yet practical. Instead, Quantum Cryptography or Quantum key distribution (QKD) is being used that employs single or entangled photons to generate shared secret key between the parties that is perfectly secure. Qubits can be encoded, for example, in the polarization states of a photon or in the spin states of electrons and atomic nuclei. The security is guaranteed by Heisenberg’s uncertainty principle. This ensures that any attempts to intercept and measure quantum transmissions, will introduce an anomalously high error rate in the transmissions and therefore will be detectable.
QKD systems have been shown to be unconditionally secure, however, this is true only for an ideal system. Communication using QKD can be delivered through fiber-optic networks, over the air, and drones to satellites.
The future milestones are to extend QKD from point-to-point configuration to National scale multi-user QKD networks and finally to Global Quantum Internet. But there’s a problem with this vision of the quantum future. At the moment, physicists can only send photons carrying quantum information over the length of a single optical fibre. Current quantum communication systems are limited to couple of hundreds of kilometers because of losses in fibers. So far the majority of experimental quantum networks was limited to peer-to-peer communications between two parties. However, practical implementation of quantum communications networks needs to address the problem of scalability to serve large numbers of users.
The quantum signals can’t be amplified like an electronic signal, therefor these networks require the development of quantum repeaters and quantum memories. However, no reliable and practical quantum memory is available yet.
Quantum Internet
The Quantum internet that will employ quantum entanglement to enable quantum communication applications between any two points on the earth. It will consist of quantum computers as end devices sending qubits which will be processed with quantum routers, repeaters, gateways, hubs, and other quantum tools.
Quantum Internet will enable many new applications Such as: transmitting large volumes of data across immense distances with high security, multiply the power of quantum computers and quantum sensors by linking them together, such as synchronization of atomic clocks all over the globe, and detecting gravitational waves.
The Quantum Internet will provide a powerful platform for communications among quantum computers and other quantum devices. It will further enable a quantum version of the Internet-of-Things (IoT) as progress is being made in developing quantum sensors. Finally, quantum networks can be the most secure networks ever built – completely invulnerable if constructed properly.
Qantum networking in the real world is being driven by three research programs and commercialization efforts: Quantum Key Distribution (QKD) adds unbreakable coding of key distribution to public key encryption. Cloud/network access to quantum computers is core to the business strategies of leading quantum computer companies. Quantum sensor networks promise enhanced navigation and positioning; more sensitive medical imaging modalities, etc. Although a fully realized quantum network is still a far-off vision, recent breakthroughs in transmitting, storing and manipulating quantum information have pointed to its accelerated development.
Quantum Internet will be based on Quantum key distribution (QKD) that uses quantum mechanics to guarantee secure communication. Where the Internet carries bits, the Quantum Internet will carry qubits, which can be in a superposition of both 0 and 1 at the same time. It enables two parties to produce a shared random secret key known only to them, which can then be used to encrypt and decrypt messages. QKD is said to be nearly impossible to hack, since any attempted eavesdropping would change the quantum states and thus could be quickly detected by data flow monitors. This technology offers extremely high security, but its application is currently restricted to metropolitan area networks.
Whereas qubits encoded using a photon’s polarization can be sent over optical fibers (as is done 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 in a detector, making for an unreliable transmission channel.
Extending the quantum network relies on two ways one is use of quantum satellite which can connect over large distances, China has already launched a quantum satellite. Another method is to use optical amplifiers as Robert Thew, who co-leads the Quantum Technologies Group at the University of Geneva explains, “In classical communication, amplifiers are used to regenerate the signal. However, in the quantum regime this adds too much noise and destroys the coherence of the quantum states” .
A research team at the University of Delft in the Netherlands has laid out a roadmap for a quantum internet. Led by Stephanie Wehner, David Elkouss and Ronald Hanson, the trio have set out what is necessary to establish a quantum internet, how it will interact with the current internet, and where it could take us. The researchers say a quantum internet is not designed to replace the current internet but complement it by offering various advantages.
These include much more secure remote access to the cloud, stronger security identification methods, secure messaging and more accurate time synchronization across devices. The capabilities of a quantum internet would grow as it develops through six stages A quantum internet is also capable of developing in parallel to quantum computers which are only necessary to reach its final stage.
“No one yet realises what the quantum internet will enable us to do said Prof. Ronald Hanson,”For example, people have calculated that you can increase the baseline of telescopes by using quantum entanglement. So, two telescopes quite far apart could have better precision than each of them individually would have. You could envision using this quantum internet to create entanglement between atomic clocks with different positions in the world and this would increase the accuracy of timekeeping locally.’
Quantum teleportation enabled by Quantum teleportation
Fortunately, there is a more robust way to exchange quantum information—via the use of another property of quantum systems, called entanglement. When two particles or quantum systems interact, they can get entangled. Once entangled, both systems are described by a single quantum state, so measuring the state of one system instantly influences the state of the other, even if they are kilometers apart.
Quantum teleportation is an essential quantum operation by which we can transfer an unknown quantum state to a remote location with the help of quantum entanglement and classical communication. Quantum teleportation is the idea that quantum states – and they contain information of course – disappear on one side and then reappear at the other side. What is interesting there is that, since the information does not travel on a physical carrier, it’s not encoded in a pulse of light, or in a letter, it does not travel between sender and receiver, it cannot be intercepted. The information disappears on one side and reappears on the other side.
Nasa researchers explain transportation: “If a hypothetical particle called Photon 1 is entangled with Photon 2, the latter can be sent to a distant location, and they still will remain linked, So, if in the second location Photon 2 meets a third particle, Photon 3, and interacts with it, the state which Photon 3 transfers to Photon 2 will automatically be teleported to its entangled twin as well, Photon 1. This is a ‘disembodied transfer,’ meaning that Photons 1 and 3 never interact. Harnessing this type of system could revolutionize encrypted messaging, allowing senders to transmit ‘disembodied’ information to the desired recipient that would be impossible for an eavesdropper to intercept.”
‘Quantum teleportation is the most fundamental operation that can be done on the quantum internet. So, to get entanglement distributed over long distances you are actually teleporting the entanglement from one node to the other. ‘In a classical network you send your data package, and there is an address contained in that, and the router will read off that information and send it on to the next node. We don’t want to do that with these quantum signals. We want to send these quantum signals by teleportation so they don’t have to go through the (optical) fibre, they disappear in one side and reappear in the next.’
Quantum teleportation advances
In 2016, Engineers at Nasa’s Jet Propulsion Lab, the University of Calgary, and the National Institute of Standards and Technology in Boulder, Colorado Researchers achieved quantum teleportation over the farthest distance yet outside of the lab, by sending the quantum state of a photon across 3.7 miles (8.2 Kms) in a metropolitan network. The experiments were conducted using ‘dark’ cables under the city of Calgary in Canada, and mark a major step toward the ultimate goal of a quantum Internet as researchers finally begin tests in real-world contexts. This means the Quantum Internet could be run over currently installed fiber optic networks. “This bring us closer to a future Quantum Internet that can connect powerful quantum computers with a security ensured by the laws if quantum mechanics,” said Quantumrun, Marcel.li Grimau Puigibert (one of the key players in the Calgary experiment)
Chinese team, led by Professor Pan Jianwei and Professor Zhang, have also been able achieve “full” quantum teleportation of photons over a optical fibre network 12.5km apart.Zhang at the University of Science and Technology of China, said the team’s work was only a small step towards the construction of a quantum network. Many technical hurdles, such as storage for the extremely fragile quantum data, remained and it was difficult to predict when a global quantum internet would be operational.
French physicist Frederic Grosshans while commenting on the two experiments in the scientific journal Nature Photonics said, “The two experiments clearly showed that teleportation across metropolitan distances was technologically feasible.”The Chinese and Canadian teams used different approaches to carry out their experiments. The Chinese team demonstrated a fuller version of the quantum network with higher reliability, but the Canadian approach was more efficient, according to Grosshans. The Chinese method “comes at the price of a low rate of two teleported photons per hour, which would strongly limit its practical applications if it could not be improved”, he said. The Canadian method “allows a faster teleportation rate of 17 photons per minute”, but their low accuracy during transmission “also limits its immediate practical applications.”
Longer distances have been achieved in the past, but only in lab settings. A team at the National Institute of Standards and Technology in the US reported in 2015 that it had achieved quantum teleportation over a fibre optical network more than 100km in length, but the whole cable was coiled within a laboratory. Scientists have also teleported photons through the air over 100km, but the technology can only be used at night and in remote areas because too many of the particles are generated by other sources including natural light.Using a cable shields the photons from interference and is viewed by researchers as a more practical way of harnessing the technology.
In January 2019, Lanyon’s team in Innsbruck reported setting the record for creating entanglement between matter and light over 50 kilometers of optical fiber. Lanyon’s team is part of Europe’s Quantum Internet Alliance, coordinated by Stephanie Wehner of the Delft University of Technology in 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 communications satellite—as well as in the U.S. Last December the U.S. government enacted the National Quantum Initiative Act, which will lavishly fund a number of research hubs dedicated to quantum technologies, including quantum computers and networks. In 2018 NASA initiated the development of a National Space Quantum Laboratory that would use lasers on the International Space Station to achieve secure communications between ground stations. Separately, a joint team between the U.K. and Singapore is making rapid progress toward launching its own quantum communications satellite next year. And Japan and India are also pursuing such work.
Teleportation/ Entanglement is exploited by parallel computing, quantum communication, cryptography technology and distributed computing . The distribution of quantum states over long distances is essential for future applications such as Regional, National or global scale quantum networks based on Quantum Key Distribution. Such quantum internet will be useful for distributed quantum computing, distributed cryptographic protocols and dramatically lowering communication complexity. Short-distance quantum teleportation will play a role in transporting quantum information inside quantum computers. Quantum repeaters allow entanglement between quantum devices over long distances. Most experts predict repeaters will start to prototype in real-world applications in about five years, but this is far from certain.
https://www.youtube.com/watch?v=30YqMaeh-n8
“In our experiments, we overcome this limitation by exploiting a teleportation-based approach, which can be thought of as a lossless channel.”The team, which includes researchers from the University of Geneva and Delft University of Technology, has demonstrated the heralded photon amplification technique over a simulated distance of 50 km, reporting its results in the journal Quantum Science and Technology.
As the researchers highlight, one of the major applications of heralded photon amplification is for so-called device-independent quantum key distribution – an approach aimed at certifying the security of a connection with minimal assumptions about the system itself and the technology that is exploited.
At the heart of the approach is the conceptually simple idea of sending a single photon on a 50/50 beam-splitter to generate entanglement. Repeating the process in succession and monitoring the output from single photon detectors provides the building blocks for studying quantum communication protocols. Taking this a step further, it’s possible to distribute the entanglement between two locations, generating a unique key for encrypting data transmission.
“The single photon, or path entangled, scheme we are using is also closely connected to quantum repeaters in terms of how entanglement is distributed in these long distance and fully-quantum network solutions,” commented Thew. “Our next step is to develop compact and more efficient heralded photon sources that can be more easily deployed, allowing us to push these sorts of experiments into real-world networks.”
A promising alternative for long distance quantum states distribution is the use of quantum repeaters. NTT/NIST has utilized teleportation technique could be used make quantum repeaters. Quantum Repeaters can be thought of as being analogous to the optical amplifiers that provide an economic and compact solution for long distance classical communication. However, whereas the idea of amplifiers is to regenerate the classical optical signal, what these Quantum Repeater links do is to create sections of lossless transmission line over which the quantum state is teleported. .” The key technology for implementing quantum repeaters is quantum memories that allow the storage of quantum states.
The researchers expect that the highly efficient multifold photon measurement using the SNSPDs will pave the way toward advanced quantum communication systems based on multiphoton quantum states such as the Greenberger–Horne–Zeilinger state and the cluster state over optical fiber. Increasing the dimensionality of quantum entanglement is a key enabler for high-capacity quantum communications and key distribution, quantum computation and information processing, imaging and enhanced quantum phase measurement.
In January 2019, Lanyon’s team in Innsbruck reported Entanglement sent over 50 km of optical fiber
The quantum internet promises absolutely tap-proof communication and powerful distributed sensor networks for new science and technology. However, because quantum information cannot be copied, it is not possible to send this information over a classical network. Quantum information must be transmitted by quantum particles, and special interfaces are required for this. The Innsbruck-based experimental physicist Ben Lanyon, who was awarded the Austrian START Prize in 2015 for his research, is researching these important intersections of a future quantum Internet. Now his team at the Department of Experimental Physics at the University of Innsbruck and at the Institute of Quantum Optics and Quantum Information of the Austrian Academy of Sciences has achieved a record for the transfer of quantum entanglement between matter and light. For the first time, a distance of 50 kilometers was covered using fiber optic cables. “This is two orders of magnitude further than was previously possible and is a practical distance to start building inter-city quantum networks,” says Ben Lanyon.
Converted photon for transmission
For matter, Lanyon’s team used a so-called trapped ion—a single calcium ion confined to an optical cavity using electromagnetic fields. Using laser beams, the researchers write a quantum state onto the ion and simultaneously excite it to emit a photon in which quantum information is stored. The ion ends up encoding a qubit as a superposition of two energy states, while also emitting a photon, with a qubit encoded in its polarization states. As a result, the quantum states of the atom and the light particle are entangled. But the challenge is to transmit the photon over fiber optic cables. “The photon emitted by the calcium ion has a wavelength of 854 nanometers and is quickly absorbed by the optical fiber,” says Ben Lanyon. His team therefore initially sends the light particle through a nonlinear crystal illuminated by a strong laser. Thereby the photon wavelength is converted to the optimal value for long-distance travel: the current telecommunications standard wavelength of 1550 nanometers. The researchers from Innsbruck then send this photon through a 50-kilometer-long optical fiber line. Their measurements show that atom and light particle are still entangled even after the wavelength conversion and this long journey.
Even greater distances in sight
As a next step, Lanyon and his team show that their methods would enable entanglement to be generated between ions 100 kilometers apart and more. Two nodes send each an entangled photon over a distance of 50 kilometers to an intersection where the light particles are measured in such a way that they lose their entanglement with the ions, which in turn would entangle them. With 100-kilometer node spacing now a possibility, one could therefore envisage building the world’s first intercity light-matter quantum network in the coming years: only a handful of trapped ion-systems would be required on the way to establish a quantum internet between Innsbruck and Vienna, for example.
Meanwhile Hanson’s team at Delft has demonstrated how to entangle a different type 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 carbon atom in the gem’s crystalline structure, leaving a vacancy in the crystal lattice adjacent to the nitrogen atom. The team used lasers to manipulate the spin of one “free” electron in the diamond NV center, placing the electron in a superposition of spin states, thus encoding one qubit. The process also results in the emission of a photon. The photon is in a superposition of being emitted in one of two consecutive time slots. “The photon is always there, but in a superposition of being emitted early or late,” Hanson says. The qubit stored in the electron’s spin and the qubit stored in the photon’s presence or absence in the time slots are now entangled.
Delft scientists Quantum entanglement advances
By exploiting the power of quantum entanglement it is theoretically possible to build a quantum internet that cannot be eavesdropped on. However, the realization of such a quantum network is a real challenge: you have to be able to create entanglement reliably, ‘on demand’, and maintain it long enough to pass the entangled information to the next node. So far, this has been beyond the capabilities of quantum experiments.
In 2018, Scientists at QuTech in Delft have now been the first to experimentally generate entanglement over a distance of two metres in a fraction of a second, ‘on demand’, and subsequently maintain this entanglement long enough to enable -in theory- further entanglement to a third node. ‘The challenge is now to be the first to create a network of multiple entangled nodes: the first version of a quantum internet’, professor Hanson states.
In 2015 the Delft team placed two spatially separated matter nodes made of diamond NV centers about 1.3 kilometers apart, linked by optical fiber. The group then transmitted an entangled photon from each node to a point roughly midway on the path between these two nodes. There the team swapped the entanglement, causing the two NV centers to become entangled. But just 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 every kilometer they travel. “It makes it impossible to go beyond a few kilometers,” Hanson says.
By exploiting the power of quantum entanglement it is theoretically possible to build a quantum internet that cannot be eavesdropped on. However, the realization of such a quantum network is a real challenge: you have to be able to create entanglement reliably, ‘on demand’, and maintain it long enough to pass the entangled information to the next node. So far, this has been beyond the capabilities of quantum experiments.
In 2018, Scientists at QuTech in Delft have now been the first to experimentally generate entanglement over a distance of two metres in a fraction of a second, ‘on demand’, and subsequently maintain this entanglement long enough to enable -in theory- further entanglement to a third node. ‘The challenge is now to be the first to create a network of multiple entangled nodes: the first version of a quantum internet’, professor Hanson states.
So, in May 2019, the Delft team reported a remedy similar to that developed by the Innsbruck team, also using nonlinear crystals and lasers to convert the photon to telecom wavelengths. In 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.
NIST Team’s Quantum Teleportation, step towards Unhackable Global Quantum Internet
Earlier, Researchers at the National Institute of Standards and Technology (NIST), “teleported” or transferred quantum information carried in light particles over 100 kilometers (km) of optical fiber, four times farther than the previous record. The experiment confirmed that quantum communication is feasible over long distances in fiber based on photon entanglement distribution. Quantum key distribution (QKD) over optical fiber with schemes based on attenuated laser light have resulted in the exponential decrease in the key rate caused by fiber loss
Quantum teleportation over optical fiber has been challenging also because the low photon detection efficiencies of typical telecom-band single-photon detectors. In these experiments, however, the ultra sensitive photon sensors allowed for more precise detection. ‘The superconducting detector platform, which has been pioneered by JPL and NIST researchers, makes it possible to detect single photons at telecommunications wavelengths with nearly perfect efficiency and almost no noise,’ said Daniel Oblak, of the University of Calgary’s Instutite for Quantum Science and Technology. ‘This was simply not possible with earlier detector types, and so experiments such as our, using existing fiber-infrastructure, would have been close to impossible without JPL’s detectors.’
Moving forward, the researchers will build repeaters to teleport entangled photons across longer distances. With ‘super-sensitive photon detectors,’ they say repeaters could even send entangled photons across the country. And eventually, space-related communications could achieve teleportation without the use of repeaters, with photons instead fired into space with lasers, and the states teleported from Earth. “By using advanced superconducting detectors, we can use individual photons to efficiently communicate both classical and quantum information from space to the ground,” Shaw said. “We are planning to use more advanced versions of these detectors for demonstrations of optical communication from deep space and of quantum teleportation from the International Space Station.”
Researchers Achieve Long-Distance Teleportation of Entanglement
Quantum entanglement could allow users to send data through a network and know immediately whether that data had made it to its destination without being intercepted or altered. This allows us to create a private key between two remote legitimate users, say the sender Alice and the receiver Bob, by transmitting the photons over a quantum channel, and performing a protocol, BB84 for example, to distill a final, shared secret key.
Zeilinger and his team has generated entanglement between independent qubits over a record distance of 143 kilometers, linking the Canary Islands of La Palma and Tenerife. For the teleportation of entanglement, they made use of phenomenon called Bell-state measurement, by which it is possible to entangle two photons by performing a joint measurement on them.
Assume you have two pairs of entangled photons, “0” and “1” in the receiving station and “2” and “3” in the transmitting station. Now, assume you send photon 3 from the transmitter to the receiver, and perform a Bell-state measurement simultaneously on photon 3 and on photon 1, due to which, photons 3 and 1 become entangled. But surprisingly, photon 2, which stayed home, is now also entangled with photon 0, at the receiver. The entanglement between the two pairs has been swapped, and a quantum communication channel has been established between photons 0 and 2, although they’ve never been formally introduced. Entanglement swapping in conjunction with quantum memory will be an important component of future secure quantum links with satellites, says Thomas Scheidl, a member of Zealander’s research group.
Photon encoding based on time-bin qubit
Various quantum states can be used to carry information; the NTT/NIST experiment used quantum states that indicate when in a sequence of time slots a single photon arrives. As a quantum information carrier, they use a photon encoded as a time-bin qubit i.e. which of time slots a single photon arrives, instead of polarization qubit because it is generally difficult to preserve a polarization state in a long fiber.
NIST experiment added quantum information to a photon in its position in a very small slice of time of only 1 nanosecond, “early” or “late” in time bin. A special crystal splits one input photon to two entangled photons, a helper photon and an output photon. The “output photon” is sent over 102 km of optical fibre.
They then determined the state of entangled “helper photon” by bouncing it off a photon that they has been generated with a known state of “early” or “late”time bin. Pair of detectors through their timing difference are able to determine the state of helper photon. Once they’d worked out the state of the helper photon, they knew the state of the output photon as they are both entangled, and they use another pair of detectors on the other end to confirm this and that state has indeed been teleported over 102 kms.
China building Satellite based worldwide quantum Network.
Since the first experimental demonstrations using photonic qubits and continuous variables, the distance of photonic quantum teleportation over free-space channels has continued to increase and has reached >100 km. European physicists have been able to teleport photons between the two Canary Islands of La Palma and Tenerife off the Atlantic coast of North Africa, a distance of almost 150 kilometres. Next step was to teleport it to satellite.
The launch of the Chinese satellite Micius in 2016 solely dedicated to quantum information science, arguably represents the nation’s lead in an emerging contest among great powers at the frontiers of physics. “In its two-year mission, QUESS is designed to establish ‘hack-proof’ quantum communications by transmitting uncrackable keys from space to the ground,” Xinhua news agency said. said.
In 2017 the team, along with a group of researchers in Austria, was able to employ the satellite to perform the world’s first quantum-encrypted virtual teleconference between Beijing and Vienna. Despite being a huge milestone, this method was not bulletproof against hacking. Micius itself was the weak point: The satellite “knew” the sequences of photons, or keys, for each location, as well as a combined key for decryption. If, somehow, a spy had carefully eavesdropped on its activity, the integrity of the teleconference could have been compromised.
To overcome this problem, the new demonstration by Pan and his colleagues ensured that Micius would not “know” anything. The trick was to avoid using the satellite as a communications relay. Instead the team relied on it solely for simultaneously transmitting a pair of secret keys to allow two ground stations in China, located more than 1,120 kilometers apart, to establish a direct link. “We don’t need to trust the satellite,” Pan says. “So the satellite can be made by anyone—even by your enemy.” Each secret key is one of two strings of entangled photon pairs. The laws of quantum physics dictate that any attempt to spy on such a transmission will unavoidably leave an errorlike footprint that can be easily detected by recipients at either station.
This is the first time the technique—called entanglement-based quantum-key distribution—has been demonstrated using a satellite. (The 2017 test also distributed quantum keys. It did not utilize entanglement to the same degree, however.) “When the satellite was launched, that was a huge milestone,” says Shohini Ghose, a physicist at Wilfrid Laurier University in Ontario, who was not involved in the new study. “But [the researchers] didn’t have the level of error-detection rates that are required to actually use that entanglement to do key distribution.”
The error-detection rate is vital because distinguishing between a real error and an errorlike footprint from eavesdropping is crucial for security. In addition, a high rate could mean that the keys that two ground stations receive differ from each other—a scenario that would render secure communications impossible. To improve the fidelity of their communications system, the scientists focused on boosting the light-gathering efficiency of telescopes at each of the two ground stations that monitored Micius’s transmissions—updating filtering systems and optical components to reach the necessary low error rate required for quantum-key distribution.
The satellite will enable secure communications between Beijing and Urumqi, Xinhua said. “The newly-launched satellite marks a transition in China’s role – from a follower in classic information technology development to one of the leaders guiding future achievements,” Pan Jianwei, the project’s chief scientist, told the agency. Quantum communications holds “enormous prospects” in the field of defense, it added. China then plans to put additional satellites into orbit China hopes to complete a QKD system linking Asia and Europe by 2020, and have a worldwide quantum Network.
However realistic National and Global Quantum would likely be a hybrid one based on both free space and fiber optic links. The fiber network could complement free space network in urban settings where line of sight is blocked by buildings e.t.c. China has also developed Beijing-Shanghai quantum link connects Beijing to Jinan to Hefei to Shanghai, a distance of more than 1,200 miles. Several major Chinese banks are already using the link to transfer their most sensitive data.However, even though the newly opened quantum link is technically quantum, it is still not 100 percent secure. Photons, or light, can only go through about 100 kilometers of optic fiber before getting too dim to reliably carry data. As a result, the signal needs to be relayed by a node, which decrypts and re-encrypts the data before passing it on. This process makes the nodes susceptible to hacking. There are 32 of these nodes for the Beijing-Shanghai quantum link.
This is still a significant security improvement over traditional fiber optics, said Yu-Ao Chen, a quantum physicist at the University of Science and Technology of China in Shanghai. “Before the Beijing-Shanghai line, hackers could in principle wiretap anywhere along the entire length of the optic fiber. Now, the number of vulnerable points has been narrowed down to just 32,” he said. Chen works in the research group in charge of China’s large-scale quantum communication projects, which is led by Jian-Wei Pan, known in China by his audacious nickname, “Father of Quantum.”
For a quantum communication system to be 100 percent secure, the nodes themselves would have to be hack-proof as well. Scientists are already working on a solution: the quantum repeater. A quantum repeater essentially serves the same purpose as an ordinary relay node, except it works in a slightly different way. A network using quantum repeaters is shaped more like a family tree than a linear chain. In this family tree-shaped game of telephone, the quantum repeater is the parent who distributes identical pairs of quantum keys between two children, therefore doubling the possible distance between users. Moreover, these “parents” can also have their own “parents,” which can then double the key-sharing distance between the children at the bottom for every extra level created atop the family tree. This in effect increases the distance a quantum message can be sent without ever having to decrypt it.
Although the principle is straightforward, each additional layer of parents creates a new set of technological challenges, such as additional noise in the data. A recent paper by Chen and his colleagues in Physical Review Letters reports how they built an experimental system up to the grandparent layer, quadrupling the theoretical distance limit for a hack-proof quantum communication link. But for international distances over thousands of miles, one’ll need to go higher up the family tree, perhaps up to the great-great-great-great grandparent level.
That situation does not mean that the satellite-based system is inherently better than the ground-based one. “It’s kind of apples and oranges,” says Paul Kwiat, a physicist at the University of Illinois at Urbana-Champaign, who was also not involved in the study. “The satellite has a couple of problems. One is there aren’t many [quantum research] satellites that are flying at the moment. Two, those satellites are not always parked over your own telescopes that you want.” Relying on a satellite’s passage overhead means secure communications can only take place at certain times of day. And even then, the technique presently requires other factors, such as reasonably clear skies, to ensure a ground station can receive a key. “I think it’s not a good strategy to say you’re trying to decide which of these two you want to buy,” Kwiat says. Instead, he adds, a hybrid system utilizing local fiber networks linked by satellites could be the best way forward.
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.
The Innsbruck and Delft teams each worked with only one type of matter for storing and entangling qubits. But real-life quantum networks may use different types of materials in each node, depending on the exact task at hand—for example, quantum computation or quantum sensing. And quantum nodes, besides manipulating qubits, may also have to store them for brief periods, in so-called quantum memories. “It’s still not clear what’s going to be the right platform and the right protocol,” says Marcelli Grimau Puigibert of the University of Basel in Switzerland. “It’s always good to be able to connect different hybrid systems.”
To this end, Puigibert, working with Wolfgang Tittel’s team at the University of Calgary, recently showed how to entangle qubits stored in two different types of materials. They started with a source that emits a pair of entangled photons, one at a wavelength of 794 nm and the other at 1,535 nm. The 794-nm photon interacts with a lithium-niobate crystal doped with thulium, so that the photon’s state becomes stored in 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 particular 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 prior to emitting those photons, thus preserving entanglement over time.
Still, 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.
A Dutch team has succeeded in exchanging qubits between distant nodes with no direct connection between sender and receiver reported in May 2020
Explaining the relevance of this primordial quantum network, Hanson says: “Regarding quantum communication, our work shows how teleportation can be used in a real network environment, with nodes that have no direct connection. In a future quantum internet, such teleportation will be the main way to transfer quantum information over large distances. Our network can be viewed as a modular quantum computer [where the nodes are the modules]; our work shows that nodes can exchange quantum information, even if they are not on a single chip.”
Cabello attempts to explain the achievement in simplified terms. “You have a quantum state in a city that could be Seville and you want to send it to another city, say Madrid,” he says. “You need there to be a state of entangled qubits between Seville and Madrid. That is standard teleportation protocol. The interesting thing about the experiment is that the entanglement can only be established at a certain distance. Let’s say, in the example of the cities, it is 500 kilometers. If you want to send qubits from Seville to San Sebastián, you have to overcome the distance limitation. That’s what Hanson has managed: it’s no longer Seville-Madrid, it’s Seville-San Sebastián. The distance has been doubled.”
According to García Ripoll, “sending not only classical information [bits] but also quantum states [arbitrary qubit states] requires a mechanism to distribute an entangled state between two distant points and a quantum memory [memory qubit] to store the information to be transmitted while establishing this communication channel based on entanglement.”
Hanson’s experiment uses Nitrogen-Vacancy (NV) centers, “a type of diamond impurity that acts like a qubit and can be optically manipulated,” says García Ripoll. “Through the emission of photons, this qubit makes it possible to create long-distance entanglement.” An NV-center is a defect whereby a carbon atom in the diamond crystal lattice is replaced by a nitrogen atom (N) and a neighboring vacancy (V).
A single NV can detect a magnetic moment of a single molecule and has wide applications in quantum technology. According to García Ripoll, “the NV-center or color center can also talk to the magnetic moments of surrounding atoms and, in [Hanson’s] experiment, they use this to gain a quantum memory by passing the NV information to a nearby nuclear spin [in a carbon-13 isotope]. The information to be sent can be kept safe for a long time, freeing the NV to perform the task of establishing entanglement with another communication node.
Here we overcome these challenges by a set of key innovations and achieve qubit teleportation between non-neighbouring network nodes. Our quantum network consists of three nodes in a line configuration, Alice, Bob and Charlie. Each node contains a NV centre in diamond. Using the NV electronic spin as the communication qubit, we are able to generate remote entanglement between each pair of neighbouring nodes. In addition, Bob and Charlie each use a nearby 13C nuclear spin as a memory qubit. The steps of the teleportation protocol are : To prepare the teleporter, we use an entanglement swapping protocol mediated by Bob, similar to a quantum repeater protocol, to establish entanglement between Alice and Charlie. Once successful preparation of the teleporter is heralded, the input qubit state is prepared on Charlie and finally teleported to Alice.
“Apart from the quality of the experiment, the demonstration of a sophisticated quantum communication setup with three nodes and very elaborate communication algorithms, lays the groundwork for its extension to scalable entanglement distribution and quantum communication setups that are very promising,” García Ripoll adds.
These results are enabled by key innovations in the qubit readout procedure, active memory qubit protection during entanglement generation and tailored heralding that reduces remote entanglement infidelities.
Quantum Internet Roadmap
The first stage in the process is to build a fast and reliable small network of nodes that can transmit and receive quantum entangled messages. This requires a physical channel to send the message such as a fiber optic cable, a quantum repeater capable of extending the distance information can be sent, and end nodes to receive the messages.
The Delft group is building such a network between four cities in the Netherlands and hopes to replicate the achievements of the ARPANET (the precursor to the modern internet) by sending the first message between Delft University and Amsterdam in 2020. Countries such as China have also been building first stage quantum networks such as the Beijing-Shanghai quantum link for security purposes.
In a world in which the privacy and security of the internet are rapidly eroding in the face of surveillance capitalism, aggressive state espionage, new technological challenges (such as AI and the Internet of Things) and economic incentivisation for speed over security, could a quantum internet act as a partial cure to such dire strategic trends?
The short answer is yes. By using near faultless quantum encryption there is an opportunity for small networks to regain the confidentiality and integrity of their information. The recent use of internet traffic rerouting and cloud hopping to conduct industrial espionage against Western countries including Australia and the United States could be mitigated by quantum secure remote access protocols and the use of quantum internets.
In the final stage of a quantum internet, the creation of quantum byzantine agreements could also help decentralised networks organise and share information safely even when there is a malicious actor hiding amongst them. This is because the arrangement of the system is resilient enough to accommodate up to a third of the actors in the system being bad whilst simultaneously allowing good actors verify their information and carry out their message.
The long answer is that this is a partial technical solution to two human problems. One, the age-old security problem of states stealing and sponsoring proxies to acquire knowledge from competitors. Using a quantum internet will raise the cost for attackers but it will not deter them as the geo-strategic or business imperatives (or a civil-military fusion of the two) for compromising communications will continue to drive their actions. If the stages of a quantum internet can grant increasingly absolute communications security, as it has been theorised to do, what is of a higher likelihood is that it will simply shift attackers’ attention to the humans on either end of the node.
Two, the new-age problem of structural deficiencies in internet security created by digital business models that require huge amounts of data and whose speed of technology iteration undermines the security of the flow of new technologies and infrastructure that fuel the internet’s expansion. The complexity of this problem cannot be answered by one technology and requires multiple solutions to be sought across government and industry.
In the darkness of the web there stands a path of light. The photons that entangle a quantum message, whilst not a silver bullet to problems of cybersecurity, can provide a much greater level of security than what we have now. The scientific and engineering challenges along the six stages will be difficult to surmount. However, by offering a unified approach across industries with a common plan, the Delft team have brought the possibility of a quantum internet several steps closer to being a new part of the web.
References and resources also include:
- http://www.dailymail.co.uk/sciencetech/article-3837340/Quantum-teleportation-breakthrough-Nasa-reveals-dark-cable-experiment-Calgary.html
- http://www.scmp.com/news/china/policies-politics/article/2020643/chinese-canadian-scientists-achieve-breakthrough
- https://phys.org/news/2017-06-physicists-amplifier-quantum-toolbox.html
- https://horizon-magazine.eu/article/no-one-yet-realises-what-quantum-internet-will-enable-us-do-prof-ronald-hanson_en.html
- https://projectqsydney.com/the-road-to-a-quantum-internet/
- https://www.scientificamerican.com/article/the-quantum-internet-is-emerging-one-experiment-at-a-time/
