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Quantum information is the enabling factor behind a number of emerging technologies that many physicists expect to have a huge impact on society in future: powerful quantum computers, (almost) perfectly secure quantum cryptography and the quantum internet that will distribute these capabilities round the planet. Quantum cryptography is an emerging technology in which two parties may simultaneously generate shared, secret cryptographic key material using the transmission of quantum states of light.
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. Qubits can be encoded, for example, in the polarization states of a photon or in the spin states of electrons and atomic nuclei. Quantum cryptography is an emerging technology in which two parties may simultaneously generate shared, secret cryptographic key material using the transmission of quantum states of light. Qubits are already being used for creating secret keys—random strings of 0s and 1s—that can then be used to encode classical information, an application called quantum key distribution (QKD).
Quantum Internet will be based on Quantum key distribution (QKD) that uses quantum mechanics to guarantee secure communication. 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.
In July 2018 Alberto Boaron of the University of Geneva and his colleagues reported distributing secret keys using QKD over a record distance of more than 400 kilometers of optical fiber, at 6.5 kilobits per second. In contrast, commercially available systems, such as the one sold by the Geneva-based company ID Quantique, provide QKD over 50 kilometers of fiber. 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.
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. Practical implementation of quantum communications networks, however, needs to address the problem of scalability to serve large numbers of users.
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” .
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.
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.
Implementation of quantum Internet require a network of quantum routers linked by fibers. These routers must receive quantum information, store it, and then send it on through the network. Similarly to classical computer networks, their quantum counterparts would require routing protocols to direct the signal from its source to destination. Devices implementing these routing protocols are called quantum routers and have recently been subject of an intense research.
First Demonstration of A Quantum Router
Guiding the photons into another fibre is a process called routing, which uses a control signal to determine the destination and route of a data signal. A classical router simply reads the data in the control signal and routes the data signal accordingly.
But in the quantum world, reading a control signal also destroys it. So it’s only been possible to route quantum data signals using classical control signals. And although that’s handy, it doesn’t allow the routing process to exploit the full power of quantum information.
In 2012, , Xiuying Chang and a few buddies at Tsinghau University in China announce that they have built and tested the first quantum router to use a quantum control signal to determine the route of a quantum data signal. “We…realize the first proof-of-principle demonstration of a genuine quantum router,” they say.
In this new device, the information is encoded in the polarisation of photons, either horizontal or vertical. The Chinese group begin by creating a single photon that is in a superposition of both horizontal and vertical polarisation states.
They then convert this single photon into a pair of lower energy photons that are entangled, a process called parametric down conversion. Both of these photons are also in a superposition of polarisation states.
The router works by using the polarisation of one of these photons as the control signal to determine the route of the other, the data signal. The device is simple, little more than a collection of half mirrors for guiding photons and waveplates for rotating their polarisation.
First, let’s follow the route of the data photon which is determined by a set of half mirrors that send it one way or the other, depending on its polarisation. The trick is to set up the router so that the polarisation of the control photon influences this route.
The Chinese group do this by rotating the polarisation of the control photon using half and quarter wave plates as the data photon reaches the half mirrors. The quantum phenomenon of entanglement then ensures that the data photon is routed accordingly. In effect, the router works like a logic gate.
Of course, the routing success is a probabilistic like all other quantum phenomena. Chang and co finish their experiment by verifying logic-gate like characteristics of the router and ensuring that both photons are still entangled after passing through it.
That’s an interesting step forward but the new router has significant limitations. The most significant of these is that it can handle only one quantum bit or qubit at a time. And because the process of parametric down conversion cannot handle more qubits, it cannot be scaled to more qubits.
Quantum routers implementations
Quantum routers have been investigated both theoretically and experimentally for various experimental platforms. Not these entire implementations can, however, be considered as fully quantum. In some cases, the routing information is classical and thus the router only semi-quantum in a sense that it classically routes a quantum state.
Implementations rely on non-linear interaction or combine various non-optical physical platforms making them impractical for realistic quantum networks due to ineffective and noisy interfaces. There are implementations that unavoidably disturb the inserted signal state and thus cannot even be considered quantum routers at all. While the cross-system interaction (e.g. light-atom interaction) introduces experimental challenges, the purely optical implementations face different shortcomings such as scalability issues or low success rates. A general quantum state fusion protocol implemented by Vitelli et al. meets all the requirements for a quantum router, but was not designed as such and operates with a rather low success probability of 1/8 (while applying feed-forward corrections).
Czech Researchers led by Karol Bartkiewicz, in their 2018 nature paper, “Implementation of an efficient linear-optical quantum router”, report on experimental implementation of a linear-optical quantum router. Our device allows single-photon polarization-encoded qubits to be routed coherently into two spatial output modes depending on the state of two identical control qubits. The polarization qubit state of the routed photon is maintained during the routing operation. The success probability of our scheme can be increased up to 25% making it the most efficient linear-optical quantum router developed to this date.
Nanomechanical Router Could Open Way for Scalable Quantum Networks
In April 2019, University of Copenhagen researchers reported to have developed a nanocomponent, called a nanomechanical router that emits quantum information carried by photons and routes the photons in different directions inside a photonic chip. The microscopic-size component could provide a way to scale up quantum technology.
The router is based on two coupled waveguides whose distance is adjusted on demand by an external voltage. The researchers showed controllable two-port routing of single photons emitted from quantum dots embedded in the same chip. They observed a maximum splitting ratio >23 decibels (dB), a low insertion loss of 0.67 dB, and a response time below 1 microsecond (μs).
The work merges two research disciplines — nano-optomechanics and quantum optics — to develop an approach to photon routing that can be implemented for many material systems, wavelengths, and temperatures, including for active photonic chips containing quantum emitters. The size of the component, one-tenth the width of a human hair, makes it promising for scaling up applications.
“Bringing the worlds of nanomechanics and quantum photonics together is a way to scale up quantum technology,” said professor Leonardo Midolo. “Until now, we have been able to send off individual photons. However, to do more advanced things with quantum physics, we will need to scale systems up. …To build a quantum computer or quantum internet, you don’t just need one photon at a time, you need lots of photons simultaneously that you can connect to each another.”
To build a quantum computer or a quantum internet, many nanomechanical routers will need to be integrated in the same chip. About 50 photons will be required to provide enough power to achieve what is known as “quantum supremacy.” According to Midolo, the new nanomechanical router makes “quantum supremacy” a realistic goal. “We have calculated that our nanomechanical router can already be scaled up to 10 photons, and with further enhancements, it should be able to achieve the 50 photons,” he said.
The invention is also an advance in technology for controlling light in a chip. Existing technology allows for only a few routers to be integrated on a single chip due to the large device footprint. The nanomechanical routers, in contrast, are so small that several thousands can be integrated in the same chip.
“Our component is extremely efficient,” Midolo said. “It is all about being able to emit as many photons at once, without losing any of them. No other current technique allows for this.”
