Quantum teleportation experiments have been successfully completed in China and Canada

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Experiments on quantum teleportation over a distance of more than eight kilometers have been successfully carried out in China and Canada. These experiments in the city were carried out independently by scientists from both countries.

According to the South China Morning Post, previously such experiments were carried out only in laboratory conditions. Quantum teleportation is the transmission over a distance of a quantum state of matter, which is destroyed at the point of departure and then recreated at the point of reception without direct transfer of the particle itself.

A team of researchers from the University of Science and Technology of China teleported photons over a distance of 12.5 km in the city of Hefei (eastern Chinese province of Anhui). For this, conventional fiber optic networks were used.

Canadian scientists conducted a similar experiment in the city of Calgary (southwestern Alberta) at a distance of 8.2 km.

Specialists from the two countries used different approaches. The Chinese teleported only two photons per hour through their channel, but with higher reliability. Canadians were able to transmit up to 17 particles per minute, but their technology is less accurate and has a number of limitations for use in practice.

Last year, American scientists managed to send a photon over a distance of more than 100 km, but only within the laboratory - through a fiber optic cable wound there in turns, reports

System for preparing entangled states and transmitted states for teleportation

The QUESS Quantum Communications Satellite (aka Mo Tzu) mission team reported the first successes in teleporting photons from the Earth's surface into orbit. As part of a month-long experiment, physicists managed to teleport 911 photons over a distance of 500 to 1,400 kilometers. These are record distances for quantum teleportation. A preprint of the study was published on the arXiv.org server, and MIT Technology Review briefly reported on it.

Quantum teleportation involves transferring the quantum state of one particle to another particle without directly transferring the first particle in space. To teleport, for example, the polarization of a photon would require a pair of quantum entangled particles. One of the entangled particles must be kept by the sender of the quantum state, and the second by the recipient. The sender then makes a measurement simultaneously on the transmitted particle and one of the particles of the entangled pair. Quantum entanglement is designed in such a way that two particles behave as a single system - the entangled particle at the recipient feels that a measurement has been taken with its pair and changes its state. Knowing the measurement result on the sender's side (it can be sent via a regular channel), you can receive an exact copy of the sent particle - directly from the recipient. You can read more about this in our material on the quantum alphabet: "".

Previously, the distance for teleportation was limited to tens of kilometers - in 2012, Austrian physicists teleported photon states between La Palma and Tenerife (143 kilometers). The new work overcomes this milestone and improves it several times.

One of the main problems for teleportation - the distribution of entangled photons between the sender (on Earth) and the recipient (satellite) - has already been solved by physicists. The work on creating an entangled pair separated by 1200 kilometers was published a month ago in the magazine Science. Using these pairs, all that remained was to experimentally demonstrate teleportation itself.

Experimental design

Ji-Gang Ren et al. / arXiv.org, 2017

In the new work, the authors used an entangled photon generator installed not on a satellite, but on Earth, at the Ngari Observatory (Tibet). It created over four thousand entangled pairs per second, one photon from each being sent by a laser beam to a satellite that flew over the generator every midnight. First, scientists showed that quantum entanglement persists between the Earth and the satellite, and then they teleported the polarization of a photon. In reality, to reliably test teleportation, scientists needed to create not one, but two entangled pairs of photons.

The greatest losses were associated with turbulence and heterogeneity of the Earth's atmosphere. These effects lead to a broadening of the beam of entangled photons and their scattering - which means fewer particles reach the satellite.

In total, 911 particles were successfully teleported - and during the entire experiment, millions of photon pairs were prepared and transmitted. The authors note that teleportation accuracy reaches 80 percent, and losses range from 41 to 52 decibels (one photon in 100 thousand flies). If you transmit a similar signal over a 1200-kilometer optical fiber with a loss level of 0.2 decibels per kilometer, then the transmission of even one photon will take 20 times longer than the lifetime of the Universe.

Quantum teleportation is one of the important data transmission techniques in quantum telecommunications. It is necessary when developing a global “quantum Internet” with ideally protected communication channels (at the level of physical laws prohibiting the cloning of quantum states). Last year, quantum teleportation protocols for physics on urban fiber optic lines.

Vladimir Korolev

Conducted a satellite experiment on the transfer of quantum states between pairs of entangled photons (so-called quantum teleportation) over a record distance of more than 1200 km.

The phenomenon (or entanglement) occurs when the states of two or more particles are interdependent (correlated), which can be separated to arbitrarily large distances, but at the same time they continue to “feel” each other. Measuring the parameter of one particle leads to instantaneous destruction of the entangled state of another, which is difficult to imagine without understanding the principles of quantum mechanics, especially since particles (this was specially shown in experiments on violation of the so-called Bell inequalities) do not have any hidden parameters in which information about the state of the “companion” would be stored, and at the same time, an instantaneous change in state does not lead to a violation of the principle of causality and does not allow useful information to be transmitted in this way.

To transmit real information, it is additionally necessary to involve particles moving at a speed not exceeding light speed. Entangled particles can be, for example, photons that have a common progenitor, and the dependent parameter is, say, their spin.

Not only scientists involved in fundamental physics, but also engineers designing secure communications are showing interest in transmitting the states of entangled particles over increasingly long distances and under the most extreme conditions. It is believed that the phenomenon of particle entanglement will provide us with, in principle, unhackable communication channels in the future. “Protection” in this case will be the inevitable notification of the conversation participants that a third party has intervened in their communication.

Evidence of this will be the inviolable laws of physics - the irreversible collapse of the wave function.

Prototypes of devices for implementing such secure quantum communication have already been created, but ideas are also emerging to compromise the operation of all these “absolutely secure channels,” for example, through reversible weak quantum measurements, so it is still unclear whether quantum cryptography will be able to leave the prototype testing stage without whether all developments will turn out to be doomed in advance and unsuitable for practical use.

Another point: the transmission of entangled states has so far only been carried out over distances not exceeding 100 km, due to the loss of photons in the optical fiber or in the air, since the probability that at least some of the photons will reach the detector becomes vanishingly small. From time to time, reports appear about the next achievement along this path, but it is not yet possible to cover the entire globe with such a connection.

So, earlier this month, Canadian physicists announced successful attempts to communicate via a secure quantum channel with an aircraft, but it was only 3-10 km from the transmitter.

The so-called quantum repeater protocol is recognized as one of the ways to radically improve signal propagation, but its practical value remains in question due to the need to solve a number of complex technical issues.

Another approach is precisely the use of satellite technology, since the satellite can remain in line of sight to different very distant places on Earth at the same time. The main advantage of this approach would be that most of the photon path would be in a virtual vacuum, with almost zero absorption and no decoherence.

To demonstrate the feasibility of satellite experiments, Chinese experts conducted preliminary ground tests that demonstrated successful bidirectional propagation of entangled photon pairs through an open medium at distances of 600 m, 13 and 102 km with an effective channel loss of 80 dB. Experiments have also been carried out on the transfer of quantum states on moving platforms under conditions of high loss and turbulence.

After detailed feasibility studies with the participation of Austrian scientists, a $100 million satellite was developed and launched on August 16, 2016 from the Jiuquan Satellite Launch Center in the Gobi Desert using a Long March 2D launch vehicle into an orbit at an altitude of 500 km.

The satellite was named “Mo Tzu” in honor of the ancient Chinese philosopher of the 5th century BC, the founder of Moism (the doctrine of universal love and state consequentialism). For several centuries in China, Mohism successfully competed with Confucianism until the latter was adopted as the state ideology.

The Mozi mission is supported by three ground stations: Delinghe (Qinghai Province), Nanshan in Urumqi (Xinjiang) and the GaoMeiGu Observatory (GMG) in Lijiang (Yunnan Province). The distance between Delinghe and Lijian is 1203 km. The distance between the orbiting satellite and these ground stations ranges from 500-2000 km.

Because entangled photons cannot be simply “amplified” like classical signals, new techniques had to be developed to reduce attenuation in transmission links between Earth and satellites. To achieve the required communication efficiency, it was necessary to simultaneously achieve minimal beam divergence and high-speed and high-precision targeting of detectors.

Having developed an ultra-luminous cosmic source of two-photon entanglement and high-precision APT (acquiring, pointing, and tracking) technology, the team established “quantum coupling” between pairs of photons separated by 1203 km, scientists conducted the so-called Bell test to test locality violations (the ability to instantly influence the state of a remote particles) and obtained a result with a statistical significance of four sigma (standard deviations).

Diagram of the photon source on the satellite. The thickness of the KTiOPO4 (PPKTP) crystal is 15 mm. A pair of off-axis concave mirrors focuses the pump laser (PL) at the center of the PPKTP crystal. The output of a Sagnac interferometer uses two dichromatic mirrors (DM) and filters to separate signal photons from the pump laser. Two additional mirrors (PI), remotely controlled from the ground, are used to finely adjust the beam direction for optimal beam collection efficiency. QWP - quarter-wave phase section; HWP - half-wave phase section; PBS - polarizing beam splitter.

Compared to previous methods using the most common commercial telecommunications fibers, the efficiency of the satellite connection was many orders of magnitude higher, which, according to the study authors, opens the way to practical applications previously unavailable on Earth.

MOSCOW, July 12 - RIA Novosti. Physicists from Shanghai announced the success of the first “space” quantum teleportation, transferring information about the state of a particle from the Mo Tzu quantum satellite to a tracking station on Earth, according to an article posted in the electronic library arXiv.org

“We announce the first quantum teleportation of single photons from an observatory on Earth to a satellite in low-Earth orbit, 1,400 kilometers away from it. The successful implementation of this task opens the way to ultra-long-range teleportation and is the first step towards the creation of a quantum Internet,” Jian writes -Wei Pan (Jian-Wei Pan) from the University of Shanghai and his colleagues.

The phenomenon of quantum entanglement is the basis of modern quantum technologies. This phenomenon, in particular, plays an important role in secure quantum communication systems - such systems completely eliminate the possibility of unnoticed “wiretapping” due to the fact that the laws of quantum mechanics prohibit “cloning” the state of light particles. Currently, quantum communication systems are actively being developed in Europe, China, and the USA.

In recent years, scientists from Russia and foreign countries have created dozens of quantum communication systems, the nodes of which can exchange data over fairly large distances, amounting to about 200-300 kilometers. All attempts to expand these networks internationally and intercontinentally have encountered insurmountable difficulties related to the way light fades as it travels through fiber optics.

For this reason, many teams of scientists are thinking about moving quantum communication systems to the “cosmic” level, exchanging information via satellite, allowing them to restore or strengthen the “invisible connection” between entangled photons. The first spacecraft of this kind is already present in orbit - it is the Chinese Mo Tzu satellite, launched into space in August 2016.

This week, Pan and his colleagues described the first successful quantum teleportation experiments carried out on board the Mo-Zu and at a communications station in the town of Ngari in Tibet, built at an altitude of four kilometers to exchange information with the first quantum satellite.

Quantum teleportation was first described at a theoretical level in 1993 by a group of physicists led by Charles Bennett. According to their idea, atoms or photons can exchange information at any distance if they were “entangled” at the quantum level.

To carry out this process, a regular communication channel is required, without which we cannot read the state of entangled particles, which is why such “teleportation” cannot be used to transmit data over astronomical distances. Despite this limitation, quantum teleportation is extremely interesting to physicists and engineers because it can be used for data transmission in quantum computers and for data encryption.

Guided by this idea, scientists entangled two pairs of photons in a laboratory in Ngari, and transferred one of the four “entangled” particles aboard the Mo-Dza using a laser. The satellite simultaneously measured the state of both this particle and another photon, which was on board at that moment, as a result of which information about the properties of the second particle was instantly “teleported” to Earth, changing the way the “ground” photon, confused with the first, behaved particle.

In total, as Chinese physicists say, they managed to “entangle” and teleport over 900 photons, which confirmed the correctness of the “Mo-Zu” work and proved that two-way “orbital” quantum teleportation is possible in principle. In a similar way, as scientists note, it is possible to transmit not only photons, but also qubits, memory cells of a quantum computer, and other objects of the quantum world.

Last year, a Long March 2D rocket took off from the Gobi Desert and placed the Mo Tzu satellite into orbit at a point synchronous with the Sun, so it circles the Earth every day. Mozi is a highly sensitive satellite designed to transmit quantum information. It can detect the quantum states of individual photons released from the surface of our planet.

Today, Mo Tzu's team announced their unique achievement: they managed to create the first ground-to-ground satellite quantum network. This network was used to teleport the first object in history from Earth into its orbit. Teleportation is carried out by scientists who conducted experiments in the field of optical physics. This process is based on the strange phenomenon of entanglement, during which two photons form one point in time and space. From a technical point of view, they are described by a single wave function.

The peculiarity of quantum entanglement is that these two photons exist at the same point, even if they are kilometers apart. Thus, a change in the state of one instantly affects the state of the other. Back in the 90s of the last century, scientists realized that they could use this phenomenon to teleport objects from one point in the Universe to another.

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The idea is to "download" information into one photon, then the other becomes identical to the first. This is teleportation

Such experiments have been carried out many times in laboratory conditions on Earth, but this is the first time they have been tested in interstellar space. Teleportation has enormous implications for a range of technologies related to quantum networks and computing.

In fact, there is no maximum distance for the teleportation of photons, but the connection created between them is too fragile and can be destroyed due to foreign matter appearing in the atmosphere or in the optical fiber. To confirm their theory, scientists carried out experiments all the time at a greater distance, and now they entered orbit. True, for this it was necessary to build a station in Tibet at an altitude of 4 thousand meters.

As part of the experiment, entangled pairs of photons were created, which were launched at a speed of 4000 m/s