Last month, two interesting developments in the field of quantum technology took place at once: Chinese scientists teleported photons of light from a ground station to a space satellite and an annual conference of leading experts in quantum physics was held in Moscow. Business Insider was able to capture Dr. Eugene Polzik of the Niels Bohr Institute, one of the leading experts in quantum teleportation, and questioned him on a variety of issues, including the outstanding success of his Chinese colleagues.
“Teleportations of this kind have been carried out in laboratory conditions since 1997, but Chinese scientists have managed to achieve this amazing technological effect at a great distance,” Polzik said.
In 2012, a team of European scientists successfully teleported photons between the two Canary Islands. The distance between the transmitting and receiving devices was 141 kilometers. Chinese researchers managed to break this record in July, when they successfully teleported photons over a distance of 500 kilometers.
We have long dreamed of such a technology from Star Trek, although our intuition has always said that teleportation is basically impossible. However, the physics of our real world, in which we live every day, bears little resemblance to the physics of the quantum world. Here, the laws of a falling stone from a cliff face and governing electrons and individual photons of light are completely different from what we are used to seeing. Therefore, in such a bizarre world, almost everything is possible, including teleportation. How to understand all this? We start with quantum entanglement.
What is quantum entanglement?
Sometimes two quantum particles turn out to be mirror-linked. Whatever happens to one of these particles, the same will happen to the other. Even if they are separated by great distances. They are still two separate objects, but they are identical in everything. When two particles share their states with each other, then such particles are called entangled.
“Suppose I created a pair of entangled photons,” Polzik explains.
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“I keep one, and send the other with a laser to an orbiting space satellite, hoping that the photon will reach its destination. Teleportation can be considered successful only when the two-photon entanglement state is separated between the transmitting and receiving stations."
The main technical difficulty of the teleportation process is the transfer of a photon to a certain distance from the entangled partner particle. In the case of the Chinese experiment, one photon was in a laboratory on Earth, and the second was successfully sent to an orbiting satellite. The changes that have occurred with the photon on Earth as part of the manipulations of scientists have also affected the photon in space - this is quantum teleportation in its purest form.
How to understand whether the satellite received the desired photon, and not some random particle of light?
This is relatively easy to do thanks to a process called spectral filtering. It allows scientists to identify and track individual photons of light by labeling them with a unique identification number.
“You know the frequency of the photon you are sending, you know its directionality. The satellite is aimed at the source of dispatch located on Earth. If you have very good optical equipment on both sides, then this optics sees only the source, and nothing else,”Polzik continues.
The spectral filtering method is indifferent to “noise” in the form of other photons. For example, in the same experiment in the Canary Islands, the transmission was carried out under a clear sunny sky.
There was a transfer of millions of photons to the satellite, but only 900 reached the destination. Why?
The further you try to send the entangled photon, the less efficient this process becomes. Moreover, the Earth's atmosphere is in constant motion, so losing photons on their way into outer space is easy.
“Even if there was no atmosphere, you still need to focus the beam of light so that it is directed towards the satellite. If you shine a laser pointer on your palm, the point of light will be small, but if you just remove the laser, the point becomes larger - this is the law of diffraction,”says Polzik.
From the ground, it is quite difficult for light to break through to space (to an optical receiver installed on an orbiting satellite). It distorts a lot, so most of the photons just go nowhere.
“Successful teleportation can only be achieved over a very short period of time. In a general sense, this is very impractical, but nevertheless, ways of using this technology can be found,”continues Polzik.
Is Quantum Teleportation an Instant Transfer of Data?
Not really. Teleportable objects do not disappear and then reappear somewhere else. Scientists use entanglement to transfer information about the quantum state of one photon to another. Without this information, the photon will have to physically cover the entire distance between the transmitter and receiver. Again, information is not transmitted instantly. This is possible only when the sender measures the quantum state of his photon, thereby changing the state of the photon at the receiver. Because of quantum entanglement, essentially one photon "becomes" another photon.
So what is all this for?
Quantum teleportation is capable of proving the concept of the possibility of creating an ultra-secure world communication network. Like a key that opens a lock, a message transmitted over a quantum network will reach only the addressee who possesses the correctly entangled photon, which will allow this message to be received and read.
Albert Einstein once called quantum entanglement “spooky long-range action,” but this long-range action is the fundamental component that makes everything work. And one day he may become the driver of our secure communication in the future.
Nikolay Khizhnyak