Photon teleportation: Less ‘beam me up’, more 007
Wednesday, Jul 12, 2017, 06:58 AM | Source: Pursuit
By Steven Prawer
Chinese researchers in the Tibetan mountains have, for the first time, beamed 'entangled' particles of light between a satellite and the ground and used 'spooky' quantum physics in which the particles can instantly share their information despite being separated by over 1,200km.
It sounds like something out of Star Trek, but maybe something out of James Bond is more accurate. That is because what this is actually about isn't teleportation, as it is popularly understood, but secret codes, communications security and what the researchers have called the creation of a "quantum internet."
And along the way they have also proved some frankly weird physics that challenges our commonsense.
"What these researchers have done is technologically a tour de force," says Melbourne University physicist Professor Steven Prawer.
"They have allowed entanglement to occur over a large distance by using satellites, potentially opening the way for two people within the scope of the same satellite to communicate with complete security using quantum key distribution."
Stop there. What is quantum physics, quantum key distribution and entanglement?
Quantum physics describes the forces at work at the atomic level – the particles that make up atoms such as electrons, or in the case of light particles, photons. But this world is profoundly different from the one we are used to, because a single particle can be in two or more states at the same time; a phenomenon called superposition. That is, a particle can be in two positions at once, or have a horizontal and vertical polarisation at once. It is only when scientists measure a particle that this uncertain state is resolved and it takes on a single state.
Entanglement is a separate phenomenon in which two paired particles will share the same position once their superposition is measured. Once you measure the position of one of the entangled particles, the paired particle will be in the same state, apparently regardless of how far apart they physically might be. For example, if an entangled particle is in a horizontal polarisation, then its pair will also be horizontal when it is measured.
"So although we don't know before the measurement whether the photon is horizontally or vertically polarised, once we measure one, we can be certain that its entangled twin is in exactly the same state," says Professor Prawer.
If we substitute horizontal and vertical for the digits 0 and 1 we have the basis of a computer code that can be secretly shared between two people to ensure their communications are secure.
Scientists can generate entanglement by simply splitting a particle in two. A single photon can be split by beaming ultra violet light through a crystal, and the pair of photons emerging from the crystal will be entangled.
It is the use of this property of entanglement that physicists refer to as teleportation, but it has nothing to do with Star Trek's Scotty beaming anyone up. It is actually using entanglement to exchange information between two separated particles instantly, even when they far apart.
The use of this entanglement to create security codes is what scientists call quantum key distribution.
Quantum key distribution has got people excited because advances in quantum physics in computer applications threatens to make redundant the mathematical codes we currently rely on to keep our data secret, whether it is our bank details or national security information.
Computing is the big opportunity for harnessing quantum physics because superposition can be used to crunch huge amounts of data like never before. Unlike a computer bit – which is either 0 or 1 – a quantum bit is in two states simultaneously which means calculations can be made all in parallel. While a classic computer has to crunch through all possibilities in order to crack our current codes, which can be made effectively secure by using large numbers, a powerful quantum computer has the potential to simply blitz the calculations needed to crack a code.
"The quantum computer stores all the possibilities at the same time, working through all the possibilities in parallel," says Professor Prawer.
Today we only have weak quantum computers, but even if powerful quantum computers are still years or decades away, they are coming and that it is a worry for security agencies and governments. Quantum key distribution is seen as a way to make the coming quantum computers secure.
"Just as quantum physics is creating this security problem, people are looking at quantum key distribution as the way to guarantee security," says Professor Prawer.
A code based on sequences of single particles, like pulses of photons, can be made unbreakable. That is because if someone were to attempt to interfere (listen in) they would disturb the state of the single photon, immediately resolving its uncertain state. When that happens both the parties at either end of the connection (whether wireless, fibre, or copper) would know someone was tying to listen in. In this way they can establish beyond doubt whether a line is secure before exchanging the quantum key to a randomly generated code sequence, called the protocol. Once both parties have the protocol, or key, they will be able to understand each other, but the information they would subsequently exchange would be gobbledygook to an eavesdropper who would have no way of finding the code.
Based on what the Chinese researchers have now shown is feasible, Professor Prawer says it could work like this. "If you wanted to have a secure conversation you would both order up a stream of entangled photons from a satellite and that would create the protocol, and once we have the key we can say bye to the satellite and communicate securely over normal optical fibre links."
One of the difficulties in exchanging the quantum key is that the signal is just one particle, such as single photon. This very weak signal can get lost over a long distance as it interacts with optical fibres or with matter in the air. Until now, scientists have only been able to transmit entangled photons over a distance of just over 100km. But this latest research, led by a team from the University of Science and Technology of China, have solved this problem by using the vacuum in space to transmit entangled photons from space to ground stations that are over more than 1,200km apart.
"What they have done is a major technological achievement of measurement and accuracy," says Professor Prawer.
Of course at this stage you will need a strong laser telescope to receive your laser beam stream of single photons that would otherwise fly all over the place. But Professor Prawer says this could eventually be overcome if the technology developed to using microwaves rather than light. In that way the quantum signal could be received like any other wave signal from a satellite, he says.
But despite this latest achievement, Professor Prawer says he doubts that quantum key distribution has much application. He points out that the big problem in communications security isn't really eavesdroppers interfering with signals; it is leaks at either end of the line – the source and the receiver.
He is referring to phishing, in which people are tricked into giving away their information, or hacking, in which computers are hijacked and information stolen.
Indeed, cracking codes is so difficult now that governments are resorting to introducing legislation to force Internet providers to give up the codes that security agencies can't break.
Professor Prawer also notes that mathematicians are already at work trying to create new code forms that even a quantum computer would struggle to decrypt.
"So the number of cases that this technology is actually useful for and essential, is likely to be very small," says Professor Prawer.
But he says the achievement is still profound because it has proven the weirdness of entanglement, and by implication the weirdness of the universe.
Even Albert Einstein called entanglement 'spooky', but because it seemed we could only make it happen over relatively short distances, there had always been some common sense suspicion that the entangled particles are somehow communicating to influence each other's state. But by achieving entanglement at more than 1000km, Professor Prawer says there just isn't the time available for one particle to communicate a message to the other to influence its state.
It means that entangled particles must be independent of each other while still resolving themselves in the same way without any apparent cause and effect.
"We don't understand why this happens because our commonsense is assailing us to say this just doesn't make sense. The idea that there is no cause and effect violates our sensibilities.
"But this really does confirm that 'spooky' action at a distance is really how the world works."
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