Back in the 1980s, quantum physicists discovered that the strange rules of quantum mechanics allowed information to be sent from one part of the universe to another with complete privacy. This so-called “quantum cryptography” would be perfect, they said, because the security of the message would be guaranteed by the laws of physics themselves.
Within a few years, researchers demonstrated the technique in the lab, and today quantum cryptography is becoming commercially viable thanks to companies such as ID Quantique in Geneva, Switzerland.
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But the entire mechanism is a little counterintuitive. The private message is not sent using quantum mechanics at all. Instead, physicists use quantum processes to send a code called a one-time pad that is used to encrypt the original message. The encrypted message is then sent over an ordinary telecommunications channel and decoded in the usual way. The technique is called quantum key distribution.
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Alice and Bob can exchange perfectly secure messages using the new quantum technique.
Computer scientists know that a message encoded using a one-time pad cannot be broken. So the security comes from the ability to send the one-time pad with perfect privacy, which is what this approach guarantees.
And that raises an interesting question. If it’s possible to send the one-time pad securely using quantum mechanics, why not just send the original message that way?
Today, Wei Zhang at Tsinghua University in Beijing and a few pals say they have done just this. The new process is called quantum secure direct communication, and the Chinese team have used it through 500 meters of fiber-optic cable for the first time.
The reason physicists have relied on one-time pads in the past is simple. At issue is whether a message has been overheard. Physicists can check this because quantum particles cannot be measured without destroying the information they contain.
So when photons are transmitted, if they are arrive in the same state they were sent in, an eavesdropper cannot have extracted the information they contain. But if they arrive in a different state, that is clear evidence that the information has leaked into the environment and the message is not secure.
(In practice, physicists can be sure that a message is secure as long as this leakage is below some critical threshold.)
The problem is that the leakage becomes apparent only after it has occurred. So an eavesdropper would already have the information by the time physicists found out about the ruse.
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That’s why they use this process to send a one-time pad, a set of random numbers that can be used to encrypt a message. If the one-time pad is overheard, physicists simply disregard it and send another, until they can be sure that the process was completely private.
But physicists would dearly love to do away with the one-time pad if they could find a way to ensure the secrecy of a message before it is sent. And some years ago, theorists worked out a way to do this.
The method exploits the quantum phenomenon of entanglement. This occurs when quantum particles are so closely linked that they share the same existence—for example, when they are both created at the same time and place.
When this happens, the particles remain linked, even when they are separated by vast distances. And a measurement on one particle immediately influences the state of the other.
So the trick is to create a set of entangled particles, such as photons, and encode information in their polarization state. So vertical polarization could represent a 1 and horizontal polarization a 0, for instance.
The sender, Alice, keeps one half of each pair and sends the others to Bob, who then has a set of photons that are entangled with Alice’s photons.
Bob separates his photons randomly into two groups. He measures the polarizations of one set and sends the results back to Alice. She then checks whether the states have changed during transmission—in other words, whether Eve has been listening in.
If not, then Alice and Bob know Eve cannot have seen the other photons either, because they have been separated at random. And that means Alice and Bob can use the remaining photons to transmit data using the normal process of quantum communication, which is perfectly private.
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And that’s exactly what Zhang and co have done. One reason the experiment is difficult is that the photons have to be stored while this checking process is ongoing. Zhang and co do this by sending the photons around a two-kilometer loop of optical fiber and carrying out the checks as quickly as possible. The longer it takes, the more likely the photons are to be absorbed or scattered by the optical fiber.
The results clearly show the potential of the technique. “This fibre based QSDC system has the potential to realize a transmission rate close to security key rates of current commercial quantum key distribution systems,” say Zhang and co. “The advantage [is] that the QSDC system could transmit not only secure keys but also the information directly.”
Of course, various improvements are needed to make this kind of system commercially viable. But the work is an important stepping stone toward entirely quantum-based secure communication. Banks, governments, and military agencies will be watching eagerly.
