Quantum encryption is here, but the laws of physics can do much more than protect privacy
By Davide Castelvecchi
From Science News, August 16, 2008
A stylish new way of surfing the Internet is coming to Vienna this fall. Researchers plan to flip the switch on the next step toward a quantum version of the Internet. They will build a network allowing users to send each other messages as virtually unbreakable ciphers, with privacy protected by the laws of quantum physics.
The Vienna net is admittedly just a prototype for research purposes. It is also not yet a true quantum version of the Internet. Although it can transmit ordinary data with quantum security, it can’t transfer quantum information, which encodes the states of objects that obey quantum rules. Such a breakthrough might be years off, but it’s getting closer.
Truth be told, it’s not completely clear what a fully quantum Internet would be good for. In fact, at first it even sounds like a really bad idea. Quantum information is notoriously wobbly. An object tends to live in a superposition of states — for example, an electron can spin in two directions at once, or an atom can be simultaneously in two different places — until interaction with the rest of the world forces the object to pick one state. Quantum reality is a limbo of coexisting possibilities.
And because any measurement done of a quantum system changes the system’s state irreversibly, quantum information is different every time it’s read. That makes it impossible, for example, to copy, broadcast or back up quantum data.
But the eccentric physics could also impart unique strengths to networks. While each data bit in an ordinary computer takes the value 0 or 1, the units of quantum information, called quantum bits, or qubits for short, can take both values simultaneously. A quantum Internet could transfer software and data between future (and futuristic) quantum computers, which could outperform ordinary computers by running multiple operations at once, in superposition. And the network could lead to new kinds of social interactions — such as letting quantum physics pick a presidential candidate who pleases the most voters or allowing people to donate to a cause based on whether others donate as well — and do so with absolute secrecy.
Perhaps — and this inches toward Star Trek territory — some day a quantum net could even “beam up” a physical object. All the information needed to re-create the object, such as its shape and energy, would be transferred elsewhere, leaving just chaos behind.
In the meantime, when the switch is flipped October 8, the Vienna net will demonstrate how quantum physics can keep ordinary information, such as an e-mail or the balance of a checking account, safe from prying eyes.
This latest step toward the quantum Internet is a limited network backbone that will often run at the speed of a 1980s modem. To plug into it, a user would need to buy expensive gear and link an optical fiber to one of the backbone’s five nodes. But it’s a step.
Meanwhile, most of the basic technical ingredients of a truly quantum Internet have now been demonstrated, at least in the lab. In particular, researchers have created various types of “quantum memory,” in which light pulses traveling through an optical fiber essentially slow to a halt, a crucial requirement for the quantum version of an Internet router. So it may be just a matter of time before scientists can start beaming up stuff — or at least data.
“I’m optimistic that within a few years we’ll be able to build at least a lab demonstration of a quantum network,” says Mikhail Lukin of Harvard University.
A solid quantum key
In tunnels stretching under Vienna and the Danube river, pulses of light will be beamed this October along tens of kilometers of existing optical fibers owned by German engineering conglomerate Siemens. A collaboration of more than 40 universities, companies and research institutions will piece together technologies to link five Siemens buildings, four of them scattered across the city and one 85 kilometers away in the town of St. Pölten.
The building-to-building connections will use a number of quantum encryption systems to pass the information, many of them inspired by a version of quantum encryption first proposed in 1991 by Artur Ekert, now at the National University of Singapore. With Ekert’s procedure, the sender and the receiver, conventionally called Alice and Bob, use both a quantum connection and a classical one, which could be the good-old Internet or a phone line.
Through the quantum connection, Alice and Bob establish a common encryption key — a secret sequence of data bits that Alice will use to scramble her message, and Bob to unscramble it. Alice can then send her scrambled message to Bob through the classical connection, for example as an e-mail attachment.
To someone who doesn’t know the key, Alice’s message would look like a random sequence of bits. Even the most sophisticated computer imaginable wouldn’t be able to crack it. But Bob knows the key, so he can unscramble the message.
Keeping the key secret as they create it is the crucial part, and here’s where Ekert exploits quantum physics — specifically, a weird phenomenon called quantum entanglement. In quantum physics, each of two objects can exist in its own state, or the objects’ states can be entangled, meaning that, while separate, they are not independent of each other.
Take photons, the elementary particles that form electromagnetic radiation, including light. Photons wiggle sideways as they zip along an optical fiber. Two photons can wiggle in independent directions, called linear polarizations. But two photons can also be entangled, so that, for example, when one photon is polarized vertically, the other must be polarized horizontally, and vice versa.
In Ekert-style encryption, a laser device creates pairs of entangled photons and sends (along the fiber-optic cable) one photon from each entangled pair to Alice and the other one to Bob.
Because photons in each pair have correlated polarizations, Alice and Bob could now turn that information into a common key, which for example could contain a 0 for each vertically polarized photon and a 1 for each horizontally polarized one.
However, Alice and Bob also want to be sure the photons they are using haven’t been intercepted by an eavesdropper, inevitably referred to as Eve. Any Eve who intercepts the photons, trying to steal the key, will change the photons’ states, or even destroy them, since it’s impossible to measure the state of a quantum system without changing it irreversibly. Alice and Bob, over the phone, will then compare notes on their test photons. If they notice discrepancies, they’ll know Eve was there, so they’ll throw away the key and start again.
Quantum encryption systems are now available commercially. Some are owned by banking institutions, for example, and one was used last fall in Switzerland to transmit electoral data from an electronic polling station. So far, though, these links have mostly been point-to-point rather than networks with multiple users.
With a network of quantum-encrypted lines such as the one being built in Vienna, users will just need to link to the node closest to them. When one user wants to send a secret message to another, the message will travel in encrypted form from the first user to an entry node. There, the message will be decrypted and then encrypted again (using a new key) to be sent to the next node. The same will happen at every node in between, until the message reaches its destination.
Privacy will be guaranteed, as long as the locations of the sender, the receiver and the intermediate nodes stay protected from intrusion. (By routing messages through multiple nodes simultaneously and using some mathematical tricks, the network will actually guarantee privacy even if one of the nodes is broken into.)
This piecemeal encryption — a solution also adopted on a smaller scale in a Boston-area quantum network laid out in 2003 — is needed because of a fundamental limitation with transmitting photons.
Sharing an encryption key between any two users requires sending single photons — entangled photons in the case of Ekert’s scheme. But something as small as a photon easily gets lost or absorbed even in the highest-quality optical fiber, says Norbert Lütkenhaus of the University of Waterloo in Canada, a physicist who helped design Vienna’s quantum net. “You lose one-half of the photons every 15 kilometers,” he says.
Establishing a key thus becomes exponentially slower as the distance increases. Lütkenhaus calculates that 25 kilometers is still a good distance for decently efficient quantum communication, but beyond that distance a different solution is needed.
In the case of ordinary optical communications, the problem of photon loss is easily solved by adding “repeaters” along the line — gadgets that receive weakened laser pulses and replace them with stronger ones. But ordinary repeaters don’t work for quantum systems such as single photons. For one thing, as Lukin points out, “If you sent a single photon, if it’s lost, there’s nothing left to amplify.” And if the photon does arrive at the node, the laws of quantum physics forbid fully copying its quantum state, so some of the photon’s information will inevitably be lost. In particular, if the photon was entangled with another photon somewhere else, the entanglement will be lost.
However, in 2001 Lukin and his collaborators envisioned a way to get around this problem by creating entangled pairs from photons that are far apart. If realized, their scheme would enable long-distance, quantum-encrypted communication.
If photons can be entangled over long distances, they could enable people to interact in ways that just aren’t possible within the realm of classical physics.
One power of entanglement is that it makes quantum teleportation possible. That’s a nearly magical way of transferring the quantum state of an object onto another object, possibly far away. Say Alice has a photon X, which she wants to teleport to Bob. Alice also has a photon Y, which is entangled with a photon Z owned by Bob. Alice then makes her two photons interact. That way, the state of X becomes entangled with the state of Y, and thus with the state of Z.
Alice then destroys X and Y by measuring their states, and she calls up Bob to tell him the results. Using that information, Bob can now twist the state of Z to make it identical to the original state of X. Alice has sacrificed the two photons in her possession, but as a result, Bob now has an exact copy of the original photon, photon X.
Lukin’s idea to create long-distance entanglement relies on yet another trick called entanglement swapping. In entanglement swapping, each of two sources produces a pair of entangled photons. The photons from the first source, say A and B, are not entangled with those from the second source, say C and D. Next, B and C are brought to the same detector. There, B and C interact and are destroyed, causing A and D to become entangled even though they were never close to each other.
Repeated applications of entanglement swapping over a chain of nodes can create pairs of entangled photons that are farther and farther from each other. Eventually, all photons are destroyed, except for the ones at the opposite ends of the chain. Those two end up entangled.
The method seems fail-safe on paper, but in practice, at each step at least some of the photons have a high chance of getting lost. But if one could somehow store pairs of photons that have successfully been entangled while other pairs are still being generated, long-distance entanglement would become possible at a reasonable speed. The key to quantum networking, then, is the ability to keep entangled photons in a sort of quantum RAM.
Catch and release
In 2001, Lukin and his collaborators, and an independent Harvard group led by Lene Hau, created the first rudimentary quantum memory, essentially by slowing light to a crawl inside clouds of atoms (Sience News: 1/27/01, p. 52). Since then, several groups have performed ever-more-advanced quantum-memory tricks. For example, groups led by Lukin, Alex Kuzmich of the Georgia Institute of Technology in Atlanta and Jeff Kimble of the California Institute of Technology in Pasadena were able to take a photon emitted by one atom cloud and store it in another atom cloud. And last September, Christopher Monroe and his team at the University of Maryland in College Park were able to entangle two qubits made of single ions.
Most recently, in the March 6 Nature, a team led by Kimble described what may be the most advanced kind of quantum memory to date. The researchers captured two entangled photon states in atom clouds and were able to release the states on demand. The photon states remained entangled during the capture and release. “We put entanglement into matter and then read it out,” says Kimble’s coauthor Julien Laurat, who was then a colleague of Kimble’s at Caltech but is now at the Pierre and Marie Curie University in Paris.
First, Kimble, Laurat and their colleagues shot photons one at a time at a semitransparent mirror. In this situation each photon, presented with the choice of bouncing off or zipping through, will not make up its mind right away. Instead, it will split its path into two, a superposition of both possibilities. Only when forced to interact, for example by running into a detector, will the photon appear all in one place or in the other. Because these two measurements are mutually exclusive rather than independent, the two paths are entangled states.
Next, the researchers trapped each of the virtual photons in a cloud of cesium atoms. Using a laser pulse, the physicists turned the clouds transparent, to allow the photons in. When the physicists turned the laser off, the clouds went back to opaque, trapping the photons inside. That forced the photons to virtually come to a stop, as their quantum states became enmeshed with the quantum states of the clouds. So the clouds themselves became entangled.
The team was able to store the quantum information — preserve the entanglement — for up to 10 microseconds. A second laser pulse made the gas transparent again, allowing the two virtual photons to escape and continue on their paths. The physicists were able to check that the two photon states were still entangled.
What’s missing now, Laurat says, is the ability to entangle two separate qubits by entanglement swapping. Still, Lukin says, the Caltech result was “an important step.”
In another recent result, Kuzmich and his collaborators induced a cloud of atoms to emit two photons at once, with wavelengths that were each optimized for different tasks — for transmission through an optical fiber and for storage in another qubit. Typically, single photons emitted by atom clouds tend to have wavelengths too short for efficient telecommunications, Kuzmich says.
According to Lukin, eventually, a practical quantum memory will need to store information on some kind of solid support. In this respect, he says, single-atom impurities in artificial diamond are one of the most promising candidates, since they would require no sophisticated laboratory to handle (Science News: 4/5/08, p. 216).
Most of the pieces needed to put together a quantum Internet now exist, and the challenge will be to make them work together efficiently. With the best technology available so far, a working prototype might end up costing as much as $100 million, and might be able to send just one qubit per minute, Kuzmich says.
A more sensible question might be: What would a quantum Internet be good for? So far, the main motivation for researchers has been to provide secure communications. But a quantum Internet might some day do things that, until recently, would have sounded like complete science fiction.