Keeping Secrets With Quantum Mechanics

"Human ingenuity cannot concoct a cipher which human ingenuity cannot resolve."

—Edgar Allan Poe, 1841

Unbreakable codes are the holy grail of secure communications—and the promise of quantum cryptography.


It wasn't long after humans created symbols to communicate in written form that other symbols were devised to hide meaning. And ever since writing went secret, people have worked to lift that veil of secrecy. But today the stakes are ever-higher. Throughout most of cryptography's roughly 4,000-year history, the efforts to scramble and unscramble words occurred at the highest levels of society. Governments, military groups, and even noblemen had secrets to keep. Nowadays encryption touches nearly everyone in modern society—all who use credit cards or cloud storage, send email, or bank online—or offline, as banks send electronic communications internally. All that information is kept secure by complex algorithms such as the factorization of the product of two huge prime numbers.


So how safe is our info?


Edgar Allan Poe's argument that humans can crack any human-devised cipher "nicely summarized the history of code-making and code-breaking, all the way from ancient Greece until the advent of quantum cryptography," says quantum physicist Artur Ekert, a professor at Oxford University. "Quantum cryptography is different."


And what's different is the difference between math and physics.


"When it comes to the laws of physics no human ingenuity can find a way around them," Ekert says. "There is no way to cheat gravity by levitation or otherwise, and, by the same token, there is no way to cheat quantum phenomena."


Because information always has a physical representation, "the same tool physicists use to understand the nature of reality," says Ekert, one of the inventors of quantum cryptography, "can be used to protect information."


That tool is quantum mechanics—and its inherent secrecy and bizarreness, what Einstein called "spooky action at a distance."


Quantum Cryptography: It Takes Two to Entangle


Say Alice and Bob need to pass information to each other secretly. They can create a quantum key (in binary code, the language of computers) by taking advantage of quantum entanglement, a state in which two subatomic particles maintain a correlation with each other even if they are separated.


In essence, to create their key, which is theoretically uncrackable, Alice and Bob measure certain characteristics of a set of photons and then share with each other some (but not all!) of the measured data. If they find a low error rate they know—thanks to quantum theory—that the remaining data is secure. (See video below for more detail on how this works.) A high error rate indicates eavesdropping and so they would scrap that key.


Of course, in practice, the code is not unbreakable. Simple human error can often aid a resourceful hacker. If Eve hacks into Alice's computer and reads Alice's messages as she types them, quantum cryptography cannot safeguard that.


"Quantum cryptography mostly just addresses Eve trying to tap into the fiber-optics connecting Alice and Bob," explains Daniel Gottesman, a quantum computation researcher at Canada's Perimeter Institute and a participant in this year's quantum program on May 31.


So when it comes to keeping our information and data more secure, quantum cryptography may hold the key. The quantum code that may one day lock our data up is theoretically uncrackable, says Ekert, who adds, "in theory there is no difference between theory and practice, but in practice there is."


So even quantum when it smashes into the real world doesn't look to be totally uncrackable. Still, scientists like Gottesman and Ekert are pretty excited by the prospect.


"It is groundbreaking and fascinating stuff," said Ekert in an email. "It may take a few more years to see it in action, but eventually it will take over classical methods of encryption."