Original version of This story Featured in Quanta Magazine.
For example, say you want to send a private message, vote secretly, or sign a document securely. When you use a computer to perform these tasks, you rely on encryption to keep your data safe. Because encryption must withstand attacks from code breakers using their own computers, modern encryption methods rely on assumptions about mathematical problems that are difficult for computers to solve.
But as cryptographers laid the mathematical foundations of this approach to information security in the 1980s, several researchers discovered that computational difficulty isn’t the only way to keep secrets. Quantum theory, originally developed to understand the physics of atoms, turned out to have deep connections to information and cryptography. Researchers found ways to base the security of a few specific cryptographic tasks directly on the laws of physics. But these tasks were strange exceptions. For all other tasks, there seemed to be no alternative to classical computational approaches.
By the end of the 2000s, quantum cryptography researchers thought that was the end of the story. But in the past few years, the field has once again undergone major changes.
“There’s a reordering of what we believe is possible with quantum cryptography,” said Henry Yuen, a quantum information theorist at Columbia University.
In a series of recent papers, researchers have shown that most cryptographic tasks can be performed securely even in a virtual world where virtually everything is computationally easy – it’s only the difficulty of the special computational problems involved in quantum theory itself that matters.
“The assumptions required can be much weaker,” says Fermi Ma, a quantum cryptographer at the Simons Institute for Theory of Computation in Berkeley, Calif. “This gives us new insight into just how hard computation is.”
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The story begins in the late 1960s, when Stephen Wiesner, a physics graduate student, began thinking about the disruptive nature of measurements in quantum theory. Measuring a system governed by the rules of quantum physics changes the quantum state that mathematically describes its makeup. This quantum measurement perturbation was an obstacle for most physicists. Wiesner, who took an unorthodox, information-centric view of quantum theory, wondered if he could make it useful, perhaps as a built-in tamper-proof feature for sensitive data.
But Wiesner’s ideas were too ahead of his time, so he left academia after graduate school. Luckily, he discussed his ideas with his friend, physicist Charles Bennett. Bennett tried for a decade to interest others in the subject, but without success. Finally, in 1979, while swimming off the coast of Puerto Rico during a conference, Bennett met computer scientist Gilles Broussard. Together, the two wrote a groundbreaking paper that described a new approach to an important cryptographic task. Their protocol was based on quantum measurement disturbances and required no assumptions about the difficulty of the computational problem.
