Synopsis: Security Breach for Qubit Communication

Theorists predict that quantum information can’t be securely “locked” in a vault.

Data can be protected from spies via cryptography. Let’s say a person—Alice—has a message she wants to keep secret. Alice decides to lock the message in a bank’s vault. But Alice doesn’t trust that the bank’s owner—Bob—won’t open the vault and read her secret, so she encrypts the message before she hands it over. Without the decryption key, Bob can’t read the coded message, and Alice’s secret stays safe. But a team of researchers now predicts a failure in this “masking” protocol when the message is stored in a quantum system, such as a qubit, leaving the secret readable by Bob.

The masking process stands true when encrypting classical information, such as a bit that can be either a 0 or a 1. In this case, the original message exists only in the correlations between the key and the coded message. Kavan Modi from Monash University in Australia and his colleagues, however, found that masking can fail when the information is stored in a quantum system that is in a superposition of 0 and 1.

In the team’s scenario, Alice uses a quantum key to encrypt a qubit, which she leaves in Bob’s bank. Bob should need Alice’s key to read the coded qubit. But the team’s calculations show that remnants of the original message always remain in the coded qubit itself, allowing Bob to read it whenever he wants. The team doesn’t yet understand why masking fails for quantum information, but they did find that Bob’s “spying” can be thwarted if a third person—and a third qubit—takes part in the encryption process.

This research is published in Physical Review Letters.

–Katherine Wright

Katherine Wright is a Contributing Editor for Physics.


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