Sending Messages with a Quantum Seal

In today’s information society, the transfer, processing, and protection of information has become an essential part of our daily lives. In many cases, a recipient must confirm the origin of a message, and this usually relies on a so-called digital signature, which is a secret code sent along with the message that identifies the sender. To guard against forgery, classical digital signature schemes rely on computationally intractable problems, like factoring extremely large numbers, but there’s no proof that these codes couldn’t be cracked. Quantum digital signatures (QDS) [1] could offer unconditional security by using quantum states that cannot be fully characterized by someone hoping to forge a signature. The price to pay for going quantum is that these quantum states are fragile and thus short-lived. Previous QDS schemes required using quantum memories, which are currently not robust enough. However, another scheme has recently been proposed that directly measures—rather than stores—quantum states in order to obtain partial information about them [2]. Reporting in Physical Review Letters, Robert Collins of Heriot-Watt University, UK, and his colleagues have successfully implemented a variant of this memory-free QDS protocol in an optical system [3].