Quantum fluctuations at the physical layer of encryption enable ultrasecure communications with highly competitive performance metrics.

Perfect information- theoretical security requires that the meaning of an encrypted message transmitted from point A to point B be statistically independent of the ciphertext in which that message is embedded. In other words, possession and analysis of the ciphertext must yield no information about the message sent. This article briefly describes cryptographic protocols exhibiting perfect, or nearperfect, security before addressing a new quantum data encryption protocol that employs quantum noise of light at the physical layer to buttress security based on mathematical complexity. This new protocol is called Keyed Communication in Quantum Noise, or KCQ. KCQ does not presently guarantee flawless informationtheoretical security; however, because of KCQ’s physical-layer encryption in the quantum noise of light, some scientists believe that it enables better security than current secure communications systems based solely on mathematical complexity.

Virtually no electronic communication transmitted between individuals, in uniform or not, can be assured perfect information-theoretical security in the ideal sense defined above. However, communications using either the “one-time pad” or the quantum key generation protocol, called BB84, are notable exceptions. Scientists can prove one-time pad transmissions to be perfectly secure, and they believe BB84 provides near-perfect security.^1,2 A onetime pad encrypts a plaintext message by combining it with a random bitstream generated by an automated physical process. Cryptographers refer to this encrypting random bitstream as a secret key. The recipient can decrypt the resultant ciphertext, also a randomly generated bitstream, using the same secret key that encrypted the message, thus recovering the plaintext content. In cryptographic systems with perfect security, a plaintext item might very well be a fresh key, uncompromisable even in the hands of an enemy who has managed to obtain the ciphertext and secret key used for secure distribution to allies. The onetime pad is a symmetric cipher, wherein parties (allies) A and B share a secret key. In contrast, BB84 becomes a symmetric cipher through its inherent generation of a fresh key, engendered on the spot in a two-way communications link between A and B. That fresh session key can subsequently encrypt other keys for distribution, as indicated in Figure 1. Since each transmission event employs just a single photon, however, BB84 is extremely sensitive to noise and loss. Necessary error correction and privacy amplification mechanisms require additional bits, reducing the effective bit rate and severely limiting the effective transmission range in both wired and wireless implementations. Consequently, communications engineers cannot easily incorporate the BB84 protocol into existing networks.

KCQ is an alternative, newer physical-layer quantum communication protocol— one that has received much less attention from the commercial press than single-photon quantum protocols such as BB84 have garnered.^3,4,5,6 KCQ employs the radiation states of multiple photons emitted by ordinary lasers as the information transport medium. These radiation states are called coherent states of light. In terms of quantum mechanics, they are fuzzy waves in that their amplitudes, phases, and polarization states do not exist in crisply measured quantities. Rather, those observable characteristics are stochastic (random) variables possessing mean values and equally important variances from those mean values. The measurement fluctuation in amplitude, phase, and polarization is called quantum noise. Truly a fundamental physical random process, quantum noise is irreducible; it cannot be filtered away, not even in principle.