Counterfactual Quantum Cryptography

Quantum cryptography allows one to distribute a secret key between two remote parties using the fundamental principles of quantum mechanics. The well-known established paradigm for the quantum key distribution relies on the actual transmission of signal particle through a quantum channel. In this Letter, we show that the task of a secret key distribution can be accomplished even though a particle carrying secret information is not in fact transmitted through the quantum channel. The proposed protocols can be implemented with current technologies and provide practical security advantages by eliminating the possibility that an eavesdropper can directly access the entire quantum system of each signal particle.

Figure 1

Schematic of the proposed QKD system. A single-photon pulse entering a Michelson-type interferometer is split into two pulses by a beam splitter BS and travels through two paths $a$ and $b$. The interferometer is adjusted using an optical delay OD. Therefore, if the bit values chosen at random by Alice and Bob are different, the two split pulses are recombined in the BS and the single photon is detected at detector $D2$ with certainty as a result of constructive interference. However, if the two bit values are equal, a split pulse going through path $b$ is blocked by detector $D3$. Consequently, the interference is destroyed and the photon can be detected at detector $D1$ with a finite probability. In this case, the photon has been completely isolated from the outside of Alice’s secure station, as it has traveled through only path $a$.