How it works
Single photons are transmitted across a free-space optical link using quantum state preparation. Each photon carries a randomly prepared quantum state. Any attempt to intercept alters the state in a detectable way.
A gain-switched laser generates photon pairs with intrinsically random phase. Optical elements convert these into quantum states, removing the need for high-speed modulation.
The receiver measures each photon in a randomly selected basis, determining the bit value according to the chosen setting.
The sender and receiver reveal only their basis choices. Measurements taken in matching bases are kept, forming the raw key.
A small portion of the raw key is compared to calculate the quantum bit error rate (QBER). If the QBER is acceptable, errors are corrected via an authenticated classical channel.
The reconciled key is passed through a hashing function to remove any partial information available to an eavesdropper. The final symmetric key is ready for for use in VPN, HSM, or OTP systems.
Tamper-evident by physics
Any tap raises the error rate (QBER). If it exceeds the threshold, the session is dropped and no key is produced.
Mini‑glossary
- Photon
- A single particle of light.
- Basis
- The measurement setting used to read a photon.
- Sifting
- The process of retaining only the bits for which the sender and receiver used the same basis.
- Privacy amplification
- A hashing process that removes any information an eavesdropper could have gained.