Experimental setup elements in an optical laboratory.
The security of modern cryptographic systems used today is based on unproven mathematical assumptions that could be disproved at any time. Moreover, future quantum computers will be able to systematically break our public key cryptography. In contrast, quantum cryptography is based on fundamental laws of quantum mechanics: the no-cloning theorem, which states that no unknown quantum state can be copied perfectly and the Heisenberg uncertainty, which states that not all properties of a quantum system can be readout simultaneously. The encoding in single photons is crucial, as only then the information is protected from eavesdropping attacks.
Many implementations of quantum cryptography utilize weak coherent states. In order to reduce the amount of multi-photon pulses, a very low mean photon number is used, which implicates that most pulses are actually empty and carry no information. Instead, we are using single photons emitted from fluorescent defects in solid-state crystals. By using a true single photon source, we can enhance the data rate significantly. While our current experiments are still in a laboratory, in the near future we will also test free space links in the field and establish quantum links between distant buildings.