Quantum Cryptography


A future quantum computer will be able to solve certain complex problems such as the factorization of large numbers much faster than a regular computer. This leaves many current cryptographic schemes insecure. Specifically, the RSA algorithm which is used in today's browsers will be rendered unsafe.

Fortunately, quantum mechanics provides a solution to this problem as well: Quantum Cryptography. An encrypted communication even sent over an classical channel can be proven to be absolutely safe if a random cryptographic key is used by both parties. This is true once the key is as long as the message and it is only used once (one-time-pad). The task at hand is to devise a means of distributing random keys in a secure manner over a quantum channel. Thus, quantum cryptography really means quantum key distribution (QKD). Many protocols exist that enable such a QKD. An example is the BB84 protocol.

The problem is that one needs powerful sources of entangled photons or single photon sources which produce single photons on demand. We are interested in:

  1. generation of entangled photons in parametric downconversion and related schemes
  2. single photon generation in doped photonic crystals


Prof. Dr. Thomas Walther

Laser und Quantenoptik
Institut für Angewandte Physik
Fachbereich 05 - Physik
Technische Universität Darmstadt
Schlossgartenstr. 7
D-64289 Darmstadt

+49 6151 16-20831 (Sekretariat)

+49 6151 16-20834




Optics in our Time

Open Access Buch über den aktuellen Stand der Optik in Grundlagenforschung und Anwendungen

Im Springer Verlag ist zur Feier des "Jahr des Lichts 2015" ein Open Access Buch... weiter......
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