At the begining of the last century probably the most striking physical theory have appeared: Quantum Mechanics. During decades it has become the one of the best tested theories and it has influenced many practical domains. However, in some sense it is still the least understood theory. The reason is its rather contra-intuitive nature. Nevertheless, a present-day quantum-optical technology enables us to test experimentally even the most intimate - but the most fundamental - features of quantum mechanics like quantum interference and quantum entanglement.

Quantum interference is a manifestation of the wave behavior of particles (or indivisible energy quanta). Perhaps everybody knows that a strong "classical" coherent radiation interferes - i.e. its amplitudes are added in a destructive or constructive way. But even if the radiation is so dim that there is at most one photon with a high probability in the interferometer one can still observe an interference pattern. A contra-intuitive fact is that the single photon must go somehow through both the arms of the interferometer and interfere with itself. However, if one is able to determine which path the photon chose interference disappears. Interference is the essentional component of many experiments carried out in our laboratory. Quantum cryptography and quantum phase estimation can serve as examples.

Entanglement is perhaps the most misterious quantum phenomena. It is a kind of quantum correlation that is stronger, in a certain sense, than any classical one. If some quantum system, consisting of several subsystems, is in an entangled state (even in a pure entangled state) its individual subsystems cannot be described by pure quantum states. Using Schrodinger's words: The best possible knowledge of the whole does not include the best possible knowlwdge of its parts. Entangled states can be used to test so called Bell inequalities in order to judge between quantum theory and local-realistic theories with hidden variables. They can also serve for quantum key distribution and quantum teleportation. Entanglement is an essential ingredient in quantum computation and information processing. Experimental quantum optics offers an efficient tool for preparing entangled pairs of photons - the spontaneous process of down conversion in non-linear optical crystals. It enables us to create pairs of photons with entangled wavelengths, directions and/or polarizations. In the following figure you can see the optical field rising from KDP crystal.

An important part of the research in our laboratory represents quantum cryptography. Quantum cryptography is a method for secure communication. Better say, it is a way how to solve the problem of secret key distribution. Its security depends only on the validity of quantum theory. I.e., it is guarantied directly by the laws of physics. This is a substantial difference from any classical cryptographic techniques.

The quantum key distribution procedure (QKD) allows two parties to establish a common random secret key. It takes advantage of the fact that quantum mechanics does not allow us to distinguish non-orthogonal states with certainty. Within the framework of classical physics, information encoded into a property of a classical object, can be acquired without affecting the state of the object. However, if information is encoded into a property of a quantum object, any attempt to discriminate its non-orthogonal states inevitably changes the original state with a nonzero probability. And since eavesdropping is also governed by the laws of quantum mechanics, these changes cause errors in transmissions and reveal the eavesdropper. QKD cannot prevent from eavesdropping, but it enables legitimate users to discover it. If any eavesdropping is detected, the key is simply thrown away and a new one is generated. No leakage of information occurs, since the key is just a random sequence.

Next Figures show quantum identification system build in our laboratory in recent past.

Miloslav Dusek