The effectiveness of "Quantum machines"

The more power a steam engine can get out of a given amount of fuel, the better it is. But the maximal efficiency of every machine is limited by the laws of physics. This is also true for the efficiency of "quantum machines" which are built to transfer certain quantum states into other quantum states. But until now it was unknown how big the best possible efficiency might be in practice. Scientists of the Technische Universität München and Harvard University have now discovered fundamental boundaries for the efficiency of such systems under realistic conditions and they could also experimentally realize the predicted maximum using a model system.

A controlled steering of quantum states with minimum of losses is already nowadays very important for modern spectroscopy. In structure determination by nuclear magnetic resonance, for example, the spin quantum states of atomic nuclei are transferred to other nuclei by radio frequency irradiation at the presence of high magnetic fields. Additionally the manipulation of quantum states are an essential requirement for the realisation of future technologies like quantum information processing. But quantum states can never be manipulated without losses as they can not be totally isolated from their environment. This "loss of information" to the environment is called relaxation or decoherence and is comparable to friction losses of mechanical machines. 

Although the quantum mechanics is already over 100 years old, the question of the best possible efficiency of a "quantum machine" in the presence of relaxation was so far an unsolved problem. This is comparable to the time before the discovery of fundamental boundaries of steam engine efficiency for the conversion of heat into mechanical energy. More than 100 years after the discovery of the steam engine this still remained an open question: "Despite of various reseach projects about heat engines, ... the progress regarding the theory about them is still quite limited and attempts to improve them are most of the time driven by chance", wrote Sadi Carnot 1824. Nowadays it is clear, that the maximum efficiency of a heat engine does not depend on the skills of the enginers but on the fundamental laws of the thermodynamic co-founded by Carnot.

With the technique presented in the actual edition of the Proceedings of the National Academy of Science (USA) by Prof. Navin Khaneja (Harvard), Dr. Burkhard Luy and Prof. Steffen Glaser (TU München) it is now for the first time possible to theoretically determine the physical boundaries for manipulation of quantum states in presence of realistic relaxation effects. It was surprising for the scientists, that the efficiency of hitherto existing methods was far below the so far unknown boundary.

The research group from Harvard and Munich has been able to implement its findings also practically and they could increase the efficiency of a basic quantum machine to the maximal achievable value. At the Bavarian Nuclear Magnetic Resonance Center in Garching they optimized an innovative experiment for the transfer of nuclear spin states in an organic molecule (a formic acid salt) by radio frequncy irradiation. This technique is supposed to speed-up very much the nuclear magnetic resonance spectroscopy of biomolecules and to make it more sensitive and therefore simplyfies the structure determination of the structures of big molecules.

Original publication: Boundary of quantum evolution under decoherence N. Khaneja, B. Luy, S. Glaser, Proc. Natl. Acad. Sci. USA 100, 13162-13166, 2003.

Preprint: quant-ph/0302060