Experimental algorithmic cooling of spins - a novel polarization-enhancement method

Full Text:Algorithmic cooling is a promising new spin-cooling approach devised by Boykin, Mor, Rowchodhury, Vatan and Vrijen (PNAS, Vol. 99, p. 3388, 2002). This approach suggests data compression methods in open systems. It reduces the entropy of spins on long molecules to a point far beyond Shannon's bound on reversible entropy manipulations, thus increasing their polarization. The algorithm recursively employs two steps: The first is an adiabatic entropy compression of the computation qubifs of the system. The second step is an isothermal heat transfer from the system to the environment through a set of reset qubifs that reach thermal relaxation rapidly. Interestingly, the interaction with the environment, usually a most undesired interaction, is used here to our benefit, allowing a cooling mechanism, which is useful for initializing NMR systems in general and NMR quantum computers in particular. A later work, ''Algorithmic cooling of spins: a practicable method for increasing polarization'' (e-print: quant-ph/0401135, to be published in IJQI) presented a much improved algorithm, yielding a significant spin-polarization increase already on small molecules. To allow experimental algorithmic cooling, the thermalization time of the reset qubits must be much shorter than the thermalization time of the computation qubits. We investigated the effect of the paramagnetic material Chromium Acetylacetonate on the thermalization times of computation qubits (carbons) and reset qubit (hydrogen). We report here the accomplishment of an improved ratio of the thermalization times from TI(H)/TI(C) of approximately 5 to around 15. The magnetic ions from the Chromium Acetylacetonate interact with the reset qubits, reducing their ther-malization time, while their effect on the less exposed computation qubits is found to be weaker. An experimental demonstration of non-adiabatic cooling by thermalization in a nuclear magnetic resonance apparatus (based on using these magnetic ions) yielded cooling beyond Shannon's bound