Density-Matrix Algorithm for Phonon Hilbert Space Reduction in the Numerical Diagonalization of Quantum Many-Body Systems

Combining density-matrix and Lanczos algorithms we propose a new optimized phonon approach for finite-cluster diagonalizations of interacting electron-phonon systems. To illustrate the efficiency and reliability of our method, we investigate the problem of bipolaron band formation in the extended Holstein Hubbard model.Comment: 14 pages, 6 figures, Workshop on High Performance Computing in Science and Engineering, Stuttgart 200

Quantum oscillations in a molecular magnet

Bertaina S, Gambarelli S, Mitra T, Tsukerblat B, Müller A, Barbara B. Quantum oscillations in a molecular magnet. NATURE. 2008;453(7192):203-206.The term 'molecular magnet' generally refers to a molecular entity containing several magnetic ions whose coupled spins generate a collective spin, S ( ref. 1). Such complex multi- spin systems provide attractive targets for the study of quantum effects at the mesoscopic scale. In these molecules, the large energy barriers between collective spin states can be crossed by thermal activation or quantum tunnelling, depending on the temperature or an applied magnetic field(2-4). There is the hope that these mesoscopic spin states can be harnessed for the realization of quantum bits 'qubits', the basic building blocks of a quantum computer - based on molecular magnets(5-8). But strong decoherence(9) must be overcome if the envisaged applications are to become practical. Here we report the observation and analysis of Rabi oscillations ( quantum oscillations resulting from the coherent absorption and emission of photons driven by an electromagnetic wave(10)) of a molecular magnet in a hybrid system, in which discrete and well-separated magnetic V-15(IV) clusters are embedded in a self- organized non- magnetic environment. Each cluster contains 15 antiferromagnetically coupled S = 1/ 2 spins, leading to an S = 1/ 2 collective ground state(11-13). When this system is placed into a resonant cavity, the microwave field induces oscillatory transitions between the ground and excited collective spin states, indicative of long-lived quantum coherence. The present observation of quantum oscillations suggests that low- dimension self- organized qubit networks having coherence times of the order of 100 mu s ( at liquid helium temperatures) are a realistic prospect