Small quantum networks operating as quantum thermo- dynamic machines

– We show that a 3-qubit system as studied for quantum information purposes can alternatively be used as a thermodynamic machine when externally driven in finite time and interfaced between two split baths. The spins are arranged in a chain where the working spin in the middle exercises Carnot cycles the area of which defines the exchanged work. The cycle orientation (sign of the exchanged work) flips as the difference of bath temperatures goes through a critical value. Back in 1824 Sadi Carnot published his ”Reflections on the Motive Power of Fire”, proposing ideas for a combustion engine [1]. His reasoning was based on his now famous ideal gas cycle analysis. With the advent of nano physics the intriguing question arises: Down to which scales can one reasonable define thermodynamic variables and thermodynamic machines? First attempts on the quantum level have been made in [2, 3]. Meanwhile such miniature machines have attracted quite some attention [4–9]. Artificial autonomous nanomotors powered by visible light have recently been demonstrated experimentally [10]. Nanomechanical devices have been of growing interest in the last couple of years [11]; the importance of cooling processes is being recognized with respect to quantum information processing [12]. Furthermore, it has been realized that strange analogies appear to exist between the basic concepts of quantum information theory like specific entanglement measures on the one hand side and thermodynamic concepts on the other side [13], the origin of which is still fairly unclear. Two level systems (TLS) like spins or qubits are the essential ingredient for quantum computation. Much effort has been directed towards control of small clusters and chains of qubits in quantum optical systems [14], nuclear magnetic resonance [15] and solid state systems [16]. A serious problem in any such realisation is the interaction of the respective quantum network with its environment. Here we argue that structurally identical quantum networks can be used as quantum information processors as well as quantum thermodynamic engines! This may not only shed new light on that intricate relationship, but may also give new directions for the application of such few-TLS structures, for which a number of different implementations are becoming available now. (∗) E-mail: [email protected]