Keynote Speaker
 Jörg Kotthaus Ludwig-Maximilians-Universität München |
Title Non-equilibrium interaction between quantum devices Abstract Non-equilibrium interactions between quantum devices are unavoidable during read-out processes, as exemplified by the readout of the state of charge and spin qubits realized in semiconductor quantum dots. Here one often employs a biased quantum point contact (QPC) as charge detector and has to consider potential back-action processes acting on the quantum dot circuitry(1). Recent experiments are reviewed that illuminate such back-action processes and attempt to clarify their physical origins. In particular we discuss the action of a biased QPC as emitter of energy quanta onto an electrically separated but close-by detector circuit containing either a double quantum dot(2) or an individual QPC(3). In either case current driven through the strongly biased QPC emitter results in a driven current in the adjacent and unbiased detector circuit. Employing a QPC as detector the driven current is always opposite to the drive current in the emitter QPC(3). If the detector circuit contains a double quantum dot the direction of the driven current depends on the asymmetry between discrete quantum levels in the double quantum dot and is interpreted as a quantum ratchet phenomenon2. In either case the origin of the driven current is explained by preferred absorption of quanta with a typical energy of 1 meV in the detector circuit. To further study such non-equilibrium interactions a mesoscopic circuit is developed in which injection across a tunable barrier ballistically emits electrons with a well defined excess energy(4). The emitter circuit is separated by a large potential barrier, impenetrable for electrons, from a detector circuit in which energy quanta transferred across this barrier can excite electrons out of equilibrium. Another tunable energy barrier within the detector circuit is used to analyze the acquired excess energy of those electrons, again yielding typical energy values of about 1 meV, even though the excess energies in the emitter circuit reach values up to several ten meV. The observations are well explained by the relevant energy quanta being acoustic interface phonons emitted by inelastic back-scattering of the injected electrons with momentum transfer of about twice the Fermi momentum. These results demonstrate that acoustic phonons emitted by non-equilibrium electrons can become an important source of decoherence via back-action. The work reviewed is based on collaborative work with Vadim Khrapai, Stefan Ludwig, Georg Schinner and Daniela Taubert. The heterostructures employed in the studies were kindly provided by H. P. Tranitz and W. Wegscheider. Financial support of the Deutsche Forschungsgemeinschaft via SFB 631 and DIP DE 730/5-1, the excellence cluster "Nanosystems Initiative Munich" (NIM), and the Alexander von Humboldt foundation is gratefully acknowledged. References: |
