A serial triple quantum dot ͑TQD͒ electrostatically defined in a GaAs/ AlGaAs heterostructure is characterized by using a nearby quantum point contact as charge detector. Ground-state stability diagrams demonstrate control in the regime of few electrons charging the TQD. An electrostatic model is developed to determine the ground-state charge configurations of the TQD. Numerical calculations are compared with experimental results. In addition, the tunneling conductance through all three quantum dots in series is studied. Quantum cellular automata processes are identified, which are where charge reconfiguration between two dots occurs in response to the addition of an electron in the third dot.

No abstract

We study a double quantum dot (DQD) coupled to a strongly biased quantum point contact (QPC), each embedded in independent electric circuits. For weak interdot tunneling we observe a finite current flowing through the Coulomb blockaded DQD in response to a strong bias on the QPC. The direction of the current through the DQD is determined by the relative detuning of the energy levels of the two quantum dots. The results are interpreted in terms of a quantum ratchet phenomenon in a DQD energized by a nearby QPC.

Controlling coherent interaction at avoided crossings is at the heart of quantum information processing. The regime between sudden switches and adiabatic transitions is characterized by quantum superpositions that enable interference experiments. Here, we implement periodic passages at intermediate speed in a GaAs-based two-electron charge qubit and observe Landau-Zener-Stückelberg-Majorana (LZSM) quantum interference of the resulting superposition state. We demonstrate that LZSM interferometry is a viable and very general tool to not only study qubit properties but beyond to decipher decoherence caused by complex environmental influences. Our scheme is based on straightforward steady state experiments. The coherence time of our two-electron charge qubit is limited by electron-phonon interaction. It is much longer than previously reported for similar structures.LZSM interferometry is a double-slit kind experiment which, in principle, can be realized with any qubit, while the specific measurement protocol might vary. Our system is a charge qubit based on two-electron states in a lateral double quantum dot (DQD) embedded in a twodimensional electron system (2DES) (Fig. 1). Source and drain leads at chemical potentials µ S,D , each tunnel coupled to one dot, allow current flow by single-electron tunneling. Applying the voltage V = (µ S − µ D )/e = 1 mV across the DQD (Fig. 1B) we use this current to detect the steady-state properties of the driven system. We interprete the singlets, S 11 (one electron in each dot) and S 20 (two electrons in the left dot), as qubit states. They form an avoided crossing (Fig. 1C), described by the Hamiltonianwhere we consider a variable energy detuning (t) and a constant inter-dot tunnel coupling tuned to ∆ 13 µeV, corresponding to a clock speed of ∆/h 3.1 GHz, where h is the Planck constant. Let us first discuss a single sweep through the avoided crossing at = 0: as shown back in 1932 independently by Landau, Zener, Stckelberg, and Majorana it brings the qubit into a superposition state [1][2][3][4], the electronic analog to the optical beam splitter. The probability to remain in the initial qubit state, P LZ = exp(−π∆ 2 /2 v), thereby grows with the velocity v = d /dt, here assumed to be constant [1][2][3][4]. Because the relative phase between the split wavepackets depends on their energy evolutions, repeated passages by a periodic modulation (t) =¯ +A cos(Ωt), give rise to so-called LZSM quantum interference [2][3][4][5][6][7][8][9][10][11][12][13][14][15]. We present a breakthrough which * These authors contributed equally to this work.makes LZSM interferometry a powerful tool: it is based on systematic measurements together with a realistic model, which explicitly includes the noisy environment. We demonstrate how to decipher the detailed qubit dynamics and directly determine its decoherence time T 2 based on straightforward steady state measurements. Keeping the experiment simple we detect the dccurrent I through the DQD. It involves electron tunneling giving rise to the confi...