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We present experimental results in which the unexpected zero-two transition of a circuit composed of two inductively coupled transmons is observed. This transition shows an unusual magnetic flux dependence with a clear disappearance at zero magnetic flux. In a transmon qubit the symmetry of the wave functions prevents this transition to occur due to selection rule. In our circuit the Josephson effect introduces strong couplings between the two normal modes of the artificial atom. This leads to a coherent superposition of states from the two modes enabling such transitions to occur.

We present an experimental and theoretical analysis of the self- and cross-Kerr effect of extended plasma resonances in Josephson junction chains. We calculate the Kerr coefficients by deriving and diagonalizing the Hamiltonian of a linear circuit model for the chain and then adding the Josephson non-linearity as a perturbation. The calculated Kerr-coefficients are compared with the measurement data of a chain of 200 junctions. The Kerr effect manifests itself as a frequency shift that depends linearly on the number of photons in a resonant mode. By changing the input power on a low signal level, we are able to measure this shift. The photon number is calibrated using the self-Kerr shift calculated from the sample parameters. We then compare the measured cross-Kerr shift with the theoretical prediction, using the calibrated photon number., Comment: 10 pages, 9 figures

We analyze properties of excitations due to voltage pulses applied to a 1D noninteracting electron gas, assuming that the integral of the voltage over time is equal to the unit of flux. We show that the average charge transfer due to such pulses does not depend on the pulse shape. For pulses with a Lorentzian profile, we prove the single-particle nature of the electron and the hole excitations., Comment: 7 pages, 2 figures

The metallic tip of a scanning force microscope operated at 300 mK is used to locally induce a potential in a fully controllable double quantum dot defined via local anodic oxidation in a GaAs/AlGaAs heterostructure. Using scanning gate techniques we record spatial images of the current through the sample for different numbers of electrons on the quantum dots (i.e., for different quantum states). Owing to the spatial resolution of current maps, we are able to determine the spatial position of the individual quantum dots, and investigate their apparent relative shifts due to the voltage applied to a single gate.

Incorporating a variable capacitance diode into a radio-frequency matching circuit allows us to in-situ tune the resonance frequency of an RF quantum point contact, increasing the versatility of the latter as a fast charge sensor of a proximal quantum circuit. The performance of this method is compared in detail to conventional low-frequency charge detection. The approach is also applicable to other RF-detection schemes, such as RF-SET circuits., Comment: 4 pages, 3 figures

The detection of the quantum dot charge state using a quantum point contact charge detector has opened a new exciting route for the investigation of quantum dot devices in recent years. In particular, time-resolved charge detection allowed the precise measurement of quantum dot shot noise at sub-femtoampere current levels, and the full counting statistics of the current. The technique can be applied to different material systems and holds promise for the future application in quantum dot based quantum information processing implementations. We review recent experiments employing this charge detection technique, including the self-interference of individual electrons and back-action phenomena.

We measure the shot noise of a quantum point-contact using a capacitively coupled InAs double quantum dot as an on-chip sensor. Our measurement signals are the (bidirectional) interdot electronic tunneling rates which are determined by means of time-resolved charge sensing. The detector frequency is set by the relative detuning of the energy levels in the two dots. For nonzero detuning, the noise in the quantum point contact generates inelastic tunneling in the double dot and thus causes an increase in the interdot tunneling rate. Conservation of spectral weight in the dots implies that this increase must be compensated by a decrease in the rate close to zero detuning, which is quantitatively confirmed in our experiment., Comment: 5 pages, 4 figures