We present the experimental realization of a quantum dot (QD) operating as a high-frequency noise detector. Current fluctuations produced in a nearby quantum point contact (QPC) ionize the QD and induce transport through excited states. The resulting transient current through the QD represents our detector signal. We investigate its dependence on the QPC transmission and voltage bias. We observe and explain a quantum threshold feature and a saturation in the detector signal. This experimental and theoretical study is relevant in understanding the backaction of a QPC used as a charge detector. DOI: 10.1103/PhysRevLett.96.176601 PACS numbers: 72.70.+m, 73.21.La, 73.23.ÿb On chip noise detection schemes, where device and detector are capacitively coupled within submillimeter length scales, can benefit from large frequency bandwidths. This results in a good sensitivity and allows one to study the quantum limit of noise, where an asymmetry can occur in the spectrum between positive and negative frequencies. The asymmetry, caused by the difference in the occurrence probability of emission and absorption processes, can be probed using quantum detectors [1]. In this Letter, we investigate the transport through a quantum dot (QD) under the influence of high-frequency irradiation generated by a nearby quantum point contact (QPC). The QPC current fluctuations induce photoionization, taking the QD out of Coulomb blockade, thereby allowing sequential tunneling through an excited state [2,3]. By studying this transient current while changing the QPC parameters, we show that we can perform high-frequency shot noise detection in the 20 -250 GHz frequency range.The granularity of the electrons and the stochastic nature of their transport lead to unavoidable temporal fluctuations in the electrical current, i.e., shot noise [4]. For systems where transport is completely uncorrelated, such as vacuum diodes [5], noise is characterized by a Poissonian value of the power spectral density S I 2eI dc . Here we use the QPC as a well-known noise source. When the QPC is driven out of equilibrium, i.e., by applying an electrochemical potential difference between the source and the drain of the QPC, a net current will flow if the QPC is not pinched off. At zero temperature (k B T eV QPC ), the stream of incident electrons is noiseless, and shot noise, due to particle partition, dominates. The electrons are either transmitted or reflected, with a probability depending on the QPC transmission T. The power density can be written as S I 2eI dc F, where F P N i 0 T i 1 ÿ T i = P N i 0 T i is the Fano factor and the summation is over transport channels with transmissions T i . In this case, correlations in the transport are introduced by the Pauli exclusion principle, resulting in a suppression of noise below the Poissonian value. Thus, shot noise vanishes if all the 1D quantum channels either fully transmit (In many recent experiments, QPCs are used as charge detectors [7]. In this context, our experiment provides information regarding the ba...

Recent experiments by Hayashi et al. [Phys. Rev. Lett. 91, 226804 (2003)] demonstrate coherent oscillations of a charge quantum bit (qubit) in laterally defined quantum dots. We study the intrinsic electron-phonon decoherence and gate performance for the next step: a system of two coupled charge qubits. The effective decoherence model contains properties of local as well as collective decoherence. Decoherence channels can be classified by their multipole moments, which leads to different lowenergy spectra. It is shown that due to the super-Ohmic spectrum, the gate quality is limited by the single-qubit Hadamard gates. It can be significantly improved, by using double-dots with weak tunnel coupling.

Abstract. This is an introduction to elementary decoherence theory as it is typically applied to superconducting qubits.Abbreviations: SQUID -superconducting quantum interference device; qubit -quantum bit; TSS -two state system

We perform a nonperturbative analysis of a charge qubit in a double quantum dot structure coupled to its detector. We show that strong detector-dot interaction tends to slow down and halt coherent oscillations. The transitions to a classical and a low-temperature quantum overdamping (Zeno) regime are studied. In the latter, the physics of the dissipative phase transition competes with the effective shot noise.Comment: 5 pages, 4 figure