The electrical noise of mesoscopic devices can be strongly influenced by the quantum motion of electrons. To probe this effect, we have measured the current fluctuations at high frequency (5 to 90 gigahertz) using a superconductor-insulator-superconductor tunnel junction as an on-chip spectrum analyzer. By coupling this frequency-resolved noise detector to a quantum device, we can measure the high-frequency, nonsymmetrized noise as demonstrated for a Josephson junction. The same scheme is used to detect the current fluctuations arising from coherent charge oscillations in a two-level system, a superconducting charge qubit. A narrow band peak is observed in the spectral noise density at the frequency of the coherent charge oscillations.

An on-chip detection scheme for high frequency signals is used to detect noise generated by a quantum dot formed in a single wall carbon nanotube. The noise detection is based on photon assisted tunneling in a superconductor-insulator-superconductor junction. Measurements of shot noise over a full Coulomb diamond are reported with excited states and inelastic cotunneling clearly resolved. Super-Poissonian noise is detected in the case of inelastic cotunneling. DOI: 10.1103/PhysRevLett.96.026803 PACS numbers: 73.63.Kv, 73.23.Hk, 73.63.Fg, 74.40.+k The study of shot noise, i.e., nonequilibrium current fluctuations due to the discreteness of charge carriers, is an important tool for studying correlations induced in mesoscopic transport by different types of interactions [1,2]. Current is characterized by Poissonian shot noise when transport is determined by an uncorrelated stochastic process. Electron-electron interactions, such as Coulomb repulsion or resulting from the Pauli exclusion principle, can correlate electron motion and suppress shot noise. The noise power density is defined as the Fourier transform of the current-current correlator S I ! R 1 ÿ1 d e i! h I t I t i. This definition is valid for both positive and negative frequencies !, corresponding to energy absorption or emission by the device [3][4][5]. When jeVj j@!j; k B T (V is the voltage bias and T the temperature), shot noise dominates over other types of noise and the power density has a white spectrum that can be expressed as S I ÿ! S I ! FeI. Here I is the average current and the Fano factor F indicates the deviation from Poissonian shot noise for which F 1. If the noise detector cannot distinguish between emission and absorption processes, a symmetrized version S symm I ! S I ! S I ÿ! is used. The Schottky formula S symm I 2eI refers to this symmetrized case.For electron transport through a quantum dot (QD), shot noise can be either suppressed or enhanced with respect to the Poissonian value. First, for resonant tunneling, when a QD ground state is aligned between the Fermi levels in the leads, the Fano factor can vary between 1=2 and 1. The exact value is determined by the ratio of tunneling rates between the dot and the two leads [6]. For strongly asymmetric barriers, transport is dominated by the most opaque one and shot noise is Poissonian. If the barriers are symmetric, the resonant charge state is occupied 50% of the time and a F 1=2 shot-noise suppression is predicted. Second, when the QD is in Coulomb blockade, first-order sequential tunneling is energetically forbidden. Transport can still occur via cotunneling processes [7], elastic or inelastic. These are second-order processes, with a virtual intermediate state, allowing electron transfer between the leads. The elastic process leaves the QD in its ground state and transport is Poissonian. Inelastic cotunneling switches the system from a ground to an excited state and can lead to super-Poissonian noise with a Fano factor up to F 3 [8]. Experiments have shown shot-noise suppress...

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...

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