Counting statistics of single electron transport in a quantum dot

We have measured the full counting statistics of current fluctuations in a semiconductor quantum dot (QD) by real-time detection of single electron tunneling with a quantum point contact. This method gives direct access to the distribution function of current fluctuations. Suppression of the second moment (related to the shot noise) and the third moment (related to the asymmetry of the distribution) in a tunable semiconductor QD is demonstrated experimentally. With this method we demonstrate the ability to measure very low current and noise levels.     

Detecting charge noise with a Josephson junction: a problem of thermal escape in presence of non-Gaussian fluctuations

Motivated by several experimental activities to detect charge noise produced by a mesoscopic conductor with a Josephson junction as on-chip detector, the switching rate out of its zero-voltage state is studied. This process is related to the problem of thermal escape in presence of non-Gaussian fluctuations. In the relevant case of weak higher than second order cumulants, an effective Fokker-Planck equation is derived, which is then used to obtain an explicit expression for the escape rate. Specific results for the rate asymmetry due to the third moment of current noise allow to analyze experimental data and to optimize detection circuits.     

Cryogenics and Einstein Telescope

The dominant noises which limit the present sensitivity of the gravitational wave detectors are the thermal noise of the suspended mirrors and the shot noise. For the third generation of gravitational wave detectors as the Einstein Telescope (ET), the reduction of the shot noise implies to increase the power stored in the detector at 1 MW level and, at the same time, to compensate the huge optic distortion due to induced thermal lensing. At low temperature it is possible to reduce both these effects. However, lowering the temperature of the test masses without injecting vibration noise from the cooling system is a technological challenge. We review here the thermal noise impact on the ultimate ET sensitivity limit and we discuss possible cryogenic configurations to cool the mirror.     

Observation of shot-noise-induced asymmetry in the Coulomb blockaded Josephson junction

We have investigated the influence of shot noise on the IV curves of a single mesoscopic Josephson junction. We observe a linear enhancement of zero-bias conductance of the Josephson junction with increasing shot-noise power. Moreover, the IV curves become increasingly asymmetric. Our analysis on the asymmetry shows that the Coulomb blockade of Cooper pairs is strongly influenced by the non-Gaussian character of the shot noise.     

Observation of photon-assisted noise in a diffusive normal metal-superconductor junction

We report measurements of nonequilibrium noise in a diffusive normal metal-superconductor (N-S) junction in the presence of both dc bias and high-frequency ac excitation. We find that the shot noise of a diffusive N-S junction is doubled compared to a normal diffusive conductor. Under ac excitation of frequency nu the shot noise develops features at bias voltages |V| = hnu/(2e), which bear all the hallmarks of a photon-assisted process. Observation of these features provides clear evidence that the effective charge of the current carriers is 2e, due to Andreev reflection.     

Stochastic dynamics of a Josephson junction threshold detector

We generalize the stochastic path integral formalism by considering Hamiltonian dynamics in the presence of general Markovian noise. Kramers' solution of the activation rate for escape over a barrier is generalized for non-Gaussian driving noise in both the overdamped and underdamped limit. We apply our general results to a Josephson junction detector measuring the electron counting statistics of a mesoscopic conductor. The activation rate dependence on the third current cumulant includes an additional term originating from the backaction of the measurement circuit.     

Temperature-dependent third cumulant of tunneling noise

Poisson statistics predicts that the shot noise in a tunnel junction has a temperature independent third cumulant e(2)I, determined solely by the mean current I. Experimental data, however, show a puzzling temperature dependence. We demonstrate theoretically that the third cumulant becomes strongly temperature dependent and may even change sign as a result of feedback from the electromagnetic environment. In the limit of a noninvasive (zero-impedance) measurement circuit in thermal equilibrium with the junction, we find that the third cumulant crosses over from e(2)I at low temperatures to -e(2)I at high temperatures.     

Effect of electron-phonon scattering on shot noise in nanoscale junctions

We investigate the effect of electron-phonon inelastic scattering on shot noise in nanoscale junctions in the regime of quasiballistic transport. We predict that when the local thermal energy of the junction is larger than its lowest vibrational mode energy eV(c), the inelastic contribution to shot noise (conductance) increases (decreases) with bias as V (sqrt[V]). The corresponding Fano factor thus increases as sqrt[V]. We also show that the inelastic contribution to the Fano factor saturates with increasing thermal current exchanged between the junction and the bulk electrodes to a value which, for V > V(c), is independent of bias. These predictions can be readily tested experimentally.     

Optimal control of shot noise and Fano factor by external fields

A method is devised to control the current, shot noise and Fano factor in a molecular junction using external fields. The tunneling of electrons through a molecular junction weakly coupled to two leads in the presence of a time-dependent external field is studied using a quantum master equation approach. By combining optimal control theory and assuming a predefined time-dependent current pattern, an external field can be determined which does generate a current pattern close to the requested one. With this approach the current flow pattern in time can be chosen in an almost arbitrary fashion. The same technique can be applied to control the shot noise. For minimizing the current, the corresponding shot noise decreases but does not vanish. By minimizing the shot noise, the corresponding current also approaches zero for the present model of spinless electrons. Within certain limits the proposed strategy even works well for the control of the Fano factor.     

Subharmonic Shapiro steps and noise in high-T c superconductor Josephson junctions

An experimental investigation is made of the subharmonic Shapiro steps observed on the I-V curves of high-T _c superconductor Josephson junctions and on the bias-voltage dependences of the rf noise and detector response when the junctions are subjected to external submillimeter radiation. Structures of this type are ordinarily described by a nonsinusoidal current-phase relation, which is why subharmonic steps appear. Numerical modeling of the processes occurring in a Josephson junction by means of a simple current-phase relation, as in the case of an SNS junction, gives good agreement with experiment. The width of the characteristic Josephson generation line of the junction was estimated on the basis of the noise dependences and the selective detector response. The width can be explained by taking into account the shot noise of the tunneling component of the conductivity. A model of the conductivity of a high-T _c superconductor Josephson junction, consisting of a tunnel junction with microshorts possessing metallic conductivity, is discussed. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 426–430 (10 September 1998)     

Versatile wideband balanced detector for quantum optical homodyne tomography

We present a comprehensive theory and an easy to follow method for the design and construction of a wideband homodyne detector for time-domain quantum measurements. We show how one can evaluate the performance of a detector in a specific time-domain experiment based on the electronic spectral characteristic of that detector. We then present and characterize a high-performance detector constructed using inexpensive, commercially available components such as low-noise high-speed operational amplifiers and high-bandwidth photodiodes. Our detector shows linear behavior up to a level of over 13 dB clearance between shot noise and electronic noise, in the range from DC to 100 MHz. The detector can be used for measuring quantum optical field quadratures both in the continuous-wave and pulsed regimes with standard commercial mode-locked lasers.     

Shot-noise-driven escape in hysteretic Josephson junctions

We have measured the influence of shot noise on hysteretic Josephson junctions initially in the macroscopic quantum tunneling regime. The escape threshold current into the resistive state decreases monotonically with increasing average current through the scattering conductor, which is another tunnel junction. Escape is predominantly determined by excitation due to the wideband shot noise. This process is equivalent to thermal activation (TA) over the barrier at effective temperatures up to about 4 times the critical temperature of the superconductor. The presented TA model is in excellent agreement with the experimental results.     

半导体/磁性d波超导隧道结中的散粒噪声

考虑到磁性d波超导体中的能隙与磁交换能的依赖关系,通过求解Bogoliubov-de Gennes(BdG)方程,利用Blonder-Tinkham-Klapwijk理论方法研究半导体/磁性d波超导隧道结中的散粒噪声.计算结果表明:磁性d波超导结中的磁交换能h0可导致散粒噪声在零偏压处的双峰和能隙处的峰出现劈裂,劈裂的宽度为2h0;磁交换能h0同时对散粒噪声及噪声功率与平均电流的比值有抑制作用.     

Study on Spin-Dependent Shot Noise in Parabolic-Well Magnetic Tunneling Junction

On the basis of the Landauer-Büttiker scattering formalism and transfer matrix method, we investigated the spin-dependent shot noise in parabolic-well with two ferromagnetic contacts (F/PW/F). The quantum size and Rashba spin-orbit interaction are discussed simultaneously. The results indicate that the shot noise is periodic function of the parabolic-well width. The oscillation frequencies of the shot noise decrease with the increasing of the parabolic-well depth, and increase with the increasing of the Rashba spin-orbit coupling strength. The amplitude and peak to valley ratio of the shot noise are strongly dependent on the magnetization configuration of the junction.     

电子器件散粒噪声测试方法研究

本文分析了超导量子干涉器(SQUID)和超导-绝缘-超导(SIS)约瑟夫森结散粒噪声测试方法的应用局限性,提出了常规器件的散粒噪声测试方案.针对常规电子器件散粒噪声特性,研究了噪声测试基本条件,并建立了低温测试系统.通过采用双层屏蔽结构和超低噪声前置放大器,实现了较好的电磁干扰屏蔽和极低的背景噪声.在10 K温度下对常规二极管散粒噪声进行了测试,通过理论和测试结果对比分析,验证了测试系统的准确和可信性. 关键词： 散粒噪声 电子器件 噪声测试     

Shot Noise in Ferromagnetic Superconductor Tunnel Junctions

In this paper, the superconducting order parameter and the energy spectrum of the Bogoliubov excitations are obtained from the Bogoliubov-de Gennes (BdG) equation for a ferromagnetic superconductor (FS). Taking into account the rough interface scattering effect, we calculate the shot noise and the differential conductance of the normal-metal insulator ferromagnetic superconductor junction. It is shown that the exchange energy E_h in FS can lead to splitting of the differential shot noise peaks and the conductance peaks. The energy difference between the two splitting peaks is equal to 2E_h. The rough interface scattering strength results in descent of conductance peaks and the shot noise-to-current ratio but increases the shot noise.     

Phonon-Assisted Resonant Tunnelling through a Triple-Quantum-Dot： a Phonon-Signal Detector

We study the effect of electron-phonon interaction on current and zero-frequency shot noise in resonant tunnelling through a series triple-quantum-dot coupling to a local phonon mode by means of a nonperturbative mapping technique along with the Green function formulation. By fixing the energy difference between the first two quantum dots to be equal to phonon frequency and sweeping the level of the third quantum dot, we find a largely enhanced current spectrum due to phonon effect, and in particular we predict current peaks corresponding to phonon-absorption and phonon-emission assisted resonant tunnelling processes, which show that this system can be acted as a sensitive phonon-signal detector or as a cascade phonon generator.     

Spin-current shot noise as a probe of interactions in mesoscopic systems

It is shown that the spin-resolved current shot noise can probe attractive or repulsive interactions in mesoscopic systems. This is illustrated in two physical situations: (i) a normal-superconducting junction where the spin-current noise is found to be zero, and (ii) a single-electron transistor where the spin-current noise is found to be Poissonian. Repulsive interactions may also lead to weak attractive correlations (bunching of opposite spins) in conditions far from equilibrium. Spin-current shot noise can also be used to measure the spin relaxation time T1, and a setup is proposed in a quantum dot geometry.     

Superconductor-metal transition in an ultrasmall Josephson junction biased by a noisy voltage source

Shot noise in a voltage source changes the character of the quantum (dissipative) phase transition in an ultrasmall Josephson junction: The superconductor-insulator transition transforms into the superconductor-metal transition. In the metallic phase, the IV curve probes the voltage distribution generated by shot noise, whereas in the superconducting phase, it probes the counting statistics of electrons traversing the noise junction.     

Computer-based photon-counting lock-in for phase detection at the shot-noise limit

We implement a simple computer-based photon-counting lock-in that combines the signal-to-noise benefits of photon counting with lock-in detection. We experimentally specify the flatness and the noise characteristics of a flexible software implementation. The noise of amplitude and phase of the small signal is at the limit of photonic shot noise; from 1000 counted photons we reach an amplitude resolution of 4.5% and a phase resolution of 13 degrees . The photon-counting lock-in reduces illumination noise, detector dark count noise, and can suppress background. In particular, phase detection is useful to image the delay characteristics in microscopic systems by use of fluorescent probes that are designed to report membrane potential, temperature, or concentration in a chemical reaction.