We have measured the full counting statistics (FCS) of current fluctuations in a semiconductor quantum dot (QD) by real-time detection of single electron tunneling with a quantum point contact (QPC). 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.

Understanding the influence of vibrational motion of the atoms on electronic transitions in molecules constitutes a cornerstone of quantum physics, as epitomized by the Franck-Condon principle 1,2 of spectroscopy. Recent advances in building molecular-electronics devices 3 and nanoelectromechanical systems 4 open a new arena for studying the interaction between mechanical and electronic degrees of freedom in transport at the single-molecule level. The tunneling of electrons through molecules or suspended quantum dots 5,6 has been shown to excite vibrational modes, or vibrons 7-9,6 . Beyond this effect, theory predicts that strong electron-vibron coupling dramatically suppresses the current flow at low biases, a collective behaviour known as Franck-Condon blockade 10 . Here we show measurements on quantum dots formed in suspended single-wall carbon nanotubes revealing a remarkably large electron-vibron coupling and, due to the high quality and unprecedented tunability of our samples, admit a quantitative analysis of vibronmediated electronic transport in the regime of strong electron-vibron coupling. This allows us to unambiguously demonstrate the Franck-Condon blockade in a suspended nanostructure. The large observed electron-vibron coupling could ultimately be a key ingredient for the detection of quantized mechanical motion 11,12 . It also emphasizes the unique potential for nanoelectromechanical device applications based on suspended graphene sheets and carbon nanotubes. In a polar semiconductor, a conduction electron deforms the surrounding lattice to form a polaron state 13 . The formation of this quasi-particle, by combining an electron and a cloud of lattice vibrations, or phonons, strongly influences the transport properties. The possibility for localization of strongly coupled polarons was suggested by Landau more than 70 years ago 13 . Recently, Koch et al. predicted that a related trapping of heavy polarons can occur in a quantum dot (QD) formed in a mechanically suspended nanostructure 10 . In such a nanoelectromechanical system (NEMS), the vibrational modes of the nanostructure can be strongly affected by the presence of electrons in the QD, as they deform the embedding medium. For strong electron-phonon coupling, the deformation effectively blocks electronic transport, termed Franck-Condon (FC) blockade. By analysing electronic transport through a suspended carbon nanotube (CNT) quantum dot over a wide range of electronic states, we are able to highlight generic features of vibron-assisted electronic transport, and unambiguously confirm the FC blockade scenario.Scanning electron microscope images and a scheme of our suspended CNT quantum dot device are shown in Figs. 1a, 1b and 1c. The CNT is electrically and mechanically connected to both source (S) and drain (D) contacts, while the central electrode acts as a suspended top-gate (TG). A quantum dot in the CNT is formed between defects 14 , which are presumably created during the release process and act as local barriers. The double top-and back-gat...

We investigate the triplet-singlet relaxation in a double quantum dot defined by top gates in an InAs nanowire. In the Pauli spin blockade regime, the leakage current can be mainly attributed to spin relaxation. While at weak and strong interdot coupling relaxation is dominated by two individual mechanisms, the relaxation is strongly reduced at intermediate coupling and finite magnetic field. In addition we observe a characteristic bistability of the spin-nonconserving current as a function of magnetic field. We propose a model where these features are explained by the polarization of nuclear spins enabled by the interplay between hyperfine and spin-orbit mediated relaxation.

We use a double quantum dot as a frequency-tunable on-chip microwave detector to investigate the radiation from electron shot-noise in a near-by quantum point contact. The device is realized by monitoring the inelastic tunneling of electrons between the quantum dots due to photon absorption. The frequency of the absorbed radiation is set by the energy separation between the dots, which is easily tuned with gate voltages. Using time-resolved charge detection techniques, we can directly relate the detection of a tunneling electron to the absorption of a single photon.The interplay between quantum optics and mesoscopic physics opens up new horizons for investigating radiation produced in nanoscale conductors [1,2]. Microwave photons emitted from quantum conductors are predicted to show non-classical behavior such as anti-bunching [3] and entanglement [4]. Experimental investigations of such systems require sensitive, high-bandwidth detectors operating at microwave-frequency [5]. On-chip detection schemes, with the device and detector being strongly capacitively coupled, offer advantages in terms of sensitivity and large bandwidths. In previous work, the detection mechanism was implemented utilizing photon-assisted tunneling in a superconductor-insulator-superconductor junction [6,7] or in a single quantum dot (QD) [8].Aguado and Kouwenhoven proposed to use a double quantum dot (DQD) as a frequency-tunable quantum noise detector [9]. The idea is sketched in Fig. 1(a), showing the energy levels of the DQD together with a quantum point contact (QPC) acting as a noise source. The DQD is operated with a fixed detuning δ between the electrochemical potentials of the left and right QD. If the system absorbs an energy E = δ from the environment, the electron in QD1 is excited to QD2. This electron may leave to the drain lead, a new electron enters from the source contact and the cycle can be repeated. The process induces a current flow through the system. Since the detuning δ may be varied continuously by applying appropriate gate voltages, the absorbtion energy is fully tunable.The scheme is experimentally challenging, due to low current levels and fast relaxation processes between the QDs [10]. Here, we show that these problems can be overcome by using time-resolved charge-detection techniques to detect single electrons tunneling into and out of the DQD. Apart from giving higher sensitivity than conventional current measurement techniques, the method also allows us to directly relate a single-electron tunneling event to the absorbtion of a single photon. The system can thus be viewed as a frequency-selective single- * Electronic address: simongus@phys.ethz.ch

We use time-resolved charge detection techniques to investigate single-electron tunneling in semiconductor quantum dots. The ability to detect individual charges in real-time makes it possible to count electrons one-by-one as they pass through the structure. The setup can thus be used as a high-precision current meter for measuring ultra-low currents, with resolution several orders of magnitude better than that of conventional current meters. In addition to measuring the average current, the counting procedure also makes it possible to investigate correlations between charge carriers. Electron correlations are conventionally probed in noise measurements, which are technically challenging due to the difficulty to exclude the influence of external noise sources in the experimental setup. Using real-time charge detection techniques, we circumvent the problem by studying the electron correlation directly from the counting statistics of the tunneling electrons. In quantum dots, we find that the strong Coulomb interaction makes electrons try to avoid each other. This leads to electron anti-bunching, giving stronger correlations and reduced noise compared to a current carried by statistically independent electrons.The charge detector is implemented by monitoring changes in conductance in a near-by capacitively coupled quantum point contact. We find that the quantum point contact not only serves as a detector but also causes a back-action onto the measured device. Electron scattering in the quantum point contact leads to emission of microwave radiation. The radiation is found to induce an electronic transition between two quantum dots, similar to the absorption of light in real atoms and molecules. Using a charge detector to probe the electron transitions, we can relate a single-electron tunneling event to the absorption of a single photon. Moreover, since the energy levels of the double quantum dot can be tuned by external gate voltages, we use the device as a frequency-selective single-photon detector operating at microwave energies. The ability to put an on-chip microwave detector close to a quantum conductor opens up the possibility to investigate radiation emitted from mesoscopic structures and give a deeper understanding of the role of electron-photon interactions in quantum conductors.A central concept of quantum mechanics is the wave-particle duality; matter exhibits both wave-and particle-like properties and can not be described by either formalism alone. To investigate the wave properties of the electrons, we perform experiments on a structure containing a double quantum dot embedded in the Aharonov-Bohm ring interferometer. Aharonov-Bohm rings are traditionally used to study interference of electron waves traversing different arms of the ring, in a similar way to the double-slit setup used for investigating interference of light waves. In our case, we use the time-resolved charge detection techniques to detect electrons one-by-one as they pass through the interferometer. We find that the individual pa...

There is an intimate connection between the acquisition of information and how this information changes the remaining uncertainty in the system. This trade-off between information and uncertainty plays a central role in the context of detection. Recent advances in the ability to make accurate, on-chip measurements of individual-electron current through a quantum dot 1-8 (QD) have been enabled by exploiting the sensitivity of a second current, passing through a nearby quantum point contact (QPC), to the fluctuating charge on the QD 4-8 . An important characteristic of QPC detectors is their minimal influence on the systems they probe. Here we show that even the operation of an effectively non-invasive QPC detector can statistically alter the system's behaviour. By observing a particular QPC current, the statistical distribution of the QD conditional current undergoes a substantial change in comparison to that expected for unconditional shot noise 9 . These results are in almost perfect agreement with a theoretical model we develop to predict the joint current probability distribution and conditional transport statistics of interacting nanoscale systems.Noise is generally due to randomness, which can be classical or quantum in nature. Telegraph noise, where there is random switching between two stable states 10 , originates from such diverse phenomena as thermal activation of an unstable impurity 11-13 , non-equilibrium activation of a bistable system 14-17 , switching of magnetic domain orientation [18][19][20] , or a reversible chemical reaction in a biological ion channel 21 .In nanoscale conductors, where charge motion is quantum coherent over distances comparable to the system size, shot noise and telegraph noise have recently been shown to be two sides of the same coin 6,7,22,23 . A quantum dot (QD) is sufficiently small that it is effectively zero dimensional, and behaves as an artificial atom, holding a small number of electrons. Figure 1a shows the sample used in the experiment reported here. The QD is marked by the dotted circle 24 . An extra electron can tunnel into the QD from the source lead (S), stay in the QD for a random amount of time and then tunnel out into the drain lead (D) if the applied voltage bias exceeds the temperature. This singleelectron transport produces a fluctuating electrical current. In order to detect the statistical properties of this current, a sensitive electrometer with a bandwidth much higher than the tunnelling rates is required. The electrometer is a nearby quantum point contact (QPC) that is capacitively coupled to the QD via the Coulomb interaction. The voltage-biased QPC detector transports many electrons through a narrow constriction in the surrounding two-dimensional electron gas (represented with an arrow). The resistance of the QPC is susceptible to changes in the surrounding electrostatic environment, and can therefore be used to sense the presence (or absence) of an extra electron on the QD 4 . When the extra electron tunnels into or out of the QD, the current I ...

We experimentally demonstrate the validity of nonequilibrium fluctuation relations by using a quantum coherent conductor. In equilibrium the fluctuation-dissipation relation leads to the correlation between current and current noise at the conductor, namely, the Johnson-Nyquist relation. When the conductor is voltage biased so that the nonlinear regime is entered, the fluctuation theorem has predicted similar nonequilibrium fluctuation relations, which hold true even when the Onsager-Casmir relations are broken in magnetic fields. Our experiments qualitatively validate the predictions as the first evidence of this theorem in the nonequilibrium quantum regime.

We have performed nonlinear transport measurements as a function of a perpendicular magnetic field in a semiconductor Aharonov-Bohm ring connected to two leads. While the voltage-symmetric part of the conductance is symmetric in magnetic field, the voltage-antisymmetric part of the conductance is not symmetric. These symmetry relations are compatible with the scattering theory for nonlinear mesoscopic transport. The observed asymmetry can be tuned continuously by changing the gate voltages near the arms of the ring, showing that the phase of the nonlinear conductance in a two-terminal interferometer is not rigid, in contrast to the case for the linear conductance.PACS numbers: 73.50.Fq, A mesoscopic ring can be used as an electron interferometer in order to compare the electronic phase of electrons traveling through both arms of the ring using the Aharonov-Bohm (AB) effect. However, it has been shown that a two-terminal ring does not allow to measure directly this phase difference in the linear transport [1]: the two-terminal conductance shows AB oscillations with a phase constrained to 0 or π [2,3,4]. This phase rigidity is a consequence of microreversibility [5] showing that the linear conductance of a two-terminal system must be symmetric in magnetic field [1,6]. A direct measurement of the phase difference is possible only in an open multi-terminal geometry [7,8].While the Onsager-Casimir relations hold close to equilibrium (linear conductance), there is no fundamental reason why far from equilibrium the nonlinear conductance should still follow this symmetry, i.e., one could expect G(V, B) = G(V, −B). It is then natural to ask whether the phase rigidity would still hold for the nonlinear transport in a two-terminal ring.In a phase coherent diffusive system, nonlinear conductance is expected when the bias voltage is larger than E T /e, where E T is the Thouless energy [9]. Models developed for non-interacting electrons predict an effect symmetric in magnetic field, which has been observed experimentally through bias voltage induced universal conductance fluctuations [10,11]. The possibility to observe magnetic field asymmetric nonlinear transport has been addressed only very recently both theoretically [12,13] and experimentally [14,15,16,17]. The models proposed there rely on effects of electron-electron interactions in noncentrosymmetric systems. Such behavior could be also expected in AB rings, for which symmetry breaking occurs due to asymmetries in the phase accumulated in each arm of the ring.Here we address the question of the magnetic field symmetries of the nonlinear conductance in a ring used as an Aharonov-Bohm interferometer. We have performed nonlinear d.c. transport measurements in a ring connected to two terminals. The current is fitted by a polynomial function of the bias voltage, with each coefficient of the decomposition showing AB oscillations as a function of magnetic field. While the odd coefficients are symmetric in magnetic field and show strong h/2e oscillations, the even coefficient...