Quantum measurement has challenged physicists for almost a century. Classically, there is no lower bound on the noise a measurement may add. Quantum mechanically, however, measuring a system necessarily perturbs it. When applied to electrical amplifiers, this means that improved sensitivity requires increased backaction that itself contributes noise. The result is a strict quantum limit on added amplifier noise 1-6 . To approach this limit, a quantum-limited amplifier must possess an ideal balance between sensitivity and backaction; furthermore, its noise must dominate that of subsequent classical amplifiers 7 . Here, we report the first complete and quantitative measurement of the quantum noise of a superconducting single-electron transistor (S-SET) near a double Cooper-pair resonance predicted to have the right combination of sensitivity and backaction 8 . A simultaneous measurement of our S-SET's charge sensitivity indicates that it operates within a factor of 3.6 of the quantum limit, a fourfold improvement over the nearest comparable results 9 .The two mesoscopic devices most commonly used to electrically measure spin-and charge-based quantum systems are the singleelectron transistor (SET) and quantum point contact (QPC). These devices operate according to the same scheme: the electrometer is biased by a source-drain voltage V sd and the current I through it is measured. Motion of charges near the electrometer causes its differential conductance G d to change, resulting in changes in I . The ultimate sensitivity of an electrometer operated in this way is therefore set by the non-equilibrium current noise (shot noise) present in I (t ). The same current fluctuations also determine its backaction, and, therefore, its proximity to the quantum limit.Classically, current noise is described by a spectral density S I sym (ω) that is symmetric in frequency ω. Quantum mechanically, however, we must distinguish between positive frequency noise, which transfers energy from a measured system to the electrometer, and negative frequency noise, which transfers energy from the electrometer to the measured system. A simple Fermi's golden rule calculation of, for example, an electrometer coupled to a qubit prepared in its ground state shows this 10 . The transition rate for the qubit to be promoted to its excited state is proportional to S I (−ω 0 ), where S I (ω) = +∞ −∞ dt e iωt I (t )I (0) is the unsymmetrized quantum noise spectrum of the electrometer current andhω 0 is the separation in energy between the ground and excited states. Similarly, the rate at which a system prepared in its excited state decays to the ground state is given by S I (+ω 0 ). To make a complete measurement of the quantum noise of an electrometer, one must obtain information regarding both S I (+ω 0 ) and S I (−ω 0 ).Rather than couple our S-SET electrometer to a two-level system to carry out our quantum noise measurements, we instead couple it to another canonical quantum system, namely a harmonic oscillator consisting of an on-chip superconductin...

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Systems with coupled mechanical and optical or electrical degrees of freedom have fascinating dynamics that, through macroscopic manifestations of quantum behaviour, provide new insights into the transition between the classical and quantum worlds. Of particular interest is the back-action of electrons and photons on mechanical oscillators, which can lead to cooling and amplification of mechanical motion. Furthermore, feedback, which is naturally associated with back-action, has been predicted to have significant consequences for the noise of a detector coupled to a mechanical oscillator. Recently it has also been demonstrated that such feedback effects lead to strong coupling between single-electron transport and mechanical motion in carbon nanotube nanomechanical resonators. Here we present noise measurements which show that the mesoscopic back-action of electrons tunnelling through a radio-frequency quantum point contact causes driven vibrations of the host crystal. This effect is a remarkable macroscopic manifestation of microscopic quantum behaviour, where the motion of a mechanical oscillator-the host crystal, which consists of on the order of 10(20) atoms-is determined by statistical fluctuations of tunnelling electrons.

PurposeThis study was done to investigate the inhibition effects of miR-30a-3p on mitotic arrest deficient 2 like 1 (MAD2L1) expression and the proliferation of gastric cancer cells.Patients and methodsCluster analysis and the TCGA database were used to screen the key genes highly expressed in gastric cancer. Based on the LinkedOmics website, the correlation between the miR-30a-3p and the cell cycle-related target gene MAD2L1 in gastric cancer was analyzed. The mRNA and protein expression levels were detected with the quantitative real-time PCR and Western blot analysis. The cell proliferation and cell cycle were also detected and analyzed.ResultsBioinformatics analysis showed that MAD2L1 was highly expressed in tumor tissues compared with normal tissues. Compared with normal tissues, the miR-30a-3p was significantly decreased in the gastric cancer tissues. Moreover, MAD2L1 was significantly negatively correlated with the miR-30a-3p expression. Furthermore, over-expression of miR-30a-3p decreased the expression of MAD2L1 at the protein level, which inhibited the proliferation of AGS and BGC-823 gastric cancer cells. In addition, the cell cycles of AGS and BGC-823 cells were arrested at the G0/G1 phase.ConclusionMAD2L1 is a pro-oncogene which is up-regulated in gastric cancer. The miR-30a-3p can down-regulate the MAD2L1 expression, inhibiting the proliferation of gastric cancer cells and affect the cell cycle.

We investigate Fano resonances and sensing enhancements in a simple Au/TiO2 hybrid metasurface through the finite-different time-domain (FDTD) simulation and coupled mode theory (CMT) analysis. The results show that the Fano resonance in the proposed simple metasurface is caused by the destructive interaction between the surface plasmon polaritons (SPPs) and the local surface plasmon resonances (LSPRs), the quality factor and dephasing time for the Fano resonance can be effectively tuned by the thickness of Au and TiO2 structures, the length of each unit in x and y directions, as well as the structural defect. In particular, single Fano resonance splits into multiple Fano resonances caused by a stub-shaped defect, and multiple Fano resonances can be tuned by the size and position of the stub-shaped defect. Moreover, we also find that the sensitivity in the Au/TiO2 hybrid metasurface with the stub-shaped defect can reach up to 330 nm/RIU and 535 nm/RIU at the Fano resonance 1 and Fano resonance 2, which is more than three times as sensitive in the Au/TiO2 hybrid metasurface without the stub-shaped defect, and also higher than that in the TiO2 metasurface reported before. These results may provide further understanding of Fano resonances and guidance for designing ultra-high sensitive refractive index sensors.