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Optical quantum networks can connect distant quantum processors to enable secure quantum communication and distributed quantum computing. Superconducting qubits are a leading technology for quantum information processing but cannot couple to long-distance optical networks without an efficient, coherent, and low noise interface between microwave and optical photons. Here, we demonstrate a microwave-to-optical transducer using an ensemble of erbium ions that is simultaneously coupled to a superconducting microwave resonator and a nanophotonic optical resonator. The coherent atomic transitions of the ions mediate the frequency conversion from microwave photons to optical photons and using photon counting we observed device conversion efficiency approaching 10 −7 . With pulsed operation at a low duty cycle, the device maintained a spin temperature below 100 mK and microwave resonator heating of less than 0.15 quanta. Interfacing superconducting quantum information processors with long-distance optical networks would require coherent interfacing between microwave and optical photons. Here, the authors show a chip-integrated microwave-to-optical transducer based on rare earth ion ensembles.

Closer to an exotic goal Placing a macroscopic object in its quantum-mechanical ground state of motion is an exciting experimental prospect. If achieved, it should reveal counter-intuitive physical behaviour — such as the existence of the system in two locations simultaneously. Rocheleau et al . come tantalizingly close to this goal. They have cooled a nanomechanical resonator to a point where the probability of it residing in its motional ground state is 0.21 (which in itself should be sufficient to enable direct measurement of some anticipated quantum phenomena), and have identified the experimental hurdles that need to be overcome to push the system more fully into this exotic quantum regime. Placing a macroscopic object in its quantum-mechanical ground state of motion is an exciting experimental target that should reveal counterintuitive physical behaviour — such as the existence of states in which the mechanical system is located in two places simultaneously. A nanomechanical resonator is now cooled to a point where the probability of its residing in the quantum ground state of motion is 0.21; this level of cooling should allow a series of fundamental quantum mechanical observations. Cold, macroscopic mechanical systems are expected to behave contrary to our usual classical understanding of reality; the most striking and counterintuitive predictions involve the existence of states in which the mechanical system is located in two places simultaneously. Various schemes have been proposed to generate and detect such states 1 , 2 , and all require starting from mechanical states that are close to the lowest energy eigenstate, the mechanical ground state. Here we report the cooling of the motion of a radio-frequency nanomechanical resonator by parametric coupling to a driven, microwave-frequency superconducting resonator. Starting from a thermal occupation of 480 quanta, we have observed occupation factors as low as 3.8 ± 1.3 and expect the mechanical resonator to be found with probability 0.21 in the quantum ground state of motion. Further cooling is limited by random excitation of the microwave resonator and heating of the dissipative mechanical bath. This level of cooling is expected to make possible a series of fundamental quantum mechanical observations including direct measurement of the Heisenberg uncertainty principle and quantum entanglement with qubits.

According to quantum mechanics, a harmonic oscillator can never be completely at rest. Even in the ground state, its position will always have fluctuations, called the zero-point motion. Although the zero-point fluctuations are unavoidable, they can be manipulated. Using microwave frequency radiation pressure, we have manipulated the thermal fluctuations of a micrometer-scale mechanical resonator to produce a stationary quadrature-squeezed state with a minimum variance of 0.80 times that of the ground state. We also performed phase-sensitive, back-action evading measurements of a thermal state squeezed to 1.09 times the zero-point level. Our results are relevant to the quantum engineering of states of matter at large length scales, the study of decoherence of large quantum systems, and for the realization of ultrasensitive sensing of force and motion.

Nanomechanical vibrations Fabricating tiny mechanical structures whose vibrational motion is purely quantum mechanical is a long-standing goal in physics, both from a fundamental perspective and in view of the applications that they could potentially enable. A parallel — and equally important — goal is the development of a scheme for observing and controlling such tiny motions. LaHaye et al . have made important progress in this direction by coupling a tiny mechanical resonator to a superconducting two-level quantum system (qubit). The state of the superconducting qubit can be measured through its influence on the vibrations of the resonator. Such a coupled device configuration should ultimately enable the preparation and measurement of exotic quantum states of motion. Fabricating tiny mechanical structures where the vibrational motion is purely quantum mechanical is a long-standing goal in physics, and a parallel goal is the development of a scheme for observing and controlling such tiny motions. By coupling a tiny mechanical resonator to a superconducting two-level quantum system (qubit), the state of the superconducting qubit can be measured through its influence on the vibrations of the resonator, a demonstration of nanomechanical read-out of quantum interference. The observation of the quantum states of motion of a macroscopic mechanical structure remains an open challenge in quantum-state preparation and measurement. One approach that has received extensive theoretical attention 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 is the integration of superconducting qubits as control and detection elements in nanoelectromechanical systems (NEMS). Here we report measurements of a NEMS resonator coupled to a superconducting qubit, a Cooper-pair box. We demonstrate that the coupling results in a dispersive shift of the nanomechanical frequency that is the mechanical analogue of the ‘single-atom index effect’ 14 experienced by electromagnetic resonators in cavity quantum electrodynamics. The large magnitude of the dispersive interaction allows us to perform NEMS-based spectroscopy of the superconducting qubit, and enables observation of Landau–Zener interference effects—a demonstration of nanomechanical read-out of quantum interference.

•Network model with STDP explains EEG connectivity change after dual-site tACS.•Effects are predicted to depend on tACS frequency and conduction delays.•EEG data confirm dependence on conduction delays between regions.•Model can be used to estimate and maximize experimental effects. Transcranial alternating current stimulation (tACS), applied to two brain sites with different phase lags, has been shown to modulate stimulation-outlasting functional EEG connectivity between the targeted regions. Given the lack of knowledge on mechanisms of tACS aftereffects, it is difficult to further enhance effect sizes and reduce variability in experiments. In this computational study, we tested if spike-timing-dependent plasticity (STDP) can explain stimulation-outlasting connectivity modulation by dual-site tACS and explored the effects of tACS parameter choices. Two populations of spiking neurons were coupled with synapses subject to STDP, and results were validated via a re-analysis of EEG data. Our simulations showed stimulation-outlasting connectivity changes between in- and anti-phase tACS, dependent on both tACS frequency and synaptic conduction delays. Importantly, both a simple network entraining to a wide range of tACS frequencies as well as a more realistic network that spontaneously oscillated at alpha frequency predicted that the largest effects would occur for short conduction delays between the stimulated regions. This finding agreed with experimental EEG connectivity modulation by 10Hz tACS, showing a clear negative correlation of tACS effects with estimated conduction delays between regions. In conclusion, STDP can explain connectivity aftereffects of dual-site tACS. However, not all combinations of tACS frequency and application sites are expected to effectively modulate connectivity via STDP. We therefore suggest using appropriate computational models and/or EEG analysis for planning and interpretation of dual-site tACS studies relying on aftereffects. [Display omitted]

By coupling a single-electron transistor to a high-quality factor, 19.7-megahertz nanomechanical resonator, we demonstrate position detection approaching that set by the Heisenberg uncertainty principle limit. At millikelvin temperatures, position resolution a factor of 4.3 above the quantum limit is achieved and demonstrates the near-ideal performance of the single-electron transistor as a linear amplifier. We have observed the resonator's thermal motion at temperatures as low as 56 millikelvin, with quantum occupation factors of$N_{TH} = 58$. The implications of this experiment reach from the ultimate limits of force microscopy to qubit readout for quantum information devices.

Quantum fluctuations of the light field used for continuous position detection produce stochastic back-action forces and ultimately limit the sensitivity. To overcome this limit, the back-action forces can be avoided by giving up complete knowledge of the motion, and these types of measurements are called \"back-action evading\" or \"quantum nondemolition\" detection. We present continuous two-tone back-action evading measurements with a superconducting electromechanical device, realizing three long-standing goals: detection of back-action forces due to the quantum noise of a microwave field, reduction of this quantum back-action noise by 8.5 ± 0.4 decibels (dB), and measurement imprecision of a single quadrature of motion 2.4 ± 0.7 dB below the mechanical zero-point fluctuations. Measurements of this type will find utility in ultrasensitive measurements of weak forces and nonclassical states of motion.

At low temperatures, the electron gas of graphene is expected to show both very weak coupling to thermal baths and rapid thermalization, properties which are desirable for use as a sensitive bolometer. We demonstrate an ultrasensitive, wide-bandwidth measurement scheme based on Johnson noise to probe the thermal-transport and thermodynamic properties of the electron gas of graphene, with a resolution of 2mK/Hz and a bandwidth of 80 MHz. We have measured the electron-phonon coupling directly through energy transport, from 2–30 K and at a charge density of 2×1011cm−2 . We demonstrate bolometric mixing and utilize this effect to sense temperature oscillations with a period of 430 ps and determine the heat capacity of the electron gas to be 2×10−21J/(K·μm2) at 5 K, which is consistent with that of a two-dimensional Dirac electron gas. These measurements suggest that graphene-based devices, together with wide-bandwidth noise thermometry, can generate substantial advances in the areas of ultrasensitive bolometry, calorimetry, microwave and terahertz photo-detection, and bolometric mixing for applications in fields such as observational astronomy and quantum information and measurement.

It's all done with mirrors Cooling of mechanical resonators is the focus of much research effort because of possible applications in ultra-high precision measurements such as gravitational wave detection. It is also of fundamental interest as using this technique it may be possible to observe a transition between classical and quantum behaviour of a mechanical system. Three groups report advances in this area. Gigan et al . and Arcizet et al . used radiation pressure to freeze out thermal vibrations of tiny mechanical microresonators, or micromirrors. In the right conditions, the mirrors cool from room temperature to about 10 K without outside influence. Once the technique is refined it should be possible to achieve further cooling and to observe the quantum ground state of a micromirror experimentally. In the third paper, Dustin Kleckner and Dirk Bouwmeester use optical feedback to cool a micromirror to sub-kelvin temperatures. Experiments where a tiny mirror, a mechanical microresonator, within an optical cavity undergoes 'self-cooling' is detailed. Under the right, finely tuned conditions, the thermal vibration of the mirror freezes out without outside influence. It cools down by a factor of 30, from room temperature to about 10 kelvin. Cooling of mechanical resonators is currently a popular topic in many fields of physics including ultra-high precision measurements 1 , detection of gravitational waves 2 , 3 and the study of the transition between classical and quantum behaviour of a mechanical system 4 , 5 , 6 . Here we report the observation of self-cooling of a micromirror by radiation pressure inside a high-finesse optical cavity. In essence, changes in intensity in a detuned cavity, as caused by the thermal vibration of the mirror, provide the mechanism for entropy flow from the mirror’s oscillatory motion to the low-entropy cavity field 2 . The crucial coupling between radiation and mechanical motion was made possible by producing free-standing micromirrors of low mass ( m  ≈ 400 ng), high reflectance (more than 99.6%) and high mechanical quality ( Q  ≈ 10,000). We observe cooling of the mechanical oscillator by a factor of more than 30; that is, from room temperature to below 10 K. In addition to purely photothermal effects 7 we identify radiation pressure as a relevant mechanism responsible for the cooling. In contrast with earlier experiments, our technique does not need any active feedback 8 , 9 , 10 . We expect that improvements of our method will permit cooling ratios beyond 1,000 and will thus possibly enable cooling all the way down to the quantum mechanical ground state of the micromirror.

The objective of this experiment was to quantify intakes, duodenal flows, and ruminal apparent synthesis (AS) of B-vitamins in lactating dairy cows fed diets varying in forage and nonfiber carbohydrate (NFC) contents. Eight (4 primiparous and 4 multiparous) ruminally and duodenally cannulated Holstein cows were assigned to 4 dietary treatments in a replicated 21-d period, 4×4 Latin square design with a 2×2 factorial treatment arrangement. Diets, fed as TMR, contained (DM basis) 2 levels of forage (35 and 60%) and 2 levels of NFC (30 and 40%). The forage portion of the diets contained 50% corn silage, 33% alfalfa hay, and 17% grass hay. Soybean hulls and beet pulp (2:1) and corn meal and ground barley (2:1) were included to achieve desired NFC concentrations. No supplemental B-vitamins were fed. B-vitamin AS was calculated as the amount of a specific B-vitamin flowing to the duodenum minus its daily orts-corrected intake. Dry matter and organic matter intakes were higher for cows fed the 35% forage diets and the 40% NFC diets. Increasing dietary forage content decreased ruminal AS of pyridoxine, folic acid, and B12. Increasing dietary NFC content increased ruminal AS of nicotinic acid, nicotinamide, niacin, pyridoxal, B6, and folic acid but decreased AS of B12. Across diets, amounts of B-vitamins synthesized were highest for niacin, followed by riboflavin, B12, thiamin, B6, and folic acid. Biotin AS values were negative for all diets, suggesting either no ruminal synthesis or that destruction by ruminal microflora was greater than synthesis. B-vitamin intake, duodenal flow, and ruminal synthesis are influenced by dietary forage and NFC contents.