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A Bell test is a randomized trial that compares experimental observations against the philosophical worldview of local realism , in which the properties of the physical world are independent of our observation of them and no signal travels faster than light. A Bell test requires spatially distributed entanglement, fast and high-efficiency detection and unpredictable measurement settings. Although technology can satisfy the first two of these requirements, the use of physical devices to choose settings in a Bell test involves making assumptions about the physics that one aims to test. Bell himself noted this weakness in using physical setting choices and argued that human 'free will' could be used rigorously to ensure unpredictability in Bell tests. Here we report a set of local-realism tests using human choices, which avoids assumptions about predictability in physics. We recruited about 100,000 human participants to play an online video game that incentivizes fast, sustained input of unpredictable selections and illustrates Bell-test methodology. The participants generated 97,347,490 binary choices, which were directed via a scalable web platform to 12 laboratories on five continents, where 13 experiments tested local realism using photons, single atoms, atomic ensembles and superconducting devices. Over a 12-hour period on 30 November 2016, participants worldwide provided a sustained data flow of over 1,000 bits per second to the experiments, which used different human-generated data to choose each measurement setting. The observed correlations strongly contradict local realism and other realistic positions in bi-partite and tri-partite 12 scenarios. Project outcomes include closing the 'freedom-of-choice loophole' (the possibility that the setting choices are influenced by 'hidden variables' to correlate with the particle properties), the utilization of video-game methods for rapid collection of human-generated randomness, and the use of networking techniques for

We demonstrate the amplitude stabilization of a 2.85 THz quantum cascade laser with a graphene loaded split-ring-resonator array, acting as an external amplitude modulator. The transmittance of the modulator can be actively changed by modifying the graphene conductivity via electrostatic back-gating. The modulator operates at room temperature and is capable of actively modulating the quantum cascade laser power level and thus stabilizing the power output via a proportional-integral-derivative feedback control loop. The stability was enhanced by more than 10 times through actively tuning the modulation. Furthermore, this approach can be used to externally control the laser power with a high level of stability

We report a frequency locking experiment of a 3.5 THz third-order distributed feedback quantum cascade laser (QCL) by using a molecular absorption line of methanol (CH[subscript 3]OH) gas. With the help of the absorption line, the frequency noise of the QCL is transformed into measurable amplitude fluctuation. We first present the study of the noise of the THz QCL with the contribution from both the frequency and amplitude domain, by using a NbN superconducting hot-electron bolometer as a power detector. We then present the frequency locking measurement with a lock-in amplifier registering the derivate curve of the absorption line and a proportional-integral-derivative (PID) controller generating the feedback signal. The linewidth of the QCL in the free-running mode was found to be around 900 KHz. This linewidth is reduced to below 17 KHz (full width at half maximum) with a Gaussian-like shape when the control loop is active. Because of the frequency stabilization the noise power spectral density of the QCL shows a reduction of more than 20 dB at frequencies below 30 Hz., China Exchange Programme, Seventh Framework Programme (European Commission) (AMSTAR+ Project of RadioNet), NWO of the Netherlands

We report a set of measurements to demonstrate a new type of surface emitting distributed feedback (DFB) quantum cascade laser (QCL) operated at 3.5 THz as a local oscillator by pumping a superconducting hot-electron bolometer (HEB) mixer. Based on the Bragg gratings incorporated into the waveguide, the second order DFB surface emitting THz QCL shows single mode emission at 3.555 THz, which is only 4 GHz off from the hydroxyl (OH) line. Because of the radiation being emitted from the surface, the far-field beam is much improved, with a divergent far field beam pattern only in one direction. We also notice that in the far field beam pattern, unlike conventional metal-metal waveguide QCLs, there are no interference patterns. All these factors make it possible to fully pump a superconducting NbN HEB mixer at 60 K, and even at 70 K based on the estimated power., China Exchange Programme, Seventh Framework Programme (European Commission) (AMSTAR+ Project of RadioNet), NWO of the Netherlands

We report frequency locking of two 3.5-THz third-order distributed feedback (DFB)quantum cascade lasers(QCLs) by using methanol molecular absorption lines, a proportional-integral-derivative controller, and a NbN bolometer. We show that the free-running linewidths of the QCLs are dependent on the electrical and temperature tuning coefficients. For both lasers, the frequency locking induces a similar linewidth reduction factor, whereby the narrowest locked linewidth is below 18 kHz with a Gaussian-like shape. The linewidth reduction factor and the ultimate linewidth correspond to the measured frequency noise power spectral density., United States. National Aeronautics and Space Administration, National Science Foundation (U.S.)

We demonstrate an experimental scheme to simultaneously stabilize the frequency and amplitude of a 3.5 THz third-order distributed feedback quantum cascade laser as a local oscillator. The frequency stabilization has been realized using a methanol absorption line, a power detector, and a proportional-integral-derivative (PID) loop. The amplitude stabilization of the incident power has been achieved using a swing-arm voice coil actuator as a fast optical attenuator, using the direct detection output of a superconducting mixer in combination with a 2nd PID loop. Improved Allan variance times of the entire receiver, as well as the heterodyne molecular spectra, are demonstrated., United States. National Aeronautics and Space Administration, National Science Foundation (U.S.), Chinese Academy of Sciences (CAS-KNAW Joint PhD Training Programme), Royal Netherlands Academy of Arts and Sciences (CAS-KNAW Joint PhD Training Programme), National Natural Science Foundation (China) (Grant 11127903), National Natural Science Foundation (China) (Grant 10933005), Chinese Academy of Sciences (Key Laboratory of Radio Astronomy), European Commission (AMSTAR project RadioNet under FP7, NWO, and NATO SFP)

A frequency tunable terahertz heterodynespectrometer, based on a third-order distributed feedback quantum cascade laser as a local oscillator, has been demonstrated by measuring molecular spectral lines of methanol (CH[subscript 3]OH) gas at 3.5 THz. By varying the bias voltage of the laser, we achieved a tuning range of ~ 1 GHz of the lasing frequency, within which the molecular spectral lines were recorded. The measured spectra show excellent agreement with modeled ones. By fitting we derived the lasing frequency for each bias voltage accurately. The ultimate performance of the receiver including the resolution of noise temperature and frequency is also addressed., United States. National Aeronautics and Space Administration, National Science Foundation (U.S.)

A terahertz (THz) heterodyne spectrometer is demonstrated based on a quantum cascade laser(QCL) as a local oscillator (LO) and an NbN hot electron bolometer as a mixer, and it is used to measure high-resolution molecular spectral lines of methanol (CH[subscript 3]OH) between 2.913–2.918 THz. The spectral lines are taken from a gas cell containing methanol gas and using a single-mode QCL at 2.9156 THz as an LO, which is operated in the free running mode. By increasing the pressure of the gas, line broadening and saturation are observed. The measured spectra showed good agreement with a theoretical model.

We report a new experiment on a high-resolution heterodyne spectrometer using a 3.5 THz quantum cascade laser (QCL) as local oscillator (LO) and a superconducting hot electron bolometer (HEB) as mixer by stabilizing both frequency and amplitude of the QCL. The frequency locking of the QCL is demonstrated by using a methanol molecular absorption line, a proportional-integral-derivative (PID) controller, and a direct power detector. We show that the LO locked linewidth can be as narrow as 35 KHz. The LO power to the HEB is also stabilized by means of swing-arm actuator placed in the beam path in combination of a second PID controller.© (2012) SPIE.

High-resolution heterodyne spectrometers operating at above 2 THz are crucial for detecting, e.g., the HD line at 2.7 THz and oxygen OI line at 4.7 THz in astronomy. The potential receiver technology is a combination of a hot electron bolometer (HEB) mixer and a THz quantum cascade laser (QCL) local oscillator (LO).Here we report the first highresolution heterodyne spectroscopy measurement of a gas cell using such a HEB-QCL receiver. The receiver employs a 2.9 THz free-running QCL as local oscillator and a NbN HEB as a mixer. By using methanol (CH3OH) gas as a signal source, we successfully recorded the methanol emission line at 2.92195 THz. Spectral lines at IF frequency at different pressures were measured using a FFTS and well fitted with a Lorentzian profile. Our gas cell measurement is a crucial demonstration of the QCL as LO for practical heterodyne instruments. Together with our other experimental demonstrations, such as using a QCL at 70 K to operate a HEB mixer and the phase locking of a QCL such a receiver is in principle ready for a next step, which is to build a real instrument for any balloon-, air-, and space-borne observatory.