1It has recently become possible to encode the quantum state of superconducting qubits and the position of nanomechanical oscillators into the states of microwave fields 1,2 . However, to make an ideal measurement of the state of a qubit, or to detect the position of a mechanical oscillator with quantum-limited sensitivity requires an amplifier that adds no noise. If an amplifier adds less than half a quantum of noise, it can also squeeze the quantum noise of the electromagnetic vacuum. Highly squeezed states of the vacuum serve as an important quantum information resource. They can be used to generate entanglement or to realize back-action-evading measurements of position 3,4 . Here we introduce a general purpose parametric device, which operates in a frequency band between 4 and 8 GHz. It is a subquantum-limited microwave amplifier, it amplifies quantum noise above the added noise of commercial amplifiers, and it squeezes quantum fluctuations by 10 dB.With the emergence of quantum information processing with electrical circuits, there is a renewed interest in Josephson parametric devices 5,6,7,8,9 . Previous work with Josephson parametric amplifiers demonstrated that they can operate with subquantum-limited added noise and modestly squeeze vacuum noise 10,11,12,13,14 . Earlier realizations of Josephson parametric amplifiers (JPAs) were only capable of amplifying signals in a narrow frequency range, were not operated with large enough gain to make the noise of the following, conventional amplifier negligible or were too lossy to be subquantum limited 5 . For related reasons, the degree of squeezing of the vacuum noise was never larger than 3 dB. We create a new type of parametric amplifier in which we embed a tunable, low-loss, and nonlinear metamaterial in a microwave cavity. The tunability of the metamaterial allows us to adjust the amplified band between 4 and 8 GHz, and the cavity isolates the gain medium from low-frequency noise, providing the stability required to achieve high gains and large squeezing.A single mode of a microwave field with angular frequency ω can be decomposed in two orthogonal components, referred to as quadratureŝ V (t) ∝X 1 cos ωt +X 2 sin ωt whereX 1 andX 2 are conjugate quantum variables obeying the commutation relation [X 1 ,X 2 ] = i/2. The proportionality constant depends on the details of the mode 15,16,17 .As a consequence of the commutation relation, the uncertainties inX 1 andX 2 are subject 2 to the Heisenberg constraint ∆X 1 ∆X 2 ≥ 1/4, where ∆X 2 j is the variance of the quadrature amplitudeX j . A mode is "squeezed" if for one of the quadratures ∆X j < 1/2 (ref. 17). An amplifier that transforms both input quadratures by multiplying them by a gain √ G must add at least half a quantum of noise for the output signal to obey the commutation relation 18 ; if it adds exactly half a quantum of noise, it is quantum limited. On the other hand, an amplifier which transforms the input signal by multiplying one quadrature by √ G and multiplying the other quadrature by 1/ √ G would...

We present new arcminute-resolution maps of the Cosmic Microwave Background temperature and polarization anisotropy from the Atacama Cosmology Telescope, using data taken from 2013–2016 at 98 and 150 GHz. The maps cover more than 17,000 deg2, the deepest 600 deg2 with noise levels below 10μK-arcmin. We use the power spectrum derived from almost 6,000 deg2 of these maps to constrain cosmology. The ACT data enable a measurement of the angular scale of features in both the divergence-like polarization and the temperature anisotropy, tracing both the velocity and density at last-scattering. From these one can derive the distance to the last-scattering surface and thus infer the local expansion rate, H 0. By combining ACT data with large-scale information from WMAP we measure H 0=67.6± 1.1 km/s/Mpc, at 68% confidence, in excellent agreement with the independently-measured Planck satellite estimate (from ACT alone we find H 0=67.9± 1.5 km/s/Mpc). The ΛCDM model provides a good fit to the ACT data, and we find no evidence for deviations: both the spatial curvature, and the departure from the standard lensing signal in the spectrum, are zero to within 1σ; the number of relativistic species, the primordial Helium fraction, and the running of the spectral index are consistent with ΛCDM predictions to within 1.5–2.2σ. We compare ACT, WMAP, and Planck at the parameter level and find good consistency; we investigate how the constraints on the correlated spectral index and baryon density parameters readjust when adding CMB large-scale information that ACT does not measure. The DR4 products presented here will be publicly released on the NASA Legacy Archive for Microwave Background Data Analysis.

We present the temperature and polarization angular power spectra of the CMB measured by the Atacama Cosmology Telescope (ACT) from 5400 deg2 of the 2013–2016 survey, which covers >15000 deg2 at 98 and 150 GHz. For this analysis we adopt a blinding strategy to help avoid confirmation bias and, related to this, show numerous checks for systematic error done before unblinding. Using the likelihood for the cosmological analysis we constrain secondary sources of anisotropy and foreground emission, and derive a “CMB-only” spectrum that extends to ℓ=4000. At large angular scales, foreground emission at 150 GHz is ∼1% of TT and EE within our selected regions and consistent with that found by Planck. Using the same likelihood, we obtain the cosmological parameters for ΛCDM for the ACT data alone with a prior on the optical depth of τ=0.065±0.015. ΛCDM is a good fit. The best-fit model has a reduced χ2 of 1.07 (PTE=0.07) with H 0=67.9±1.5 km/s/Mpc. We show that the lensing BB signal is consistent with ΛCDM and limit the celestial EB polarization angle to ψ P  =−0.07̂±0.09̂. We directly cross correlate ACT with Planck and observe generally good agreement but with some discrepancies in TE. All data on which this analysis is based will be publicly released.

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Fully controlled coherent coupling of arbitrary harmonic oscillators is an important tool for processing quantum information 1 . Coupling between quantum harmonic oscillators has previously been demonstrated in several physical systems using a two-level system as a mediating element 2,3 . Direct interaction at the quantum level has only recently been realized by means of resonant coupling between trapped ions 4,5 . Here we implement a tunable direct coupling between the microwave harmonics of a superconducting resonator by means of parametric frequency conversion 6,7 . We accomplish this by coupling the mode currents of two harmonics through a superconducting quantum interference device (SQUID) and modulating its flux at the difference (∼7 GHz) of the harmonic frequencies. We deterministically prepare a single-photon Fock state 8 and coherently manipulate it between multiple modes, effectively controlling it in a superposition of two different 'colours'. This parametric interaction can be described as a beamsplitter-like operation that couples different frequency modes. As such, it could be used to implement linear optical quantum computing protocols 9,10 on-chip 11 . The ability to create and manipulate quantum number states in a linear resonator is an important task in cavity quantum electrodynamics (QED; ref. 1). Early theory 6,7 predicted that parametric frequency conversion could be a way to implement a tunable direct coupling between quantized modes of different energies. Classically, two harmonic oscillators coupled through a time-varying element, modulated at the difference of the resonator frequencies, will periodically exchange energy. At the quantum level, this can be used to swap the quantum states of two harmonic modes. In optics, the efficiency and quantum coherence of frequency up-conversion have been demonstrated using a pumped nonlinear crystal to couple light at different wavelengths [12][13][14] . However, in these experiments it is challenging to access the state dynamics because strong coupling rates are difficult to obtain 15 . In hybrid mechanical systems, strong parametric coupling, based on frequency conversion, has been recently achieved 16,17 , creating the possibility for the manipulation of quantum states of mesoscopic mechanical resonators 18 . In superconducting circuits, parametric processes have been used mainly to couple superconducting quantum bits (qubits) at their optimal points 19 , or to make quantum-limited microwave amplifiers [20][21][22][23] , yet little has been done with frequency conversion. Several circuit designs that enable frequency conversion between linear resonators have been proposed 21,24,25 . This particular interaction can be combined with the powerful tools already available in circuit QED (ref. dynamics of the parametric frequency conversion of a single photon between the first three internal resonant modes of a superconducting cavity, the state of which is prepared and read out with a superconducting qubit. Our circuit consists of a quarter-wave (λ/4) c...