Simultaneous ideal quantum measurements of multiple single-photon-level signals would advance applications in quantum information processing, metrology, and astronomy but require the first amplifier to be simultaneously broadband, quantum limited, and directional. However, conventional traveling-wave parametric amplifiers support broadband amplification at the cost of increased added noise and are not genuinely directional due to non-negligible nonlinear backward-wave generation. In this work, we introduce a new class of amplifiers that encode the information in the Floquet modes of the system. Such Floquetmode amplifiers prevent information leakage and overcome the trade-off between quantum efficiency (QE) and bandwidth. Crucially, Floquet-mode amplifiers strongly suppress the nonlinear forward-backward wave coupling and are therefore genuinely directional and readily integrable with qubits, clearing another major obstacle toward broadband ideal quantum measurements. Furthermore, Floquet-mode amplifiers are insensitive to out-of-band impedance mismatch, which otherwise may lead to gain ripples, parametric oscillations, and instability in conventional traveling-wave parametric amplifiers. Finally, we show that a Floquet-mode Josephson traveling-wave parametric amplifier implementation can simultaneously achieve > 20 dB gain and a QE of η/η ideal > 99.9% of the quantum limit over more than an octave of bandwidth. The proposed Floquet scheme is also widely applicable to other platforms, such as kinetic inductance traveling-wave amplifiers and optical parametric amplifiers.

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The nondegenerate optical parametric amplifier (OPA) has many uses for investigating quantum fluctuations of light. Among them are EPR-type quadrature-phase correlations,1 QND measurement,2 and phase-sensitive quantum amplification.3 To study these problems, a reliable high-gain low-loss optical parametric amplifier (OPA) is crucial. Although OPAs operated at 1064 nm with KTP crystals pumped by a frequency doubled YAG laser exist,1 their performance is compromised because of problems with beam walk-off and polarization mixing for pairs of critically phase-matched KTP crystals. To avoid such problems, we have constructed a different nondegenerate OPA, which is operated at 1080 nm and consists of a single KTP crystal with type II noncritical phase matching. The pump beam at 540 nm for the amplifier is obtained from intracavity frequency doubling of a single-mode Nd: YAP laser with a similar KTP crystal as in the OPA.

We present an efficient, accurate, and comprehensive analysis framework for generic, multi-port nonlinear parametric circuits, in the presence of dissipation from lossy circuit components, based on "quantum-adapted" X-parameters. We apply this method to Josephson traveling-wave parametric amplifiers (JTWPAs) -a key component in superconducting and spin qubit quantum computing architectures -which are challenging to model accurately due to their thousands of linear and nonlinear circuit components. X-parameters are generated from a harmonic balance solution of the classical nonlinear circuit and then mapped to the field ladder operator basis, so that the energy associated with each of the multiple interacting modes corresponds to photon occupancy, rather than classical power waves. Explicit relations for the quantum efficiency of a generic, multi-port, multi-frequency parametric circuit are presented and evaluated for two distinct JTWPA designs. The gain and quantum efficiency are consistent with those obtained from Fourier analysis of time-domain solutions, but with enhanced accuracy, speed, and the ability to include real-world impairments, statistical variations, parasitic effects, and impedance mismatches (inand out-of-band) seamlessly. The unified flow is implemented in Keysight's PathWave Advanced Design System (ADS) and independently in an open-source simulation code, JosephsonCircuits.jl, from the MIT authors.