We propose a resonant electromagnetic detector to search for hidden-photon dark matter over an extensive range of masses. Hidden-photon dark matter can be described as a weakly coupled "hidden electric field," oscillating at a frequency fixed by the mass, and able to penetrate any shielding. At low frequencies (compared to the inverse size of the shielding), we find that the observable effect of the hidden photon inside any shielding is a real, oscillating magnetic field. We outline experimental setups designed to search for hidden-photon dark matter, using a tunable, resonant LC circuit designed to couple to this magnetic field. Our "straw man" setups take into consideration resonator design, readout architecture and noise estimates. At high frequencies, there is an upper limit to the useful size of a single resonator set by 1/ν. However, many resonators may be multiplexed within a hidden-photon coherence length to increase the sensitivity in this regime. Hidden-photon dark matter has an enormous range of possible frequencies, but current experiments search only over a few narrow pieces of that range. We find the potential sensitivity of our proposal is many orders of magnitude beyond current limits over an extensive range of frequencies, from 100 Hz up to 700 GHz and potentially higher.

Josephson parametric amplifiers have become a critical tool in superconducting device physics due to their high gain and quantum-limited noise. Traveling wave parametric amplifiers (TWPAs) promise similar noise performance while allowing for significant increases in both bandwidth and dynamic range. We present a TWPA device based on an LC-ladder transmission line of Josephson junctions and parallel plate capacitors using low-loss amorphous silicon dielectric. Crucially, we have inserted λ/4 resonators at regular intervals along the transmission line in order to maintain the phase matching condition between pump, signal, and idler and increase gain. We achieve an average gain of 12 dB across a 4 GHz span, along with an average saturation power of -92 dBm with noise approaching the quantum limit.The Josephson parametric amplifier [1][2][3][4][5][6][7] (JPA) is a critical tool for high fidelity state measurement in superconducting qubits [8][9][10] as it allows parametric amplification with near quantum-limited noise [11]. Despite its success, the JPA has typically been used only for single frequency measurements due to lower bandwidth and saturation power. A promising approach to scaling superconducting qubit experiments is frequency multiplexing [12][13][14], which requires additional bandwidth and dynamic range for each measurement tone. Simultaneous amplification of up to five multiplexed tones has been achieved with a JPA [15-17] but was only possible with the Impedance-transformed parametric amplifier [18] (IMPA). This highly engineered JPA provides much larger bandwidth and saturation power but pushes the resonant design to its low Q limit.To extend this frequency multiplexed approach for future experiments, we have adopted the distributed design of the traveling wave parametric amplifier (TWPA) [19]. Fiber-optic TWPAs have already demonstrated high gain, dynamic range, and bandwidth while reaching the quantum-limit of added noise [20,21]. In this letter we present a microwave frequency TWPA with 4 GHz of bandwidth and an order of magnitude more saturation power than the best JPA. This device is compatible with scaling to much larger qubit systems through multiplexed measurement, and may find applications outside quantum information such as astrophysics detectors [12,22] At microwave frequencies the TWPA can be thought of as a transmission line where the propagation velocity is controlled by varying the individual circuit parameters of inductance or capacitance per unit length [24,25]. This is typically achieved by constructing a signal line with a current dependent (nonlinear) inductance. Like the JPA, a large enough pump tone will modulate this inductance, coupling the pump (ω p ) to a signal (ω s ) and idler (ω i ) tone via frequency mixing such that ω s + ω i = 2ω p . Unlike the JPA however, the TWPA has no resonant structure so gain, bandwidth, and dynamic range are determined by the coupled mode equations of a nonlinear transmission line [23]. In addition to allowing more bandwidth and saturation power...

We discuss the limits of electromagnetic searches for axion and hidden-photon dark matter, subject to the Standard Quantum Limit (SQL) on amplification. We begin by showing the signalto-noise advantage of scanned resonant detectors over purely resistive broadband detectors. Building on this calculation, we discuss why the detector circuit should be driven by the dark-matter signal through a reactance (an equivalent inductance or capacitance); examples of such detectors include single-pole resonators, which are used broadly in axion and hidden-photon detection. Focusing thereafter on reactively coupled detectors, we develop a framework to optimize dark matter searches using prior information about the dark matter signal. Priors can arise, for example, from cosmological or astrophysical constraints, constraints from previous direct-detection searches, or preferred search ranges. We define integrated sensitivity as a figure of merit in comparing searches over a wide frequency range and show that the Bode-Fano criterion sets a limit on integrated sensitivity. We show that when resonator thermal noise dominates amplifier noise, substantial sensitivity is available away from the resonator bandwidth. The optimization of this sensitivity is found to be closely related to noise mismatch with the amplifier and the concept of measurement backaction. Additionally, we show that the optimized one-pole resonator is close to the Bode-Fano limit. The Bode-Fano constraint establishes the single-pole resonator as a near-ideal method for single-moded dark-matter detection. We optimize time allocation in a scanned tunable resonator search using priors. Combining our insights into integrated sensitivity and time allocation, we derive quantum limits on resonant search sensitivity. At low frequencies, the application of our optimization may enhance scan rates by a few orders of magnitude. We show that, in contrast to some previous work, resonant searches benefit from quality factors above one million, which corresponds to the characteristic quality factor (inverse of fractional bandwidth) of the dark-matter signal. We also show that the optimized resonator is superior, in signal-to-noise ratio, to the optimized reactive broadband detector at all frequencies at which a resonator may practically be made. Finally, we discuss prospects for evading the quantum limits using backaction evasion, photon counting, squeezing, entanglement, and other nonclassical approaches, in the context of directions for further investigation.

We introduce the DM Radio, a dual search for axion and hidden photon dark matter using a tunable superconducting lumped-element resonator. We discuss the prototype DM Radio Pathfinder experiment, which will probe hidden photons in the 500 peV (100 kHz)-50 neV (10 MHz) mass range. We detail the design of the various components: the LC resonant detector, the resonant frequency tuning procedure, the differential SQUID readout circuit, the shielding, and the cryogenic mounting structure. We present the current status of the pathfinder experiment and illustrate its potential science reach in the context of the larger experimental program.

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