This paper reports on a cavity haloscope search for dark matter axions in the galactic halo in the mass range 2.81-3.31 µeV . This search excludes the full range of axion-photon coupling values predicted in benchmark models of the invisible axion that solve the strong CP problem of quantum chromodynamics, and marks the first time a haloscope search has been able to search for axions at mode crossings using an alternate cavity configuration. Unprecedented sensitivity in this higher mass range is achieved by deploying an ultra low-noise Josephson parametric amplifier as the first-stage signal amplifier.Axions are a hypothesized particle that emerged as a result of the Peccei-Quinn solution to the strong CP problem [1][2][3]. In addition, axions are a leading darkmatter candidate that could explain 100% of the darkmatter in the Universe [4][5][6][7][8]. There are a number of mechanisms for the production of dark-matter axions in the early Universe [5,6,9,10]. For the case where U PQ (1) becomes spontaneously broken after inflation, cosmological constraints suggest an axion mass on the scale of 1 µeV or greater [11][12][13][14][15][16]. Two benchmark models for the axion are the Kim-Shifman-Vainshtein-Zakharov (KSVZ) [17,18] and Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) [19,20] models. Of the two, the DFSZ model is especially compelling because of its grand unification properties [19].The Axion Dark Matter eXperiment (ADMX) searches for dark-matter axions using an axion haloscope [21], which consists of a microwave resonant cavity inside a magnetic field. In the presence of an external magnetic field, axions inside the cavity can convert to photons with frequency f = E/h, where E is the total energy of the axion, including the axion rest mass energy, plus a small kinetic energy contribution. The power expected from the conversion of an axion into microwave photons in the ADMX experiment is extremely low, O(10 −23 W ), requiring the use of a dilution refrigerator and an ultra low-noise microwave receiver to detect the photons.In limits set in a previous paper, ADMX became the only axion haloscope to achieve sensitivity to both benchmark axion models for axion masses between 2.66 and 2.81 µeV [22]. This paper reports on recent operations which extend the search for axions at DFSZ sensitivity to 2.66-3.31 µeV .The ADMX experiment consists of a 136-liter cylindrical copper-plated cavity placed in a 7.6-T field produced by a superconducting solenoid magnet. The magnet and cavity configuration are similar to the configuration described in Ref. [23,24]. A magnetic field-free region above the cavity is maintained by a counter-wound bucking magnet above the cavity. Field sensitive receiver components, such as a Josephson parametric amplifier (JPA) and circulators, are located there, and the JPA is protected by additional passive magnetic shielding.The resonant frequency of the cavity is set by two copper tuning rods that run parallel to the axis of the cavity and can be positioned between near the center of the cavity and the...

The creation of a quantum network requires the distribution of coherent information across macroscopic distances. We demonstrate the entanglement of two superconducting qubits, separated by more than a meter of coaxial cable, by designing a joint measurement that probabilistically projects onto an entangled state. By using a continuous measurement scheme, we are further able to observe single quantum trajectories of the joint two-qubit state, confirming the validity of the quantum Bayesian formalism for a cascaded system. Our results allow us to resolve the dynamics of continuous projection onto the entangled manifold, in quantitative agreement with theory.

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