The length of time that a quantum system can exist in a superposition state is determined by how strongly it interacts with its environment. This interaction entangles the quantum state with the inherent fluctuations of the environment. If these fluctuations are not measured, the environment can be viewed as a source of noise, causing random evolution of the quantum system from an initially pure state into a statistical mixture--a process known as decoherence. However, by accurately measuring the environment in real time, the quantum system can be maintained in a pure state and its time evolution described by a 'quantum trajectory' determined by the measurement outcome. Here we use weak measurements to monitor a microwave cavity containing a superconducting quantum bit (qubit), and track the individual quantum trajectories of the system. In this set-up, the environment is dominated by the fluctuations of a single electromagnetic mode of the cavity. Using a near-quantum-limited parametric amplifier, we selectively measure either the phase or the amplitude of the cavity field, and thereby confine trajectories to either the equator or a meridian of the Bloch sphere. We perform quantum state tomography at discrete times along the trajectory to verify that we have faithfully tracked the state of the quantum system as it diffuses on the surface of the Bloch sphere. Our results demonstrate that decoherence can be mitigated by environmental monitoring, and validate the foundation of quantum feedback approaches based on Bayesian statistics. Moreover, our experiments suggest a new means of implementing 'quantum steering'--the harnessing of action at a distance to manipulate quantum states through measurement.
The act of measurement bridges the quantum and classical worlds by projecting a superposition of possible states into a single (probabilistic) outcome. The timescale of this 'instantaneous' process can be stretched using weak measurements, such that it takes the form of a gradual random walk towards a final state. Remarkably, the interim measurement record is sufficient to continuously track and steer the quantum state using feedback. Here we implement quantum feedback control in a solid-state system, namely a superconducting quantum bit (qubit) coupled to a microwave cavity. A weak measurement of the qubit is implemented by probing the cavity with microwave photons, maintaining its average occupation at less than one photon. These photons are then directed to a high-bandwidth, quantum-noise-limited amplifier, which allows real-time monitoring of the state of the cavity (and, hence, that of the qubit) with high fidelity. We demonstrate quantum feedback control by inhibiting the decay of Rabi oscillations, allowing them to persist indefinitely. Such an ability permits the active suppression of decoherence and enables a method of quantum error correction based on weak continuous measurements. Other applications include quantum state stabilization, entanglement generation using measurement, state purification and adaptive measurements.
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...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.