Detection of Weak Microwave Fields with an Underdamped Josephson Junction

Abstract

We have constructed a microwave detector based on the voltage switching of an underdamped Josephson junction, that is positioned at a current antinode of a λ/4 coplanar waveguide resonator. By measuring the switching current and the transmission through a waveguide capacitively coupled to the resonator at different drive frequencies and temperatures we are able to fully characterize the system and assess its detection efficiency and sensitivity. Testing the detector by applying a classical microwave field with the strength of a single photon yielded a sensitivity parameter of 0.5 in qualitative agreement with theoretical calculations.PACS numbers: 07.57. Kp, 74.78.Na, 85.25.Cp The light emission by single, microscopic quantum systems displays a number of non-classical features which have been exploited in fundamental investigations in quantum physics and which may result in applications in metrology, quantum communication and computing. Potential applications, however, would suffer from the rather weak coupling between atoms and single optical photons. This has stimulated efforts to study the same features with macroscopic artificial atoms. A particularly successful system relies on solid state superconducting circuits. Due to the Josephson non-linearity such circuits have an anharmonic excitation spectrum and may be restricted to an effective two-level systems which can interact resonantly with microwave fields. Besides the stronger coupling of superconducting circuits, an additional advantage is that they can be designed and fabricated on chip-scale, thereby allowing the integration in and scaling to larger systems with multiple components.Essential quantum optical effects with superconducting qubits, such as vacuum Rabi splitting [1], resonance fluorescence of a single artificial atom [2], and single atom lasing [3] have already been observed. Microwave fields can be amplified, detected and fully characterized in homodyne set-ups [4]. The effective coupling to transmission wave guides has made it possible to efficiently monitor the emitted radiation and verify the validity of the quantum trajectories of qubits conditioned on the detection record [5,6], as well as to apply feedback and stabilize coherent superposition states of the qubit [7].Quantum optics benefits from high efficiency single photon detectors. It relies on the energy of the individual photons being sufficient to exploit the photoelectric effect and liberate an electron which can be amplified and detected [8]. Transition edge sensors [9] and superconducting nanowire single photon detectors [10,11] also require a sufficiently large energy of the incident photon to heat and thus modify the current through the detector. The energy of microwave photons is too low to allow detection by these methods, and for this energy range both controllable single photon sources and efficient single photon detectors are still under development.When working in the single photon regime, it is an obvious choice to use the resonant coupling to qubit systems. Indeed...

Application and fabrication aspects of sub-micrometer-sized Josephson junctions

“…Therewith the resonator frequency will shift to lower values making the sensitivity frequency dependent. For even higher detunings already a value of 0.5 of sensitivity at a single photon level is achieved for the discussed device [36].…”

Microwave photon detection by an Al Josephson junction

“…There are, at least, two different approaches for practical realization of the single-photon detectors based on Josephson junctions, both having their advantages and disadvantages. The first approach relies on a continuous current sweep at a constant repetition rate and the measurements of the switching current distributions, from which the response and sensitivity can be determined [13][14][15]. In particular, in [15] the tunneling properties of the current-biased Josephson junction coupled with a resonator directly depend on the number of microwave photons in the resonator.…”

On a simple derivation of the very low damping escape rate for classical spins by modifying the method of Kramers

“…Versions of the Kramers escape rate theory are still being employed in innovative scientific research. For example, this theory as applied to Josephson junctions [6] has recently been used in the design of single photon detectors [7]. The current-voltage characteristics of Josephson junctions can be modelled as a Brownian particle in a tilted periodic potential, where the degree of the tilt is proportional to the bias current and the escape from a potential well corresponds to the creation of a non-zero voltage across the junction.…”