Flux Qubit-Based Detector: A Game Changer for Microwave Photon Detection and Quantum Physics

A flux qubit-based detector of microwave photons uses a superconducting qubit to detect microwave photons, a challenging task due to the small energies of these photons. The detector uses a flux qubit, characterized by two levels belonging to different potential wells, to measure changes in the direction of magnetic flux when a photon arrives. This technology has applications in various fields of physics, including measuring superconducting qubits, entangling two remote superconducting qubits, imaging with a small number of photons, and searching for dark matter axions. The research has potential implications for quantum computing and photodetection.

What is a Flux Qubit-Based Detector of Microwave Photons?

A flux qubit-based detector of microwave photons is a device that uses a flux qubit, a type of superconducting qubit, to detect microwave photons. This technology is used in various fields of physics, including measuring superconducting qubits, entangling two remote superconducting qubits, imaging with a small number of photons, and searching for dark matter axions.

The detection of microwave photons is challenging due to the small energies of these photons. Different solutions have been proposed to overcome these difficulties, but no prevailing technology exists. Some detectors of microwave photons are based on photon-assisted tunneling in semiconducting double quantum dot circuits. Another research direction uses superconducting circuits with Josephson junctions, which are nonlinear inductors and allow making superconducting qubits.

The usual single-photon detectors are destructive, meaning that a photon is absorbed, transitioning the qubit from the ground state to the excited one. This transition induces a click of the detector. However, quantum non-demolition detection of microwave photons has also been reported.

How Does a Flux Qubit-Based Detector Work?

A flux qubit-based detector works by using a flux qubit, a type of superconducting qubit, to detect microwave photons. A flux qubit is characterized by two levels that belong to different potential wells, meaning they are characterized by opposite directions of a magnetic flux or supercurrent flowing in the loop.

When a photon arrives, it changes the direction of the magnetic flux, and this change is measured. A flux qubit-based single-photon detector has been studied both theoretically and experimentally. In these studies, the dressed states of a superconducting qubit represented an artificial Λ-type three-level system. The signal pulse was in a weak coherent state with a mean photon number of order 0.1.

The parameters which correspond to near-perfect absorption condition were found. At these values of parameters, the reflection of the input signal is reduced. In this paper, the authors study a flux qubit-based detector of microwave photons in a semiclassical approximation, which means that the input signal is in a coherent state.

What is the Principle of Operation of the Detector?

The principle of operation of the flux qubit-based detector of microwave photons involves the use of a flux qubit as a multilevel system. Some experimental studies of a flux qubit-based photon detector of this kind were presented, and the scheme of weak continuous measurement for readout of the flux qubit states was proposed.

Using numerical calculations, the authors plot the dynamics of qubits’ populations for readout and reset stages of detection. The stationary Hamiltonian of a flux qubit is written down, and stationary energy levels and eigenfunctions are obtained numerically.

The superconducting flux qubit can be described by an electric circuit that contains a Josephson junction and an inductance and is pierced by an external magnetic flux. The full magnetic flux in the loop and the external magnetic flux are related by a transcendental equation, which is due to the existence of a shielding current and holds for radiofrequency SQUID.

What is the Significance of the Superconducting Flux Qubit?

The superconducting flux qubit is a crucial component of the flux qubit-based detector of microwave photons. It can be described by an electric circuit that includes a Josephson junction and an inductance and is pierced by an external magnetic flux.

The full magnetic flux in the loop and the external magnetic flux are related by a transcendental equation, which is due to the existence of a shielding current and holds for radiofrequency SQUID. The energy associated with the capacitance can be considered as a kinetic energy.

There are two potential energy terms, i.e., associated with the inductance and the Josephson junction energy. The superconducting flux qubit is a multilevel system, and the dynamics of the occupations of the levels of the multilevel system for readout and reset stages are calculated.

What are the Future Implications of this Research?

The research on the flux qubit-based detector of microwave photons has significant implications for the field of physics. The theoretical description of the driven-dissipative dynamics of qudits, including applications such as single-photon detection, can be useful.

The detection of microwave photons is used in various fields of physics, and the development of more efficient and effective detectors can have a significant impact on these fields. The research also contributes to the understanding of superconducting qubits and their potential applications.

The authors conclude by providing an approximate analytical description of the reset stage dynamics, which can be useful for further research and development in this area. The research is a significant contribution to the field of quantum physics and has potential applications in various areas, including quantum computing and photodetection.