In these years, the study of quantum techniques has been growing so rapidly. New ideas for quantum calculations are being developed, and even the quantum computer will be available in the near future. New quantum tools can be the key to access the physics we have not ever seen.

One example is the application of the two-level qubit to searches of dark matter (DM) of dark photons. When a dark photon enters the qubit system, it behaves as an effective electric field and then excites the qubit. Direct measurements of the excitation of a qubit are already achieved. However, in order to claim the significance, we have to achieve a high signal-to-noise ratio which is, in the simplest setup, proportional to the number of the excited qubits.

The gain can be improved by introducing another quantum nature to such an experiment. By constructing the quantum circuit combining single systems, the signal-to-noise ratio proportional to the square of the number of the excited qubits is obtained. Behind this magic, coherent accumulations of the phase are achieved due to the entanglement effect. Thanks to this enhancement, the sensitivity to dark photon DM can be improved by about two orders of magnitude compared to the case without a circuit.

Further exciting news we have is that such equipment is coming to the real world. Superconducting qubit is realized as an ultra-cold non-linear LC circuit, the readout system is developed using a cavity. The coherence time is important for achieving good readout and it is now rapidly improving. All the system has already been assembled at the University of Tokyo,
meaning that it is ready to operate.

The characterization of the system is now ongoing and discussion of further improvements and enhancement starts. We should see fantastic results in the near future!

Reported by Nagisa Hiroshima