Superconducting Light-Emitting Diodes

Cooper pairs form spin singlet states and are regarded as entangled electron pairs. Extracting entangled electrons has been actively studied by the use of superconductor-normal metal junctions. We have proposed to convert Cooper pairs to entangled photon pairs via interband radiative recombination of Cooper pairs penetrated into a semiconductor by the proximity effect. We fabricated a superconducting (SC) light emitting diode, where a superconductor is used for the n-type electrode. We observed light output enhancement and recombination lifetime shortening below the SC critical temperature. Our observations were very well explained with an analysis based on a time-dependent perturbation theory of the radiative recombination. Superconductivity was included via the Bogoliubov transformation. We present our measurements on the Cooper-pair-enhanced luminescence from InAs quantum dots. We demonstrate the observation of sharp edge in the luminescence spectra that reflects the SC density of states.

I. Introduction

Quantum information and communications technology (Quantum ICT) is expected to be able to realize secure communication networks, high-capacity low-power communications, and so on [1]. In this new paradigm, superposition states of the bit signals “0” and “1”, referred to as quantum bits (qubits), play an essential role. The qubits are conveyed by quantum states of light, photonic qubits, for communications, and are converted into matter states of qubits for processing and storing. Matter state qubits include atom, molecule, nuclear spin, semiconductor, and superconductor systems. Superconducting (SC) qubits are of particular interest from the viewpoint of large-scale integration and well-established method of circuit quantum electrodynamics for electromagnetic-field control in the strong coupling regime [2] . Actually, coherent control of quantum states has been demonstrated in a single Cooper-pair box (CPB) [3] and also with SC flux qubits [4]. Transmon qubits [5], [6], which are qubits coupled with transmission cavities and are the optimum operations of the CPB, have demonstrated the coherence time approaching 0.1 ms [2], [7]. Deterministic quantum teleportation was also demonstrated with strong interactions between SC qubits held by Alice and Bob employing transmon qubits [8].