Electrically-Pumped Wavelength-Tunable GaAs Quantum Dots Interfaced with Rubidium Atoms

Abstract

We demonstrate the first wavelength-tunable electrically pumped source of nonclassical light that can emit photons with wavelength in resonance with the D₂ transitions of ⁸⁷Rb atoms. The device is fabricated by integrating a novel GaAs single-quantum-dot light-emitting diode (LED) onto a piezoelectric actuator. By feeding the emitted photons into a 75 mm long cell containing warm ⁸⁷Rb vapor, we observe slow-light with a temporal delay of up to 3.4 ns. In view of the possibility of using ⁸⁷Rb atomic vapors as quantum memories, this work makes an important step toward the realization of hybrid-quantum systems for future quantum networks.

Keywords: GaAs quantum dots; atomic vapors; light emitting diodes; single photon source; strain actuators.

Figure 1

Sketch of the electrically pumped wavelength-tunable Q-LED. The n-i-p diode contains GaAs QDs embedded in Al_0.4Ga_0.6As barriers and is integrated on a PMN-PT piezoelectric actuator, which provides variable strain fields to tune the photon emission energy. V_d is the bias voltage applied to the diode and V_p is the voltage applied to the actuator.

Figure 2

(a) Typical current–voltage (I–V) characteristics of the QD LED, with a diode turn-on voltage of ∼2.3 V. (b) Evolution of the electroluminescence (EL) spectra of a single QD embedded in the tunable-LED with the magnitude of the applied voltage V_d. The intensity (in CCD counts per second) is color-coded. In the spectra, the brightest line stems from the neutral exciton (X) transition, which is well separated from the group of low-energy states, which are ascribed to charged and neutral multiexcitonic (MX) states. (c) Color-coded microelectroluminescence (μ-EL) spectra of the exciton in (a), whose wavelength is scanned across the D₂ transitions of the ⁸⁷Rb cloud (at a temperature T_Rb = 70 °C) by applying variable stress on the Q-LED membrane. The white dotted line indicates the middle wavelength of the D₂ transitions. The transmitted intensity drops at 16.05 kV/cm and 16.40 kV/cm due to the absorption by the atomic vapor (region indicated by dashed circle). (d) Transmitted intensity as a function of the electric field applied to the piezoelectric actuator (F_p), obtained by extracting the peak intensity values from the corresponding spectra in (c), as described in the text. The two dips correspond to the two hyperfine lines of the ground state of the D₂ transitions.

Figure 3

(a) EL spectra of a second QD, acquired with a double spectrometer equipped with two 1200 l/mm gratings (spectral resolution ∼30 μeV). The two spectra correspond to two orthogonal polarization directions. Only the bright line marked by arrows (with a line width of 58 μeV) is used in the following measurement. (b) HBT setup used to extract the second-order correlation function, g⁽²⁾(τ), of the EL emitted by a single QD embedded in the Q-LED. A 75 mm long ⁸⁷Rb vapor cell is inserted in one of the arms of the setup. (c) Black: g⁽²⁾(τ) measurement of the EL emission when the photon energy is tuned off resonance with respect to the ⁸⁷Rb D₂ transitions. Red and blue: g⁽²⁾(τ) values when the photon energy is tuned on resonance with the ⁸⁷Rb D₂ lines and the Rb cell temperature is 96 (red), and 100 °C (blue). The corresponding minimum values of g⁽²⁾(τ), reached at a T_Rb-dependent delay time τ_c(T_Rb) are 0.15, 0.44, and 0.53 for the data displayed in of the black, red, and blue, respectively. The gradual increase of τ_c and of g⁽²⁾(τ_c) with increasing T_Rb comes from the dispersion of the atomic optical medium. This leads to slow light and, due to the finite line width of our photon source, to time-jitter and consequent broadening of the g⁽²⁾(τ) curve.