Phonon-Assisted Two-Photon Interference from Remote Quantum Emitters

Abstract

Photonic quantum technologies are on the verge of finding applications in everyday life with quantum cryptography and quantum simulators on the horizon. Extensive research has been carried out to identify suitable quantum emitters and single epitaxial quantum dots have emerged as near-optimal sources of bright, on-demand, highly indistinguishable single photons and entangled photon-pairs. In order to build up quantum networks, it is essential to interface remote quantum emitters. However, this is still an outstanding challenge, as the quantum states of dissimilar "artificial atoms" have to be prepared on-demand with high fidelity and the generated photons have to be made indistinguishable in all possible degrees of freedom. Here, we overcome this major obstacle and show an unprecedented two-photon interference (visibility of 51 ± 5%) from remote strain-tunable GaAs quantum dots emitting on-demand photon-pairs. We achieve this result by exploiting for the first time the full potential of a novel phonon-assisted two-photon excitation scheme, which allows for the generation of highly indistinguishable (visibility of 71 ± 9%) entangled photon-pairs (fidelity of 90 ± 2%), enables push-button biexciton state preparation (fidelity of 80 ± 2%) and outperforms conventional resonant two-photon excitation schemes in terms of robustness against environmental decoherence. Our results mark an important milestone for the practical realization of quantum repeaters and complex multiphoton entanglement experiments involving dissimilar artificial atoms.

Keywords: Quantum dots; entanglement; quantum optics; resonant two-photon excitation; two-photon interference.

Figure 1

Spectrum and power dependent studies. (a) Spectrum of a GaAs QD under phonon-assisted TPE for optimized detuning of the laser energy and pulse length (E_L = 1.5901 eV, τ_p = 10 ps) and a pulse area of 6π. X and XX are clearly visible and the residual lines are attributed to laser scattering (mostly suppressed with notch filters without any polarization rejection) as well as weakly excited charged states (inset: sketch of the phonon-assisted two-photon excitation scheme used in this work. The dashed-orange lines represent the vibrational quasicontinuum coupled to the biexciton state). (b) Population of the X state as a function of the laser detuning for varying excitation power. While the traditional TPE (blue) suffers from a steep drop in inversion efficiency, a stable plateau can be observed exploiting the QD phonon sideband (green). The measured data for high excitation powers (7π) are interpolated with an asymmetric-modulated Gaussian function while the low power data (π-pulse) is fitted with a Lorentzian function. The two excitation regimes are distinguished considering the full width at half-maximum (fwhm) of the excitation laser (∼0.2 meV). In particular, the dashed line indicates the configuration in which the detuning is set to a value that equals the fwhm. (c) Power dependent studies of the resonant TPE with (blue) and without (red) white light. The envelope of the Rabi oscillation is modeled with a single exponential damping. The results of the phonon-assisted excitation scheme are shown as green circles.

Figure 2

Comparison of entanglement. (a) XX–X cross-correlation measurements under phonon-assisted TPE for different polarization detection bases: rectilinear (H,V), diagonal (D,A) and circular basis (R,L). (b) Fidelity to the expected Bell state for all the different excitation methods.

Figure 3

Two-photon interference using the same, randomly chosen, QD. The two-photon interference measurement in co-polarized configuration is performed on the same QD for (a) the standard TPE (b) the white-light-assisted TPE and (c) the phonon-assisted TPE, as schematically illustrated on top of each panel. The envelope function (bold) is the sum of 5 Lorentzian peaks fitted to the Hong-Ou-Mandel quintuplet. The resulting interference visibilities are reported in each panel.

Figure 4

Two-photon interference from remote QDs. (a) Illustration of the interference of single photons from remote GaAs QD sources (ice cubes). A pulse-shaped femtosecond laser is split (at BS₁) and excites both QDs throughout excitation beam splitters (BS_A and BS_B, respectively) via the phonon-assisted TPE. The stream of photons from one of the QDs is mechanically delayed by a excitation-located delay line (DL) to ensure perfect timing coincidence for photons meeting at the interference beam splitter BS₂. Strain tuning (c-clamp) of one QD allows for the precise frequency matching of the emitted photons, which are either co- or cross-polarized (using the polarizers P). Single-photon counters (D₁ and D₂) are used to assess the quality of the interference by performing cross-correlation measurements at the two outputs of BS₂. (b) Color-coded photoluminescence spectra of the X emission of QD A and QD B as a function of the piezo voltage (V_p) applied to QD A to achieve color coincidence with the X transition of QD B. (c) Second-order correlation for remote X photons excited in the phonon-assisted TPE scheme in co- and cross-polarized configuration. (d) Remote two-photon interference visibility as a function of detuning between the two X photons.