Direct Photonic Coupling of a Semiconductor Quantum Dot and a Trapped Ion

Coupling individual quantum systems lies at the heart of building scalable quantum networks. Here, we report the first direct photonic coupling between a semiconductor quantum dot and a trapped ion and we demonstrate that single photons generated by a quantum dot controllably change the internal state of a ${\mathrm{Yb}}^{+}$ ion. We ameliorate the effect of the 60-fold mismatch of the radiative linewidths with coherent photon generation and a high-finesse fiber-based optical cavity enhancing the coupling between the single photon and the ion. The transfer of information presented here via the classical correlations between the ${\sigma }_z$ projection of the quantum-dot spin and the internal state of the ion provides a promising step towards quantum-state transfer in a hybrid photonic network.

Figure 1

Experimental setup and optical transitions of the atomic and solid-state nodes. (a) The QD is located inside a magneto-optical cryostat operating at 4.2 K and 4.2 T. Single photons generated resonantly at 935 nm are sent to the atomic node via a 50 m single-mode fiber. The ion is placed inside a high-finesse optical cavity resonant with the ion transition at 935 nm. (b) Relevant level schemes of a neutral InAs QD (left) and a ${}^{174}{\mathrm{Yb}}^{+}$ ion (right). The $|0⟩-|+1⟩$ transition of the QD is on resonance with the ${{}^{2}D}_{3/2}-{}^{3}D[3/2{]}_{1/2}$ transition of ${}^{174}{\mathrm{Yb}}^{+}$. At 4.2 T the $|0⟩-|-1⟩$ transition is detuned by 75 GHz and is not addressed. The $S\text{−}P$ transition of ${{}^{174}\mathrm{Yb}}^{+}$ at 369 nm is used for laser cooling and state readout of the ion. The number insets (in parentheses) are the cavity-modified (natural) ${{}^{174}\mathrm{Yb}}^{+}$ branching ratios. (c) Absorption spectra of the QD (left) and the ion (right) transitions centered at 935 nm. The 60-fold mismatch in the radiative linewidths is further exacerbated to 93 including power broadening and spectral wandering effects.

Figure 2

Optical pumping of the ion by QD photons. (a) The ion is prepared in the $|-3/2⟩$ state of the ${{}^{2}D}_{3/2}$ manifold by optical pumping at 369 and 935 nm simultaneously [28]. Subsequently, the QD is excited for a time $T$ and the generated photons are sent to the ion. The protocol cycle ends with the optical readout of the ion state. (b) Probability of the ion to absorb a QD photon during $T$ in the weak excitation regime ($I=0.5I_{\mathrm{sat}}$). The solid line is an exponential fit with a time constant $\tau =1.08(4)\mathrm{ms}$. The top axis indicates the mean photon number for a given $T$ reaching the ion-cavity system and has 10% statistical error.

Figure 3

Efficiency of ion-state transfer as a function of the QD-ion detuning and QD driving intensity. (a) Spectral dependency of the absorption probability per received photon. The solid line displays the numerical model. The slight asymmetry is due to the detuning of the QD transition from the excitation laser frequency ${\omega }_L$ induced by finite nuclear spin polarization and the presence of fast dephasing at high excitation powers, characteristic for this sample [32]. (b) Absorption probability of the ion per received photon as a function of the QD excitation intensity. Insets show the QD photon spectrum $\mathcal{S}(\omega )$ at two different excitation intensities and the ion absorption line $\mathcal{L}(\omega )$. The convolution of the two, normalized by the number of QD photons, gives rise to the solid line. Including the imperfect suppression of the excitation laser (ratio of QD to laser photons is $70:1$ at $I_{\mathrm{sat}}$) in the numerical modeling predicts the dashed line. The error bars are statistical errors. We note that the ion-cavity coupling rate for the results in (b) is slightly higher than that for the measurements in (a).

Figure 4

Correlations between QD-spin projection and the state of the ion. (a) At 0.7 T magnetic field the 20 GHz Zeeman splitting between the two transitions of a negatively charged QD ensures that only the $|\uparrow ⟩-|\Uparrow \downarrow \uparrow ⟩$ transition is resonant with the ion. The spin is prepared by optical pumping for a finite duration of $\tau$. Then, the $|\uparrow ⟩-|\Uparrow \downarrow \uparrow ⟩$ transition is driven for a fixed time of 600 ns and the generated photons are sent to the ion. (b) The measured spin-projection dependence of the ion-state transfer rate which has been normalized to maximum state-transfer rate of 318 Hz (solid circles). The solid (dashed) curve is the expected dependence including (excluding) imperfect laser rejection. Uncertainties on the QD spin-up projection are within the width of the data points.