Visible-to-Telecom Quantum Frequency Conversion of Light from a Single Quantum Emitter

We demonstrate efficient ($>30\%$) quantum frequency conversion of visible single photons (711 nm) emitted by a quantum dot to a telecom wavelength (1313 nm). Analysis of the first- and second-order coherence before and after wavelength conversion clearly proves that pivotal properties, such as the coherence time and photon antibunching, are fully conserved during the frequency translation process. Our findings underline the great potential of single photon sources on demand in combination with quantum frequency conversion as a promising technique that may pave the way for a number of new applications in quantum technology.

Figure 1

Frequency down-conversion of nonclassical light emitted by a single QD. (a) Experimental setup. The confocal microscope consists of a liquid helium (LHe) continuous flow cryostat containing the QD sample at a temperature of 12 K. A bias-voltage of 3.20–3.47 V is applied to the QDs which are additionally optically excited using an average power of 160 nW at 590 nm from a pulsed fs optical parametric oscillator (OPO) with 80 MHz repetition rate [35]. Optical excitation and collection of photoluminescence (PL) emitted by a QD are performed using a $100\times$ microscope objective with numerical aperture $\mathrm{NA}=0.8$. We collect up to $188400\mathrm{photons}/\mathrm{s}$ into a single-mode fiber (SMF600). A silica etalon can be inserted optionally for narrow-band filtering of the PL. Two long-pass filters prevent residual excitation light from entering the fiber. For frequency down-conversion, the visible photons are coupled into the Zn:PPLN ridge waveguide together with a strong pump beam at 1550 nm provided by a continuous-wave OPO (see the Supplemental Material [27]). The converted photons are spatially separated from the strong pump light and from residual visible photons by a prism and a pinhole and are coupled into a standard telecom fiber (SMF28). To suppress residual pump light and noise background around the target wavelength ${\lambda }_{\mathrm{out}}$, we additionally use a spectral filtering setup composed of a fiber-optic circulator, a fiber Bragg grating (FBG) centered at 1312.714 nm ($-1.0\mathrm{dB}$ reflection bandwidth: 0.755 nm) and two $1310\mathrm{nm}/1550\mathrm{nm}$ wavelength division multiplexers (WDM1 and WDM2). SMF: single mode fiber, PC: polarization control, HWP: half-wave plate, PBS: polarizing beam splitter, BC: beam combiner, AL: aspheric lens. (b) Total efficiency and SNR of the frequency conversion setup calculated from measured data using single photon input from a QD. (c) Spectral acceptance bandwidth of the DFG process measured for 2 mW input power around 710.74 nm from a tunable cw Ti:sapphire laser. For further details, see the Supplemental Material [27].

Figure 2

PL maps with corresponding spectra. (a),(c),(e) Same $6\times 6\mu \mathrm{m}$ detail of the QD sample. Maps (a) and (c) were recorded by scanning the $xy$ position of the sample and directly detecting the visible PL from the QD using a Si-APD (a) without and (c) with etalon filtering. Map (e) was obtained by scanning the same sample area but detecting the down-converted light at 1313 nm with a SSPD. (b),(d),(f) Spectra that were measured by setting the $xy$ position of the sample to the point corresponding to the maximum intensity of the central QD. (b),(d) Visible PL spectrum without and with etalon filtering, respectively. The two prominent lines in (b), separated by 1.45 nm (3.54 meV), are attributed to exciton ($X$) and biexciton ($XX$) transitions. (f) Converted spectrum illustrating the effect of spectral filtering by a combination of DFG acceptance curve and FBG.

Figure 3

Single-photon temporal coherence and lifetime before and after frequency down-conversion. (a) Visibility of first-order interference fringes vs time delay for PL from the QD at 711 nm (lower curve: data points and fit) and for converted light at 1313 nm (upper curve: data points and fit) deduced from measurements of the $g^{(1)}(\tau )$ correlation function. The insets show examples of interference fringes at fixed delay time (left inset: interference at 711 nm; right inset: interference at 1313 nm). (b) Lifetime of single photons measured via time-correlated single photon counting for original visible light and converted telecom light.

Figure 4

Conservation of photon antibunching under frequency down-conversion: simulation and measurement. (a) $g^{(2)}$ correlation function of the single photons at 711 nm emitted by the QD (integration time $t_{\mathrm{int}}=745\mathrm{s}$). (b) $g^{(2)}$ correlation function of the converted light at 1313 nm ($t_{\mathrm{int}}=6085\mathrm{s}$). (c) $g^{(2)}$ cross-correlation function measured with a hybrid HBT setup, i.e., one half of the visible photons is sent directly to a visible photon detector, the other half undergoes frequency down-conversion and is then detected with an IR photon detector ($t_{\mathrm{int}}=9440\mathrm{s}$). Black circles in (a)–(c) represent measured data while the solid curves are obtained from Monte Carlo simulations (see the Supplemental Material [27]). No background has been subtracted from the experimental data in all plots.