Electro-optic frequency shift of single photons from a quantum dot

Abstract

Quantum dots (QDs) are a promising source of single photons mainly due to their on-demand operation. However, their emission wavelength depends on their size and immediate surroundings in the solid-state environment. By applying a serrodyne electro-optic phase modulation, we achieve a spectral shift up to 0.01 nm (3.5 GHz) while preserving the purity and indistinguishability of the photons. This method provides an efficient and scalable approach for tuning the emission wavelength of QDs without relying on nonlinear frequency mixing or probabilistic processes. Our results show that the electro-optic phase modulation enables stable and tunable spectral shifts, making it suitable for applications such as quantum communication, quantum key distribution, and primarily integrating remote quantum dot sources into large-scale quantum networks.

Keywords: electro-optic phase modulation; quantum dots; quantum interface; quantum photonics; single-photon source; spectral shift.

Figure 1:

Schematic of the experimental setup: The QD is resonantly excited using a pulsed ps Ti:Sapphire laser, and the QD emission is collected into a polarization maintaining fiber (PM fiber). The collected emission is passed through a fiber-coupled phase modulator (EOPM), which is driven by a sawtooth RF waveform implementing a serrodyne frequency shift. For spectral measurements, the output fiber end of the EOPM is connected to a spectrometer at port (1). For single-photon purity measurements, the output of the EOPM is connected at port (2), skipping the interferometer setup (dashed-black box). For photon indistinguishability measurements, the output is connected at port (3) to an unbalanced in-fiber Mach–Zehnder interferometer, with a 26 ns delay. The EOPM acts passively when no voltage is applied, allowing comparison between shifted and unshifted photons.

Figure 2:

Spectra of the QD emission. The green trace shows the emission spectrum of the QD under pulsed-resonant excitation, while the black trace shows the background with detuned QD bias. The blue and red traces show the electro-optically blue-shifted and red-shifted spectra, respectively. A clear spectral shift of approximately 3.5 GHz (0.01 nm) is observed when the electro-optic phase modulator (EOPM) is activated. Solid-lines are Gaussian fits to the data. A background spectrum with the laser off is subtracted from all the spectra.

Figure 3:

Second-order correlation measurements of the single-photon source. The green trace shows the correlation histogram when the electro-optic phase modulator (EOPM) is off, indicating a photon purity of 88.7 % ± 2.1 %. The red and blue traces represent the results for red-shifted and blue-shifted photons, respectively, with a similar g ⁽²⁾(0) values within the error bars, confirming that the frequency shift does not degrade the single-photon purity.

Figure 4:

Photon indistinguishability: Hong–Ou–Mandel (HOM) interference. (a) HOM interference visibility of 53.1 % ± 5.6 % is observed for copolarized (green trace) and cross-polarized (orange trace) consecutive photons from the QD when the electro-optic phase modulator (EOPM) is off. (b) HOM visibility of 49.5 % ± 5.2 % and 46.9 % ± 6.1 % is observed for blue-shifted and red-shifted photons, respectively. These results confirm that the frequency shift does not degrade photon indistinguishability, with small differences falling within experimental uncertainty. The counts were accumulated for 15 min in all measurements.

Figure 5:

Tunable spectral shift using different modulation frequencies of serrodyne signal. (a) Normalized emission spectra of the QD for three different modulation frequencies 2.28 GHz (solid lines), 4.56 GHz (dotted lines), and 5.32 GHz (dashed lines). The blue and red shifts are achieved by changing the sign of the slope of serrodyne modulation. The magnitude of the spectral shift increases monotonically with the modulation frequency. The lines represent Gaussian fits to the data. (b) Plot of the measured spectral shift as a function of modulation frequency, showing good agreement with realistic simulations. The solid lines are derived from simulations of serrodyne electro-optic shift. Parameters of the modulation were estimated from specifications of the amplifier and the modulator. Frequency response of the modulator and finite bandwidth of the AWG were incorporated to make simulations experimentally realistic.