Abstract Quantum networks typically operate in the telecom wavelengths to take advantage of low-loss transmission in optical fibres. However, bright quantum dots (QDs) emitting highly indistinguishable quantum states of light, such as InGaAs QDs, often emit photons in the near infrared thus necessitating frequency conversion (FC) to the telecom band. Furthermore, the signal quality of quantum emissions is crucial for the effective performance of these networks. In this work we report a method for simultaneously implementing spectral purification and frequency shifting of single photons from QD sources to the c-band in a periodically poled lithium niobate waveguide. We consider difference frequency generation in the counter-propagating configuration to implement FC with the output emission bandwidth in units of GHz. Our approach establishes a clear path to integrating high-performance single-emitter sources in a hybrid quantum network.
Abstract Quantum networks typically operate in the telecom wavelengths to take advantage of low-loss transmission in optical fibres. However, bright quantum dots (QDs) emitting highly indistinguishable quantum states of light, such as InGaAs QDs, often emit photons in the near infrared thus necessitating frequency conversion (FC) to the telecom band. Furthermore, the signal quality of quantum emissions is crucial for the effective performance of these networks. In this work we report a method for simultaneously implementing spectral purification and frequency shifting of single photons from QD sources to the c-band in a periodically poled lithium niobate waveguide. We consider difference frequency generation in the counter-propagating configuration to implement FC with the output emission bandwidth in units of GHz. Our approach establishes a clear path to integrating high-performance single-emitter sources in a hybrid quantum network.
On-demand indistinguishable single-photon sources are essential for quantum networking and communication. Semiconductor quantum dots are among the most promising candidates, but their typical emission wavelength renders them unsuitable for use in fiber networks. Here, we present quantum frequency conversion of near-infrared photons from a bright quantum dot to the telecommunication C-band, allowing integration with existing fiber architectures. We use a custom-built, tunable 2400 nm seed laser to convert single photons from 942 nm to 1550 nm in a difference-frequency generation process. We achieve an end-to-end conversion efficiency of ≃35%, demonstrate count rates approaching 1 MHz at 1550 nm with g(2)(0)=0.043(1), and achieve Hong-Ou-Mandel (HOM) visibilities of 60%. We expect this scheme to be preferable to quantum dot sources directly emitting at telecom wavelengths for fiber-based quantum networking.
Single-photon sources based on quantum dots have been shown to exhibit almost ideal properties such as high brightness and purity in terms of clear anti-bunching as well as high two-photon interference visibilities of the emitted photons, making them promising candidates for different quantum information applications such as quantum computing, quantum communication and quantum teleportation. However, as most single-photon sources also quantum dots typically emit light at wavelengths of electronic transitions within the visible or the near infrared range. In order to establish quantum networks with remote building blocks, low-loss single photons at telecom wavelengths are preferable, though. Despite recent progress on emitters of telecom-photons, the most efficient single-photon sources still work at shorter wavelengths. On that matter, quantum frequency down-conversion, being a nonlinear optical process, has been used in recent years to alter the wavelength of single photons to the telecom wavelength range while conserving their nonclassical properties. Characteristics such as lifetime, first-order coherence, anti-bunching and entanglement have been shown to be conserved or even improved due to background suppression during the conversion process, while the conservation of indistinguishability was yet to be shown. Here we present our experimental results on quantum frequency down-conversion of single photons emitted by an InAs/GaAs quantum dot at 903.6 nm following a pulsed excitation of a p-shell exciton at 884 nm. The emitted fluorescence photons are mixed with a strong pump-field at 2155 nm inside a periodically poled lithium niobate ridge waveguide and converted to 1557 nm. Common issues of a large background due to Raman-scattered pump-light photons spectrally overlapping with the converted single photons could largely be avoided, as the pump-wavelength was chosen to be fairly longer than the target wavelength. Additional narrowband spectral filtering at the telecom regime as a result of the small conversion bandwidth and using a high-performance fiber-Bragg-grating solely left the detector dark counts as the only noise source in our setup. Therefore, we could achieve conversion efficiencies of more than 20 %. In order to test the indistinguishability, sequentially emitted photons were fed into a Mach-Zehnder interferometer and spatially as well as temporally overlapped at the output beam splitter. Cross-correlation measurements between both output-ports of the beam splitter exhibit two-photon interference contrasts of more than 40 % prior to and after the down-conversion step. Accordingly, we demonstrate that the process of quantum frequency conversion preserves photon indistinguishability and can be used to establish a versatile source of indistinguishable single photons at the telecom C-Band. Furthermore our scheme allows for converting photons in a wavelength band from 900 nm to 910 nm to the same telecom target wavelength. This enables us to test indistinguishability of frequency-converted photons, originally stemming from different sources with dinstinguishable wavelengths.
We demonstrate a quantum interface that simultaneously converts the wavelength, bandwidth, and duration of single-photon-level pulses, enabling compatibility between disparate quantum systems. Using difference frequency generation in a lithium niobate waveguide driven by a highly chirped pump, we transform pulses from 798 nm, 5 GHz, 150 ps (quantum dot-like) to 1300 nm, 35 GHz, < 25 ps (telecom standard). This nonlinear optical time lens achieves over 80% internal conversion efficiency and compresses output pulses below detector resolution. The device offers a compact solution for integrating quantum emitters, memories, and telecom networks, with potential for further pulse shape control.
We demonstrate a quantum interface that simultaneously converts the wavelength, bandwidth, and duration of single-photon-level pulses, enabling compatibility between disparate quantum systems. Using difference frequency generation in a lithium niobate waveguide driven by a highly chirped pump, we transform pulses from 798 nm, 5 GHz, 150 ps (quantum dot-like) to 1300 nm, 35 GHz, < 25 ps (telecom standard). This nonlinear optical time lens achieves over 80% internal conversion efficiency and compresses output pulses below detector resolution. The device offers a compact solution for integrating quantum emitters, memories, and telecom networks, with potential for further pulse shape control.
Signal photons emitted by quantum nodes typically fall outside the low-loss telecom window of optical fibers, leading to severe transmission losses. Quantum frequency conversion (QFC) offers an effective optical interface that bridges quantum nodes with telecom-band channels, enabling long-distance quantum communication. In this work, we demonstrate a compact, fiber-integrated QFC system with low noise and a high signal-to-noise ratio (SNR). Using a periodically poled lithium niobate (PPLN) waveguide, input photons at 637.2 nm are down-converted to telecom photons at 1588.3 nm. Our system achieves a total conversion efficiency of approximately 9%, with pump-induced noise suppressed to 154 Hz. For input photon rates of 32.7, 118.0, and 327.7 kHz, the corresponding SNRs are 12.3, 43.9, and 117.8, respectively. We further develop a theoretical model to simulate the entanglement fidelity between nitrogen-vacancy (NV) center spins and the frequency-converted telecom photons. At the emission rate of an NV center, our QFC system maintains an expected fidelity exceeding 52% over a transmission distance of 100 km. These findings highlight the potential of our QFC system for scalable, long-distance quantum networking.
A source of bright and pure single photons is an essential tool for photonic quantum technologies, predicted to enable secure communication and networking. Self-assembled quantum dots are among the leading candidates to realise such a source. The currently highest-performing quantum dots emit at wavelengths unsuitable for fibre transmission, with telecom quantum dots lagging in performance. An intermediate step is to use quantum frequency conversion. Here we report to our knowledge the brightest quantum dot based source of telecom photons by frequency converting a near-infrared quantum dot embedded in a micropillar cavity to the telecom C-band. In a single-photon BB84 protocol, this source is capable of producing asymptotic keys rates of 1 kbps at over 150 km of optical fibre.