Quantum communication networks are used for QKD and metrological applications. We present research connecting two nodes ≈ 20 kilometers apart over the municipal fiber network using semiconductor quantum dots emitting at 1550 nm.

Quantum communication networks are used for QKD and metrological applications. We present research connecting two nodes ˜ 20 kilometers apart over the municipal fiber network using semiconductor quantum dots emitting at 1550 nm.

Quantum communication networks are used for QKD and metrological applications. We present research connecting two nodes ≈ 20 kilometers apart over the municipal fiber network using semiconductor quantum dots emitting at 1550 nm.

Photonic integrated circuits (PICs) present significant benefits with respect to table-top optical systems regarding footprint, stability, and power consumption. Among the materials used to fabricate PICs, thin film lithium niobate (TFLN) is one of the most attractive ones, as its χ(2) nonlinearity and electro-optic properties allow to implement on-chip light generation and routing [1]. On-chip detection of light has also been demonstrated on TFLN, based on the waveguide integration of superconducting nanowire single photon detectors (SNSPDs) [1]. Combining efficient detectors with TFLN nanophotonic waveguides holds promises for the realization of quantum photonics experiments fully on-chip. On the other hand, the sensitivity of SNSPDs changes with the wavelength of the detected photons [2], setting a boundary to the longest detectable wavelength and limiting the use of the wide transparency window of TFLN. However, this wavelength dependency in the response of SNSPDs can be leveraged to achieve new on-chip functionalities. In this work, by performing a straightforward analysis of the light signal measured at different bias currents [2], we operate hairpin SNSPDs on TFLN as waveguide-integrated wavelength-meters in the telecom bandwidth.

Lithium niobate, because of its nonlinear and electro-optical properties, is one of the materials of choice for photonic applications. The development of nanostructuring capabilities of thin film lithium niobate (TFLN) permits fabrication of small footprint, low-loss optical circuits. With the recent implementation of on-chip single-photon detectors, this architecture is among the most promising for realizing on-chip quantum optics experiments. In this Letter, we report on the implementation of superconducting nanowire single-photon detectors (SNSPDs) based on NbTiN on 300 nm thick TFLN ridge nano-waveguides. We demonstrate a waveguide-integrated wavelength meter based on the photon energy dependence of the superconducting detectors. The device operates at the telecom C- and L-bands and has a footprint smaller than 300 × 180 μm2 and critical currents between ∼12 and ∼14 μA, which ensures operation with minimum heat dissipation. Our results hold promise for future densely packed on-chip wavelength-multiplexed quantum communication systems.

Quantum communication networks will connect future generations of quantum processors, enable metrological applications, and provide security through quantum key distribution. We present a testbed that is part of the municipal fiber network in the greater Stockholm metropolitan area for quantum resource distribution through a 20 km long fiber based on semiconductor quantum dots emitting in the telecom C-band. We utilize the service to generate random numbers passing the NIST test suite SP800-22 at a subscriber 8 km outside of the city with a bit rate of 23.4 kbit/s.

Entangled photons are an integral part in quantum optics experiments and a key resource in quantum imaging, quantum communication, and photonic quantum information processing. Making this resource available on-demand has been an ongoing scientific challenge with enormous progress in recent years. Of particular interest is the potential to transmit quantum information over long distances, making photons the only reliable flying qubit. Entangled photons at the telecom C-band could be directly launched into single-mode optical fibers, enabling worldwide quantum communication via existing telecommunication infrastructure. However, the on-demand generation of entangled photons at this desired wavelength window has been elusive. Here, we show a photon pair generation efficiency of 69.9 +/- 3.6% in the telecom C-band by an InAs/GaAs semiconductor quantum dot on a metamorphic buffer layer. Using a robust phonon-assisted two-photon excitation scheme we measure a maximum concurrence of 91.4 +/- 3.8% and a peak fidelity to the Phi(+) state of 95.2 +/- 1.1%, verifying on-demand generation of strongly entangled photon pairs and marking an important milestone for interfacing quantum light sources with our classical fiber networks.

Entangled photon generation at 1550 nm in the telecom C-band is of critical importance as it enables the realization of quantum communication protocols over long distance using deployed telecommunication infrastructure. InAs epitaxial quantum dots have recently enabled on-demand generation of entangled photons in this wavelength range. However, time-dependent state evolution, caused by the fine-structure splitting, currently limits the fidelity to a specific entangled state. Here, we show fine-structure suppression for InAs quantum dots using micromachined piezoelectric actuators and demonstrate generation of highly entangled photons at 1550 nm. At the lowest fine-structure setting, we obtain a maximum fidelity of 90.0 +/- 2.7% (concurrence of 87.5 +/- 3.1%). The concurrence remains high also for moderate (weak) temporal filtering, with values close to 80% (50%), corresponding to 30% (80%) of collected photons, respectively. The presented fine-structure control opens the way for exploiting entangled photons from quantum dots in fiber-based quantum communication protocols.

We present self-assembly of InAs/InAlAs quantum dots by the droplet epitaxy technique on vicinal GaAs(111)A substrates. The small miscut angle, while maintaining the symmetries imposed on the quantum dot from the surface, allows a fast growth rate thanks to the presence of preferential nucleation sites at the step edges. A 100nm InAlAs metamorphic layer with In content >= 50% directly deposited on the GaAs substrate is already almost fully relaxed with a very flat surface. The quantum dots emit at the 1.3 mu m telecom O-band with fine structure splitting as low as 16 mu eV, thus making them suitable as photon sources in quantum communication networks using entangled photons.

We investigate the degree of indistinguishability of cascaded photons emitted from a three-level quantum ladder system; in our case the biexciton-exciton cascade of semiconductor quantum dots. For the three-level quantum ladder system we theoretically demonstrate that the indistinguishability is inherently limited for both emitted photons and determined by the ratio of the lifetimes of the excited and intermediate states. We experimentally confirm this finding by comparing the quantum interference visibility of noncascaded emission and cascaded emission from the same semiconductor quantum dot. Quantum optical simulations produce very good agreement with the measurements and allow us to explore a large parameter space. Based on our model, we propose photonic structures to optimize the lifetime ratio and overcome the limited indistinguishability of cascaded photon emission from a three-level quantum ladder system.