Michael Jetter

Bringing quantum light sources into real‐world telecommunication networks requires moving beyond the laboratory and developing sources that operate at the telecom C‐band. Epitaxial quantum dots (QDs) provide deterministic sources of quantum light at these wavelengths. Here, we demonstrate an important step toward quantum communication networks: a state‐of‐the‐art semiconductor QD source, exhibiting a record coincidence rate for entangled‐photon emission in the telecom C‐band, integrated into an optimised rack‐based setup exhibiting remote controle. This setup includes a tunable pulsed laser for QD excitation, enabling operation from quasi‐resonant to resonant two‐photon excitation, all‐optical excitation suppression, and QD signal coupling into single‐mode fibres. Overall, the setup achieves more than 65% transmission for both exciton and biexciton photons. To demonstrate the usability of our stand‐alone source, we transmit single photons through a deployed 36 km‐long fibre link and verify that their single‐photon purity is preserved. Moreover, based on the measured entangled‐photon pair rate, we estimate that the source could enable entangled QKD protocols with a projected secret key rate 3.4 cps after 150 km of optical fibres. These results demonstrate the potential of quantum‐dot‐based telecom light sources for integration into existing fibre infrastructures, paving the way toward the realisation of a quantum internet.

The ability to reliably inscribe information on single photons is a crucial requirement for quantum applications such as quantum communication and measurement-based photonic quantum computation. Experimental implementations generally employ phase modulators in single-pass or Mach-Zehnder-interferometer configurations to encode information on photonic qubits. However, these approaches are intrinsically sensitive to environmental influences, which limits the achievable quantum error rates in practice, posing a major bottleneck in the transition of quantum applications with deterministic sources from the laboratory to the field. Inspired by the adaptation of Sagnac interferometer encoders for laser-based applications, we report a demonstration of a polarization encoder for single-photon qubits based on a free-space Sagnac interferometer. The device showcases inherent phase stability and overcomes previous error-rate limitations: Telecom-wavelength photons emitted by a quantum dot are modulated at a rate of 152 MHz and with a quantum bit error rate of 0.69 ± 0.02 % , marking the lowest value reported to date for MHz-rate information encoding on single photons by a factor of > 3 . This combination of low error rate, high modulation speed, stability, and operation with telecom-wavelength sub-Poissonian photons is an essential requirement for bringing quantum applications based on deterministic photon sources from the laboratory to the field and a significant step toward unlocking the full potential of a future quantum Internet.

Quantum emitters promise to emit exactly one photon when pumped by a laser pulse. However, even in ideal systems, re-excitation during the laser pulse duration causes the consecutive emission of two photons, thus limiting the single-photon purity. Although the probability and characteristics of re-excitation are largely determined by the excitation scheme, until now, only resonant driving has been studied. Here, we demonstrate qualitative differences for phonon-assisted excitation and resolve the distinct temporal and spectral profile of each of the photons. These uncover correlations between the emission time and wavelength, resulting in an asymmetric two-photon spectrum. On a fundamental level, we show that this spectrum provides direct access to the Rabi frequency of an incoherently driven quantum dot. On the application side, we leverage the asymmetry to selectively suppress multiphoton noise, ensuring high single-photon purity regardless of the laser pulse length and thus enhancing implementations across quantum cryptography and quantum computing.

Quantum key distribution (QKD) with deterministic single photon sources has been demonstrated over intercity fiber and free-space channels. The previous implementations relied mainly on polarization encoding schemes, which are susceptible to birefringence, polarization-mode dispersion and polarization-dependent loss in practical fiber networks. In contrast, time-bin encoding offers inherent robustness and has been widely adopted in mature QKD systems using weak coherent laser pulses. However, its feasibility in conjunction with a deterministic single-photon source has not yet been experimentally demonstrated. In this work, we construct a time-bin encoded QKD system employing a high-brightness quantum dot (QD) single-photon source operating at telecom wavelength. Our proof-of-concept experiment successfully demonstrates the possibility of secure key distribution over fiber link of 120 km, while maintaining extraordinary long-term stability over 6 h of continuous operation, that is highest secure key rate among the time-bin QKDs based on single-photon sources. This work provides the first experimental validation of integrating a QD single-photon source with time-bin encoding in a telecom-band QKD system. This development signifies a substantial advancement in the establishment of a robust and scalable QKD network based on solid-state single-photon technology.

Modern semiconductor device technology expands to new application fields and increasing integrability even to hybrid system approaches. Thus, membrane technologies emerge to be a new standard for light-emitting devices as well. However, the fabrication of membranes often results in a shift of the emission wavelength of the active region compared to the as-deposited structure, especially when containing quantum dots (QDs). In this study, we investigate the wavelength shift of an AlGaInP heterostructure with InP QDs during the GaAs substrate removal step. For this, the substrate of a fivefold InP QD layer is selectively etched in such a way that a single sample provides areas with different remaining substrate thicknesses in a range within 1 μm. With photoluminescence measurements, a continuous blueshift of the emission spectrum of the QD ensemble can be observed. We explain these findings by the change of the strain within the entire structure, which impacts the confinement energies within the QDs.

Quantum cryptography based on the application of single photons offers a secure communication based on the laws of physics. However, given the sophisticated and challenging implementation of true-single photon sources, the majority of implementations takes advantage of the decoy state protocol, which uses weak coherent pulses instead. For polarization-based protocols, a typical sender configuration includes one or multiple laser sources with external optical elements to realize four different polarization states and two intensity levels (signal and decoy). In this regard, the application of eight independent lasers with fixed polarization makes an external state preparation unnecessary and allows for a higher signal rate, while requiring otherwise identical optical properties of all laser devices such as wavelength and linewidth. Here, we demonstrate a solution on the basis of a compact, monolithic VCSEL array operating at room temperature. The array consists of eight VCSELs with integrated subwavelength gratings for polarization control. Orthogonal polarization suppression ratios (OPSRs) of up to 19.7 dB (19.1 dB) are reached for the rectilinear (diagonal) basis. By careful adjustment of the individual VCSEL’s operating point, a wavelength matching of all VCSELs is achieved at an emission wavelength of 851.2 nm.

Photonic cluster states are highly entangled states that allow for photonic quantum computing and memory-less quantum repeaters. Their generation has been recently demonstrated using semiconductor quantum dots emitting at the 900 nm wavelength range. However, a similar demonstration at the communication-optimal telecom range has remained elusive. A key ingredient that is still missing is an appropriate optical excitation method. A central requirement of such a method is to allow an arbitrary spin initialization of quantum-dot excitonic complexes. In this work, we report on developing such a method based on a quasiresonant p -shell excitation for an InAs/InGaAs quantum dot emitting at 1535 nm. We show qubit writing of a neutral exciton and spin-preserving excitation of a negative trion. Using the Larmor precession of the negative trion under an externally applied magnetic field, we determine the in-plane g factors of both the electron and the hole in the investigated quantum dot. In addition, we measure a lower bound on the hole coherence time, T 2 * > 6.4 ns , boosting its candidacy as a sound photon entangler for more advanced quantum photonic schemes.