Thomas Kauten

Deterministic photon pairs and coherent optical control of a single quantum dot.

The strong confinement of semiconductor excitons in a quantum dot gives rise to atomlike behavior. The full benefit of such a structure is best observed in resonant excitation where the excited state can be deterministically populated and coherently manipulated. Because of the large refractive index and device geometry it remains challenging to observe resonantly excited emission that is free from laser scattering in III/V self-assembled quantum dots. Here we exploit the biexciton binding energy to create an extremely clean single photon source via two-photon resonant excitation of an InAs/GaAs quantum dot. We observe complete suppression of the excitation laser and multiphoton emissions. Additionally, we perform full coherent control of the ground-biexciton state qubit and observe an extended coherence time using an all-optical echo technique. The deterministic coherent photon pair creation makes this system suitable for the generation of time-bin entanglement and experiments on the interaction of photons from dissimilar sources.

Time-bin entangled photons from a quantum dot

Long distance quantum communication is one of the prime goals in the field of quantum information science. With information encoded in the quantum state of photons, existing telecommunication fibre networks can be effectively used as a transport medium. To achieve this goal, a source of robust entangled single photon pairs is required. Here, we report the realization of a source of time-bin entangled photon pairs utilizing the biexciton-exciton cascade in a III/V self-assembled quantum dot. We analyse the generated photon pairs by an inherently phase-stable interferometry technique, facilitating uninterrupted long integration times. We confirm the entanglement by performing quantum state tomography of the emitted photons, which yields a fidelity of 0.69(3) and a concurrence of 0.41(6) for our realization of time-energy entanglement from a single quantum emitter.

Efficiency vs. multi-photon contribution test for quantum dots

The development of linear quantum computing within integrated circuits demands high quality semiconductor single photon sources. In particular, for a reliable single photon source it is not sufficient to have a low multi-photon component, but also to possess high efficiency. We investigate the photon statistics of the emission from a single quantum dot with a method that is able to sensitively detect the trade-off between the efficiency and the multi-photon contribution. Our measurements show, that the light emitted from the quantum dot when it is resonantly excited possess a very low multi-photon content. Additionally, we demonstrated, for the first time, the non-Gaussian nature of the quantum state emitted from a single quantum dot.

Observation of genuine three-photon interference

Three photons can display qualitatively new interference phenomena such as genuine three-photon interference. Here we show how to isolate three-photon interference with more than 90 % visibility, completely suppressing two-photon and single-photon interference.

Obtaining tight bounds on higher-order interferences with a 5-path interferometer

Within the established theoretical framework of quantum mechanics, interference always occurs between pairs of trajectories. Higher order interferences with multiple constituents are, however, excluded by Borns rule and can only exist in generalized probabilistic theories. Thus, high-precision experiments searching for such higher order interferences are a powerful method to distinguish between quantum mechanics and more general theories. Here, we perform such a test in optical multi-path interferometers. Our results rule out the existence of higher order interference terms to an extent which is more than four orders of magnitude smaller than the expected pairwise interference, refining previous bounds by two orders of magnitude. This establishes the hitherto tightest constraints on generalized interference theories.

Hybrid waveguide-bulk multi-path interferometer with switchable amplitude and phase

We design and realise a hybrid interferometer consisting of three paths based on integrated as well as on bulk optical components. This hybrid construction offers a good compromise between stability and footprint on one side and means of intervention on the other. As experimentally verified by the absence of higher-order interferences, amplitude and phase can be manipulated in all paths independently. In conjunction with single photons, the setup can, therefore, be applied for fundamental investigations on quantum mechanics.

Measurement and modeling of the nonlinearity of photovoltaic and Geiger-mode photodiodes

While in most cases the absolute accuracy, resolution, and noise floor are the only relevant specifications for the dynamic range of a photodetector, there are experiments for which the linearity plays a more important role than the former three properties. In these experiments nonlinearity can lead to systematic errors. In our work we present a modern implementation of the well-known superposition method and apply it to two different types of photodetectors.

Invited Article: Time-bin entangled photon pairs from Bragg-reflection waveguides

Semiconductor Bragg-reflection waveguides are well-established sources of correlated photon pairs as well as promising candidates for building up integrated quantum optics devices. Here, we use such a source with optimized non-linearity for preparing time-bin entangled photons in the telecommunication wavelength range. By taking advantage of pulsed state preparation and efficient free-running single-photon detection, we drive our source at low pump powers, which results in a strong photon-pair correlation. The tomographic reconstruction of the state’s density matrix reveals that our source exhibits a high degree of entanglement. We extract a concurrence of 88.9(1.8)% and a fidelity of 94.2(9)% with respect to a Bell state.Semiconductor Bragg-reflection waveguides are well-established sources of correlated photon pairs as well as promising candidates for building up integrated quantum optics devices. Here, we use such a source with optimized non-linearity for preparing time-bin entangled photons in the telecommunication wavelength range. By taking advantage of pulsed state preparation and efficient free-running single-photon detection, we drive our source at low pump powers, which results in a strong photon-pair correlation. The tomographic reconstruction of the state’s density matrix reveals that our source exhibits a high degree of entanglement. We extract a concurrence of 88.9(1.8)% and a fidelity of 94.2(9)% with respect to a Bell state.

Individually shuttered waveguide multi-path interferometer

Multi-mode interferometers can be used for a wide range of applications. They allow a higher precision for a phase measurement compared to a simple two-mode version [1, 2]. In quantum optics, schemes for enhancement of non-classical visibility [3] as well as better resilience to photon loss [4] have been proposed. All these schemes depend on a good stability of the used interferometer, which gives integrated optics, such as laser-written waveguides [5], a natural advantage over bulk-optics. Furthermore, waveguide interferometers always ensure perfect mode overlap at the recombiner, yielding higher interference contrast. For some experiments investigating fundamentals of quantum mechanics there is also a need for an individual manipulation and switching of each path [6-10].

Single quantum dots as photon pair emitters

Summary form only given. A realization of time-bin entanglement obtained from quantum dots would join the strength of long distance transmission that characterizes this type of entanglement with the single photon state purity of the quantum dot emission. Compared with polarization entanglement from quantum dots this scheme does not require elimination of the fine structure splitting responsible for partial distinguishability of the quantum dot cascades [1].Here, we present our results on coherent and resonant excitation of quantum dots, creation of photon pairs, scattering free emission and measurement of the high purity of the emitted photons [2, 3]. In addition, we will show our measurements of the generated time-bin entanglement. To excite a single quantum dot we performed two-photon resonant excitation of the biexciton state. Here, we exploited the biexciton binding energy in order to use the laser light which was not resonant to any photon emitted from the quantum dot (Fig. 1a). Our measurements were performed on a single self-assembled InAs/GaAs quantum dot. We confirmed the resonant nature of the excitation by observing Rabi oscillations (Fig. 1b). Additionally, we performed Ramsey interference measurement and determined the coherence time of the ground-biexciton state superposition. These measurements show that we can coherently transfer the phase of the excitation laser onto the quantum dot system, a necessary requirement to obtain time-bin entanglement.To test the statistics of the emitted state we measured the auto-correlation of the emitted photons (Fig. 3c). Here, the auto-correlation parameter at zero delay was measured to be 0.012(1) without and 0.0073(8) with background subtraction. In addition, we performed a measurement to characterize the state emitted from a quantum dot using a witness-based criterion described in [4]. With this measurement we experimentally confirmed that the resonantly excited quantum dot emits a non-Gaussian state of light (Fig. 1d). This measurement is the first of this nature ever performed on a semiconductor single photon emitter.