Quantum photonic devices require methods to efficiently generate photonic qubits and to create strong photon-photon interactions. Quantum dots can provide both of these crucial functionalities. They are ideal single photon sources that exhibit both high efficiency and indistinguishability. Furthermore, coupling them to high Q cavities with small mode volumes enables the strong coupling regime where a nonlinearities can enter the single photon regime.
Quantum photonic devices require methods to efficiently generate photonic qubits and to create strong photon-photon interactions. Quantum dots can provide both of these crucial functionalities. They are ideal single photon sources that exhibit both high efficiency and indistinguishability. Furthermore, coupling them to high Q cavities with small mode volumes enables the strong coupling regime where a nonlinearities can enter the single photon regime.
A single photon source is an enabling technology in device-independent quantum communication, quantum simulation, linear optics-based and measurement-based quantum computing. These applications employ many photons and place stringent requirements on the efficiency of single photon creation. The scaling on efficiency is typically an exponential function of the number of photons. Schemes taking full advantage of quantum superposition also depend sensitively on the coherence of the photons, i.e. their indistinguishability. \nThis thesis reports a single-photon source with a high end-to-end efficiency. We employ gated quantum dots in an open, tunable microcavity. The gating provides control of the charge and electrical tuning of the emission frequency, the high-quality material and careful design of the system ensures low noise, and the tunability of the microcavity compensates for the lack of control in quantum dot position and emission frequency. Transmission through the top mirror is the dominant escape route for photons from the microcavity, and this output is well-matched to a single-mode fiber. With this design, we can create a single photon at the output of the final optical fiber on-demand with a probability of 57% and with an average two-photon interference visibility of 97.5%. Coherence persists in trains of thousands of photons with single photon creation at a repetition rate of 1 GHz.
An optical source that produces single-photon pulses on demand has potential applications in linear optics quantum computation, provided that stringent requirements on indistinguishability and collection efficiency of the generated photons are met. We show that these are conflicting requirements for anharmonic emitters that are incoherently pumped via reservoirs. As a model for a coherently pumped single photon source, we propose cavity-assisted spin-flip Raman transitions in a single electron charged quantum dot embedded in a microcavity. We demonstrate that using such a source, arbitrarily high collection efficiency and indistinguishability of the generated photons can be obtained simultaneously with increased cavity coupling. We analyze the role of errors that arise from distinguishability of the single-photon pulses in linear optics quantum gates by relating the gate fidelity to the strength of the two-photon interference dip in photon cross-correlation measurements. We find that performing controlled phase operations with error <1 requires nanocavities with Purcell factors ${F}_{P}\ensuremath{\geqslant}40$ in the absence of dephasing, without necessitating the strong coupling limit.
An efficient single-photon nonlinearity is a key resource for photonic quantum technologies. Single-photon nonlinearities have been demonstrated with quantum dots (QDs) across a range of cavity and waveguide geometries. However, none of these realizations simultaneously provide high-efficiency direct fiber coupling and compatibility with scalable on-chip integration. The nanobeam cavity addresses these limitations through its in-plane geometry, which enables highly efficient, direct fiber coupling alongside scalable homogeneous and heterogeneous on-chip integration. Here, we report a single-photon nonlinearity from a nanobeam cavity coupled to a single quantum dot. The nanobeam interfaces directly with a single-mode fiber with an efficiency of 60%. Driven by resonant picosecond pulses, the platform achieves a single-photon nonlinearity with a low threshold of 0.24 incident photons per pulse. The system operates in the strong-coupling regime, exhibiting a coupling rate of g/2π = 29.1 GHz and a cooperativity of C = 3.98. These results establish the nanobeam cavity-QD system as a compact, chip-integrable, and fiber-compatible building block for scalable photonic quantum technologies.
The interaction of light with matter is at the heart of quantum optics, which itself enables insight into the fundamental aspects of quantum mechanics. First experimental access to this research field has been realized by coupling atoms to light. Here, the transition between discrete energy states of the atom is associated with the absorption and emission of a photon, a single quantum of the electromagnetic field. As a central aspect of quantum optics, the light-emitter interaction can be significantly enhanced by placing the emitter in optical cavity that is on resonance with the emitter. In recent years, this has led to a rapidly evolving research field known as cavity quantum electrodynamics (CQED). In CQED two different regimes are distinguished: the strong and the weak coupling regimes. In the strong coupling regime, the emitted photon is reflected from the cavity mirrors and eventually reabsorbed by the emitter. In contrast, the weak coupling regime describes the irreversible emission, where the photon leaks out of the cavity before it can be reabsorbed. Both the weak and strong coupling regimes enabled fundamental experiments for a better understanding of quantum optical phenomena. \n \n \n \nCQED grants access to the quantum world and hence offers potentially revolutionizing applications, particularly in the field of quantum information processing. A central aspect for the successful implementation of quantum applications is the system's scalability. Unfortunately, placing atoms deterministically inside a cavity remains technologically elaborate and hence minimizes the prospect of scaling a atom-CQED system. \n \nA possibility to address this issue is to implement CQED in the solid state, where sophisticated fabrication strategies allow miniaturization and scalability of the system. Particularly the development of self-assembled quantum dots (QD) in semiconductors represent a promising route. QDs can be considered as artificial atoms that mimic the atomic two-level system. These structures interact strongly with light and therefore have the potential for replacing atoms in CQED. As a central advantage, QDs are naturally trapped, which greatly simplifies the deterministic incorporation into the cavity. \n \nIn recent years, many efforts have been made to couple self-assembled QDs to microcavities. Generally, the successful implementation of CQED requires a cavity with a high quality factor Q and a low mode volume. In a majority of the approaches, the high Q/small mode volume cavities were monolithically defined around the QD, embedding the QD at a fixed position inside the cavity. Both the weak and strong coupling regimes have been reached with these systems. However, for future applications they suffer from several disadvantages. The fixed position of the QD inside the cavity minimizes the prospects for spectral tunability and spatial positioning the QD inside the cavity. Furthermore, prospects for further increasing of the cavity Q-factor and minimization of the mode volume remain limited in these systems. \n \nIn this thesis the mentioned disadvantages are addressed by developing a fully tunable miniaturized Fabry-Pérot microcavity with low mode volume. The design enables both spatial positioning of the emitter inside the cavity and spectral tunability. Successful coupling of a single QD to the microcavity is demonstrated including the strong coupling regime. Further a new approach to decrease the cavity mode volume is presented, where we demonstrate weak coupling is achieved
Semiconductor quantum dots (QDs) are promising artificial atoms for quantum information processing: they can generate single photons flying quantum bits; they show single photon sensitivity promising to develop quantum gates and the spin of a carrier in a QD can be a quantum memory. The scalability of a quantum network requires efficient interfaces between stationary and flying quantum bits. In the last few years, our group has made important progresses in this direction using cavity quantum electrodynamics. With a deterministic positioning of a single QD in a microcavity, we control the QD spontaneous emission on demand [1]. With such technique highly efficient single photon sources with brightness as large as 80% are demonstrated [2]. By minimizing the charge noise around the QD in a gated structure [3], we demonstrate the generation of fully indistinguishable photon. The source brightness is shown to exceed by one or two orders of magnitude the one of a parametric down-conversion source of same quality [4]. Symmetrically, these devices perform as excellent interfaces between a flying quantum bit and a stationary one, where coherent control of a quantum bit can be done when only few photons [5]. References [1] A. Dousse, et al. , Phys. Rev. Lett. 101, 267404 (2008) [2] O. Gazzano, et al. , Nature Communications 4, 1425 (2013) [3] A. Nowak. et al., Nature Communications 5, 3240 (2014) [4] N. Somaschi, et al. Nature Photonics 10.1038/nphoton.2016.23 (2016). [5] V. Giesz, et al., Nature Communications doi:10.1038/ncomms11986 (2016)
The capability of realizing quantum photonics implementations on-chip will have a strong impact on several aspects of quantum computation and information processing. The implementation of large scale integrated quantum photonic technologies requires on-chip reconfigurable waveguide circuits as well as single-photon sources and detectors. Many successes have been made in recent years to demonstrate complex quantum circuits in many material platforms. Among all these platforms, the Silicon-on-Insulator (SOI) platform is outstanding as it is well developed for classical applications and therefore deploys a high component density, mature fabrication techniques and compatibility with the telecom regime and the complementary metal-oxide-semiconductor (CMOS) technology. However, the lack of on-demand non-classical light sources, directly incorporated in silicon chips, forces the use of nonlinear processes to generate non-classical light states: the probabilistic nature of the emission process sets a limit in the maximally achievable experimental complexity. The use of on-demand sources will have a strong impact on the device’s scalability.