Time-resolved optical charge sensing for transport measurements on single self-assembled quantum dots
Zusammenfassung der Projektergebnisse
One of the driving forces in nowadays physics research is the dream of a quantum computers and quantum networks, which could be used to solve special problems faster than ever since, simulate quantum systems or transfer encrypted data with 100 percent security. Quantum cryptography is already commercially available, while quantum computers and networks are still visions for the future with numerous challenges to solve. One of the research directions is devoted to the search of an ideal quantum system that could be used to prepare, store and manipulate the quantum state, the so-called quantum bit (qubit). In the focus are trapped cold atoms and ions, dopants in a solid state matrix (like NV centers), superconducting qubits and single photons. Another promising system is self-assembled quantum dots, which are formed by epitaxial growth in a semiconductor matrix. These quantum dots act like artificial atoms with quantum states that can be addresses by optical and electrical means. The original aim of the research project was to combine nowadays transport spectroscopy with high resolution resonant optical spectroscopy to build a new time-resolved detection scheme for charge sensing of non-equilibrium quantum states in a single self-assembled quantum dot. After lack of suitable samples, the project was redirected towards another interesting part which fits into the same direction of quantum information processing with solid-state qubits: “Waveform shaping of coherently scattered photons from a single quantum dot.” For a quantum network, the quantum state should be stored in nodes and the information should be transfer in quantum channels. The quantum channels between different nodes can be realized by single photons emitted and absorbed between the nodes. A major challenge is to build these quantum interconnects between the stationary qubit (the exciton) and the flying qubits (the photon), i.e. to generate “high-quality” flying qubits that have encoded the quantum state from the node in the polarization of the photon. Therefore, one step towards a quantum network is to generate single photon with long coherence times and high degree of indistinguishably from a single self-assembled QD. This challenge has been address with an InAs QD in a Schottky diode using resonance fluorescence (RF). The QD was driven in the so-called “Heitler regime” (low excitation power) on resonance where mainly elastically scattered photons are emitted. It has been shown in an optical heterodyne experiment that these elastically scattered photons are phase-locked to the excitation laser with mutual-coherence time exceeding three seconds. This coherence time corresponds to a mutualcoherence length of one million kilometers. The results obtained reveal that each single photon inherits the coherence properties of the excitation laser. After generation of highly coherent photons that are phase-locked to the laser, the laser light was modulated (the waveform was shaped) and this modulation was imprinted onto the single photon wavepacket by the elastic scattering process. It has been shown that the spectra of the single photons are dictated by the laser waveform, while the strong antibunching (demonstrating single photon emission) is fully sustained. Finally, the indistinguishability of these highly coherent single photon wavepackets in the regime of elastic scattering was measured in a Hong-Ou-Mandel (HOM) style two-photon interference (TPI) experiments. We were able to extract a raw contrast of the HOM-TPI experiment of 0.927 +/- 0.012; taking into account deviations from an ideal setup, e.g. interferometer visibility and beamsplitter coefficients, we extracted a photon wavepacket indistinguishability of 0.99 +/-0.02. This high value of indistinguishability confirms the expected advantage of the coherently scattered photons that exhibit a mutual coherence with the laser and can be shaped into arbitrary waveforms. It is an idea system for further investigation in the direction of single photons emitted from single selfassembled QDs and used for future quantum networks.
