Semiconductor quantum dots can be utilized to capture single electron or hole spins and they have therewith promise for various applications in fields like spintronics, spin based quantum information processing and chiral photonics. We integrate quantum dots into semiconductor microcavities to enhance light-matter interaction for ultrafast optical manipulation and read-out. Single electron and single hole spins can be statistically or deterministically loaded into the quantum dots and coherently controlled. Within the about μs-coherence times of the spins about 105 complete single qubit rotations can be performed with ultrafast optical pulses. By utilizing a Λ-type energy level system of a single quantum-dot electron spin in a magnetic field and ultrafast non-linear frequency conversion, quantum-dot spin-photon entanglement is observed.
Individual electron and hole quantum dot spin qubits can be coherently manipulated using picosecond modelocked laser pulses; an all-optical spin-echo was implemented that decouples slow environmental changes. While dephasing and decoherence mechanisms for electrons and holes are intrinsically different, similar qualitative results are obtained,
except for dynamic nuclear polarization effects that affect the controllability of electrons. In addition, we demonstrate
spin-photon entanglement in a charged InAs quantum dot, using an ultrafast downconversion technique that converts a single, spontaneously emitted photon at 900 nm into a 1560 nm photon with picosecond timing resolution. This ultrafast conversion technique allows quantum erasure of which-path frequency information in the spontaneous emission process.
Novel nanostructured III-V semiconductor devices are investigated for light detection in the near infrared
spectral region. Single-electron memories based on site-controlled InAs quantum dots embedded in a GaAs/AlGaAs
quantum-wire transistor were fabricated and studied. By using a nanohole structure template on a modulation-doped
GaAs/AlGaAs heterostructure, two single InAs quantum dots were centrally positioned in a quantum-wire transistor so
that pronounced shifts of the transistor threshold occur by charging of the QDs with single electrons. Single-electron
read and write functionalities up to room temperature were observed and the memory function can be also controlled by
light with a wavelength in the telecommunication range. Furthermore, AlGaAs/GaAs/AlGaAs double barrier resonant
tunneling diodes (RTD) with an embedded GaInNAs absorption layer have been fabricated for telecom wavelength light
detection at room temperature. The absorption layer was lattice matched grown within the GaAs system of the RTD.
We demonstrate that the devices exhibit typical RTDs characteristic and they are light sensitive at the telecom
wavelength 1.3 μm in the order of just a few nW. Routes to further reduce the detection limit are discussed whereas the
envisaged devices have prospects to deliver sensitivities approaching the quantum limit.
Electron spins in quantum dots under coherent control exhibit a number of novel feedback processes. Here, we
present experimental and theoretical evidence of a feedback process between nuclear spins and a single electron
spin in a single charged InAs quantum dot, controlled by the coherently modified probability of exciting a trion
state. We present a mathematical model describing competition between optical nuclear pumping and nuclear
spin-diffusion inside the quantum dot. The model correctly postdicts the observation of a hysteretic sawtooth
pattern in the free-induction-decay of the single electron spin, hysteresis while scanning a narrow-band laser
through the quantum dot's optical resonance frequency, and non-sinusoidal fringes in the spin echo. Both the
coherent electron-spin rotations, implemented with off-resonant ultrafast laser pulses, and the resonant narrowband
optical pumping for spin initialization interspersed between ultrafast pulses, play a role in the observed
behavior. This effect allows dynamic tuning of the electron Larmor frequency to a value determined by the pulse
timing, potentially allowing more complex coherent control operations.
The phase coherence of a physical qubit is essential for quantum information processing, motivating fast control
methods to preserve that phase. Ultrafast optical techniques allow complete spin control to be performed on a much
faster timescale than microwave or electrical control (ps vs. ns at best). Using our ultrafast control techniques, we
demonstrate our experimental approach towards a spin echo sequence on the spin of a single electron confined in a
semiconductor quantum dot (QD), increasing the observed decoherence time of a single QD electron spin from
nanoseconds to several microseconds. The ratio of the observed decoherence time to the demonstrated single-qubit gate
time exceeds 105, suggesting strong promise for future quantum information processors.
We demonstrate a complete set of fast, all-optical single-qubit operations on a single electron spin confined in a
semiconductor quantum dot (QD). Optical initialization and measurement of the single spin are accomplished by optical
pumping in a 3.4 ns timescale with 92±7% fidelity. The spin is coherently manipulated by a single red-detuned
broadband laser pulse with 4 picosecond duration. We achieve over six complete Rabi oscillations between the two spin
states by varying the rotation pulse's intensity. The fidelity of π/2 and π rotations both exceed 90%. Next we use two
sequential rotation pulses, separated by a variable time delay, to demonstrate a complete set of Ramsey fringes. This
two-pulse sequence is sufficient to achieve an arbitrary rotation of the spin and thus serves as a universal single-qubit
We report on experiments where a single quantum dot is strongly coupled to a high-Q mode of a micropillar
cavity. Photon correlation measurements confirm that the observed avoided crossing originates from strong
coupling of a single quantum dot to the cavity mode. Cross-correlations between the cavity mode and the
spectrally detuned quantum dot enabled us to assign the unexpected strong cavity emission to a coupling with
the single quantum dot. The coupled quantum dot-microcavity system displays an Purcell factor of 61 and
represents a single-photon source with an efficiency of 97%.