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.
Active depth imaging approaches have numerous potential applications in a number of disciplines, including
environmental sensing, manufacturing and defense. The high sensitivity and picosecond timing resolution of the singlephoton
counting technique can provide distinct advantages in the trade-offs between required illumination power, range,
depth resolution, and data acquisition durations. These considerations must also address requirements for eye-safety,
especially in applications requiring outdoor, kilometer range sensing. We present a scanning time-of-flight imager based
on high repetition-rate (>MHz) pulsed illumination and a silicon single-photon detector. In advanced photon-counting
experiments, we have employed the system for unambiguous range resolution at several kilometer target distance,
multiple-surface resolution based on adaptive algorithms, and a cumulative data acquisition method that facilitates
detector characterization and evaluation. We consider a range of optical design configurations and discuss the
performance trade-offs in more detail. Much of this work has been performed at wavelengths around 850nm for
convenient use with Si-based single photon avalanche diode detectors, however we will also discuss the performance at
wavelengths around 1550 nm employing superconducting nanowire single photon detectors. The extension of this depth
profiling technique to longer wavelengths will lead to relaxed eye safety requirements, reduced solar background levels
and improvements in atmospheric transmission.
Single-photon sources and detectors are key enabling technologies for photonics in quantum information science and
technology (QIST). QIST applications place high-level demands on the performance of sources and detectors; it is
therefore essential that their properties can be characterized accurately. Superconducting nanowire single-photon
detectors (SNSPDs) have spectral sensitivity from visible to beyond 2 μm in wavelength, picosecond timing resolution
(Jitter <100 ps FWHM) and the capacity to operate ungated with low dark counts (<1 kHz). This facilitates data
acquisition at high rates with an excellent signal-to-noise ratio.
We report on the construction and characterization of a two-channel SNSPD system. The detectors are mounted in a
closed-cycle refrigerator, which eliminates reliance on liquid cryogens. Our specification was to deliver a system with
1% efficiency in both channels at a wavelength of 1310 nm with 1 kHz dark count rate. A full width at half maximum
timing jitter of less than 90 ps is achieved in both channels. The system will be used to detect individual photons
generated by quantum-optical sources at telecom wavelengths. Examples include single-photon sources based on
quantum dots (emitting at 1310 nm). The SNSPD system's spectral sensitivity and timing resolution make it suited to
characterization of such sources, and to wider QIST applications.
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