Fiber-based distance sensing interferometry.

We present an interferometric displacement sensor based on a folded low-finesse Fabry-Perot cavity. The fiber-optic sensor uses a quadrature detection scheme based on the wavelength modulation of a DFB laser. This enables measuring position changes over a range of 1 m for velocities up to 2 m/s. The sensor is well suited to work in extreme environments such as ultrahigh vacuum, cryogenic temperatures, or high magnetic fields and supports multichannel applications. The interferometer achieves a repeatability of 0.44 nm(3σ) at a working distance of 20 mm, a resolution of 1 pm, and an accuracy of 1 nm.

Optical pumping of a single hole spin in a quantum dot.

The spin of an electron is a natural two-level system for realizing a quantum bit in the solid state. For an electron trapped in a semiconductor quantum dot, strong quantum confinement highly suppresses the detrimental effect of phonon-related spin relaxation. However, this advantage is offset by the hyperfine interaction between the electron spin and the 10(4) to 10(6) spins of the host nuclei in the quantum dot. Random fluctuations in the nuclear spin ensemble lead to fast spin decoherence in about ten nanoseconds. Spin-echo techniques have been used to mitigate the hyperfine interaction, but completely cancelling the effect is more attractive. In principle, polarizing all the nuclear spins can achieve this but is very difficult to realize in practice. Exploring materials with zero-spin nuclei is another option, and carbon nanotubes, graphene quantum dots and silicon have been proposed. An alternative is to use a semiconductor hole. Unlike an electron, a valence hole in a quantum dot has an atomic p orbital which conveniently goes to zero at the location of all the nuclei, massively suppressing the interaction with the nuclear spins. Furthermore, in a quantum dot with strong strain and strong quantization, the heavy hole with spin-3/2 behaves as a spin-1/2 system and spin decoherence mechanisms are weak. We demonstrate here high fidelity (about 99 per cent) initialization of a single hole spin confined to a self-assembled quantum dot by optical pumping. Our scheme works even at zero magnetic field, demonstrating a negligible hole spin hyperfine interaction. We determine a hole spin relaxation time at low field of about one millisecond. These results suggest a route to the realization of solid-state quantum networks that can intra-convert the spin state with the polarization of a photon.

Versatile multilevel soft lithography method with micrometer alignment using all-flexible rubber stamps and moiré fringe technique.

Soft lithography has gathered wide interest for the fabrication of unconventional micrometer and nanometer-sized structures and devices. Nevertheless, accurate alignment is essential to achieve multilevel soft lithography. Because of the soft nature of the stamp materials, such as soft polydimethylsiloxane, they are susceptible to mechanical distortions, which lower the registration accuracy. To reduce the distortions we backed the stamp with a polymer foil and minimized the overall forces applied to the stamp. We furthermore employed an alignment method using additive type moiré fringe technique that is easy to implement and does not require extensive processing steps. The alignment results show less than 1 µm misalignment when the stamp is brought again onto a previously structured rigid template. When performing two consecutive lithography steps by transfer printing of thin gold films, we were able to obtain average registration accuracy of 1.3 µm over an area of 400 mm(2). This method is versatile and can be used for several soft lithography techniques. Better results can be obtained with smaller moiré gratings and the use of harder materials.

Cavity cooling of a microlever.

The prospect of realizing entangled quantum states between macroscopic objects and photons has recently stimulated interest in new laser-cooling schemes. For example, laser-cooling of the vibrational modes of a mirror can be achieved by subjecting it to a radiation or photothermal pressure, actively controlled through a servo loop adjusted to oppose its brownian thermal motion within a preset frequency window. In contrast, atoms can be laser-cooled passively without such active feedback, because their random motion is intrinsically damped through their interaction with radiation. Here we report direct experimental evidence for passive (or intrinsic) optical cooling of a micromechanical resonator. We exploit cavity-induced photothermal pressure to quench the brownian vibrational fluctuations of a gold-coated silicon microlever from room temperature down to an effective temperature of 18 K. Extending this method to optical-cavity-induced radiation pressure might enable the quantum limit to be attained, opening the way for experimental investigations of macroscopic quantum superposition states involving numbers of atoms of the order of 10(14).

Hybridization of electronic states in quantum dots through photon emission.

The self-assembly of semiconductor quantum dots has opened up new opportunities in photonics. Quantum dots are usually described as 'artificial atoms', because electron and hole confinement gives rise to discrete energy levels. This picture can be justified from the shell structure observed as a quantum dot is filled either with excitons (bound electron-hole pairs) or with electrons. The discrete energy levels have been most spectacularly exploited in single photon sources that use a single quantum dot as emitter. At low temperatures, the artificial atom picture is strengthened by the long coherence times of excitons in quantum dots, motivating the application of quantum dots in quantum optics and quantum information processing. In this context, excitons in quantum dots have already been manipulated coherently. We show here that quantum dots can also possess electronic states that go far beyond the artificial atom model. These states are a coherent hybridization of localized quantum dot states and extended continuum states: they have no analogue in atomic physics. The states are generated by the emission of a photon from a quantum dot. We show how a new version of the Anderson model that describes interactions between localized and extended states can account for the observed hybridization.

Fluctuating nanomechanical system in a high finesse optical microcavity.

The idea of extending cavity quantum electrodynamics experiments to sub-wavelength sized nanomechanical systems has been recently proposed in the context of optical cavity cooling and optomechanics of deformable cavities. Here we present an experiment involving a single nanorod consisting of about 10(9) atoms precisely positioned into the confined mode of a miniature high finesse Fabry-Pérot microcavity. We show that the optical transmission of the cavity is affected not only by the static position of the nanorod but also by its vibrational fluctuation. The Brownian motion of the nanorod is resolved with a displacement sensitivity of 200 fm/square root Hz at room temperature. Besides a broad range of sensing applications, cavity-induced manipulation of optomechanical nanosystems and back-action is anticipated.

Fiber-based confocal microscope for cryogenic spectroscopy.

We describe the design and performance of a fiber-based confocal microscope for cryogenic operation. The microscope combines positioning at low temperatures along three space coordinates of millimeter translation and nanometer precision with high stability and optical performance at the diffraction limit. It was successfully tested under ambient conditions as well as at liquid nitrogen (77 K) and liquid helium (4 K) temperatures. The compact nonmagnetic design provides for long term position stability against helium refilling transfers, temperature sweeps, as well as magnetic field variation between -9 and 9 T. As a demonstration of the microscope performance, applications in the spectroscopy of single semiconductor quantum dots are presented.

Fabry-Pérot interferometry for long range displacement sensing.

We investigate different optical configurations of a low-finesse Fabry-Pérot interferometer used for displacement sensing. The different configurations of the Fabry-Pérot cavity are selected in order to achieve large measurement ranges and angular alignment tolerances and to make the interferometer applicable for targets of various reflectivity ranges. The possible working ranges and angular alignment tolerances are characterized with respect to the interference contrast which is a measure for the signal quality. The use of a confocal arrangement enables a measurement range of up to about 0.4 m, or to work with an angular tolerance of more than ±0.2°. In order to predict the optical response of arbitrary configurations of the Fabry-Pérot interferometer, we introduce a simulation method based on the Airy formula and the fiber optic coupling efficiency.

Absolute distance sensing by two laser optical interferometry.

We have developed a method for absolute distance sensing by two laser optical interferometry. A particularity of this technique is that a target distance is determined in absolute and is no longer limited to within an ambiguity range affecting usually multiple wavelength interferometers. We implemented the technique in a low-finesse Fabry-Pérot miniature fiber based interferometer. We used two diode lasers, both operating in the 1550 nm wavelength range. The wavelength difference is chosen to create a 25 µm long periodic beating interferometric pattern allowing a nanometer precise position measurement but limited to within an ambiguity range of 25 µm. The ambiguity is then eliminated by scanning one of the wavelengths over a small range (3.4 nm). We measured absolute distances in the sub-meter range and this with just few nanometer repeatability.

Easy fabrication of electrically insulating nanogaps by transfer printing.

An easy and cost-effective method to reproducibly fabricate nanogaps over a large area is introduced. Gold is evaporated on low-aspect-ratio polydimethylsiloxane (PDMS) stamps at an angle of 60°. Afterwards, the stamp is brought into contact with a silicon/silicon dioxide substrate and subsequently peeled at rates varying from 1 to 3 mm s(-1), resulting in the fabrication of nanogaps between two gold electrodes. The fabrication of insulating nanogaps with a width down to 50 nm is demonstrated.

A coherent single-hole spin in a semiconductor.

Semiconductors have uniquely attractive properties for electronics and photonics. However, it has been difficult to find a highly coherent quantum state in a semiconductor for applications in quantum sensing and quantum information processing. We report coherent population trapping, an optical quantum interference effect, on a single hole. The results demonstrate that a hole spin in a quantum dot is highly coherent.

Self-induced oscillations in an optomechanical system driven by bolometric backaction.

We have explored the nonlinear dynamics of an optomechanical system consisting of an illuminated Fabry-Perot cavity, one of whose end mirrors is attached to a vibrating cantilever. The backaction induced by the bolometric light force produces negative damping such that the system enters a regime of nonlinear oscillations. We study the ensuing attractor diagram describing the nonlinear dynamics. A theory is presented that yields quantitative agreement with experimental results. This includes the observation of a regime where two mechanical modes of the cantilever are excited simultaneously.

Optical detection of single-electron spin resonance in a quantum dot.

We demonstrate optically detected spin resonance of a single electron confined to a self-assembled quantum dot. The dot is rendered dark by resonant optical pumping of the spin with a laser. Contrast is restored by applying a radio frequency (rf) magnetic field at the spin resonance. The scheme is sensitive even to rf fields of just a few microT. In one case, the spin resonance behaves as a driven 3-level lambda system with weak damping; in another one, the dot exhibits remarkably strong (67% signal recovery) and narrow (0.34 MHz) spin resonances with fluctuating resonant positions, evidence of unusual dynamic processes.

Quantum-dot spin-state preparation with near-unity fidelity.

We have demonstrated laser cooling of a single electron spin trapped in a semiconductor quantum dot. Optical coupling of electronic spin states was achieved using resonant excitation of the charged quantum dot (trion) transitions along with the heavy-light hole mixing, which leads to weak yet finite rates for spin-flip Raman scattering. With this mechanism, the electron spin can be cooled from 4.2 to 0.020 kelvin, as confirmed by the strength of the induced Pauli blockade of the trion absorption. Within the framework of quantum information processing, this corresponds to a spin-state preparation with a fidelity exceeding 99.8%.

Voltage-controlled optics of a quantum dot.

We show how the optical properties of a single semiconductor quantum dot can be controlled with a small dc voltage applied to a gate electrode. We find that the transmission spectrum of the neutral exciton exhibits two narrow lines with approximately 2 mueV linewidth. The splitting into two linearly polarized components arises through an exchange interaction within the exciton. The exchange interaction can be turned off by choosing a gate voltage where the dot is occupied with an additional electron. Saturation spectroscopy demonstrates that the neutral exciton behaves as a two-level system. Our experiments show that the remaining problem for manipulating excitonic quantum states in this system is spectral fluctuation on a mueV energy scale.