arXiv subjects

A. Kuhn

Publications and source records attributed to A. Kuhn.

11 recordsLinked to original sources

arXiv2014

Observation of the Three-Mode Parametric Instability

Three-mode parametric interactions occur in triply-resonant optomechanical systems: photons from an optical pump mode are coherently scattered to a high-order mode by mechanical motion of the cavity mirrors, and these modes resonantly interact via radiation pressure force when certain conditions are met. Such effects are predicted to occur in long baseline advanced gravitational-wave detectors. They can pump energy into acoustic modes, leading to parametric instability, but they can also extract acoustic energy, leading to optomechanical cooling. We develop a large amplitude model of three-mode interactions that explains the ring-up amplitude saturation after instability occurs. We also demonstrate both radiation-pressure cooling and mechanical amplification in two different three-mode optomechanical systems, including the first observation of the three-mode parametric instability in a free-space Fabry-Perot cavity. The experimental data agrees well with the theoretical model. Contrary to expectations, parametric instability does not lead to loss of cavity lock, a fact which may make it easier to implement control techniques to overcome instability.

physics.optics↗

arXiv2009

Optimum pulse shapes for stimulated Raman adiabatic passage

Stimulated Raman adiabatic passage (STIRAP), driven with pulses of optimum shape and delay has the potential of reaching fidelities high enough to make it suitable for fault-tolerant quantum information processing. The optimum pulse shapes are obtained upon reduction of STIRAP to effective two-state systems. We use the Dykhne-Davis-Pechukas (DDP) method to minimize nonadiabatic transitions and to maximize the fidelity of STIRAP. This results in a particular relation between the pulse shapes of the two fields driving the Raman process. The DDP-optimized version of STIRAP maintains its robustness against variations in the pulse intensities and durations, the single-photon detuning and possible losses from the intermediate state.

quant-ph↗

arXiv2008

Coherent imaging of extended objects

When used with coherent light, optical imaging systems, even diffraction-limited, are inherently unable to reproduce both the amplitude and the phase of a two-dimensional field distribution because their impulse response function varies slowly from point to point (a property known as non-isoplanatism). For sufficiently small objects, this usually results in a phase distortion and has no impact on the measured intensity. Here, we show that the intensity distribution can also be dramatically distorted when objects of large extension or of special shapes are imaged. We illustrate the problem using two simple examples: the pinhole camera and the aberration-free thin lens. The effects predicted by our theorical analysis are also confirmed by experimental observations.

physics.optics↗

arXiv2006

Polarization-controlled single photons

Vacuum-stimulated Raman transitions are driven between two magnetic substates of a rubidium-87 atom strongly coupled to an optical cavity. A magnetic field lifts the degeneracy of these states, and the atom is alternately exposed to laser pulses of two different frequencies. This produces a stream of single photons with alternating circular polarization in a predetermined spatio-temporal mode. MHz repetition rates are possible as no recycling of the atom between photon generations is required. Photon indistinguishability is tested by time-resolved two-photon interference.

quant-ph↗

arXiv2006

Scheme for generating a sequence of single photons of alternating polarisation

Single-photons of well-defined polarisation that are deterministically generated in a single spatio-temporal field mode are the key to the creation of multi-partite entangled states in photonic networks. Here, we present a novel scheme to produce such photons from a single atom in an optical cavity, by means of vacuum-stimulated Raman transitions between the Zeeman substates of a single hyperfine state. Upon each transition, a photon is emitted into the cavity, with a polarisation that depends on the direction of the Raman process.

quant-ph↗

arXiv2005

Characterization of single photons using two-photon interference

Adiabatic passage techniques allow the generation of single photons which are very long compared to the typical detector time resolution. Therefore the detection time of a photon can be measured within the duration of the single-photon wavepacket. As a consequence, two-photon interference can be investigated in a time-resolved manner, i.e., the coincidence rate can be measured as a function of the time between photodetections. The theoretical analysis shows that this method not only gives information about the duration of single photons, but also about their coherence time. Here we discuss how to use this method for a spectral or temporal characterization of a single-photon source.

quant-ph↗

arXiv2004

Photon Statistics of a Non-Stationary Periodically Driven Single-Photon Source

We investigate the photon statistics of a single-photon source that operates under non-stationary conditions. The photons are emitted by shining a periodic sequence of laser pulses on single atoms falling randomly through a high-finesse optical cavity. Strong antibunching is found in the intensity correlation of the emitted light, demonstrating that a single atom emits photons one-by-one. However, the number of atoms interacting with the cavity follows a Poissonian statistics so that, on average, no sub-Poissonian photon statistics is obtained, unless the measurement is conditioned on the presence of single atoms.

quant-ph↗

arXiv2004

Transition from antibunching to bunching in cavity QED

The photon statistics of the light emitted from an atomic ensemble into a single field mode of an optical cavity is investigated as a function of the number of atoms. The light is produced in a Raman transition driven by a pump laser and the cavity vacuum [M.Hennrich et al., Phys. Rev. Lett. 85, 4672 (2000)], and a recycling laser is employed to repeat this process continuously. For weak driving, a smooth transition from antibunching to bunching is found for about one intra-cavity atom. Remarkably, the bunching peak develops within the antibunching dip. For saturated driving and a growing number of atoms, the bunching amplitude decreases and the bunching duration increases, indicating the onset of Raman lasing.

quant-ph↗

arXiv2003

Electrical resistivity of the Ti4O7 Magneli phase under high pressure

We have measured resistivity as a function of temperature and pressure of Ti4O7 twinned crystals using different contact configurations. Pressures over 4kbar depress the localization of bipolarons and allow the study of the electrical conduction of the bipolaronic phase down to low temperatures. For pressures P > 40 kbar the bipolaron formation transition is suppressed and a nearly pressure independent behavior is obtained for the resistivity. We observed an anisotropic conduction. When current is injected parallel to the principal axis, a metallic conduction with interacting carrier effects is predominant. A superconducting state was not obtained down to 1.2 K, although evidences of the proximity of a quantum critical point were noticed. While when current is injected non-parallel to the crystal's principal axis, we obtained a logarithmic divergence of the resistivity at low temperatures. For this case, our results for the high pressure regime can be interpreted in the framework of interacting carriers (polarons or bipolarons) scattered by Two Level Systems.

cond-mat.str-el↗

arXiv2003

Time-Resolved Two-Photon Quantum Interference

The interference of two independent single-photon pulses impinging on a beam splitter is analysed in a generalised time-resolved manner. Different aspects of the phenomenon are elaborated using different representations of the single-photon wave packets, like the decomposition into single-frequency field modes or spatio-temporal modes matching the photonic wave packets. Both representations lead to equivalent results, and a photon-by-photon analysis reveals that the quantum-mechanical two-photon interference can be interpreted as a classical one-photon interference once a first photon is detected. A novel time-dependent quantum-beat effect is predicted if the interfering photons have different frequencies. The calculation also reveals that full two-photon fringe visibility can be achieved under almost any circumstances by applying a temporal filter to the signal.

quant-ph↗

arXiv2002

Vacuum-Stimulated Raman Scattering based on Adiabatic Passage in a High-Finesse Optical Cavity

We report on the first observation of stimulated Raman scattering from a Lambda-type three-level atom, where the stimulation is realized by the vacuum field of a high-finesse optical cavity. The scheme produces one intracavity photon by means of an adiabatic passage technique based on a counter-intuitive interaction sequence between pump laser and cavity field. This photon leaves the cavity through the less-reflecting mirror. The emission rate shows a characteristic dependence on the cavity and pump detuning, and the observed spectra have a sub-natural linewidth. The results are in excellent agreement with numerical simulations.

quant-ph↗