C. Thiel

Quantum imaging with incoherent photons

We propose a technique to obtain sub-wavelength resolution in imaging with 100% visibility using incoherent light. We can in principle obtain a resolution of ¿/N using coincidence detection of N photons emitted from N excited atoms.

Generation of symmetric Dicke states of remote qubits with linear optics.

We propose a method for generating all symmetric Dicke states, either in the long-lived internal levels of N massive particles or in the polarization degrees of freedom of photonic qubits, using linear optical tools only. By means of a suitable multiphoton detection technique, erasing Welcher-Weg information, our proposed scheme allows the generation and measurement of an important class of entangled multiqubit states.

Operational determination of multiqubit entanglement classes via tuning of local operations.

We present a physical setup with which it is possible to produce arbitrary symmetric long-lived multiqubit entangled states in the internal ground levels of photon emitters, including the paradigmatic Greenberger-Horne-Zeilinger and W states. In the case of three emitters, where each tripartite entangled state belongs to one of two well-defined entanglement classes, we prove a one-to-one correspondence between well-defined sets of experimental parameters, i.e., locally tunable polarizer orientations, and multiqubit entanglement classes inside the symmetric subspace.

Sub-Rayleigh quantum imaging using single-photon sources

We present a general model to account for the multimode nature of the quantum electromagnetic field in projective photon-counting measurements. We focus on photon-subtraction experiments, where non-gaussian states are produced conditionally. These are useful states for continuous-variable quantum information processing. We present a general method called mode reduction that reduces the multimode model to an effective two-mode problem. We apply this method to a multimode model describing broadband parametric downconversion, thereby improving the analysis of existing experimental results. The main improvement is that spatial and frequency filters before the photon detector are taken into account explicitly. We find excellent agreement with previously published experimental results, using fewer free parameters than before, and discuss the implications of our analysis for the optimized production of states with negative Wigner functions.We propose a technique capable of imaging a distinct physical object with sub-Rayleigh resolution in an ordinary far-field imaging setup using single-photon sources and linear optical tools only. We exemplify our method for the case of a rectangular aperture and two or four single-photon emitters obtaining a resolution enhanced by a factor of 2 or 4, respectively.

Quantum interference and entanglement of photons that do not overlap in time.

We discuss the possibility of quantum interferences and entanglement of photons that exist at different intervals of time, i.e., one photon being recorded before the other has been created. The corresponding two-photon correlation function is shown to violate Bells inequalities.

Creating path entanglement and violating Bell inequalities by independent photon sources

We demonstrate a novel approach of violating position-dependent Bell inequalities by photons emitted via independent single photon sources in free space. We trace this violation back to path entanglement created a posteriori by the selection of modes due to the process of detection.

Operational monitoring of multi-qubit entanglement classes via tuning of local operations

We show that for a system of N emitters, incoherently radiating single photons it is possible to associate well-defined sets of experimental parameters with multiqubit entanglement classes, allowing their monitoring in an operational manner.

Quantum Interference and Entanglement of Photons which Do Not Overlap in Time

We report on quantum interferences and entanglement of photons which exist at different intervals of time. The corresponding two-photon correlation function is shown to violate Bell’s inequalities.

Nonlocality fromN>2independent single-photon emitters

We demonstrate that intensity correlations of second order in the fluorescence light of N>2 single-photon emitters may violate locality while the visibility of the signal remains below 1/{radical}(2){approx_equal}71%. For this, we derive a homogeneous Bell-Wigner-type inequality, which can be applied to a broad class of experimental setups. We trace the violation of this inequality back to path entanglement created by the process of detection.

Quantum imaging with uncorrelated single photon sources

We propose a technique to obtain sub-wavelength resolution using photons from uncorrelated single photon sources. The method employs N photons of wavelength λ spontaneously emitted from N atoms and subsequently detected by N detectors. We demonstrate that for certain detector positions the N-th order correlation function as a function of the first detector position is a pure sinusoidal oscillation with a fringe spacing of λ/N and a contrast of 100%. The result corresponds to an N-fold increase in resolution compared to classical microscopy. Our technique is also capable of imaging a distinct physical object with sub-Rayleigh resolution.