Juergen Appel

Homodyne tomography characterization and nonlocality of a dual-mode optical qubit.

A single photon, delocalized over two optical modes, is characterized by means of quantum homodyne tomography. The reconstructed four-dimensional density matrix extends over the entire Hilbert space and thus reveals, for the first time, complete information about the dual-rail optical quantum bit as a state of the electromagnetic field. The experimental data violate the Bell inequality albeit with a loophole similar to the detection loophole in photon counting experiments.

A versatile digital GHz phase lock for external cavity diode lasers

We present a versatile, inexpensive and simple optical phase lock for applications in atomic physics experiments. Thanks to all-digital phase detection and implementation of beat frequency pre-scaling, the apparatus requires no microwave-range reference input, and permits phase locking at frequency differences ranging from sub-MHz to 7 GHz (and with minor extension, to 12 GHz). The locking range thus covers ground state hyperfine splittings of all alkali metals, which makes this system a universal tool for many experiments on coherent interaction between light and atoms.

Adiabatic frequency conversion of optical information in atomic vapor

We experimentally demonstrate a communication protocol that enables frequency conversion and routing of quantum information in an adiabatic and thus robust way. The protocol is based on electromagnetically induced transparency (EIT) in systems with multiple excited levels: transfer and/or distribution of optical states between different signal modes is implemented by adiabatically changing the control fields. The proof-of-principle experiment is performed using the hyperfine levels of the rubidium D1 line.

Propagation of squeezed vacuum under electromagnetically induced transparency

We analyze the transmission of continuous-wave and pulsed squeezed vacuum through rubidium vapor under the conditions of electromagnetically induced transparency. Our analysis is based on a full theoretical treatment for a squeezed state of light propagating through temporal and spectral filters and detected using time and frequency-domain homodyne tomography. A model based on a three-level atom allows us to evaluate the linear losses and extra noise that degrade the nonclassical properties of the squeezed vacuum during the atomic interaction and eventually predict the quantum states of the transmitted light with a high precision.

Raman adiabatic transfer of optical states in multilevel atoms

We analyze electromagnetically induced transparency and light storage in an ensemble of atoms with multiple excited levels multi- configuration which are coupled to one of the ground states by quantized signal fields and to the other one via classical control fields. We present a basis transformation of atomic and optical states which reduces the analysis of the system to that of electromagnetically induced transparency in a regular three-level configuration. We demonstrate the existence of dark state polaritons and propose a protocol to transfer quantum information from one optical mode to another by an adiabatic control of the control fields.

Photons as quasicharged particles

The Schrodinger motion of a charged quantum particle in an electromagnetic potential can be simulated by the paraxial dynamics of photons propagating through a spatially inhomogeneous medium. The inhomogeneity induces geometric effects that generate an artificial vector potential to which signal photons are coupled. This phenomenon can be implemented with slow light propagating through an a gas of double-Lambda atoms in an electromagnetically-induced transparency setting with spatially varied control fields. It can lead to a reduced dispersion of signal photons and a topological phase shift of Aharonov-Bohm type.

Electromagnetically‐induced transparency and squeezed light

We report theoretical and experimental studies of the transmission and storage of squeezed vacuum under the conditions of electromagnetically induced transparency in rubidium vapor. A model based on a three‐level atom allows us to evaluate the linear losses and extra noise that degrade nonclassical properties of the squeezed vacuum during the atomic interaction and eventually predict the quantum states of the transmitted light with a high precision. Experimentally, we show that squeezing is preserved after storage in rubidium vapor for 1 μs.

Generation of Squeezed Vacuum States via Polarization Self‐Rotation

Recently, the possibility of generating squeezed vacuum states via polarization self‐rotation in hot atomic vapour has been debated. We present experimental results confirming that vacuum squeezed states can be generated using this method.

Characterization of atomic coherence decay for storage of light experiments

We report characterization of EIT resonances in the D1 line of Rb 87 under various experimental conditions. The dependence of the EIT linewidth on the power of the control field investigated. Strictly linear behavior between the ground levels as the main source of decoherence. We therefore formulated a new theory assuming pure dephasing to be the main decoherence mechanism. We also performed experiments where we created additional decoherence mechanisms by means of a counter-propagating repumper field. This field caused the ground-state population exchange, thus reproducing conditions in which the original theory is valid.

Frequency conversion and routing of quantum information in atomic media

We demonstrate a quantum communication protocol that enables frequency conversion of quantum optical information in an adiabatic way. The protocol is based on electromagnetically induced transparency in sustems with multiple excited levels. The proof-of-principle experiment is performed using the hyperfine levels of the rubidium D1 line.