H. Walther

Continuous generation of single photons with controlled waveform in an ion-trap cavity system

The controlled production of single photons is of fundamental and practical interest; they represent the lowest excited quantum states of the radiation field, and have applications in quantum cryptography and quantum information processing. Common approaches use the fluorescence of single ions, single molecules, colour centres and semiconductor quantum dots. However, the lack of control over such irreversible emission processes precludes the use of these sources in applications (such as quantum networks) that require coherent exchange of quantum states between atoms and photons. The necessary control may be achieved in principle in cavity quantum electrodynamics. Although this approach has been used for the production of single photons from atoms, such experiments are compromised by limited trapping times, fluctuating atom–field coupling and multi-atom effects. Here we demonstrate a single-photon source based on a strongly localized single ion in an optical cavity. The ion is optimally coupled to a well-defined field mode, resulting in the generation of single-photon pulses with precisely defined shape and timing. We have confirmed the suppression of two-photon events up to the limit imposed by fluctuations in the rate of detector dark counts. The stream of emitted photons is uninterrupted over the storage time of the ion, as demonstrated by a measurement of photon correlations over 90 min.

Strong-driving-assisted multipartite entanglement in cavity QED.

We propose a method of generating multipartite entanglement by considering the interaction of a system of N two-level atoms in a cavity of high quality factor with a strong classical driving field. It is shown that, with a judicious choice of the cavity detuning and the applied coherent field detuning, vacuum Rabi coupling produces a large number of important multipartite entangled states. It is even possible to produce entangled states involving different cavity modes. Tuning of parameters also permits us to switch from Jaynes-Cummings to anti-Jaynes-Cummings-like interaction.

Preparing pure photon number states of the radiation field

The quantum mechanical description of a radiation field is based on states that are characterized by the number of photons in a particular mode; the most basic quantum states are those with fixed photon number, usually referred to as number (or Fock) states. Although Fock states of vibrational motion can be observed readily in ion traps, number states of the radiation field are very fragile and difficult to produce and maintain. Single photons in multi-mode fields have been generated using the technique of photon pairs. But in order to generate these states in a cavity, the mode in question must have minimal losses; moreover, additional sources of photon number fluctuations, such as the thermal field, must be eliminated. Here we observe the build-up of number states in a high-Q cavity, by investigating the interaction dynamics of a probe atom with the field. We employ a dynamical method of number state preparation that involves state reduction of highly excited atoms in a cavity, with a photon lifetime as high as 0.2 seconds. (This set-up is usually known as the one-atom maser or ‘micromaser’.) Pure states containing up to two photons are measured unambiguously.

Attosecond Double-Slit Experiment

A new scheme for a double-slit experiment in the time domain is presented. Phase-stabilized few-cycle laser pulses open one to two windows (slits) of attosecond duration for photoionization. Fringes in the angle-resolved energy spectrum of varying visibility depending on the degree of which-way information are measured. A situation in which one and the same electron encounters a single and a double slit at the same time is observed. The investigation of the fringes makes possible interferometry on the attosecond time scale. From the number of visible fringes, for example, one derives that the slits are extended over about 500 as.

Rescattering effects in above-threshold ionization: a classical model

Recent experimental investigations of the high-order above-threshold ionization peaks near the onset of the plateau have exhibited anomalous angular distributions of the emitted photoelectrons with pronounced side lobes surrounding emission in the direction of the laser electric field. It is shown that the existence and angular position of these side lobes are consequences of the classical kinematics of electrons in laser fields.

Very-low-temperature behavior of a micromaser

We show that at very low temperatures the steady-state photon statistics of the micromaser are stronglv influenced by the existence of trapping states. The resulting resonances are a true quantum effect resulting from the granulated character of the electromagnetic field.

Quantum computation with trapped ions in an optical cavity

Two-qubit logical gates are proposed on the basis of two atoms trapped in a cavity setup and commonly addressed by laser fields. Losses in the interaction by spontaneous transitions are efficiently suppressed by employing adiabatic transitions and the quantum Zeno effect. Dynamical and geometrical conditional phase gates are suggested. This method provides fidelity and a success rate of its gates very close to unity. Hence, it is suitable for performing quantum computation.

Surrealistic Bohm Trajectories

Abstract A study of interferometers with one-bit which-way detectors demonstrates that the trajectories, which David Bohm invented in his attempt at a realistic interpretation of quantum mechanics, are in fact surrealistic, because they may be macroscopically at variance with the observed track of the particle. We consider a two-slit interferometer and an incomplete Stern-Gerlach interferometer, and propose an experimentum crucis based on the latter.

Novel miniature ion traps

Two new radiofrequency ion traps are presented, a Paul-Straubel type ion trap, and an ion trap which essentially consists of two endcaps, the “endcap trap”. The potential which is generated by these electrode configurations is analyzed. Experiments on In+ and Mg+ in these traps are reported.

Rotational state populations and angular distributions on surface scattered molecules: No on graphite

Abstract Rotational state populations and angular distributions of NO molecules were determined after the scattering of a supersonic beam from a graphite surface at different surface temperatures. The angular distributions exhibit an isotropic and a specular part. The rotational population of the scattered molecules can be described by Boltzmann distributions with identical temperatures for both electronic ground states 2 Π ½ and 2 Π 3/2 and both scattering components. The rotational temperature agrees with the surface temperature below 170 K and converges to a constant value of 250 K for surface temperatures higher than 350 K.