T. W. Hänsch

Quantum phase transition from a superfluid to a Mott insulator in a gas of ultracold atoms

For a system at a temperature of absolute zero, all thermal fluctuations are frozen out, while quantum fluctuations prevail. These microscopic quantum fluctuations can induce a macroscopic phase transition in the ground state of a many-body system when the relative strength of two competing energy terms is varied across a critical value. Here we observe such a quantum phase transition in a Bose–Einstein condensate with repulsive interactions, held in a three-dimensional optical lattice potential. As the potential depth of the lattice is increased, a transition is observed from a superfluid to a Mott insulator phase. In the superfluid phase, each atom is spread out over the entire lattice, with long-range phase coherence. But in the insulating phase, exact numbers of atoms are localized at individual lattice sites, with no phase coherence across the lattice; this phase is characterized by a gap in the excitation spectrum. We can induce reversible changes between the two ground states of the system.

Optical frequency metrology

Extremely narrow optical resonances in cold atoms or single trapped ions can be measured with high resolution. A laser locked to such a narrow optical resonance could serve as a highly stable oscillator for an all-optical atomic clock. However, until recently there was no reliable clockwork mechanism that could count optical frequencies of hundreds of terahertz. Techniques using femtosecond-laser frequency combs, developed within the past few years, have solved this problem. The ability to count optical oscillations of more than 1015 cycles per second facilitates high-precision optical spectroscopy, and has led to the construction of an all-optical atomic clock that is expected eventually to outperform todays state-of-the-art caesium clocks.

Attosecond control of electronic processes by intense light fields.

The amplitude and frequency of laser light can be routinely measured and controlled on a femtosecond (10-15 s) timescale. However, in pulses comprising just a few wave cycles, the amplitude envelope and carrier frequency are not sufficient to characterize and control laser radiation, because evolution of the light field is also influenced by a shift of the carrier wave with respect to the pulse peak. This so-called carrier-envelope phase has been predicted and observed to affect strong-field phenomena, but random shot-to-shot shifts have prevented the reproducible guiding of atomic processes using the electric field of light. Here we report the generation of intense, few-cycle laser pulses with a stable carrier envelope phase that permit the triggering and steering of microscopic motion with an ultimate precision limited only by quantum mechanical uncertainty. Using these reproducible light waveforms, we create light-induced atomic currents in ionized matter; the motion of the electronic wave packets can be controlled on timescales shorter than 250 attoseconds (250 × 10-18 s). This enables us to control the attosecond temporal structure of coherent soft X-ray emission produced by the atomic currents—these X-ray photons provide a sensitive and intuitive tool for determining the carrier-envelope phase.

Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity

Abstract We propose a new scheme for locking the frequency of a laser to a resonant reference cavity. A linear polarizer or Brewster plate is placed inside the reference cavity, so that the reflected light acquires a frequency-dependent elliptical polarization. A simple polarization analyzer detects dispersion shaped resonances which can provide the error signal for electronic frequency stabilization without any need for modulation techniques.

Cooling of gases by laser radiation

It is shown that a low-density gas can be cooled by illuminating it with intense, quasi-monochromatic light confined to the lower-frequency half of a resonance lines Doppler width. Translational kinetic energy can be transferred from the gas to the scattered light, until the atomic velocity is reduced by the ratio of the Doppler width to the natural line width.

Repetitively Pulsed Tunable Dye Laser for High Resolution Spectroscopy

A pulsed tunable dye laser with a bandwidth of less than 0.004 A, repetitively pumped by a nitrogen laser, is described. An intracavity beam expanding telescope together with a diffraction grating in Littrow mount and a tilted Fabry-Perot etalon provide convenient, very reproducible wavelength tuning and good stability. Output peak powers in the kilowatt range at 5-100 nsec pulse width and repetition rates up to 100 pps can be generated from the near-ultraviolet throughout the visible spectrum.

Laser frequency combs for astronomical observations.

A direct measurement of the universes expansion history could be made by observing in real time the evolution of the cosmological redshift of distant objects. However, this would require measurements of Doppler velocity drifts of ∼1 centimeter per second per year, and astronomical spectrographs have not yet been calibrated to this tolerance. We demonstrated the first use of a laser frequency comb for wavelength calibration of an astronomical telescope. Even with a simple analysis, absolute calibration is achieved with an equivalent Doppler precision of ∼9 meters per second at ∼1.5 micrometers—beyond state-of-the-art accuracy. We show that tracking complex, time-varying systematic effects in the spectrograph and detector system is a particular advantage of laser frequency comb calibration. This technique promises an effective means for modeling and removal of such systematic effects to the accuracy required by future experiments to see direct evidence of the universes putative acceleration.

A frequency comb in the extreme ultraviolet

Since 1998, the interaction of precision spectroscopy and ultrafast laser science has led to several notable accomplishments. Femtosecond laser optical frequency ‘combs’ (evenly spaced spectral lines) have revolutionized the measurement of optical frequencies and enabled optical atomic clocks. The same comb techniques have been used to control the waveform of ultrafast laser pulses, which permitted the generation of single attosecond pulses, and have been used in a recently demonstrated ‘oscilloscope’ for light waves. Here we demonstrate intra-cavity high harmonic generation in the extreme ultraviolet, which promises to lead to another joint frontier of precision spectroscopy and ultrafast science. We have generated coherent extreme ultraviolet radiation at a repetition frequency of more than 100 MHz, a 1,000-fold improvement over previous experiments. At such a repetition rate, the mode spacing of the frequency comb, which is expected to survive the high harmonic generation process, is large enough for high resolution spectroscopy. Additionally, there may be many other applications of such a quasi-continuous compact and coherent extreme ultraviolet source, including extreme ultraviolet holography, microscopy, nanolithography and X-ray atomic clocks.

A compact grating-stabilized diode laser system for atomic physics

We describe a compact, economic and versatile diode laser system based on commercial laser diodes, optically stabilized by means of feedback from a diffraction grating. We offer detailed information which should enable the reader to copy our set-up which uses only easily machined mechanical parts. Our system offers single-mode operation with a linewidth of a few 100 kHz, continuous scans over 25 GHz, high chirp rates (up to 9 GHz/ms) and FM-modulation up to the GHz range. We discuss radiofrequency phase-locking of two independent lasers systems, allowing well controlled fast frequency switching which overcomes the limitations imposed by acousto-optic modulators.

Collapse and revival of the matter wave field of a Bose- Einstein condensate

A Bose–Einstein condensate represents the most ‘classical’ form of a matter wave, just as an optical laser emits the most classical form of an electromagnetic wave. Nevertheless, the matter wave field has a quantized structure owing to the granularity of the discrete underlying atoms. Although such a field is usually assumed to be intrinsically stable (apart from incoherent loss processes), this is no longer true when the condensate is in a coherent superposition of different atom number states. For example, in a Bose–Einstein condensate confined by a three-dimensional optical lattice, each potential well can be prepared in a coherent superposition of different atom number states, with constant relative phases between neighbouring lattice sites. It is then natural to ask how the individual matter wave fields and their relative phases evolve. Here we use such a set-up to investigate these questions experimentally, observing that the matter wave field of the Bose–Einstein condensate undergoes a periodic series of collapses and revivals; this behaviour is directly demonstrated in the dynamical evolution of the multiple matter wave interference pattern. We attribute the oscillations to the quantized structure of the matter wave field and the collisions between individual atoms.