There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

This paper reviews recent experimental and theoretical progress concerning many-body phenomena in dilute, ultracold gases. It focuses on effects beyond standard weak-coupling descriptions, such as the Mott-Hubbard transition in optical lattices, strongly interacting gases in one and two dimensions, or lowest-Landau-level physics in quasi-two-dimensional gases in fast rotation. Strong correlations in fermionic gases are discussed in optical lattices or near-Feshbach resonances in the BCS-BEC crossover.

/pdf/many-body-physics-with-ultracold-gases-552s10zfdn.pdf

Many-Body Physics with Ultracold Gases

A Bose-Einstein condensate was produced in a vapor of rubidium-87 atoms that was confined by magnetic fields and evaporatively cooled. The condensate fraction first appeared near a temperature of 170 nanokelvin and a number density of 2.5 x 10 12 per cubic centimeter and could be preserved for more than 15 seconds. Three primary signatures of Bose-Einstein condensation were seen. (i) On top of a broad thermal velocity distribution, a narrow peak appeared that was centered at zero velocity. (ii) The fraction of the atoms that were in this low-velocity peak increased abruptly as the sample temperature was lowered. (iii) The peak exhibited a nonthermal, anisotropic velocity distribution expected of the minimum-energy quantum state of the magnetic trap in contrast to the isotropic, thermal velocity distribution observed in the broad uncondensed fraction.

/pdf/observation-of-bose-einstein-condensation-in-a-dilute-atomic-2wpb5ks2pi.pdf

Observation of Bose-Einstein Condensation in a Dilute Atomic Vapor

The phenomenon of Bose-Einstein condensation of dilute gases in traps is reviewed from a theoretical perspective. Mean-field theory provides a framework to understand the main features of the condensation and the role of interactions between particles. Various properties of these systems are discussed, including the density profiles and the energy of the ground-state configurations, the collective oscillations and the dynamics of the expansion, the condensate fraction and the thermodynamic functions. The thermodynamic limit exhibits a scaling behavior in the relevant length and energy scales. Despite the dilute nature of the gases, interactions profoundly modify the static as well as the dynamic properties of the system; the predictions of mean-field theory are in excellent agreement with available experimental results. Effects of superfluidity including the existence of quantized vortices and the reduction of the moment of inertia are discussed, as well as the consequences of coherence such as the Josephson effect and interference phenomena. The review also assesses the accuracy and limitations of the mean-field approach.

https://arxiv.org/pdf/cond-mat/9806038.pdf

Theory of Bose-Einstein condensation in trapped gases

Bose-Einstein condensation (BEC) has been observed in a dilute gas of sodium atoms. A Bose-Einstein condensate consists of a macroscopic population of the ground state of the system, and is a coherent state of matter. In an ideal gas, this phase transition is purely quantum-statistical. The study of BEC in weakly interacting systems which can be controlled and observed with precision holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles.

Bose-Einstein condensation in a gas of sodium atoms

Trapping a Rydberg atom close to a surface is an important step towards the realisation of many proposals for quantum information processing or hybrid quantum systems. One of the challenges in these experiments is posed by the electric field emanating from contaminations on the surface. Here we report on measurements of an electric field created by 87Rb atoms adsorbed on a 25 nm thick layer of SiO2, covering a 90 nm layer of Au. The electric field is measured using a two-photon transition to the and states. The electric field value that we measure is higher than typical values measured above metal surfaces, but is consistent with a recent measurement above a SiO2 surface. In addition, we measure the temporal behaviour of the field and observe that we can reduce it in a single experimental cycle, using ultraviolet light or by mildly locally heating the surface with one of the excitation lasers, whereas the buildup of the field takes thousands of cycles. We explain these results by a change in the adatom distribution on the surface. These results indicate that, while the stray electric field can be reduced, achieving field-free conditions above a silica-coated gold chip remains challenging.

Adsorbate dynamics on a silica-coated gold surface measured by Rydberg Stark spectroscopy

Experiments on ultracold gases offer unparalleled opportunities to explore quantum many-body physics, with excellent control over key parameters including temperature, density, interactions and even dimensionality. In some systems, atomic interactions can be adjusted by means of magnetic Feshbach resonances, which have played a crucial role in realizing new many-body phenomena. However, suitable Feshbach resonances are not always available, and they offer limited freedom since the magnetic field strength is the only control parameter. Here we show a new way to tune interactions in one-dimensional quantum gases using state-dependent dressed potentials, enabling control over non-equilibrium spin motion in a two-component gas of 87Rb. The accessible range includes the point of spin-independent interactions where exact quantum many-body solutions are available and the point where spin motion is frozen. This versatility opens a new route to experiments on spin waves, spin-"charge" separation and the relation between superfluidity and magnetism in low-dimensional quantum gases.

Controlling spin motion and interactions in a one-dimensional Bose gas

We report a stable field emission (FE) current from sharp tungsten (W) tips in relatively poor vacuum (up to 10−2 mbar) conditions. We use small tip–anode spacing to keep the extraction voltage low. A simple current regulator circuit, with a bandwidth of ∼1.6 kHz, was designed, which controls the voltage applied according to the emission current measured. Without a current regulator circuit, an uncleaned W tip in unbaked 6×10−7 mbar system pressure cannot emit stable FE current and short-term fluctuations at a few nA current were found to be more than 300%. The current regulator circuit improves the FE current stability dramatically. It was observed that at current level of ∼3.5 nA and regulation voltage of ∼120 V the short-term fluctuations in the current were ∼5% at 6×10−7 mbar unbaked system pressure. Subsequently, the system pressure was increased in steps up to 10−3 mbar of argon gas and it was observed that the current regulator circuit worked at almost the same efficiency. Around 10−2 mbar of Ar ga...

Stable field emission from W tips in poor vacuum conditions

Using time-resolved measurements, we demonstrate coherent collective Rydberg excitation crossing over into Rydberg blockade in a dense and ultracold gas trapped at a distance of 100 μm from a room-temperature atom chip. We perform Ramsey-type measurements to characterize the coherence. The experimental data are in good agreement with numerical results from a master equation using a mean-field approximation and with results from a super-atom-based Hamiltonian. This represents significant progress in exploring a strongly interacting driven Rydberg gas on an atom chip.

/pdf/from-coherent-collective-excitation-to-rydberg-blockade-on-43ve0xdl7b.pdf

From coherent collective excitation to Rydberg blockade on an atom chip

Whereas quantum noise in a laser is essentially white, we show that excess quantum noise is colored. The coloring is determined by both the geometry (nonorthogonality) and the dynamics (eigenvalues) of the eigenmodes of the laser resonator. Experimentally, we demonstrate these concepts by using nonorthogonal polarization modes. We also show that the induced correlations between the modes can be used to greatly reduce the excess quantum noise.

/pdf/excess-quantum-noise-is-colored-1s1mz9b9r3.pdf

N.J. van Druten

Papers

Adsorbate dynamics on a silica-coated gold surface measured by Rydberg Stark spectroscopy

Controlling spin motion and interactions in a one-dimensional Bose gas

Stable field emission from W tips in poor vacuum conditions

From coherent collective excitation to Rydberg blockade on an atom chip

Excess quantum noise is colored