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On the possibility of "correlation cooling" of ultracold ionic gases

Thomas Pohl, Thomas Pattard, Jan Michael Rost

Published Feb 3, 2004• 0 citations
OpenAlex

Abstract

Abstract. Recent experiments with ultracold neutral plasmas show an intrinsic heating effect based on the development of spatial correlations, which has been termed “correlation heating”. We investigate whether this effect can be reversed, so that imposing strong spatial correlations could in fact lead to cooling. We find that cooling is indeed possible. It requires, however, a very precise preparation of the initial state. Quantum mechanical zero-point motion sets a lower limit for cooling. PACS numbers: 32.80.Pj,52.27.Gr Submitted to: J. Phys. B: At. Mol. Opt. Phys. Letter to the Editor 2 Over the last two decades, techniques of cooling and trapping of ions and neutral atoms have made tremendous progress, making the regime of ultralow temperatures (T ≪ 1 K) accessible to experiment. Using sophisticated cooling schemes, temperatures far below 1µK have been realized, allowing, among many other applications, Bose-Einstein condensation of dilute atomic gases. Nevertheless, known cooling mechanisms either have a lower limit beyond which cooling is not possible (such as in “standard” laser cooling) or suffer from other drawbacks (such as loss of particles in evaporative

Related papers

On the possibility of correlation cooling of ultracold neutral plasmas

Thomas Pohl, Thomas Pattard, Jan Michael Rost

Recent experiments with ultracold neutral plasmas show an intrinsic heating effect based on the development of spatial correlations. We investigate whether this effect can be reversed, so that imposing strong spatial correlations could in fact lead to cooling of the ions. We find that cooling is indeed possible. It requires, however, a very precise preparation of the initial state. Quantum mechanical zero-point motion sets a lower limit for ion cooling.

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By photoionizing samples of laser-cooled atoms with laser light tuned just above the ionization limit, plasmas can be created with electron and ion temperatures below 10 K. These ultracold neutral plasmas have extended the temperature bounds of plasma physics by two orders of magnitude. Table-top experiments, using many of the tools from atomic physics, allow for the study of plasma phenomena in this new regime with independent control over the density and temperature of the plasma through the excitation process. Characteristic of these systems is an inhomogeneous density profile, inherited from the density distribution of the laser-cooled neutral atom sample. Most work has dealt with unconfined plasmas in vacuum, which expand outward at velocities of order 100 m/s, governed by electron pressure, and with lifetimes of order 100 μs, limited by stray electric fields. Using detection of charged particles and optical detection techniques, a wide variety of properties and phenomena have been observed, including expansion dynamics, collective excitations in both the electrons and ions, and collisional properties. Through three-body recombination collisions, the plasmas rapidly form Rydberg atoms, and clouds of cold Rydberg atoms have been observed to spontaneously avalanche ionize to form plasmas. Of particular interest is the possibility of the formation of strongly coupled plasmas, where Coulomb forces dominate thermal motion and correlations become important. The strongest impediment to strong coupling is disorder-induced heating, a process in which Coulomb energy from an initially disordered sample is converted into thermal energy. This restricts electrons to a weakly coupled regime and leaves the ions barely within the strongly coupled regime. This review will give an overview of the field of ultracold neutral plasmas, from its inception in 1999 to current work, including efforts to increase strong coupling and effects on plasma properties due to strong coupling.

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Numerical simulation of ultracold plasmas

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In recent experiments, ultracold neutral plasmas were produced by photoionizing small clouds of laser-cooled atoms. It has been suggested that the low initial temperature of these novel plasmas leads directly to strong correlation and order. In contrast, we argue that rapid intrinsic heating raises the electron temperature to the point where strong correlation cannot develop. The argument is corroborated by a molecular dynamics simulation of the early time plasma evolution.

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Early Time Ion Dynamics and Progress Towards Laser Cooling in an Ultracold Neutral Plasma

Clayton E. Simien

The progress toward laser cooling an ultracold plasma is presented in this thesis. Ultracold neutral plasmas are created by photo-ionizing laser cooled Strontium atoms. Initially this system is very strongly coupled, however the ions rapidly heat up leaving the plasma on the borderline of the strongly coupled regime. To counteract the effects of this heating an attempt to laser cool the ions in the system is undertaken. However, from these experiments we discovered that velocity changing collisions occurring in the system prevented laser cooling. In order to determine the collisions rate, a collision model was developed to simulate the spectra of the plasma from optical pumping experiments. These experiments and simulations in fact demonstrate that collisions are occurring at a fast enough rate to prevent ion cooling on the time scale of our experiment.

•Jan 1, 2008•1 citation
Abstract. Recent experiments with ultracold neutral plasmas show an intrinsic heating effect based on the development of spatial correlations, which has been termed “correlation heating”. We investigate whether this effect can be reversed, so that imposing strong spatial correlations could in fact lead to cooling. We find that cooling is indeed possible. It requires, however, a very precise preparation of the initial state. Quantum mechanical zero-point motion sets a lower limit for cooling. PACS numbers: 32.80.Pj,52.27.Gr Submitted to: J. Phys. B: At. Mol. Opt. Phys. Letter to the Editor 2 Over the last two decades, techniques of cooling and trapping of ions and neutral atoms have made tremendous progress, making the regime of ultralow temperatures (T ≪ 1 K) accessible to experiment. Using sophisticated cooling schemes, temperatures far below 1µK have been realized, allowing, among many other applications, Bose-Einstein condensation of dilute atomic gases. Nevertheless, known cooling mechanisms either have a lower limit beyond which cooling is not possible (such as in “standard” laser cooling) or suffer from other drawbacks (such as loss of particles in evaporative
By photoionizing samples of laser-cooled atoms with laser light tuned just above the ionization limit, plasmas can be created with electron and ion temperatures below 10 K. These ultracold neutral plasmas have extended the temperature bounds of plasma physics by two orders of magnitude. Table-top experiments, using many of the tools from atomic physics, allow for the study of plasma phenomena in this new regime with independent control over the density and temperature of the plasma through the excitation process. Characteristic of these systems is an inhomogeneous density profile, inherited from the density distribution of the laser-cooled neutral atom sample. Most work has dealt with unconfined plasmas in vacuum, which expand outward at velocities of order 100 m/s, governed by electron pressure, and with lifetimes of order 100 μs, limited by stray electric fields. Using detection of charged particles and optical detection techniques, a wide variety of properties and phenomena have been observed, including expansion dynamics, collective excitations in both the electrons and ions, and collisional properties. Through three-body recombination collisions, the plasmas rapidly form Rydberg atoms, and clouds of cold Rydberg atoms have been observed to spontaneously avalanche ionize to form plasmas. Of particular interest is the possibility of the formation of strongly coupled plasmas, where Coulomb forces dominate thermal motion and correlations become important. The strongest impediment to strong coupling is disorder-induced heating, a process in which Coulomb energy from an initially disordered sample is converted into thermal energy. This restricts electrons to a weakly coupled regime and leaves the ions barely within the strongly coupled regime. This review will give an overview of the field of ultracold neutral plasmas, from its inception in 1999 to current work, including efforts to increase strong coupling and effects on plasma properties due to strong coupling.
Abstract Plasma dynamics critically depends on density and temperature, thus well-controlled experimental realizations are essential benchmarks for theoretical models. The formation of an ultracold plasma can be triggered by ionizing a tunable number of atoms in a micrometer-sized volume of a 87Rb Bose-Einstein condensate (BEC) by a single femtosecond laser pulse. The large density combined with the low temperature of the BEC give rise to an initially strongly coupled plasma in a so far unexplored regime bridging ultracold neutral plasma and ionized nanoclusters. Here, we report on ultrafast cooling of electrons, trapped on orbital trajectories in the long-range Coulomb potential of the dense ionic core, with a cooling rate of 400 K ps−1. Furthermore, our experimental setup grants direct access to the electron temperature that relaxes from 5250 K to below 10 K in less than 500 ns.
The progress toward laser cooling an ultracold plasma is presented in this thesis. Ultracold neutral plasmas are created by photo-ionizing laser cooled Strontium atoms. Initially this system is very strongly coupled, however the ions rapidly heat up leaving the plasma on the borderline of the strongly coupled regime. To counteract the effects of this heating an attempt to laser cool the ions in the system is undertaken. However, from these experiments we discovered that velocity changing collisions occurring in the system prevented laser cooling. In order to determine the collisions rate, a collision model was developed to simulate the spectra of the plasma from optical pumping experiments. These experiments and simulations in fact demonstrate that collisions are occurring at a fast enough rate to prevent ion cooling on the time scale of our experiment.