Last week a Stanford team announced in Physical Review Letters that it has created the world’s first dipolar quantum fermionic gas from the metal dysprosium – “an entirely new form of quantum matter,” as Stanford applied physics Professor and lead author Benjamin Lev put it – represents a major step toward understanding the behavior of these systems of particles. And this understanding makes for a leap toward the supernatural-seeming applications that condensed-matter physics conjures.
* The quantum matter are simultaneously superfluids and crystals.
* The work presents an exciting opportunity to study a large spectrum of phases that have been predicted to occur in strongly dipolar quantum gases.
* The researchers have already begun developing a microscope to make use of the dipolar quantum fluid’s unique characteristics. Their “cryogenic atom chip microscope” is a magnetic probe that should measure magnetic fields with unprecedented sensitivity and resolution. This kind of probe may even allow for a more stable form of quantum computation that uses exotic quantum matter to process information, known as a topologically protected quantum computer.
* Beyond these applications, the study offers physicists another line of attack toward understanding unconventional quantum effects.
At extremely low temperatures, the properties of an atomic gas of bosons (atoms with integer spin) are dramatically different from those of a gas of fermions (atoms with half-integer spin). A confined gas of bosons can be cooled into a Bose-Einstein condensate (BEC), where all of the atoms fall into the same quantum mechanical state to form a macroscopic matter-wave. Fermionic atoms, however, avoid each other. At very low temperatures, a trapped gas of these atoms forms a Fermi sea, where the atoms fill the allowed energy levels of the gas up to the so-called Fermi level. Although the low-temperature states of boson and fermion gases result from purely quantum statistical effects, interactions between the atoms can lead to collective behavior, like superfluidity.
In Physical Review Letters, two experimental groups report they have prepared cold quantum gases in which the atoms have unusually strong magnetic dipolar interactions: In an experimental first, scientists at the University of Illinois, Urbana-Champaign, and Stanford University, California (M. Lu et al.), have shown they can cool fermionic dysprosium-161 to form a Fermi sea of particles, while a group at the University of Innsbruck, Austria (K. Aikawa et al.), have prepared a cold quantum gas of erbium-168, which is a boson. In both systems, the quantum statistics of the particles (bosons or fermions), and the dipolar interactions between them, are probed by analysing how these gases expand after being released from a trap.
Dipolar interactions in a quantum gas of bosons or fermions. (a) The interaction between two particles with large dipole moments has a characteristic d-wave symmetry. (b) In a Bose-Einstein condensate, this interaction causes the gas to implode with a similar d-wave symmetry, as shown here for a gas of erbium-168. (c) In comparison, a quantum gas of fermions is stable, as shown for dysprosium-161.
We report the first quantum degenerate dipolar Fermi gas, the realization of which opens a new frontier for exploring strongly correlated physics and, in particular, quantum liquid crystalline phases. A quantum degenerate Fermi gas of the most magnetic atom 161Dy is produced by laser cooling to 10 μK before sympathetically cooling with ultracold, bosonic 162Dy. The temperature of the spin-polarized 161Dy is a factor T/TF=0.2 below the Fermi temperature TF=300 nK. The cotrapped 162Dy concomitantly cools to approximately Tc for Bose-Einstein condensation, thus realizing a novel, nearly quantum degenerate dipolar Bose-Fermi gas mixture. Additionally, we achieve the forced evaporative cooling of spin-polarized 161Dy without 162Dy to T/TF=0.7. That such a low temperature ratio is achieved may be a first signature of universal dipolar scattering.
When the thermal energy of some substances drops below a certain critical point, it is often no longer possible to consider its component particles separately. Instead, the material becomes “strongly correlated” and its quantum effects become difficult to understand and study.
Making the material out of a gas of atoms allows what is normally only observed on a nanometer scale to become visible. These quantum gases, the coldest objects known to man, are where researchers see zero-viscosity fluids – superfluids – that are mathematical cousins of superconductors.
The invention of the key technique for cooling gases to near absolute zero netted Stanford Professor Emeritus Steven Chu a Nobel Prize in 1997. While researchers have been cooling gases into the quantum realm for two decades, creating strongly correlated quantum gases has proven a much larger challenge.
The basic cooling method hasn’t changed significantly since Chu’s days, but the techniques employed have become more extreme.
Lev, as well as Stanford graduate students Mingwu Lu and Nathaniel Burdick, heated their particles in a crucible to around 1,300 degrees Celsius and shot them into a powerful vacuum. Using the world’s most powerful continuous-wave blue laser, the particles were then cooled to within a thousandth of a degree of absolute zero. Subsequent lasers and an evaporative cooling process eventually brought the gas down to the experimental temperature of 64 nanokelvin – very, very cold.
An “impossible” goal
Until now, research efforts had focused on cooling bosons – fundamentally different from fermions, and much easier to work with – and weakly magnetic fermions. The Stanford team extended these techniques to gases made of the most magnetic atom: a fermionic isotope of dysprosium with magnetic energies 440 times larger than previously cooled gases.
The leap presented two major challenges.
The initial step – trapping particles in the “optical molasses” created by the high-powered lasers – works by exciting a particle and then allowing it to return to its initial state. The particle loses energy in this process, cooling dramatically. Typically, the substances that researchers cool in this way only have two or three energy levels, making for a simple “optical loop.” Dysprosium, on the other hand has more than 140.
“Everyone thought it was impossible for this most complex of elements,” said Lev.
Although this detail made for a daunting technological challenge, the second difficulty was thought to be even more fundamental.
The evaporative cooling process the researchers hoped to use to bring the gas down to the nanokelvin range depends on collisions between particles. Collisions both dissipate energy and knock high-energy particles out of the system.
“But the lore in the field is that identical fermions never collide,” Lev said.
Fermions obey the Pauli exclusion principle, which states that no two fermions can occupy the same state simultaneously. This inviolable rule seems to disallow direct interactions between fermionic particles.
Dysprosium’s status as the most magnetic element was crucial to overcoming this apparent impossibility. Because of the extraordinarily strong magnetic dipolar interactions between the dysprosium atoms, the particles were able to cool to below the critical temperature by colliding from afar, despite being quantum mechanically identical.
Putting the gas to work
Thus far, the result of the Lev lab’s high-tech efforts is a tiny ball of ultracold quantum dipolar fluid. But the researchers have reason to believe that the humble substance will exhibit the seemingly contradictory characteristics of both crystals and superfluids.
This combination could lead to quantum liquid crystals, or quantum-mechanical versions of the liquid crystals that make up most electronic displays. Or it could yield a supersolid – a hypothetical state of matter that would, in theory at least, be a solid with superfluid characteristics.
The researchers have already begun developing a microscope to make use of the dipolar quantum fluid’s unique characteristics. Their “cryogenic atom chip microscope” is a magnetic probe that should measure magnetic fields with unprecedented sensitivity and resolution. This kind of probe may even allow for a more stable form of quantum computation that uses exotic quantum matter to process information, known as a topologically protected quantum computer.
Beyond these applications, the study offers physicists another line of attack toward understanding unconventional quantum effects.
“It’s a problem that has stood unresolved since the mid-80s,” said Lev. “It’s worth taking a new approach.”