When it comes to furthering our overall understanding of the physical world, ultracold quantum gases are awfully promising. As the famous physicist Richard Feynman argued, to fully understand nature, we need quantum means of simulation and computation. Ultracold atomic systems have, in the last 30 years, proven to be amazing quantum simulators. The number of applications for these systems as such simulators is nothing short of overwhelming, ranging from engineering artificial crystals to providing new platforms for quantum computing. In its brief history, ultracold atomic experimental research has enhanced physicists’ understanding of a truly vast array of important phenomena.
One of the revelations of quantum mechanics is that any object can be seen as a wave (even you!) when an appropriate experimental test is used. Properties of these co-called “matter waves” depend on their temperature; at large temperatures they have short wavelengths and look and behave particlelike because all the peaks and valleys are so close together that they cannot be told apart. If we lower temperature to much less than a single kelvin, the wave nature of matter becomes more pronounced and wavelike behaviors more important. What happens then with a large collection of very cold atoms that behave like a large collection of waves? They can all align and overlap to form a single wave, something that was historically called a “macroscopic wave function.” Such a system—a condensate in physics parlance—is a fundamentally quantum state of matter.
Quantum condensates were theoretically predicted in the mid-1920s, but it was only in the late 1990s that experimental physicists kicked off a revolution (recognized by two Nobel prizes) by using lasers and magnets to reach sufficiently low temperatures for the transition to these phases of matter to happen. Light can interact with atoms and thus change their energies. Atoms also experience forces when placed in nonuniform magnetic fields. Physicists used these two properties to trap clouds of atoms such as rubidium and eventually lower their temperature to picokelvins—trillionths of a degree above absolute zero. Remarkably, experiments in which these extremely low temperatures can be reached, and quantum states of matter are engineered, fit in an average-sized room, on a large table with the ultracold atom gas frequently visible to the naked eye. The coldest places in the universe can often be found in a room on your local college campus, and they are likely controlled by a graduate student.
But it’s not just making something the coldest or the most quantum that excites physicists; it’s that ultracold atoms can be controlled and manipulated very precisely. Theoretical physicists have been especially emboldened by the possibility of engineering a quantum system by moving ultracold atoms around and fine-tuning the way in which they interact. To a theorist, a physical system such as a novel material that has some odd or unexpected property, is a frustrating black box that is hard to describe with mathematical equations.
An ultracold atomic experiment can be the exact opposite, bringing equations to life and determining whether they measure up to nature. Many minimal, prototypical models, extensively studied at the level of mathematical equations but not necessarily matched by any naturally found material, can be engineered in ultracold atomic experiments. Since the late 1990s, physicists of all sorts have embraced this idea and pushed it in every direction they could imagine.
As one example, adding counterpropagating laser beams to an ultracold atomic sample creates an optical lattice and turns the system into an artificial crystal. While a physical crystal has to be grown carefully, an ultracold artificial crystal can be changed from one shape to another by adjusting laser beams. Even more advantageously, such artificial crystals are typically very clean, and researchers can add in disorder by using more lasers. This means they can “reverse engineer” some of the effects of disorder. If a crystal is grown and then studied, it can be difficult to determine how much “dirt” in that sample actually matters for experimental outcomes. If researchers can control the disorder, then they can be very precise about determining its consequences.
From the very first ultracold atomic experiments, they have been really important for studying fluids having zero viscosity or superfluids. When does a normal fluid become a superfluid? Can something similar to sound propagate through a superfluid? What happens if a container of superfluid is rotated? Many such fundamental questions have been answered through simulations with ultracold atoms.
For instance, rotating a superfluid has been predicted to give rise to appearance of vortices—small hurricanes of quantum fluid—as a consequence of basic properties of the macroscopic wave function. Researchers are learning about quantum turbulence by observing and manipulating these vortices, thinking of them as controllable building blocks of more chaotic superfluid flows. Precise models for turbulent quantum flows have historically eluded theorists, which makes ultracold atomic simulations the first line of attack for this difficult problem.
As with studies of superfluids, many efforts have been made to simulate superconductors. They are perfect conductors having no resistance; no energy is wasted as electric current runs through them. As this is in contrast with all conductors used to supply electricity to businesses and households, it is a very active area of research to try and simulate a superconductor that does not have to be very cold. While a physicist’s notion of “very cold” may not quite match the colloquial use of the phrase (a “cold atom” in physics jargon is millions of times colder than a cold pint of ice cream in your fridge), even a few kelvin worth of difference could be meaningful for applications of superconductors outside the lab.
Theoretical physicists have debated various high-temperature superconducting models for years, and ultracold atomic studies have been one of the prime ways to put those, sometimes conflicting, theories to test. Experimental physicists can also make a superfluid of ultracold atoms become something like a superconductor in a process called “BEC-BCS crossover.” This crossover has been theorized in semiconductors and neutron stars but never unequivocally confirmed in any system other than ones consisting of ultracold atoms.
Superconductors and superfluids are both fundamentally quantum phases of matter, making up something like a quantum expansion of the liquid-solid-vapor list of phases you may have learned in school. Ultracold atomic experiments continue to simulate even more novel quantum phases of matter. One striking example from 2019 is simulation of a quantum supersolid. A supersolid, like a superfluid, flows without any friction between the atoms that make it up, but also has a periodic, crystal-like structure like solids do. It is a seemingly paradoxical state of matter whose existence was debated for almost 50 years before ultracold atomic experiments provided a definitively affirmative conclusion.
Many so-called topological phases of matter have also been realized in ultracold systems. Some of these experiments simulate, and generalize, the quantum hall effect, which was first observed in more traditional experiments with semiconductors. Since many topological states of matter have properties unaffected by disorder, they are a very promising setting for quantum computation. In this way, realizing topological models in very tunable ultracold atomic system means that physicists are not only able to simulate a new phase of matter but also immediately put it to use, getting closer to making a quantum computer.
Even if ultracold atomic systems have not been turned into quantum computation machines just yet, they can often be used to “beat” classical supercomputers in terms of enabling researchers to learn something new about fundamental physics. One example is that of many-body physics. In quantum mechanics, a system that has more than a few interacting particles is almost always a system where it is very difficult to calculate, and therefore predict, anything precisely. However, real materials consist of millions of atoms!
Ultracold atomic systems have been invaluable for studying highly interacting many-body systems, uncovering phenomena such as systems failing to reach thermal equilibrium and never losing “memory” of their initial state. Physicists often resort to computational methods and supercomputers in order to study these systems, but a simulation with ultracold atoms can be a more direct way to attack some of their questions. Failure to equilibrate is of great interest in statistical physics, and the advent of ultracold atomic experiments has reinforced it as a very active field of contemporary physics research.
As for me personally, despite being trained in the broader discipline of condensed matter physics, I spent my six years as a graduate student coming back to ultracold atoms over and over again. Mostly, I have been studying superfluid bubbles (hollow shells) made of ultracold atoms. This led me to the work of NASA scientists who launched an ultracold atom experiment into space to explore how it will be affected by extremely low gravity. This experiment is still ongoing aboard the International Space Station, and theorists like me who made predictions about what it will find are anxiously awaiting results.
In a way, it is fitting that studying hollow ultracold shells ended up having me think about space, as part of the motivation for this research lies with neutron stars. Physicists don’t really know what you would find if you could observe the inside of neutron star, but many theories suggest that it looks like an onion with layers of superconductors and superfluids. Studying superfluid shells in laboratories could then lead to a better understanding of some of these layers that reside in stars that are so far away that scientists may never be able to study them directly. Moreover, measurements of radio signals coming from neutron stars suggest that superfluid vortices within them may affect their rotation.
Ultracold atomic experiments have excelled in studying exactly those vortices with great precision. In the past few years, I have been working on mathematical arguments for what a vortex in a hollow shell of ultracold atoms might do if the whole thing started rotating. I have badgered a fair number of my experimental colleagues with questions about engineering such a system in their labs, and the fact that this is even something we can talk about, some semblance of simulating the quantum innards of a neutron star, still seems to me a little bit like science fiction.
My latest ultracold obsession came when I learned about quasiperiodicity in one-dimensional chains of ultracold atoms. The puzzle hiding behind the jargon is simple: physicists know well how structures of atoms in which they repeat with a regular period behave in nature, but what happens if that period is an irrational number? Such systems are called quasiperiodic, and studying them led Douglas Hofstadter in 1976 to discover a famous fractal plot latter dubbed as his butterfly. Hofstadter’s plot is self-similar: if you zoom in or zoom out any amount, it still looks the same.
This property implies that physical states having fractional dimensions can exist in nature, a revelation that jump-started a search for more physical systems where that can happen. Two years ago, another graduate student mentioned to me that they had simulated a quasiperiodic system in their ultracold atomic research lab, and I too have not stopped chasing the Hofstadter butterfly since. Why would nature care about the difference between rational and irrational numbers so much as to allow for fractional dimensions to be more than a mathematical oddity? Ultracold atomic studies are likely to help physicists answer that question, and I hope to be around to hear about them.
My experience as a researcher has included only a sliver of many topics in modern physics for which ultracold atomic experiments are meaningful. The possibilities are truly numerous. And the quantum simulation revolution is nowhere near over! Researchers continue to push the limits of existing technology to cool gases made up of more elements, and execute more manipulations.
Next steps? Quantum chemistry, where molecules form at ultracold temperatures. Ultracold quantum systems that are so large they cannot be called microscopic despite quantum mechanics always being assumed to only describe the smallest of objects. Ultracold systems that can be used to measure fundamental constants in tabletop experiments instead of large accelerators (like the Large Hadron Collider). Ultracold experiments where a single atom can be poked, prodded, moved around and imaged. And whatever else can give us a window into the fundamentals of our (quantum) world.
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