How Cold is Ultracold?
How Cold is Ultracold?
  • Professor Jee Woo Park
  • 승인 2021.12.14 00:49
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What is the coldest place in the Universe? The first entry from my Google search tells me that it is the “Boomerang Nebula,” which is 5,000 light-years away and has a temperature of about one Kelvin. However, for me, as a researcher in the field of ultracold quantum gases, the coldest place in the Universe is not in outer-space but on Earth in our laboratories, where we use laser beams, ultraclean vacuum systems, and a host of other technologies to trap and cool atoms to nano-Kelvin temperatures.
In this extreme temperature regime, our intuition of how Nature works dramatically breaks down. The rules that dictate the classical world give way to the more refined rules of quantum mechanics, and the atoms no longer behave as particles but as waves spread out in space. That is to say, the state of a given particle is no longer specified by its position and velocity but by a wave function that is governed by the Schrodinger equation. Furthermore, in the quantum world, the way in which atoms get along with each other takes an interesting turn. Quantum mechanically, any building block of Nature is classified as one of the two fundamental types: a boson or a fermion. For example, electrons, neutrons, and protons are all fermions, and sodium atoms that we use in our experiments are bosons. As it turns out, bosonic particles are a gregarious bunch—identical bosons like to be near each other. They like being near each other so much that at a certain critical temperature, they decide to undergo a phase transition and populate a single quantum state together in unison. This new state of matter is known as a “Bose-Einstein condensate (BEC),” and quantum physics dictates that a BEC under typical circumstances will be a superfluid that can flow without friction, very much like how electrons can flow without resistance in superconductors.
Since the first creation of a Bose-Einstein condensate from an ultracold gas of rubidium atoms in 1995, the field of ultracold quantum gases has grown explosively. By now, essentially all alkali atoms, which have a simple electronic structure similar to that of hydrogen, have been cooled to nano-Kelvin temperatures using techniques such as laser cooling and evaporative cooling, and even atoms with more complex structures such as chromium, dysprosium, and erbium have been cooled. The list includes not only bosonic atoms such as rubidium and sodium, but also fermionic atoms such as potassium-40 and lithium-6. Specifically, the ability to cool and precisely control the quantum behavior of fermionic atoms, which, in many cases, behave like artificial electrons, has opened the door to simulate the complex behavior that occurs in novel quantum materials. In this program of “quantum simulation,” one can even place these atoms in a crystal of light formed by overlapping laser beams that mimic the role of ionic crystals in solid-state materials. A state-of-the-art experiment can not only image the individual atoms in this artificial crystal but also address them individually to engineer and simulate some of the most important theoretical models in quantum materials research, such as the Fermi-Hubbard model that we hope will describe the inner workings of high-temperature superconductors.
Nowadays, the field has further evolved to explore the possibility of creating an ultracold atom-based quantum computer. By preparing an array of laser-cooled atoms and using techniques to optically address their individual quantum states, one can turn these atoms into “qubits,” which are the fundamental units of memory in a quantum computer. The information stored in atomic qubits can then be processed using their mutual interaction, similar to how a CPU processes information stored in classical bits. Developing a quantum computer based on ultracold atoms is still at its nascent stage, but the hopes are high that eventually, the field will be able to make significant breakthroughs in this direction.
By this point, you must be wondering what my research program at POSTECH consists of. My research revolves around the idea of using “ultracold dipolar molecules” as compared to atoms to create a novel platform for quantum simulation and computing. In the past years, experiments with ultracold atoms have truly revolutionized our understanding of the quantum world. However, one shortcoming of these experiments is that the underlying interaction between a pair of atoms is a simple contact interaction, which is essentially the quantum mechanical version of the collision between a pair of billiard balls. Nonetheless, interactions that govern Nature are often more complex: charged particles interact via the long-range Coulomb interaction and spins interact via the long-range and spatially anisotropic dipole-dipole interaction. From a quantum simulation standpoint, there has been a long-standing goal to incorporate these realistic types of interactions into the quantum simulation toolbox. Furthermore, from a quantum computing standpoint, the existence of long-range interaction between a pair of qubits is paramount for the efficient processing of information. As it turns out, one of the most promising routes to achieve this goal is to create an ultracold gas of molecules that have large electric dipole moments. Then, these molecules can undergo strong dipole-dipole interactions in a temperature regime where quantum mechanics fully dominates the many-body behavior of these molecular gases.
Our strategy to create an ultracold gas of molecules is to start with a mixture of ultracold atoms that will later bind together to form molecules. Once the atoms are cooled to nano-Kelvin temperatures using established techniques in atomic physics, we can then assemble molecules one by one, while maintaining the nano-Kelvin temperature of the trapped sample. In my experiment, we will bind sodium atoms, which are bosons, to potassium atoms, which can be bosons or fermions depending on the isotope, to create both bosonic and fermionic dipolar molecules at nano-Kelvin temperatures. Eventually placing these molecules in an optical lattice under a high-resolution microscope objective to observe and control their complex quantum dance will be a dream come true.
For those of you who are interested in this line of research, especially for the undergraduates, I urge you to reach out and participate in actual research as early as possible. Here at POSTECH, we have a very strong program that spans a broad range of atomic, molecular, and optical (AMO) physics and quantum optics. Be brave and explore.

Professor Jee Woo ParkDepartment of Physics
Professor Jee Woo Park
Department of Physics