2024-03-29 10:10:31
Researchers observed the dynamic phases of BCS-conductor interactions in Cavity QED by measuring the light leakage from the cavity. Credit: Steven Burrows/Rey and Thompson Groups
Researchers at JILA simulated superconductivity in strontium atoms inside an optical cavity to observe rare dynamical phases, including the elusive phase III, which has implications for quantum physics and technology development.
In physics, scientists have been fascinated by the mysterious behavior of superconductors – materials that can conduct electricity with zero resistance when cooled to extremely low temperatures. Within these superconducting systems, electrons combine into “Koper pairs” because they are attracted to each other due to vibrations in matter called phonons.
As a thermodynamic phase of matter, conductors usually exist in an equilibrium state. But recently, researchers at JILA have become interested in kicking these materials into excited states and studying the dynamics that follow. As reported in the news Nature In the paper, the theory and experiments teams of JILA and NIST fellows Anna Maria Rey and James K. Thompson, in collaboration with Prof. Robert Lewis-Swan of the University of Oklahoma, simulated superconductivity under such excited conditions using an atom-cavity system.
Instead of dealing with actual conductive materials, the scientists harnessed the behavior of strontium atoms, cooled by a laser to 10 millionths of an absolute zero and hovering in an optical cavity built of mirrors. In this simulator, the presence or absence of a Cooper pair is encoded in a two-level or qubit system. In this unique setup, photon-mediated interactions between electrons were realized between the atoms within the cavity.
Thanks to their simulation, the researchers observed three distinct phases of superconductivity dynamics, including a rare “Phase III” that exhibits sustained oscillating behavior predicted by condensed matter physics theorists but never observed before.
These findings can pave the way for a deeper understanding of superconductivity and its ability to be controlled, and offer new avenues for engineering unique conductors. Furthermore, it holds promise for improving the coherence time for quantum sensing applications, such as improving the sensitivity of optical clocks.
Identification of superconducting phases
The JILA team focused on simulating the Barden-Cooper-Schriefer model, which describes the behavior of the Cooper pair. As first author and JILA graduate student Dylan Young added: “The BCS model has been around since the 1950s and is central to our understanding of how conductors work. When condensed matter theorists began to study the out-of-equilibrium dynamics of conductors, they naturally began with this model”.
In recent decades, condensed matter theorists have predicted three distinct dynamical phases that a superconductor will experience as it evolves. In stage I, the strength of superconductivity rapidly decreases to zero. In contrast, phase II represents a steady state in which superconductivity is maintained.
However, the previously unseen third phase is the most intriguing. “The idea of phase III is that the strength of the superconductor has continuous oscillations without damping,” explained JILA graduate student and first author Anjun Chu. “In the phase III regime, instead of suppressing the oscillations, many-body interactions can lead to self-generated periodic driving of the system and stabilize the oscillations. Observing this exotic behavior requires precise control of the experimental conditions.”
To observe this elusive stage, the team leveraged the collaboration of theory from Ray’s group and experiment from Thompson’s group to create a precisely controlled experimental setup, hoping to fine-tune the experimental parameters to reach Phase III.
Creating accurate simulations in space definition
While researchers have previously attempted to observe phase III in real superconducting systems, measuring this phase remains elusive due to technical difficulties. “They didn’t have the right ‘handles’ or reading mechanisms,” Young explained. “On the other hand, our implementation in an opaque cavity system gives us access to both tunable controls and useful observations to characterize the dynamics.”
Building on previous work, the researchers trapped a cloud of strontium atoms inside an optical cavity. In this “quantum simulator”, the atoms simulated Cooper pairs and experienced a collective interaction analogous to the attraction experienced by electrons in BCS superconductors. “We think of each atom as representing a Cooper pair,” Young explained. “An excited-state atom simulates the presence of a Cooper pair, and the ground state represents the absence of one. This mapping is powerful because, as atomic physicists, we know how to manipulate atoms in ways you just can’t with Cooper pairs.”
The researchers applied this knowledge to induce different stages of dynamics in their simulation by a process known as “quenching”. As Yang elaborated: “Quenching is when we suddenly change or ‘kick’ our system to see how it reacts. In this case, we prepare our atoms in this very collective superposition state between ground and excited states. Then, we cause a shutdown by activating A laser beam that gives all the atoms different energies.”
By changing the nature of this quench, the researchers could see different dynamic phases. They even devised a trick to observe the elusive third stage, which involved splitting the cloud of atoms in two. “Using two opaque clouds with separate control of energy changes is the key idea to achieve Phase III,” commented Chu.
In superconductors, the energy levels of electrons can be split into two sectors, heavily occupied or barely occupied, separated by the Fermi level. “Our setup in spin systems does not intrinsically have a Fermi level, so we take this into account by using two atomic clouds: one cloud simulates the states below the Fermi level, while another cloud simulates the other (quantum) states,” Chu added.
To measure the dynamics of the superconductor inside the cavity, the researchers tracked the light leaking from the optical cavity in real time. Their data found different points where the simulated superconductor transitioned between phases, finally reaching phase III.
Seeing the first Phase III measurements surprised many of the team. As Thompson said: “Actually seeing the movement was extremely satisfying.” For her part in the collaboration, Ray was equally excited to see theory and experiment fit. “On the theoretical side, BCS superfluids/conductors can, in principle, be seen in actual degenerate fermionic gases, like the ones that Debbie Jean at JILA taught us how to make. However, the dynamical phases in these systems have been difficult to observe. We envisioned more in -2021 that all the BCS dynamical phases could instead be manifested in an opaque space experiment. It was so nice to see our theory predictions come true and to actually observe the dynamical phases in a real experiment!”
Basic physics with wider applications
While observing phase III in their system was a significant achievement, the team also found that the behaviors being measured could have broader implications beyond superconductivity. As Thompson elaborated, “In terms of the basic model that you use to describe it, it turns out that this BCS model has all these connections to different kinds of physics at different energy scales, temperature scales, and time scales, from superconductors to neutron stars. To quantum sensors! “
Ray added: “These observations really open a path to simulating unconventional conductors with fascinating topological properties for realizing powerful quantum computers. It would be fantastic to simulate even toy models of these complex systems in our quantum-atom-space simulator.”
For more information on this research, see Strontium Unlocks Quantum Secrets of Superconductivity.
This work was supported in part by the Quantum Systems Accelerator, part of the US Department of Energy, Office of Science, National Research Centers for Quantum Information Sciences.
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