Physicists have made a breakthrough in controlling quantum spin interactions, a crucial step towards understanding complex phenomena like superconductors and magnets. A team led by JILA and NIST Fellow Jun Ye, along with collaborators from Harvard University, used microwave pulses to tune interactions between ultracold potassium-rubidium molecules. This technique, known as Floquet engineering, allows for the creation of entangled states that could enhance quantum sensing in the future.
The researchers manipulated polar molecules, a promising platform for quantum simulations, and observed two-axis twisting dynamics within their system. This process can lead to highly entangled states, valuable for advancing sensing and precision measurements. The team’s work builds on the concept of Floquet engineering, which acts like a “quantum strobe light” to control how particles interact.
Key individuals involved in this study include Jun Ye, Calder Miller, and Annette Carroll. This research was supported by the US Department of Energy’s Office of Science, the National Quantum Information Science Research Centers, and the Quantum Systems Accelerator.
Tuning Interactions in Ultracold Molecules with Microwave Pulses
The manipulation of quantum spins is a crucial aspect of understanding various phenomena in the universe, such as superconductors and magnets. However, physicists have struggled to engineer controllable systems in the lab that replicate these interactions. Recently, a team of researchers led by JILA and NIST Fellow and University of Colorado Boulder Physics Professor Jun Ye has made a significant breakthrough in this area.
By using periodic microwave pulses in a process known as Floquet engineering, the researchers were able to tune interactions between ultracold potassium-rubidium molecules in a system suitable for studying fundamental magnetic systems. This technique allows for the creation of entangled states, which can enhance quantum sensing in the future.
The researchers manipulated ultracold potassium-rubidium molecules, which are polar and have a rich energy structure that depends on many different physical constants. As a result, they offer a promising platform for quantum simulations. The tunable molecular interactions using Floquet engineering could open new doors for understanding other quantum many-body systems.
Implementing Floquet Engineering
Floquet engineering has emerged as a useful technique for driving interactions within physical systems. This method acts like a “quantum strobe light,” which can create different visual effects, such as making objects appear to move in slow motion or even stand still, by adjusting the speed and intensity of the flashes.
By using periodic microwave pulses to drive the system, scientists can create different quantum effects by controlling how particles interact. In this experiment, the researchers used an FPGA-based arbitrary waveform generator to apply thousands of pulses, allowing them to engineer a pulse sequence that removes single particle noise and modifies the interactions in the system.
Encoding Quantum Information and Tuning Interactions
Before implementing the Floquet engineering, the researchers first encoded quantum information in the molecules’ two lowest rotational states. Using an initial microwave pulse, the molecules were put into a quantum superposition of these two “spin” states.
After encoding the information, the researchers used the Floquet engineering technique to tune specific types of quantum interactions, known as XXZ and XYZ spin models. These models describe how the particles’ inherent quantum spins interact with each other, which is fundamental to understanding magnetic materials and other many-body phenomena.
The researchers verified that their technique produced similar spin dynamics to those generated by fine-tuning of the interactions using an applied electric field. In addition, they precisely controlled the pulse sequence to realize less symmetric interactions that cannot be generated using electric fields.
Observing Two-Axis Twisting Dynamics
The researchers also observed that their technique produced two-axis twisting dynamics. Two-axis twisting involves pushing and pulling the quantum spins along two different axes, which can lead to highly entangled states. This process is valuable for advancing sensing and precision measurements, as it allows for the efficient creation of spin-squeezed states.
The concept of two-axis twisting was proposed in the early 1990s, but its realization had to wait until 2024. In addition to this work by Ye and his team, JILA and NIST Fellow and University of Colorado Boulder Physics professor James Thompson and his team used a completely different approach to working on atoms—cavity quantum electrodynamics, or cavity QED—also demonstrating two-axis twisting this year.
While the researchers did not attempt to detect entanglement in their system, they plan to do so in the future. The most logical next step is to improve their detection so they can actually verify the generation of entangled states.
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