Hybrid Quantum Systems
The laws of quantum mechanics tell us that a physical object, such as an atom or an electron, can simultaneously be in two distinct places when it is in a state known as a superposition. However, superpositions are in conflict with our everyday experience of how the world works.
This situation raises an interesting question: What is the largest material object that can be prepared in a superposition?
JILA researchers Konrad Lehnert and Cindy Regal are exploring answers to this question by making tiny micro-scale mechanical resonators (i.e., drums) to see if they can make them behave quantum mechanically. Recently, they succeeded by using electricity to achieve the exquisite control and measurement that allowed them to penetrate the mysterious quantum world.
The researchers are now using their tiny drums to store long-lived memories for the quantum states of electrical circuits. They’re also developing technology that is allowing them to transfer quantum states between two incompatible systems, such as light and electricity, via a mechanical intermediary. Lehnert and Regal are collaborating on a tiny system to explore the quantum interface between light and micromechanical motion. Achieving this goal requires nanomechanical engineering combined with Lehnert’s expertise in building tiny electrical circuits and Regal’s expertise in AMO physics.
As a first step, Regal developed tiny cold “drums of silicon nitride (Si3N4). The drums are thin enough to behave quantum mechanically when a force causes them to vibrate. And, when they resonate, billions of atoms inside them act as one. The Regal group applied tiny, localized forces to the drums to observe their vibrations and explored what happens to their quantum state(s). The goal was to apply a force to the drum in a way that kept the drum’s billions of atoms acting in unison. This achievement allowed Regal and Lehnert to create a system in which quantum information can be exchanged between different physical systems such as light and mechanical motion.
To create a quantum interface between light and mechanical motion, the researchers control both the drum and its interactions with the surrounding environment. To accomplish this, the researchers simultaneously couple the motion of a tiny square Si3N4 drum to both microwave and optical cavities. A portion of the drum is coated with aluminum, allowing it to couple with a tiny electrical circuit. Any drum motion will perturb the electrical circuit and vice versa. The remainder of the drum couples to an optical cavity, which is expected to act like the electrical circuit, except the light will be at much shorter wavelengths.
Lehnert and Regal are using this setup to reversibly mapping the quantum description of the electrical circuit onto the quantum description of a laser light field—without losing any information in the process. The goal is to achieve a coherent and simultaneous coupling of microwave fields and optical light.
Controlling Single Neutral Atoms
The Cindy Regal group has undertaken a long-term project to study the use of single trapped ultracold neutral atoms in quantum computation or quantum simulation. The starting point for this project was the first-ever cooling and trapping of a single atom of rubidium (87Rb) in a tightly focused laser beam. This experiment has identified an important source of ultracold neutral atoms that will enable future manipulations of single atoms, laying the groundwork for their use in quantum simulations and as quantum logic gates in future high-speed computers.
Single ultracold neutral atoms appear to be well suited for such applications. For instance, multiple neutral atoms can be stored in individual sites in an optical array created by with lasers. Inside an array, single neutral atoms are relatively easy to pinpoint and move around. More importantly, they do not usually interact with their surroundings as occurs with charged ions (which were the first single particles to be cooled and trapped). And, it’s easy to grab onto neutral atoms with light, which means that the researchers can use a highly focused beam of light (optical tweezers) to hold and precisely position individual atoms.
With these many advantages in mind, the Regal group began an investigation of the characteristics of single atoms in tiny micron-sized traps called optical tweezers. With a single rubidium atom (87Rb) in such a trap, the group figured out how to cool the atom to its quantum ground state.
The researchers first used two lasers to lower the energy of the atom by one quantum of motion, while also flipping its spin. Then, they shined another laser on the atom, which caused the spin to flip back, while leaving the atom in its lower energy state. Cooling the atom to its quantum ground state in three dimensions simply required repeating this two-step cycle many times.
Once the Regal group figured out how to prepare single neutral atoms in their quantum ground state, a whole new field of research opened up. In a recent experiment, the group collaborated with theorist Ana Maria Rey to demonstrate a key first step in assembling quantum matter one atom at a time. The experimental group laser-cooled two 87Rb atoms in separate optical tweezers, then, while maintaining complete control over the atoms to ensure that they were identical, the group moved the tweezers closer and closer until they were about 600 nm apart. At this distance, the trapped atoms could tunnel their way over to the other laser beam trap, if they were so inclined.
A few milliseconds into the experiment, the researchers observed that even though the two atoms started out trapped in different optical tweezers, they both turned up in either one or the other tweezer. However, it was impossible to predict which laser-beam trap would get the two atoms. The two atoms almost never appeared in separate optical tweezers again. An analysis of the experiment showed that the possibility of finding one atom in each tweezer disappeared because of destructive interference of the matter waves of the two atoms.
The group plans to place single atoms in more complicated optical focus arrays, or even near surfaces. One day, they may precisely place single cold atoms on a future quantum computer chip. In the meantime, the researchers will continue to investigate quantum tunneling between atoms in a few to many different array positions, which could lead to studies of the basic physics of exotic phenomena that occur in real materials. The arrays will also make it possible for the group to investigate logic operations between multiple atoms based upon atomic collisions or the atom’s interaction with light.