Tweezing a New Kind of Qubit

JILA has a long history in quantum research, advancing the state of the art in the field as its Fellows study various quantum effects. One of these Fellowsis Adam Kaufman. Kaufman and his laboratory team work on quantum systems that are based on neutral atoms, investigating their capacities for quantum information storage and manipulation. The researchers utilize optical tweezers—arrays of highly focused laser beams which hold and move atoms—to study these systems. Optical tweezers allow researchers exquisite, single-particle experimental control. In a new paper published in Physical Review X, Kaufman and his team demonstrate that a specific isotope, ytterbium-171 (171Yb), has the capacity to store quantum information in decoherence-resistant (i.e., stable) nuclear qubits, allows for the ability to quickly manipulate the qubits, and finally, permits the production of such qubits in large, uniformly filled arrays.

A New Kind of Qubit?

A qubit, or “quantum bit,” is the basic unit of quantum information. A qubit, thanks to its quantum properties, can be a 0, 1, or a combination of both. This makes it an essential building block for those working on quantum computing. The researchers had anticipated that their 171Yb isotope would allow access to a new kind of qubit, one based on the isotope’s nuclear spin. “The nice thing about this particular species of ytterbium is that it has this kind of natural qubit in it,” co-first author and postdoctoral researcher Alec Jenkins said. “It has a spin one-half. It's a very isolated sort of ideal two-level system within the nucleus of ytterbium, which is good for quantum applications.” Not only is the nuclear qubit novel, but it also affords unprecedented coherence (stability), making it an ideal system to work with.

Maintaining qubit coherence is a daunting challenge for any prospective quantum information platform. Although quantum computers allow capabilities inaccessible to classical computers, this comes at the cost because of the notorious fragility of qubits. For this purpose, the nuclear qubit, decoupled almost completely from the electron cloud and thus insensitive to outside interactions like noise, is nearly ideal. For this reason, Kaufman finds the nuclear qubit exciting. “What's special about this nuclear spin—compared to the spin in other atoms like rubidium which have historically been used in tweezers—is that, in certain quantum states, it can be completely isolated from the remainder of the atom,” he added. This means that the high-intensity tweezer light doesn’t disturb the qubit, allowing for a longer coherence time when the qubit is in a readable state. As a scientist working on quantum information systems, Kaufman realized the importance of this stability. “This is a breakthrough, because in many quantum information platforms with neutral atoms, these so-called ‘light shifts’ can significantly degrade qubit performance,” he explained.

Not only are the nuclear qubits long-lived and noise-resistant, they can also be manipulated very quickly, allowing for a more efficient process. “In this work, adding to the second scale quantum memory feature, we found we can do fast 100-nanosecond-scale qubit manipulations with an unconventional single-beam Raman transition method,” said graduate student Aruku Senoo. “We also showed the manipulation is quite low noise, only at the part in a thousand level.” The combination of long coherence time and fast control promises to enable the implementation of thousands of individual manipulations before the qubit is lost. Kaufman and his team hope to continue finding out what is possible  with this new type of qubit, which, if proven feasible for quantum computation, could have big implications for quantum information studies.

Loading Tweezers can be Tricky

To study the qubit, the team used a tweezer system, running into the challenge that preparing a uniformly filled array can present. Using 171Yb, the team set up a special 10 ×10 tweezer array. According to the co-first author and graduate student Joanna Lis: “The tweezers are made by focusing a light beam through an objective lens. The atoms sit in these traps and emit light, which we collect with the same objective and focus on a camera, which we use for imaging.” However, if multiple atoms are loaded into a tweezer, they will interfere strongly with each other, interfering with coherence times and adding to the overall noise levels. To prevent multiply-occupied tweezers, light-assisted-collisions can be used to cause the atoms to be lost pairwise from the trap. According to postdoctoral researcher Will McGrew, “One important part about that process is that you always lose the ytterbium atoms in twos. So, if you happen to load an even number of atoms, you would lose twos and then you'd have zero at the end.” This makes tweezer loading a statistical process. “What that means is, as you load many atoms, you've got on average, a 50% chance of loading an odd number and a 50% chance of loading an even number,” McGrew added.  “After the light-assisted collision process is completed, you end up with a 50% chance of filling your tweezer with a single atom.”

Instead of using the light-assisted collision process, which only gives a 50% chance of a full array, the researchers instead used a method called blue-shielded collisions. In this process, the atom gains some energy, not immediately getting lost in the trap. “You can cycle this multiple times, and eventually lead to one atom leaving the trap but not the other,” McGrew said. “You don't lose the atoms in twos anymore; you lose them by ones.” The key to achieving this condition was first realized and implemented in alkalis, by JILA Fellow Cindy Regal’s group. Inspired by these results, the Kaufman group explored using the narrow spectroscopic lines in ytterbium to isolate a similar cooling feature, needed to promote these blue-shielded collisions. In achieving this, it changed the statistical numbers game the researchers were playing with their tweezers, allowing them a higher percentage of filling the array. In using he 171Yb isotope with this technique, the researchers found that they could fill on average nearly 93 tweezers in the 10 ×10  array. This increased efficiency for optical tweezers allows for more qubits to be manipulated, which the researchers hope to use to continue studying quantum information processing.

Written by Kenna Castleberry, JILA Science Communicator