It's 2022 and you still don't know about neutral atom quantum computers

When you think of quantum computers, you may think of physical platforms such as superconductivity, ion traps, and optical quanta. But over the past five years, neutral atoms have emerged as dark horse candidates in the race to build quantum computers. PhotonBox previously wrote an article, "2022, Why is quantum computing taking off with neutral atom fever? that explained its advantages in quantum simulations. Recently, Physics Today also wrote a popular science article about neutral-atom quantum computers [1], highlighting the progress made so far this year.

01Neutral atom arrays

In a neutral-atom quantum processor, atoms are suspended in ultra-high vacuum by an array of tightly focused laser beams called optical tweezers. The researchers have scaled up to arrays of more than 100 base atoms, each with one valence electron, and used smaller arrays to perform quantum algorithms. Now they are exploring new quantum information processing and measurement capabilities in arrays of atoms with two valence electrons, including alkaline earth atoms in the second column of the periodic table and several other atoms with similar properties, such as ytterbium.

Arrays of optical tweezers of alkaline earth atoms have shown promise in both quantum computing and precision timing, where they can encode new types of quantum bits with long coherence times and be used as state-of-the-art atomic clocks. In the future, they may help researchers implement fault-tolerant quantum error correction protocols and exploit quantum entanglement on a large scale to further push the limits of atomic clock performance.

Researchers in the University of Colorado Boulder lab manipulate neutral atoms in an array of optical traps. The glowing green region contains the trapped atoms. Image credit: Adam Kaufman

02How atoms become quantum bits

In an optical tweezer trap, the electric field of a focused laser beam triggers a small polarization in the atom, pulling it toward the region of greatest intensity in the center of the trap. The researchers can then drag the atom around as they wish, or hold it in place and bombard it with other laser or microwave pulses to excite specific atomic jumps.

To create a neutral-atom quantum processor, the researchers generated an array of optical tweezers by splitting the incident laser beam into many beams and focusing them through a powerful microscope objective into a glass vacuum chamber. Clouds of cold atoms are loaded into the optical tweezer array and then rearranged to produce to produce a populated array with one atom in each tweezer. Optics outside the vacuum chamber allow researchers to control the precise position of each atom, quickly reconfigure the array, and adjust the trapping potential of each optical tweezer independently.

Each atomic species has an infinite number of discrete energy levels associated with different quantum states. Any pair of states can, in principle, act as a quantum bit. In practice, researchers choose a pair of long-lived low energy states that allow many sequential quantum logic operations to be performed before quantum information leaks from the quantum bit into its environment (decoherence).

Neutral atoms in low-energy states have weaker interactions with each other and can therefore be arranged in compact arrays. Proponents see this as a key advantage of neutral-atom quantum computing over more established methods, which use ions trapped in electric fields or superconducting circuits at milli-Kelvin temperatures. Scaling quantum computers from hundreds to millions of quantum bits is a challenge for all proposed architectures, but space is not an issue for neutral atoms; a millimeter-scale array can hold up to a million quantum bits.

To turn on the interaction between the quantum bits, the researchers targeted a pair of neighboring atoms with a laser pulse and excited one of them to a high-energy state called the Riedberg state, in which the valence electrons orbit away from the nucleus. The strong electric dipole interaction of the Riedberg atom prevents the laser from also exciting its neighbor, an effect known as Riedberg blocking, but it is impossible to know which atom is excited. The result is that two quantum bits that cannot be described separately share an excitation between them - a typical feature of quantum entanglement, a key phenomenon for quantum computers to outperform classical computers. In the past five years, Riedberg entanglement fidelity has improved significantly, but still lags behind captured ions and superconducting quantum bits.

Adjacent captured atoms can be induced into quantum entangled states. Image credit: Bichen Zhang

In April, two independent groups, one led by Mikhail Lukin of Harvard University [1] and the other by Mark Saffman of the University of Wisconsin-Madison [2], reported the first demonstration of a multi-step quantum algorithm in an array of rubidium atoms.

Rubidium (Rb) is an alkali metal atom that has long been a mainstay of atomic physics, in part because its single valence electron gives it a simple energy level structure similar to that of hydrogen. Rubidium-based quantum processors encode quantum bits in the hyperfine state of the atom, a dense energy level created by the interaction of the valence electron spin with the atomic nucleus spin. Hyperfine quantum bits have longer coherence times than quantum bits encoded in atomic electron leaps, but the unpaired electron spins still make them susceptible to decoherence from stray magnetic fields and residual interactions with optical tweezer light.

These sources of decoherence have stimulated research into quantum computing with phototweezer arrays of alkaline earth atoms, which have a more complex energy level structure due to their two valence electrons. The additional complexity presents new technical challenges, but also new ways to encode and manipulate quantum information, says Saffman: "As technology and laser technology mature, this is a natural progression of the field toward more complex atoms."

03Using alkaline earths

One advantage of alkaline earth atoms is that their valence electrons pair in the electronic ground state, so there are no hyperfine interactions. In contrast, in alkaline earth atoms whose nuclei have non-zero spin, the nuclear spin is isolated from the environmental perturbations that naturally couple to the electronic spin. As a result, quantum bits encoded in the nuclear spin states of alkaline earth atoms have much longer coherence times than hyperfine quantum bits.

In May, the Berkeley-based startup Atom Computing published a paper [4] describing its first-generation quantum processor, an array of optical tweezers in which quantum bits are encoded in the two nuclear spin energy levels of strontium-87 atoms. They reported coherence times longer than 20 seconds (T2*), more than three orders of magnitude larger than the typical value for alkaline hyperfine quantum bits.

In the same month, a group led by Jeff Thompson at Princeton University and Adam Kaufman at the University of Colorado at Boulder reported [5] that they had achieved control of long-lived nuclear spin quantum bits in an array of ytterbium-171 atoms.

Remarkably, the nuclear spin structure of ytterbium-171 is simpler than that of any other stable isotope of alkaline-earth-like atoms. It is a natural two-energy system, whereas the 10 different nuclear spin states of strontium-87 require additional laser light to pick out the two energy levels that can encode quantum bits.

The rich electronic energy level structure of alkaline-earth-like atoms may be useful for quantum computing in other ways, Thompson said, adding that if each atomic species is compared to a Swiss Army knife with a specific tool, then alkaline-earth atoms are luxury models with extra features. "We figured out ways to use other gadgets on this Swiss Army knife that weren't even known when we bought it," he said, referring to a technique to improve entanglement fidelity and a new quantum error correction protocol, both of which rely on the long-lived sub-stable properties of alkaline earth atoms, for which there is no counterpart in alkaline atoms.

This distinctive sub-stable state has a long lifetime: typically tens of seconds, and in some atoms even longer. This is because the decay to the ground state can only take place by strongly suppressed higher order processes. An atom prepared in a superposition of sub-stable and ground states will oscillate for a long time at the same frequency as the photons emitted in the leap between the two states, which is called a clock leap because it provides the frequency reference for the best atomic clocks in the world.

These clocks are called optical clocks because the clock leap frequencies of widely used atoms such as strontium, ytterbium and aluminum fall at or near the visible part of the electromagnetic spectrum. These clocks fall into two main different categories, and their main difference is how the atoms are captured. The optical tweezer arrays developed for neutral atom quantum processors have emerged as a third optical clock architecture that has the potential to combine some of the advantages of more mature platforms.

In an optical lattice clock at Colorado JILA, a large family of strontium atoms is trapped in a laser standing wave mode, allowing for very precise frequency measurements. Future optical clocks for optical tweezer arrays could combine the accuracy of lattice clocks with that of other state-of-the-art optical clocks. Image credit: JILA

These two main optical clock architectures have complementary advantages and disadvantages. The most accurate clocks use electric fields to trap individual aluminum ions that are well isolated from their environment, but they require long averaging times to achieve high accuracy. The most accurate optical lattice clocks, on the other hand, use lasers to capture neutral strontium atoms, but instead of the tightly focused beams used in optical tweezer arrays, they use a periodic potential formed by the interference of two counter-propagating beams to capture up to 100,000 atoms. Such a large array allows lattice clocks to accumulate accuracy quickly, but systematic effects (such as residual interactions between atoms captured at the same lattice position) limit their precision.

Optical clocks for optical tweezer arrays can help researchers get the best of both worlds: precise single-particle control comparable to ion clocks, and a clearer path for scaling to large tethers. And it can be scaled more clearly to large ensembles.Adam Kaufman, whose independent work with Manuel Endres at Caltech simultaneously developed the first generation of optical tweezer clocks, published a paper in 2020 [6] describing the second generation of tweezer clocks with an accuracy close to the lattice clock records of the time. In the future, the accuracy of optical tweezer clocks may be further improved by using quantum information protocols to generate multi-particle entangled states, which Kaufman calls "the frontier of quantum science"

The rapid progress of neutral-atom tweezer arrays in quantum science over the past five years suggests that these efforts may yield even more unexpected advantages, says Thompson: "It gives me a new sense of optimism about what we can do in the whole field of quantum technology."

Reference：

[1]https://physicstoday.scitation.org/do/10.1063/PT.6.1.20220824a/full/[2]https://www.nature.com/articles/s41586-022-04592-6[3]https://www.nature.com/articles/s41586-022-04603-6[4]https://www.nature.com/articles/s41467-022-29977-z[5]https://journals.aps.org/prx/abstract/10.1103/PhysRevX.12.021028[6]https://www.nature.com/articles/s41586-020-3009-y