What is a neutral atom quantum computer?

There’s a twist in the race to build a practical quantum computer. Several companies are betting that there is a better way to make the basic unit of a quantum computer, the qubit, than the leading method being pursued by giants like IBM and Google.

To return to basic quantum computer design for a moment, think of qubits as the quantum equivalent of the binary bits found in classical computers. But instead of storing either on or off states like bits (the famous 1 or 0), qubits store waveforms, allowing them to have a value of 1, 0, or a combination of both. In order to exhibit these quantum properties, objects must be either very small or very cold. In theory, this quality allows qubits to perform more complex calculations compared to bits. But in reality, the unique state that qubits attain is difficult to maintain, and once the state is lost, the information contained in those qubits is also lost. So how long qubits can stay in this quantum state currently sets the limit for the calculations that can be performed.

A frontrunner in the race to build a useful quantum computer is IBM, and their approach to these basic computing units is a device called superconducting qubits. This technique involves constructing small pieces of superconducting metals and insulators to create a material that behaves like an artificial atom in an ultra-cold environment (see here for a more detailed explanation).

[Related: IBM’s latest quantum chip breaks the elusive 100-qubit barrier]

But for emerging companies like QuEra, Atom Computing, and Pasqal, they want to try something new and build a neutral-atom quantum computer, which has long been seen as a promising platform. A neutral atom is an atom that contains a balanced amount of positive and negative charges.

So far, this approach has been largely tested by small businesses and university labs, but that could soon change. Working with qubits made from neutral atoms could be, in some ways, easier than making an artificial atom, experts said PopSci in 2021.

Lasers in Atom Computing’s prototype neutral atom quantum computer. Atom Computing Atom Computing

For example, QuEra uses rubidium atoms as qubits. Rubidium appears on the periodic table as one of the alkali metals with atomic number 37. To get the atom to carry quantum information, the researchers will aim a laser at it to excite it to different energy levels. Two of these levels can be isolated and referred to as the 0 and 1 values ​​for the qubit. In their excited states, atoms can interact with other nearby atoms. Lasers also act as “optical tweezers” for individual atoms, holding them in place and reducing their movement, which cools them and makes them easier to work with. The company says it can pack thousands of laser-trapped atoms into a square millimeter in flexible configurations. QuEra claims they have at times achieved coherence times in excess of 1 second (coherence time is how long the qubits retain their quantum properties). For comparison, the average coherence time for IBM’s quantum chips is about 300 microseconds.

“To assemble multiple qubits, physicists split a single laser beam into many, for example by passing it through a liquid crystal screen. This can create arrays of hundreds of tweezers, each capturing its own atom,” he reported Nature. “A major benefit of the technique is that physicists can combine multiple types of tweezers, some of which can move quickly – with the atoms they carry… This makes the technique more flexible than other platforms, such as superconductors, where each qubit interacts.” can only with its direct neighbors on the chip.”

Peer-reviewed articles have already been published testing the possibilities of running a quantum algorithm on such a technology. An article published in the magazine in January natural physics even characterized the behavior of a neutral atom trapped in optical tweezers.

Currently, QuEra can operate at around 256 qubits and is offered as part of Amazon Web Services’ quantum computing service. According to an Amazon blog post, these neutral atom-based processors are useful for “arranging atoms into diagrammatic patterns and solving certain combinatorial optimization problems.”

Meanwhile, Atom Computing, which bases its qubits on the alkaline-earth metal strontium, uses a vacuum chamber, magnetic fields, and lasers to create its array. Its prototype has caught the attention of the Pentagon’s DARPA research department and was recently funded under the agency’s US2QC (Underexplored Systems for Utility-Scale Quantum Computing) program.

Pasqal, another Paris-based quantum computing startup, has also raised some funds behind this emerging approach. Specifically acc TechCrunchit collected around 100 million euros at the end of January for the construction of its quantum computer with neutral atoms.

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