Google Presents New Quantum Algorithm "Quantum Echos"

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A research team from Google Quantum AI has developed a new quantum algorithm that is intended to enable a verifiable quantum advantage over conventional hardware. The algorithm is nicknamed Quantum Echos and is said to run 13,000 times faster on Google's quantum chip Willow than the best-known classical algorithm on the fastest supercomputers.

The group is presenting the results in a publication in the journal Nature. A manuscript published on the preprint server arXiv at the same time is intended to demonstrate the successful application of quantum echos in the field of NMR.

Recipes for Quantum Computers

Quantum computers promise an enormous computational advantage for certain tasks compared to conventional or even supercomputers. However, for quantum computers to deliver an advantage, they require special quantum algorithms that cleverly exploit quantum effects to solve a problem efficiently. The Shor algorithm, which solves the mathematical problem on which a large part of today's established encryption methods are based, is widespread. This means that quantum computers could one day succeed in cracking complex encryption in a very short time.

So-called Random Circuit Sampling (RCS) is another quantum algorithm that companies like Google often use to demonstrate the performance of their quantum chips. However, RCS was specifically designed to test the performance of quantum computers. Thus, RCS is to be understood as a pure benchmark, as the algorithm does not solve a problem that would be useful for real applications.

Not so Google's new quantum algorithm, Quantum Echos. This not only tests the complexity but also the precision of a quantum computation. The team announces that this algorithm can help solve real-world problems that play a role in drug or fusion research, for example.

Quantum Echos is also said to provide a verifiable quantum advantage. This means that other research groups can run the algorithm on their own quantum computers, reproduce the result of the Willow processor, and thus confirm it. According to their statements, Google has commissioned an independent team to find vulnerabilities in their algorithm to confirm the performance.

How Quantum Echos Works

The technical name for Quantum Echos is actually “Out-of-order Time Correlator (OTOC) Algorithm.” Simply put, Quantum Echos is an algorithm for turning back time and thus better understanding the properties of quantum computers, tracking or reversing disturbances.

To achieve this, the scientists first perform a series of operations on the quantum chip. Then they disturb one of the qubits, and this disturbance spreads throughout the quantum computer. They then reverse the operations, i.e., they perform the operations from the first step backward in reverse order—like an echo, hence the name of the algorithm. Finally, they read out the information.

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The result tells the researchers how quantum information and classical noise spread in the quantum computer. Usually, these two effects are difficult to distinguish because all elements of the quantum computer are interconnected and different effects overlap. Their algorithm tells the researchers how quantum information spreads in complex systems with many particles.

This allows researchers to gain insights into microscopic properties of the quantum system that a classical computer cannot calculate. The team sees this as a possible building block for future proofs of a true quantum advantage. However, the algorithm itself does not show a quantum advantage in the Nature publication and does not solve a real problem.

First Applications in NMR

The team proposes an actual application in an accompanying arXiv manuscript. The work is based on a collaboration with research groups from the USA, the United Kingdom, Canada, and Australia. The application comes from the field of Nuclear Magnetic Resonance (NMR). NMR is the basis for Magnetic Resonance Imaging (MRI), as well as many other precision measurement methods in chemistry, medicine, or geophysics.

In the example considered, quantum echoes can serve as a “molecular ruler” to measure distances very precisely and gain information about chemical structures. In their work, the team investigated two molecules with 15 and 28 atoms, respectively. The calculations with Google's Willow agreed with the predictions of classical NMR. However, quantum echo provides additional information that NMR alone cannot provide.

Better NMR is relevant for many fields, such as pharmaceutical research or material development, to better and faster understand the molecular structure in drugs or building materials. The results of the arXiv manuscript have not yet been reviewed by independent experts.

“This new work presents the quantum computer as a tool to uncover molecular structures, not only in NMR but perhaps also in quantum sensing in the future,” says Michel Devoret during a pre-release presentation of the results. The team uses the term "quantum scope" to indicate the further development of telescopes and microscopes.

Devoret recently won the Nobel Prize in Physics for the physical fundamental experiments that form the basis for superconducting qubits today. These are the building blocks from which companies like Google, as well as IBM and the European IQM, construct quantum computers. Devoret is a senior scientist for quantum hardware at Google Quantum AI.

A True Quantum Advantage?

As early as 2019, a research team from Google announced that with their then quantum chip, Sycamore, they had demonstrated a quantum advantage. The chip was said to have solved a problem in a few minutes that would have taken a conventional supercomputer 10,000 years. Shortly thereafter, this success was relativized when researchers found a way to solve the same calculation in a few minutes on a conventional computer using a more efficient algorithm.

“This race between classical and quantum computing is quite normal. All these speed predictions are snapshots. We expected back then that developers would accelerate classical algorithms,” says Hartmut Neven. “We expect that to happen in this case too. But quantum technology is also developing, and the performance of quantum computers will continue to increase.”

Outlook

Google is optimistic about being able to solve relevant problems using quantum computers within five years that no classical computer can solve. With the development of larger, error-corrected quantum computers, the company also expects more real-world applications for quantum computers to be discovered. Currently, the team is focusing on reaching its third milestone: a long-lived, logical qubit.

(spa)