Scientists just measured a quantum mechanical system without destroying it

There is a key aspect of quantum computing you may not have thought of before. Called “quantum measurements without demolition”, they refer to the observation of certain quantum states without destroying them in the process.

If we’re going to put together a working quantum computer, it would obviously help if we didn’t see it crash every second while the calculations are being made. Now, scientists have described a new technique for recording non-demolition quantum measurements that shows great promise.

In this case, the research involved quantum mechanical systems – relatively large objects in terms of quantum computing, but extremely tiny for us. They use mechanical motion (like vibration) to manage the quantum magic needed, and they can also be combined with other quantum systems.

“Our results open the door to realizing even more complex quantum algorithms using mechanical systems, such as quantum error correction and multimode operations,” the researchers write in their published article.

For the purposes of this study, the team assembled a thin strip of high-quality sapphire, just under half a millimeter thick. A thin piezoelectric transducer has been used to excite acoustic waves, moving units of energy such as photons that can, in theory, be subjected to quantum computing processes. Technically, this device is known as an acoustic resonator.

This was the first part of the installation. To perform the measurement, the acoustic resonator was coupled to a superconducting qubit – those basic quantum computer building blocks that can simultaneously contain both a 1 and a 0 value, and on which companies such as Google and IBM have already built rudimentary components quantum computers.

The team’s hybrid device, with the acoustic resonator chip on top of the superconducting-​qubit chip. (von Lüpke et al., Nat Phys, 2022)

By making the status of the superconducting qubit dependent on the number of photons in the acoustic resonator, the scientists were able to read that number of photons without interacting with them or transferring energy.

They describe it as similar to playing a theremin, the strange musical instrument that doesn’t need to be touched to produce sound.

Assembling the equivalent of quantum computing was no easy task: quantum states are typically very short-lived, and part of the innovation of this technique was how these states were extended longer . The team did this partly through the choice of materials and partly through a superconducting aluminum cavity that provided electromagnetic shielding.

In other experiments, they succeeded in extracting what is called the “parity measure” from the quantum mechanical system.

Measuring parity is crucial for a variety of quantum technologies, especially when it comes to correcting errors in systems – and no computer can function properly if it regularly makes errors.

“By interfacing mechanical resonators with superconducting circuits, quantum acoustodynamics of circuits can make available a variety of important tools for manipulating and measuring quantum states in motion,” write the researchers.

This is all very high level in terms of quantum physics, but the bottom line is that it is a significant advancement in one of the technologies that could eventually provide a foundation for future quantum computers, especially in terms of combination of different types of systems. whole.

A hybrid qubit-resonator device such as the one described in this study potentially offers the best of two different research areas: the computational capabilities of superconducting qubits and the stability of mechanical systems. Now scientists have shown that information can be extracted from such a device non-destructively.

There is still a lot of work to be done – once the task of measuring states has been refined and completed, these states must then be mined and manipulated to be truly useful – but the enormous potential of quantum computing systems perhaps just taken another step closer.

“Here we demonstrate the direct measurements of the phonon number distribution and parity of non-classical mechanical states”, write the researchers.

“These measurements are some of the building blocks for building acoustic quantum memories and processors.”

The research has been published in Natural Physics.

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