There's a key aspect of quantum computing you may not have thought about before. Called 'quantum non-demolition measurements', they refer to observing certain quantum states without destroying them in the process.
If we want to put together a functioning quantum computer, not having it break down every second while calculations are made would obviously be helpful. Now, scientists have described a new technique for recording quantum non-demolition measurements that shows a lot of promise.
In this case, the research involved mechanical quantum systems – objects that are relatively large in quantum computing terms, but exceedingly tiny for us. They use mechanical motion (such as vibration) to handle the necessary quantum magic, and they can be combined with other quantum systems too.
"Our results open the door for performing even more complex quantum algorithms using mechanical systems, such as quantum error correction and multimode operations," write the researchers in their published paper.
For the purposes of this study, the team put together a thin strip of high-quality sapphire, just under half a millimeter thick. A thin piezoelectrical transducer was used to excite acoustic waves, moving energy units such as phonons which can, in theory, be put through quantum computing processes. Technically, this device is known as an acoustic resonator.
That was the first part of the setup. To do the measuring, the acoustic resonator was coupled with a superconducting qubit – those basic quantum computer building blocks that can simultaneously hold both a 1 and a 0 value, and upon which companies such as Google and IBM have already built rudimentary quantum computers.
By making the status of the superconducting qubit dependent on the number of phonons in the acoustic resonator, the scientists could read that number of phonons without actually interacting with them or transferring any energy.
They describe it as similar to playing a theremin, the strange musical instrument that doesn't need to be touched to produce sound.
Putting together the quantum computing equivalent was no easy task: Quantum states are usually very short-lived, and part of the innovation in this technique was the way these states were drawn out for longer. The team did this partly through the choice of materials and partly through a superconducting aluminum cavity that provided electromagnetic shielding.
In further experiments, they managed to extract what's known as the 'parity measure' of the mechanical quantum system.
The parity measure is crucial to a variety of quantum technologies, particularly when it comes to correcting errors in systems – and no computer can operate properly if it's regularly making errors.
"By interfacing mechanical resonators with superconducting circuits, circuit quantum acoustodynamics can make a variety of important tools available for manipulating and measuring motional quantum states," write the researchers.
This is all very high-level in terms of quantum physics, but the bottom line is that this is an important step forward in one of the technologies that could eventually provide a foundation for future quantum computers, particularly in terms of combining different types of systems together.
A hybrid qubit-resonator device such as the one described in this study potentially offers the best of two different fields of research: the computational capabilities of superconducting qubits, and the stability of mechanical systems. Now scientists have shown information can be extracted from such a device in a non-destructive way.
Plenty more work needs to be done – once the task of measuring states has been refined and completed, these states then need to be exploited and manipulated to be of real use – but the huge potential of quantum computing systems may have just been brought another step closer.
"Here we demonstrate the direct measurements of phonon number distribution and parity of non-classical mechanical states," write the researchers.
"These measurements are some of the basic building blocks for constructing acoustic quantum memories and processors."
The research has been published in Nature Physics.
Editor's note (18 May 2022): A previous version of this article mentioned photons in place of phonons. The error has now been corrected.