In one study the scientists entangled two quantum bits using sound for the first time, while in another study they built the highest-quality long-range link between two qubits to date. The work, say the researchers, brings us closer to harnessing quantum technology to make more powerful computers, ultra-sensitive sensors, and secure transmissions.
“Both of these are transformative steps forward to quantum communications,” says Andrew Cleland, the John A. MacLean Sr. Professor of Molecular Engineering at the IME and UChicago-affiliated Argonne National Laboratory and co-author of a paper on the studies. “One of these experiments shows the precision and accuracy we can now achieve, and the other demonstrates a fundamental new ability for these qubits.”
One of the new studies addresses one of the main challenges for developing quantum technology: sending quantum information any substantial amount of distance, along cables or fibers. The researchers were able to build a system out of superconducting qubits that exchanged quantum information along a track nearly a meter long with extremely strong fidelity – with far higher performance than has been previously demonstrated.
“The coupling was so strong that we can demonstrate a quantum phenomenon called ‘quantum ping-pong’ — sending and then catching individual photons as they bounce back,” says Youpeng Zhong, a graduate student in Cleland’s group and the first author of the paper.
Building the right device to send the signal was one of the breakthroughs, say the researchers. The key was shaping the pulses correctly — in an arc shape, like opening and closing a valve slowly, at just the right rate.
This method of ‘throttling’ the quantum information, say the researchers, helped them achieve such clarity that the system could pass a gold standard measurement of quantum entanglement, called a Bell test. This is a first for superconducting qubits, and it could be useful for building quantum computers as well as for quantum communications.
The other study shows a way to entangle two superconducting qubits using sound, which addresses another challenge for advancing quantum technology: being able to translate quantum signals from one medium to the other. For example, say the researchers, microwave light is perfect for carrying quantum signals around inside chips.
“But,” says Cleland, “you can’t send quantum information through the air in microwaves; the signal just gets swamped.”
To get around this, the researchers built a system that could translate the qubits’ microwave language into acoustic sound and have it travel across the chip, using a receiver at the other end that could do the reverse translation. This involved some creative engineering, say the researchers.
“Microwaves and acoustics are not friends, so we had to separate them onto two different materials and stack those on top of each other,” says Audrey Bienfait, a postdoctoral researcher and first author on the study. “But now that we’ve shown it is possible, it opens some interesting new possibilities for quantum sensors.”
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