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Quantum computing, an approach to deriving information that leverages quantum mechanical effects, relies on qubits, quantum units of information that can exist in superpositions of states. To effectively perform quantum computing, engineers and physicists need to be able to measure the state of qubits efficiently.

In quantum computers based on superconducting materials, qubits are indirectly measured by a so-called readout resonator, a circuit that responds differently based on the state of a qubit. This circuit's responses are probed using a weak electromagnetic wave, which needs to be amplified to enable its detection.

To amplify these signals, also known as microwave tones, quantum technology engineers rely on devices known as amplifiers. Existing amplifiers, however, have notable limitations. Conventional amplifiers can send unwanted noise back to the qubit, disturbing its state. Superconducting parametric amplifiers introduced more recently can be very efficient, but they conventionally rely on bulky and magnetic hardware components that control the direction of signal and protect qubits from backaction noise.

Researchers at the National Institute of Standards and Technology and University of Colorado recently developed a new device that can act as both an amplifier and converter, amplifying electromagnetic signals while redirecting undesired return signals and preventing them from interacting with qubits. The new device, introduced in a paper in Nature Electronics, could greatly simplify the measurement of qubits, contributing to the upscaling of superconductor-based quantum computers.

"This paper is about combining, within the same 2-port device (one input, one output) the ability to amplify a signal propagating from input to output, and suppress the same signal propagating from output to input," Maxime Malnou, first author of the paper, told Â鶹ÒùÔº. "Such a feature is crucial in systems like superconducting quantum computers, where you want to amplify the signal emitted by qubits, and at the same time protect these qubits against any backaction coming back from the readout chain."

The amplification technique employed by Malnou and his colleagues is well-established within the quantum computing community, while the method they employed for isolation was introduced a few years ago. The primary challenge for the authors was to demonstrate that they could reliably implement both using a single device.

"We use parametric processes to drive the amplification and isolation within the device," explained Malnou. "A parametric process is a feature of a system whereby one component in the system can be modulated to add or convert energy within the system. The usual picture for parametric amplification (when you add energy) is the child on a swing: By moving its center of mass periodically, the child can increase their oscillation amplitude."

The new electronic device developed by the researchers integrates an artificial transmission line, a circuit structure made of lumped-element inductors and capacitors. By using Josephson junctions instead of ordinary inductors, the team ensured that the line is nonlinear, which ultimately enabled both amplification and frequency conversion.

"Remarkably, the parametric processes can be made directional," said Malnou. "By modulating the junctions with strong directional microwave pump tones, signals traveling forward get amplified, while signals traveling backward are converted to a different frequency, preventing them from reaching the sensitive qubits."

Malnou and his colleagues were ultimately able to realize both the amplification of signals and their isolation via frequency conversion, all in a single device. This is a notable achievement, which also brought to light some of the challenges one might encounter when combining the two.

"In superconducting quantum computers, isolating qubits from the back action of the readout chain is crucial," said Malnou. "This is conventionally achieved using passive magnetic components. Not only are these components not compatible with qubits (which are very sensitive to any magnetic field), they are also bulky. Nonetheless, they must be placed as close to the qubits as possible for optimal performance. This is an engineering challenge in practice and requires a lot of space at millikelvin temperatures (where the qubits live)."

In the future, the amplifier and converter developed by Malnou and his colleagues could replace previously introduced bulky and magnetic microwave circulators and isolators. In principle, the new device is also compatible with superconducting qubits and could thus be integrated on-chip along with these qubits.

"As part of our next studies, we plan to pursue several goals," added Malnou. "The first is to understand what happens when you perform a qubit measurement with such a device. Another one is to refine the device's design to make sure that only the two parametric processes that we leverage are permitted to happen within the device. Finally, we are also exploring other ways to obtain microwave isolation within a traveling-wave parametric amplifier."

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