MIT Chinese PhD student achieves important results in efficient readout of quantum information

Noise is inherent to any quantum system. Scientists can effectively bypass this limitation by "compressing" the noise using "parametric" amplification: the quantum phenomenon of "compression" reduces the noise affecting one variable while increasing the noise affecting its conjugate variable; while the total amount of noise remains the same, it is effectively redistributed. Researchers can then make more accurate measurements by focusing only on the low-noise variables.

Recently, a team led by MIT has developed a new superconducting parametric amplifier: it operates with the same gain as previous narrow-band compressors, while allowing quantum compression over a larger bandwidth.

Achieving quantum compression over a wide frequency bandwidth of 1.75 GHz

This paper, recently published in Nature Physics [1], demonstrates for the first time compression over a wide frequency bandwidth of up to 1.75 GHz while maintaining a significant amount of "compression" (selective noise reduction). In contrast, previous microwave parametric amplifiers typically had a bandwidth of 100 MHz or less.

This new broadband device may enable scientists to read out quantum information more efficiently and generate faster, more accurate quantum systems. By reducing errors in measurements, this architecture could be used for multi-quantum systems or other metrology applications that require extreme accuracy.

Yanjie (Jack) Qiu received his undergraduate degree from the University of California, Berkeley, and is currently a PhD student at MIT. At Berkeley, he worked on cryogenic engineering to optimize custom dilution chillers and develop quantum confinement amplifiers. During his PhD, he worked on superconducting circuits in the MIT Engineering Quantum Systems group.

Notably, the first author and corresponding author of this paper is Yanjie (Jack) Qiu [2], a Chinese PhD student in the Department of Electrical Engineering and Computer Science and the Engineering Quantum Systems Group at MIT. He says, "As the field of quantum computing grows and the number of quantum bits in these systems increases to thousands or more, we will need broadband amplification."

Scalable distributed nonlinear parametric amplifiers

Superconducting quantum circuits, such as quantum bits, process and transmit information in quantum systems - information carried through microwave electromagnetic signals composed of photons. However, these signals can be extremely weak, so researchers need to use amplifiers to increase the signal level so that precise measurements can be made.

However, the quantum nature of Heisenberg's uncertainty principle requires that a minimum amount of noise be added to the amplification process, which is the "standard quantum limit" of background noise. Fortunately, the Josephson parametric amplifier is a device that can be effectively redistributed elsewhere: the added noise is "compressed" below the fundamental limit.

Quantum information is represented by conjugate variables, such as the amplitude and phase of an electromagnetic wave. However, in many cases, researchers only need to measure one of these variables to determine the quantum state of a system. In these cases, they can "compress the noise" by reducing the noise of one variable and increasing the noise of the other. Due to Heisenberg's uncertainty principle, the total amount of noise remains constant, but its distribution can be shaped in such a way that less noise can be measured in one of the variables.

A conventional Josephson parametric amplifier is based on a resonator. It is like an echo chamber with a superconducting nonlinear element called a Josephson junction in the middle. Photons enter the echo chamber and bounce around, interacting with the same Josephson junction many times. In this environment, the system nonlinearity achieved by the Josephson junction is enhanced and leads to amplification and compression of the parametric quantities. However, as the photon crosses the same Josephson junction multiple times before leaving, the junction is stressed; therefore, the resonator-based amplifier is limited in the bandwidth and maximum signal it can accommodate.

This time, the MIT researchers took a different approach. Instead of embedding one or several Josephson junctions inside the resonator, they connected more than 3,000 junctions together to create the Josephson traveling-wave parametric amplifier. Photons interact with each other as they travel from junction to junction, causing noise compression without stressing any of the "junctions".

Experiments related to Josephson's traveling wave parametric amplifier

Their traveling wave system can tolerate higher power signals than resonator-based Josephson amplifiers without the bandwidth limitations of resonators, resulting in broadband amplification and high levels of compression.

"You can think of this system as a really long fiber, another type of distributed nonlinear parametric amplifier. And, we can push it to 10,000 nodes or more. It's a scalable system, as opposed to a resonant architecture." Qiu said.

Reduced noise, efficient readout of quantum information

A pair of pump photons will enter the device and act as an energy source. Researchers can adjust the frequency of the photons from each pump to produce compression at the desired signal frequency. For example, if they wanted to compress a 6 GHz signal, they would adjust the frequency of the pump to send photons at 5 and 7 GHz, respectively. As the photons of the pumps interact within the device, they combine to produce an amplified signal whose frequency is right in the middle of the two pumps. This is a special process of a more general phenomenon called nonlinear wave mixing (nonlinear wave mixing).

Noise reduction and efficient readout of quantum information

A pair of pump photons will enter the device and act as an energy source. Researchers can adjust the frequency of the photons from each pump to produce a compression of the desired signal frequency. For example, if they wanted to compress a 6 GHz signal, they would adjust the frequency of the pump to send photons at 5 and 7 GHz, respectively. As the photons of the pumps interact within the device, they combine to produce an amplified signal whose frequency is right in the middle of the two pumps. This is a special process of a more general phenomenon called nonlinear wave mixing (nonlinear wave mixing).

As the pump power increases, the compressed state evolves as the output field is imaged.

Qiu explains, "Compressed noise results in a two-photon quantum interference effect during parametrization."

This architecture allowed them to reduce the noise power to 10 times below the fundamental quantum limit while operating at an amplification bandwidth of 3.5 GHz: a frequency range almost two orders of magnitude higher than previous devices. Their device also demonstrates broadband generation of entangled photon pairs, which could allow researchers to read out quantum information more efficiently with a higher signal-to-noise ratio.

While Qiu and his collaborators are excited about these results, there is still room for improvement. The material they used to make the amplifier introduced some microwave loss, which could reduce performance. Looking ahead, they are exploring different fabrication methods that could improve insertion loss.

""This work is not meant to be a stand-alone project. It has great potential if applied to other quantum systems: interfacing with quantum bit systems to enhance readout, entangling quantum bits, extending the operating frequency range of the device for dark matter detection and improving its detection efficiency ......

Finally, the researchers stated [3], "This is essentially the blueprint for future work."

Reference links:

[1]https://www.nature.com/articles/s41567-022-01929-w

[2]https://www.nature.com/search?author=Jack%20Y.%20Qiu[3]https://news.mit.edu/2023/boost-quantum-signals-squeezing-noise-0209