Non-equilibrium quantum transport plays a vital role in technological advances ranging from nanoelectronics to thermal management. In essence, it deals with the coherent transfer of energy and (quasi-)particles through quantum channels between thermodynamic baths.
Thus, a complete understanding of quantum transport requires the ability to simulate and probe macroscopic and microscopic physics on equal footing.
Researchers from Singapore and China have utilized a superconducting quantum processor to examine the phenomenon of quantum transport in unprecedented detail.
Gaining deeper insights into quantum transport—encompassing the flow of particles, magnetization, energy, and information through quantum channels—has the potential to drive significant innovations in next-generation technologies such as nanoelectronics and thermal management.
Dario Poletti, an Associate Professor and Head of Cluster at Singapore University of Technology and Design (SUTD), along with co-corresponding authors, developed theoretical models of quantum transport in collaboration with Dr. Xiansong Xu and Dr. Chu Guo while they were both PhD candidates at SUTD. Xiansong currently serves as an assistant professor at Sichuan Normal University, and Chu Guo holds an assistant professor position at Henan Key Laboratory of Quantum Information and Cryptography.
They validated these models through experiments conducted with researchers from ZJU and CAS. The experiments carried out on the ZJU team’s 31-qubit quantum processor examined the flow of a spin/particle current between two sets of qubits.
“The work also shows the usefulness of quantum simulation in the NISQ era,” says Pengfei Zhang, a Postdoctoral Fellow at ZJU.
Quantum transport occurs when there is an imbalance or a state of nonequilibrium between connected systems. For instance, a difference in temperature leads to the flow of heat until the systems reach thermal equilibrium, whereas differences in voltage result in an electric current.
In a recent study, researchers investigated transport between two sets of qubits exhibiting differing magnetization levels. In one set, or bath, all qubits were initialized in the spin-down state, while in the other set, half were spin-down and half were spin-up, resulting in an average magnetization of zero.
These two baths were linked by a point contact, which serves as a weak connection between a qubit from each bath. The theorists aimed to apply principles from quantum thermalization to elucidate this quantum transport. They viewed the two baths as a combined system that would eventually reach thermal equilibrium over time. They anticipated that this lengthy process would allow for steady transport to take place.
Xiansong says, “I believe there should exist a unified picture of thermalization dynamics and nonequilibrium steady dynamics. But both the theoretical derivation and numerical verifications are not straightforward.”
In the experimental group, the theoretical approach appeared to be simple to carry out. With strong individual control over each qubit in their quantum processors, they were able to create various baths and thereby design different kinds of transport.
Earlier studies on quantum transport lacked the same level of flexibility or adjustability. The researchers examined how variations in the initial bath states and the number of qubits influenced the magnitude and stability of the current.
The initial bath states could vary by which qubits in the half-half setup were in a spin-up state versus a spin-down state. The researchers generated 60 randomly selected unique initial states for systems comprising 14, 17, and 31 qubits, then assessed the current after 200 nanoseconds. The results indicated that the current tends to converge to the same value as the size of the system increases.
“This is sometimes called ‘typicality,’” says Dario. “All that matters is the average spin polarisation, a macroscopic quantity, not the details of the individual qubits or how they are prepared.”
The researchers also investigated the stability of the current by assessing temporal variations, which appeared as a spin flow oscillating between the baths. This process included 60,000 measurements taken at five-nanosecond intervals over a range from 100 to 1,000 nanoseconds. They noted that as the system size increased, the fluctuations became considerably smaller in comparison to the main signal, indicating the emergence of the anticipated macroscopic physics.
“It became challenging to fine-tune the control parameters and precisely measure the tiny temporal fluctuation of particle current for a large system, but we overcame it by developing a calibration protocol and an error mitigation method,” Pengfei says.
The researchers aim to build upon these findings and maintain their collaboration to investigate more complex transport scenarios.
Journal reference:
- Pengfei Zhang, et al. Emergence of steady quantum transport in a superconducting processor. Nature Communications, 2024; DOI: 10.1038/s41467-024-54332-9