Recently, for the first time, scientists have successfully manipulated a quantum state called a "dark state" in a superconducting quantum circuit. The robustness of "dark state" is about 500 times that of a single superconducting quantum circuit, which can be used for quantum simulation and quantum information processing.
Image by Mathiue Juan
When superconducting qubits are coupled to waveguides, collective states are produced through photon-mediated long-range interactions. Phase-cancel interference between qubits then decouples the collective "dark state" from the waveguide environment, making it impossible to emit photons into the waveguide. This makes the "dark state" beneficial for the preparation of long-lived quantum multibody entangled states and the implementation of quantum information protocols in open quantum systems.
"These entangled quantum states are completely decoupled from the outside world," says max Zanner, lead author of the paper and university of Innsbruck, "and it can be said that these quantum states are invisible, which is why they are called 'dark states'." ”
However, when the "dark state" is decoupled from the waveguide environment, it is also decoupled from the electric field driving the waveguide, which makes controlling and manipulating the "dark state" a challenge. Previously, scientists could not manipulate them without disrupting the "dark state" invisibility.
In response, the team of Professor Gerhard Kirchmair of the Institute of Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences has developed a quantum system that can manipulate superconducting circuits in a "dark state" in microwave waveguides from the outside. The results were recently published in the journal Nature Physics.
Experimental setup, picture from the paper
The team built four superconducting qubits in a microwave waveguide and connected the control lines via two transverse inlets. Through these control lines, the "dark state" is manipulated using microwave radiation. These four superconducting circuits form a solid qubit that takes about 500 times longer to store than a single circuit. There are multiple "dark states" in this qubit at the same time, which can be applied to quantum simulation and quantum information processing. Matti Silveri, a research group on nano and molecular systems at the University of Oulu in Finland, said that "in principle, this system can be expanded arbitrarily. ”
This experiment laid the foundation for further study of the "dark state" and its potential applications. At present, scientists' research on "dark state" is mainly concentrated in the field of basic research, but there are still many outstanding questions about the nature of "dark state" quantum systems. The team's experimental idea of controlling the "dark state" can in principle be implemented not only on superconducting qubits, but also on other technology platforms. The research was supported by the Austrian Science Foundation FWF, the Finnish Academy of Sciences and the European Union.