Quantum computing is currently an extremely diverse field of research, in which different ways of encoding, manipulating and processing quantum information are being explored.
Alongside discrete-variable systems, such as those based on superconducting qubits, another approach uses continuous variables, including the quadratures of the electromagnetic field. This strategy, already widely developed in quantum optics, is attracting growing interest in the microwave domain.
In this context, a research team involving the Istituto Nazionale di Ricerca Metrologica (INRiM), Politecnico di Torino and the University of Palermo has demonstrated the programmable generation of different configurations of entangled quantum states. The results have been published in Nature Communications in the article Programmable microwave cluster states via Josephson metamaterials.
The study focuses on cluster states: quantum states composed of multiple, strongly correlated elements that are a fundamental resource for measurement-based quantum computing. In this paradigm, computation is performed not by applying a sequence of logic gates, but through a controlled series of measurements on the elements of the cluster state.
Using a Josephson Traveling-Wave Parametric Amplifier (JTWPA), the research team generated microwave radiation exhibiting quantum correlations between different frequency modes. By controlling the interactions within the device, the researchers obtained programmable four-mode cluster states.
The structure of these states can be modified by adjusting the frequencies and phases of the pump tones applied to the amplifier. This makes it possible to select which modes interact and to reconfigure the connections between them. During the experiment, the same device was used to generate linear, cyclic, star and fully connected cluster states.
The ability to generate different configurations using a single superconducting component is also particularly relevant from a scalability perspective. The JTWPA operates over a wide frequency bandwidth, within which multiple modes of the electromagnetic field can be defined.
The study does not yet represent the realisation of a large-scale measurement-based quantum computer. However, it demonstrates several of the elements required to move in this direction: the generation of multimode entanglement, control over the correlation structure, reconfigurability and the possibility of exploiting many modes within a single broadband device.
The results therefore open up a possible path towards programmable and scalable architectures for continuous-variable quantum computing in the microwave domain.