Many researchers believe the future of quantum computing lies in modular machines assembled from several connected processors. However, this requires learning to transfer fragile quantum information between chips without loss. For superconducting processors, this means creating reliable links about one meter long inside equipment cooled to near absolute zero.
A team at IBM Research has solved this problem for the first time: two large superconducting processors were connected by four separate high-quality quantum channels, all of which were installed after both chips were already in place. The work is published on the arXiv preprint server and has not yet been peer-reviewed.
Until now, quantum links between superconducting chips have mainly been limited to a single connection between small experimental processors. Such lines made it possible to transfer quantum states from one chip to another and to create entanglement, a special quantum correlation in which the states of particles become interrelated. To build larger machines, engineers will need many connections between processors already installed in cryostats (cooling devices). These connections must be compact, introduce minimal loss and withstand repeated heating and cooling cycles.
In the new work, the researchers connected two of their 120-qubit Nighthawk R1 processors. In each channel, a qubit inside one processor passed information to a special "interface" qubit. A tunable connector then routed it into an 80 cm superconducting aluminum cable, which delivered the signal to a similar set of components on the other chip. To allow the cables to be packed more densely, they were connected to the chip from above rather than from the side.
After assembling the setup, the scientists checked how accurately it transfers quantum states between the chips and performs logical operations involving qubits from both processors. The average fidelity of quantum state transfer across the four channels was 99%, and on the best channel it reached 99.29%. Two-qubit logical operations spanning both processors were performed with an average fidelity of 94.75%.
Using these operations, the team created entangled states distributed between the two processors, including states of 4 and 6 qubits, and also generated two entangled pairs simultaneously through adjacent channels. Thanks to the new connection method, all four links remained operational after three heating and cooling cycles.
The authors note that commercial application of the system will require improvements, especially regarding the retention time of quantum information. Nevertheless, they call the achieved result an important step forward: it opens the way to computers where quantum information is stored and processed across many processors, which will simplify the scaling and maintenance of future machines.