Physicists have implemented a superconducting quantum computing device using a modular approach, placing four individual nodes on a flat substrate whose configuration can be arbitrarily changed. The scientists achieved an average fidelity of 96 percent for the two-qubit gates and a fidelity of 98.74 percent for the preparation of an entangled two-qubit state. This new method for creating quantum devices will be useful for efficiently scaling systems of unlimited size. The results of the study were published in Physical Review X.
For quantum computing to be practical, scientists need devices containing a large number of qubits, each of which must in turn be connected to all the others. However, a fully-fledged device with such an architecture has not yet been implemented: for example, in the case of superconductors, it is physically impossible to connect all the qubits to each other on a single plane, as coherence loss and dephasing effects interfere.
At the same time, researchers are continuing to try to circumvent physical limitations, using an approach similar to Ethernet networking: superconducting qubits are connected via a central routing element. To achieve this, physicists use a common resonator bus or connect computing modules via a multimode ring resonator. Such approaches suffer from other drawbacks, such as the long switching time between nodes, which is close in order of magnitude to the coherence time of superconducting qubits.
American physicists led by Andrew Cleland from the University of Chicago have proposed a new modular architecture for a quantum processor, connecting each qubit of the system to a superconducting quantum interference device.
To achieve this, the scientists constructed a circuit consisting of four superconducting qubits and four independently controlled nodes, galvanically connected to a central capacitor. This enabled all-to-all qubit communication by selectively connecting pairs of qubits through corresponding switches controlled by dynamically adjusting the magnetic flux. Individual qubit modules, fabricated on sapphire substrates, were equipped with individual pins and mounted on a common motherboard, cooled to 10 millikelvin.
Physicists demonstrated the functionality and operating protocol of the switches using a quantum CZ gate. Test results showed that the average gate execution time was approximately 46 nanoseconds with a fidelity of 96 percent and an error margin of approximately 0.08 percent. The scientists then conducted numerical simulations and found that the theoretical limit for the accuracy of two-qubit gates in their device approaches 99 percent. The researchers then generated entangled Greenberger-Horn-Zeilinger states involving first two, then three and four qubits. The final state fidelity for the first case was 98.74 percent, for the second – 88.15 percent, and for the third – 75.18 percent. The physicists attributed this decrease in performance to the decoherence of individual qubits during idle periods while the algorithm was processing other circuit elements.
The authors of the work noted that the connection method they proposed could be improved by increasing the coherence time of the qubits and could subsequently be used to connect more than four modules simultaneously into star-shaped structures.
When it comes to connecting ion devices into a single quantum computer, scientists are using slightly different approaches, primarily involving the physical movement of ions. For example, we've already written about how physicists have learned to transfer ions between chips at record speed.
