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Building Big Quantum Computers by Connecting Smaller Ones
Building quantum computers that can function despite their inherently noisy components is a long-standing goal for physicists. Quantum error correcting codes promise to make this possible, but to use them, physicists need to carefully choreograph the interactions between large numbers of qubits. Such scaling to large numbers of qubits is generally anticipated to require connecting together many distinct hardware modules, as it becomes difficult to finely control many qubits within a single unit (dilution refrigerator, trapped ion chain atom array, etc).
The quality of these connections between modules, such as when realized with photonic interconnects, then become the bottleneck which determines whether quantum error correcting codes can scale. In our work, we address precisely how stringent these demands for fault tolerance are. By characterizing the patterns in the noise necessary to create a logical failure when two distinct modules are connected, we found that the fidelity requirements for quantum communication between modules are substantially relaxed compared to previously known results, with up to 10% communication noise tolerable even when qubits within each module operate near the usual threshold of 1%.
We anticipate that this architectural advance, as well as our elucidation of the failure mechanisms at play when connecting error corrected modules, will be of great interest to physicists working in a broad range of hardware fields, providing optimism and greatly relaxed target performance goals in the pursuit of modular quantum error correction.
Fig. 1
- It’s generally a good idea to split up the qubits in a quantum computer into distinct “modules” so that we only need to control a fixed number of qubits within each unit. Many identical modules then need to be interfaced somehow, such as by communicating with photons (pink cross-connect). Since this interface is very noisy compared to the carefully controlled qubits within each module, it’s important to characterize the impact of this interface noise.
- Schematic showing the interface between two modules. The fact that interface noise is confined to the boundary between modules allows us to show that its effects can be largely mitigated, and we can tolerate quite a bit of noise connecting the modules.