New protocol cuts the resources needed for long-distance quantum fan-out gates

[SortedBuilder]

Linking separate quantum processors is one of the main routes to scaling, and a new theoretical proposal aims to make one important operation cheaper. Quantum Zeitgeist reports on work by Wen-Qiang Liu of Shijiazhuang Tiedao University and Hai-Rui Wei of the University of Science and Technology Beijing. It is published in New Journal of Physics

The operation in question is the fan-out gate, which copies the state of one control qubit onto several target qubits at once. Doing that between processors that are physically apart is difficult. The new protocol uses pre-shared GHZ states, a type of multi qubit entangled state, combined with standard CNOT gates

Compared with standard methods, it removes n minus 2 CNOT gates and n minus 2 Hadamard gates, where n is the number of qubits involved. The optical versions of the design also need fewer single photons than approaches that break the gate into smaller pieces

There is a catch. The heralded success probability falls quickly as the number of qubits grows, and the scheme depends on distributing GHZ states between processors, which is hard to do in practice. The article is clear that this is a theoretical result and that scaling depends on progress elsewhere

It is incremental work, but distributed quantum computing needs exactly these kinds of building blocks. Every gate saved in a networked operation reduces errors. Experimental tests would be the natural next step

Is distributed quantum computing the best route to scale? And how soon could GHZ states be shared reliably between processors?

Quantum Gate Complexity Reduced For Long Distance Computationquantumzeitgeist.com