Quantum computing: First two-qubit logic gate in silicon

A marked improvement in coherence times has been observed by defining the quantum dots in silicon, which can be isotopically purified, such that quantum dots with single-spin fidelities above the threshold of surface codes can be realized

Meaning they get a bit less than a 10% error per operation.

the thing that will make quantum computing practical is quantum error correction. I describe it as "the hospital problem". Suppose there's a car accident. An ambulance gets sent to pick up the survivors and fix it. But an ambulance is also a vehicle and so there is a chance it will also get in an accident. You can send more ambulances to fix the situation, but those ambulances are also vehicles and could also get into accidents. The layout of the roads, frequency of hospitals, and probability of accident per meter will determine on average how many problems get fixed per ambulance. If on average an ambulance gets into less than one accident then it is worth it. Otherwise ambulances cause more accidents than they fix. The point where ambulances fix exactly as many accidents as they create is called the threshold. Once you pass the threshold, with sufficient hospitals you can make the expected number of accidents arbitrarily low (aka the probability of successful computation arbitrarily high).

When people talk about surface codes, they mean the broad class of error correction schemes which look a bit like the toric code. The toric code has a threshold value around 10% which is great! However it is literally a lattice of atoms (or other spin systems) embedded on a torus. Construction is difficult. It also doesn't scale. A toric code with a few dozen qubits works as well as one with a few hundred. In fact several no-go theorems show that any transitionally invariant lattice in less than four dimensions won't scale. My research is actually in getting around these theorems by throwing out the lattice idea entirely.

This research is important, but the real Chicago Pile of QC will be passing the threshold of an easy to implement correction scheme.

Like I said upthread, it all depends on quantum error correction. Right now they are below the threshold for some surface codes (note: in this context below a threshold is a good thing). However surface codes require a torus topology, and existing chip technology generally flat. They are above the threshold for topologically simple codes like Stein and Shor thus ruling them out. While their research is a step forward, what they have in their lab right this instant cannot be made into a scaleable quantum computer.

Interestingly enough, most quantum computers (both in theory and practice) are more like fpga's than like a classical CPU architecture. The general purpose qubits just kind of sit there and a quantum circuit is implemented on them with laser pulses. The only exception I'm aware of is the silicon waveguides group I mentioned up top.