Quantum Computing in the NISQ era and beyond

The 50-qubit barrier

Even with fault-tolerant quantum computing still a rather distant dream, we are now entering a pivotal new era in quantum technology. For this talk, I needed a name to describe this impending new era, so I made up a word: NISQ. This stands for Noisy Intermediate-Scale Quantum. Here “intermediate scale” refers to the size of quantum computers which will be available in the next few years, with a number of qubits ranging from 50 to a

4 few hundred. 50 qubits is a significant milestone, because that’s beyond what can be simulated by brute force using the most powerful existing digital supercomputers. “Noisy” emphasizes that we’ll have imperfect control over those qubits; the noise will place serious limitations on what quantum devices can achieve in the near term.

Physicists are excited about this NISQ technology, which gives us new tools for exploring the physics of many entangled particles. It might also have useful applications of interest to the business community, but we’re not sure about that. We shouldn’t expect NISQ is to change the world by itself; instead it should be regarded as a step toward more powerful quantum technologies we’ll develop in the future. I do think that quantum computers will have transformative effects on society eventually, but these may still be decades away. We’re just not sure how long it’s going to take.

Qubit ``quality''

I’ve emphasized the number of qubits as a measure of how difficult it is to do the simulation of a quantum computer on a classical device, but the number of qubits isn’t the only thing we care about. We also care about the quality of the qubits, and in particular the accuracy with which we can perform quantum gates — well-controlled entangling operations acting on pairs of qubits. With the best hardware we have now for controlling trapped ions [14] or superconducting circuits [15], the error rate per gate for two-qubit gates is above the.1% level (and often much worse). Furthermore, we don’t yet know whether error rates that low can be maintained in larger devices with many qubits; perhaps we’ll find out soon. Naively, then, and as I’ll say later this might be too naive, with these noisy devices we don’t expect to be able to execute a circuit that contains many more than about 1000 gates — that is, 1000 fundamental two-qubit operations — because the noise will overwhelm the signal in a circuit much larger than that. That limitation on circuit size imposes a ceiling on the computational power of NISQ technology. Eventually we’ll do better, using quantum error correction to scale up to larger circuits. But as I’ve already emphasized, since quantum error correction imposes a heavy overhead cost in number of qubits and number of gates, scaling up using quantum error correction is a more distant goal. When I speak of the NISQ era, I’m imagining quantum computers with noisy gates unprotected by quantum error correction.

There are other things we care about, too, aside from the number of qubits and the gate error rate. The time it takes to execute a single gate is also important for setting the time scale needed for a quantum computer to solve a problem, and it is noteworthy that superconducting circuits are about a thousand times faster than ion trap quantum processors. We need to be able to prepare and measure qubits accurately; currently the measurement error probability is about 1% for superconducting qubits, and much better than that for trapped ions. We care about the connectivity among the qubits — for which pairs of qubits in our device can we perform an accurate two-qubit gate? It is also important to know how reliably we can fabricate qubits — if we attempt to construct a many-qubit device, how many of the qubits actually perform well enough to be useful? These considerations and others, too, should be kept in mind when we make comparisons across different quantum computing platforms.