# What use cases are there for 127 qubit QPUs?

IBM have recently announced their 127 qubit Eagle processor. Other approaches, such as Rydberg arrays, have now 256 qubits, as for example in QuEra's QPU QPU.

While these are without a doubt outstanding techical acheivements, I am wondering what is their intended use case. The reason I'm asking is that as far as I know, a rough estimate that the number of qubits that can be entangled without error correction is about $$1/\sqrt{\epsilon_{2Q}}$$, where $$\epsilon_{2Q}$$ is 2 qubit gate error. Also, results such as this one that show that even circuits with width 16 are extremely limited in depth (another relevant metric here is IBM's record $$\log_2(QV)=7$$ as of today).

Given this, what kind of circuits/applications can be executed with today's error rates that would require the full > 100 qubit processor?

• I assume the intended use is simply to provide the equipment so that researchers may explore use cases. Certainly IBM hardware has played a pivotal role in the development of many error mitigation strategies over the last few years, squeezing as much as can be squeezed out of the theoretical limitations. But I say this as someone who has never been bold enough to use more than five qubits, so I'll not be bold enough to post an answer either... ^_^ Feb 14 at 1:23
– Lior
Feb 14 at 11:57

I think that the main reason behind is to tackle technical difficulties connected with building huge number of qubits. Having hundred of qubits brings about issues with interconnection, connections to microwave links for programming and measurement etc. If you have a look at decoherence times and quantum volume of the Washington, there is a little improvement even in comparison with 5 qubits processors. To sum up, currently there is no use case in real world, we only need to build prototypes to solve current technical issues and maybe to discover some new issues, previously unanticipated.

• I agree with this very much. Getting the thru silicon vias and the interposer chip technology to work on the Eagle sounds like a crazy fabrication challenge.
– Lior
Feb 14 at 11:53
• Nevertheles, I am interested in knowin if people in the field are thinking about what can be done with this computing power at current error levels. Perhaps there are NISQ algorithms that don't require full entanglement across all qubits to be effective? Perhaps there's a way to run circuits where the "corrlation length" is approximately what can be achieved with the current error rates, but weaker correlations across different qubits can still lead to an advantage?
– Lior
Feb 14 at 11:56
• @Lior: based on my experience, QAOA or VQE algorithms can be run on such noisy processor successfully. I run these algorithms only on 5 qubits instances, however, the error rates of them are similar to that of the Washington. Feb 14 at 22:04
• What’s the Washington?
– Lior
Feb 14 at 22:42
• @Lior: it is 127 qubits IBM processor. I realized that you refer to it as Eagle. But the Eagle is a name of family of processors (like Pentium). Sorry for confusion. Feb 15 at 7:17