The quantum computing startup announced Tuesday that its future quantum processor designs will differ significantly from its current offerings. Instead of building a monolithic processor like others have, Rigetti Computing builds a collection of smaller qubits on a chip that can be physically linked together into a single usable processor. This is not as multiprocessing as a modular chip design.
This move is a consequence for both Rigetti processors and quantum computing in general.
What is holding you back
Rigetti̵7;s computer uses a technology called “transmon,” which uses a superconducting wire circuit linked to a resonator. It is the same qubit technology used by big competitors like Google and IBM. The state of one transmon can influence its neighbors during computation. This is an important feature of quantum computing. To some extent, the connection topology between transmon qubits is an important factor affecting the computational power of a machine.
(This is in contrast to systems such as Honeywell’s ion trap computers, where any qubit can interact with any other system, at least with the current number of qubits.)
Two other factors that hinder performance are the error rate of each qubits and the number of qubits. Increasing the number of qubits can increase the processing power of a processor. But only if all added qubits are of sufficiently high quality that the error rate will not limit the ability to perform accurate calculations.
When the number of qubits reaches the thousands, error correction is possible. This will greatly change the process. We are currently stuck with less than 100 qubits, so this change will continue in the uncertain future.
What changed for Rigetti?
As a startup, Rigetti doesn’t have access to the different types of resources available to companies like IBM, and while Rigetti has done an impressive job building its own transmon processors, it lags behind major competitors in the number of qubits in processors. IBM’s latest has over 60 while Rigetti’s latest is 31.
That’s not a major limitation as we’re still uncertain whether useful computations can be performed on quantum processors without enough qubits to correct errors. at the same time A clear path for rapid scaling is essential for getting to the point where errors can be corrected. And we may find that some algorithms can effectively count qubits between the available number and the steps required to correct all errors.
For Rigetti, the ability to combine multiple microprocessors. which the company has shown that it can be produced Turning it into a single larger processor should allow its qubit counts to run quickly. in today’s announcement The company said it expects 80-qubit processors to be available within the next few months (for context, IBM’s roadmap includes plans for 127-qubit processors sometime this year).
Another benefit of moving away from monolithic designs is that most chips typically have at least one qubit that is either defective or unacceptably high in error rates. with modular design The company was able to mitigate the consequences of that fact. Rigetti was able to mass produce modules and assemble chips from the least defective parts.
Alternatively, companies can choose modules with low error rate qubits and create a processor equivalent to an all-star processor. The lower error rate can compensate for the impact of a lower qubit count.
Qubit technology requires components that are larger than anything in a computer chip. Optical quantum computers and transoms both need cables. The atoms in an ion trap computer are stored in space by an array of electrodes. As a result, scaling the number of qubits is not as simple as adding computational resources. given to traditional silicon chips, the physical limitations are too different.
As a result, the other quantum computing companies we’ve talked to have already been informed that they will have to find a way to combine qubits with more than one chip, so Rigetti might be able to solve problems other companies are likely to face.
But the transmon, which can be linked by wiring It’s probably a simpler technology in this regard. And the lessons learned here don’t apply to competing technologies such as trapped ions.