There's widespread agreement that most useful quantum computing will have to wait for the development of error-corrected qubits. Error correction involves distributing a bit of quantum information—termed a logical qubit—among a small collection of hardware qubits. The disagreements mostly focus on how best to implement it and how long it will take.
A key step toward that future is described in a paper released in Nature today. A large team of researchers, primarily based at Harvard University, have now demonstrated the ability to perform multiple operations on as many as 48 logical qubits. The work shows that the system, based on hardware developed by the company QuEra, can correctly identify the occurrence of errors, and this can significantly improve the results of calculations.
Yuval Boger, QuEra's chief marketing officer, told Ars: "We feel it is a very significant milestone on the path to where we all want to be, which is large-scale, fault-tolerant quantum computers.
Catching and fixing errors
Complex quantum algorithms can require hours of maintaining and manipulating quantum information, and existing hardware qubits aren't likely to ever reach the point where they're capable of handling that without causing errors. The generally accepted solution to this is to work with error-correcting logical qubits instead. These involve distributing individual qubits among a collection of hardware qubits so that an error in one of these qubits doesn't completely destroy the information.
Additional qubits can add error correction to these logical qubits. These are linked to the hardware qubits that hold the logical qubits, allowing their state to be monitored in a way that will identify when errors have occurred. Manipulation of these additional qubits can restore them to the state that was lost when the error happened.
In theory, this error correction can allow the hardware to hold quantum states for far longer than the individual hardware qubits are capable of.
The trade-off is significantly increased complexity and qubit counts. The latter should be obvious—if each logical qubit requires a dozen qubits, then you need a lot more hardware qubits to run any algorithm. Full error correction would also require repeated measurements to identify when errors have occurred, identify the type of error, and perform the necessary corrections. And all of that would have to happen while the logical qubits are also being used for running those algorithms.
There's also the actual practicalities of getting any of this to work. It's really easy (by a very relaxed definition of "easy") to understand how to perform operations on pairs of hardware qubits. It's far more difficult to understand how to do them when any individual hardware qubit holds, at most, only a fraction of a logical qubit. Adding to the complexity is that there are a variety of potential error-correction schemes, and we're still figuring out their trade-offs in terms of robustness, convenience, and qubit use.
That's not to say that there hasn't been progress. Error-corrected qubits have been demonstrated, and they do maintain quantum information better than the hardware qubits that host them. And, in a few cases, individual quantum operations (termed gates) have been demonstrated using pairs of logical qubits. And two companies (Atom Computing and IBM) have been ramping up qubit counts to provide enough hardware to host lots of logical qubits.
Like Atom Computing, QuEra's hardware uses neutral atoms, which have several advantages. Quantum information gets stored in the nuclear spin of individual atoms, which is relatively stable in terms of maintaining quantum information. And, since every atom of a given isotope is equivalent, there's no device-to-device variation as there is in qubits based on superconducting hardware. Individual atoms can be addressed with lasers instead of needing wiring, and the atoms can be moved around, potentially allowing any qubit to be linked to any other.
QuEra's current generation of hardware supports up to 280 atom-based qubits. For this to work, those atoms were moved around among several functional regions. One is simply storage, where qubits live when they're not being manipulated or measured. This holds both any logical qubits in use and a pool of unused qubits that can be mobilized over the course of executing an algorithm. There's also an "entanglement zone" where those manipulations take place and a readout zone where the state of individual qubits can be measured without disturbing qubits elsewhere in the hardware.