Credit: Quantum Circuits Inc.
Recently there have been two announcements of companies developing Quantum Error Detection technology that could provide an additional approach to combatting the effects of qubit noise and gate infidelity in a quantum computer. Although many people are working on Fault Tolerant Quantum Computing (FTQC) schemes to both detect and then correct errors, the problem with many of these approaches lies in the large number of physical qubits needed to create a logical qubit. In some implementation, the ratio of physical to logical qubits could be 1000:1 or even higher. This will require scaling the computer to a very large number of physical qubits, perhaps a million qubits or more, to form enough logical qubits to provide a solution for a commercially useful application which could not be calculated with a classical computer, also known as Quantum Advantage.
The two new pieces of research have shown a means for detecting a quantum error without going to the next level of correcting a error if one is detected. This approach can work if it is combined with a approaches known as Post Selection or else Repeat Until Success. In the Repeat Until Success approach, if an error is detected during the execution of an algorithm, the computer will try again until it success completes the algorithm with no error detected. The big advantage with this is that by only performing Quantum Error Detection, these new developments drastically reduce the number of physical qubits required and may allow processing Quantum Advantaged applications on moderately sized quantum computers that are expected to be available in the near term.
We reported earlier this year about technology being developed by Quantum Circuits Inc. that has created a new type of superconducting qubit called a Dual-Rail Qubit. Their approach encodes a qubit state into two superconducting resonators. A photon present in the top resonator, but not the bottom could represent a logical 0 and a photon present in the bottom, but not the top could represent a logical 1. If a photon is not detected in either resonator, then an erasure error has occurred which would represent that something went wrong. Although the die area required for a dual-rail qubit will be little larger than what is needed for a more traditional transmon superconducting qubit, this increase in size would still be orders of magnitude smaller than implementing a full error correction circuit with non dual-rail qubits. A good explanation of the company’s approach can be seen in a blog on the Quantum Circuits website here. The company is also working on implementing more efficient full error correction circuit using the dual-rail qubit, but we will have to wait to see what they come up with.
Another approach that does not require special hardware qubits has been published by Q-CTRL. Their approach combines their error mitigation technology with a coding that include a few additional ancillary flag qubits that utilize sparse parity checks to detect errors. In their recent paper they reported creating a 75 error-detected qubit GHZ state, the largest anyone has seen so far, using 84 total qubits. Since 9 of those qubits were used as the ancillaries, this would would represent a low overhead rate of about 12%. In the same paper, the company also described implementation of a long range CNOT circuit using teleportation. For more about Q-CTRL’s research in this area, read the recent paper they posted on arXiv here.
So as researchers continue to advance both the quantum hardware and also quantum algorithms there is still plenty of room for continued innovation with the hopes that Quantum Advantage can be achieved sooner than later.
November 30, 2024
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