The team showed that they could perform multiple rounds of error detection and correction on each data qubit in the circuit and that they could detect two distinct types of qubit errors. Ryan-Anderson and colleagues used ten of these physical-qubit ions to make a single error-tolerant logical qubit: seven of them were data qubits, and the other three were ancilla qubits. Such trapped-ion quantum circuits are already being developed for commercial quantum computers. To read a qubit, the team hits the ion with a laser pulse and measures its fluorescence. The qubits are embodied in the quantum electronic states of the ions, which can be manipulated by laser beams. The system allows each ion to be moved into proximity with any other ion to enable the entanglement operations. The researchers used qubits made from single ytterbium ions held in an electromagnetic trap. The algorithms and hardware developed by Ryan-Anderson and his colleagues now address these problems. This process allows some information about the data qubits to be inferred without measuring them directly.Īlthough the basic idea has been around since the 1990s, putting it into practice with schemes for rapidly and repeatedly diagnosing and fixing the errors in a real quantum circuit with good-quality qubits posed a massive technological challenge. Like most other QEC approaches, it involves additional so-called ancilla qubits, whose job is to signal errors in the “data qubits.” In each of a series of operations, an ancilla qubit is entangled with a subset of data qubits, and then the ancilla qubit is measured. They use a QEC scheme first proposed in 1996 by Andrew Steane of the University of Oxford, UK. ×Ī team of researchers at Quantinuum (formerly Honeywell Quantum Solutions) in Broomfield, Colorado, has now implemented a method that overcomes some of these limitations. The Quantinuum chip inside its vacuum chamber. Honeywell Quantum Solutions/Quantinuum Ready for action. Another problem is that they have generally been retrospective-the final result is “post-corrected”-but not all errors can be corrected after-the-fact. But these schemes have typically allowed only one round of error correction for each logical qubit in a calculation. Various QEC methods have been proposed in which several physical qubits are quantum mechanically entangled to make a single logical qubit that has some resistance to errors. The best of these quantum circuits may still outperform classical computers for certain types of calculations, but without quantum error correction (QEC), the power and scope of quantum computing remain limited. That’s why today’s quantum computers have only a relatively small number of “noisy” qubits. The problem gets harder to manage as the number of qubits increases. If qubit errors are not corrected, they will gradually accumulate and overwhelm the calculation with random noise, limiting the number of steps a quantum algorithm can reliably perform. So the states must remain unknown, and a fundamental principle forbids the copying of an unknown quantum state. But that can’t be done in quantum computers, since the computation relies on the qubits adopting quantum states that are neither 0 nor 1, and measuring them destroys those delicate states. Both features are needed to make the basic elements-the logical qubits-of a fully error-tolerant quantum computer that can be scaled up and used for applications beyond the specialized ones that these machines have tackled so far.Įrror correction is straightforward on classical computers: by keeping several copies of each bit, a random error (such as a 1 flipping to a 0) can be identified and corrected using a simple majority rule. It also allows error correction to be conducted several times on a single quantum bit (qubit) during the calculation. Researchers have now demonstrated a technique that allows errors to be detected and corrected in real time as the computation proceeds. Random errors incurred during computation are one of the biggest obstacles to unleashing the full power of quantum computers. The ion-trap chip used for the error-correction scheme holds ten single-ion qubits lined up in a row. Honeywell Quantum Solutions/Quantinuum Quantum computation without all the noise.
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