Quantinuum researchers have successfully demonstrated the entangling of logical qubits in a fault-tolerant circuit using real-time quantum error correction. The study is the first experimental comparison of different quantum error correction codes in similar environments. It presents a collection of several different experiments:
- The first illustration of entangling gates between two logical qubits is carried out in a fully fault-tolerant environment using real-time error correction.
- The first illustration of a logical entangling circuit with higher fidelity than the corresponding physical circuit.
This accomplishment proves that logical qubits can outperform physical qubits, a crucial step toward fault-tolerant quantum computers.
“Quantinuum’s trapped-ion quantum computing roadmap is designed around continuous upgrades, enabled by our flexible architecture and our precision control capabilities. This combination provides for outstanding, first-of-its-kind achievements that help accelerate the entire industry.”
Tony Uttley, president and COO of Quantinuum
According to the research paper’s co-author, David Hayes, this research brings quantum computing closer to the moment encoded circuits will outperform more basic functions.
The researchers used both the H1-1 and the H1-2 quantum computers, Powered by Honeywell, to compare the Five-Qubit error code and the Distance Three Color Code in these tests. The System Model H1 uses a quantum charged coupled device architecture and a trapped-ion design (QCCD).
“People have worked with error-corrected qubits before, but they haven’t reached this sort of special point where the encoded operation is working better than the primitive operation,” “The other thing that’s new here is that in other experiments, we’re doing the error correction while we’re doing the operations. An important next step for us is to get the error rate induced by the error correction itself down further.”
DAVID Hayes
Due to the machine’s architecture, quantum researchers can investigate a variety of quantum error codes than on other quantum hardware designs. In addition to its natural adaptability, the design has all-to-all connectivity. It is easier to transfer information along chains of ions without making numerous mistakes since all the qubits are related.
Quantum error correction is one of the key pillars of development for Quantinuum and other businesses in the quantum computing industry. All types of technology, including servers in data centers and space probes delivering communications back to Earth, require error correction. Errors hinder quantum computers from generating accurate output and subsequently overload the systems.
The researchers at Quantinuum are working toward achieving fault tolerance, which would allow errors to be suppressed to arbitrarily low levels. According to Natalie Brown, a research co-author and an experienced physicist at Quantinuum, most classical error correction principles fail with quantum computers because of the basic nature of quantum mechanics.
“It becomes very difficult to suppress noise to very small levels, and that becomes a problem in quantum computing,” “The most promising candidate was this quantum error correction, where we take the physical qubits, make a logical qubit.”
Natalie Brown
The most recent study, which builds on 2021 research with one logical qubit, shows the Quantinuum team’s advancement with quantum error correction and two logical qubits. For the experiments, the Five-Qubit Code and the Color Code were tested, with two error codes that were well known to quantum scientists. The Five-Qubit Code does not allow for a fault-tolerant transversal gate using only two logical qubits.
However, using a transversal CNOT gate, which is inherently fault-tolerant, is permitted under the Color Code. Researchers were able to break down an initially fault-intolerant logical gate operation into smaller, individually fault-tolerant portions using “pieceable” fault tolerance. The Five-Qubit Code tested on H1-2, while the Color Code tested on H1-1. H1-2 can use up to 12 qubits, and H1-1 can use up to 20.
Both computers use the same surface electrode ion trap to manage ytterbium ions as qubits. To evaluate the Five-Qubit Code and understand the effects of fault-tolerant design and circuit depth, the researchers conducted five tests using various combinations of circuit parts.
Due to the amount of CNOT operations needed, the team discovered that the additional circuitry, including boosting fault tolerance, had a detrimental effect on the overall integrity of the logical process. The researchers conducted seven tests to test these algorithms’ potential for fault tolerance. Due in part to its ability to utilize a transversal CNOT gate, the Color Code demonstrated significantly superior performance.
The Color Code was also beneficial to the State Preparation and Measurement circuits owing to the addition of fault-tolerant circuitry with a significant reduction of error rates: 99.94% for the logical qubits compared to 99.68% for the physical qubits. Since the logical CNOT is transversal and automatically fault resistant, this was the only extra circuitry needed to make the circuit fault resilient from end to end.
According to the researchers, “the Color Code will provide a better platform for computing than the qubit efficient five-qubit code” due to its comparatively inexpensive fault-tolerant hardware. The researchers discovered that the Five-Qubit Code would only be applicable in systems with far lower physical error rates than quantum computers now have. According to Hayes, the team’s next task will be to surpass the breakeven mark and present evidence of their labor.
“While there is still more work to be done to establish it truly, he added, “We are getting evidence that we’re really darn close to that point, but there’s a lot of work that needs to be done to actually prove it,” he said. “Just getting right there is not good enough, you have to actually get past it.”
DAVID Hayes
Another benefit of this experiment is that a new classical processor with improved capabilities crucial to scalable algorithmic decoders has been developed. The experiments’ decoders were built using WebAssembly after being partially coded in Rust (Wasm). Wasm was chosen because it offers a productive, secure, and portable classical language with callable functions from quantum programs.
“Our systems have very long coherence times, which is super advantageous when integrating in the classical compute real-time decision making,”
DAVID HAYES
The support for these features means that various scalable algorithmic decoders can be ergonomically implemented in high-level languages that compile to Wasm and called from quantum programs. Long coherence time and the ability to do mid-circuit measurements and reset qubits allow for real-time decision-making during the execution of the quantum circuit, another benefit of the trapped ion design.