Why Are Logical Qubits Important?

Insider Brief

• Logical qubits enhance stability by combining multiple physical qubits, providing reliable error correction for long-running quantum computations.
• Recent advances have shown that scientist are making significant progress in using logical qubit techniques to build scalable quantum systems.
• While logical qubits require multiple physical qubits, they increase overall computational power, enabling more complex and error-resistant quantum applications.

Just as most people are becoming more familiar with qubits — a unit information that’s a quantum take on classical bits — suddenly the term logical qubit appears. And it seems even more important than regular qubits.

So what are logical qubits — and why are logical qubits so important? We’ll offer a look at this new quantum buzzword that should be part of the qauntum vocabulary real soon.

Logical qubits are destined to play a pivotal role in overcoming one of the most significant barriers to achieving large-scale, reliable quantum systems: error-prone physical qubits. While today’s Noisy Intermediate-Scale Quantum (NISQ) machines are an impressive leap toward this new quantum era of computing, they struggle with maintaining coherence in qubits due to noise and interference. This is where logical qubits come into play, offering a more stable and scalable solution for real-world quantum applications.

The Problem with Physical Qubits

Physical qubits, which serve as the building blocks for quantum computers, are notoriously delicate. They are susceptible to errors caused by environmental interference, often called “noise.” In classical computing, bits are stable and either 0 or 1, but qubits exist in a superposition, meaning they can be in simultaneous probabilistic states. If this state sounds vulnerable and delicate, it is. While the unique property of superposition allows quantum computers to theoretically process vast amounts of information in parallel, it also makes qubits prone to errors, which grow exponentially as more qubits are added.

As simply put by Microsoft in their work with Quantinuum on logical qubits, “The physical qubits are too noisy and error-prone, making the machines impractical for real-world applications.”

Enter Logical Qubits

Logical qubits solve this problem by combining multiple physical qubits into a single, more stable unit. This process, called quantum error correction, allows logical qubits to detect and correct errors without collapsing the quantum state. Essentially, several physical qubits work together to maintain the integrity of the information stored in a single logical qubit, which remains coherent for longer computational tasks.

In April 2024, Microsoft made a significant leap in this technique by announcing that their Azure Quantum platform had demonstrated the most reliable logical qubits, with an error rate 800 times better than physical qubits. This achievement marked a critical step in making quantum computers more reliable and scalable for real-world applications, from chemistry simulations to AI-enhanced problem-solving. They improved on this advance with news of recent work, again in collaboration with Quantinuum, to apply the improved qubit virtualization system to create and entangle 12 highly reliable logical qubits. (Read more in-depth about this here and, for a more technical look, go here.)

According to the companies, this is the largest number of entangled logical qubits on record, which signals significant progress toward building scalable quantum machines capable of solving classically intractable problems.

For instance, Microsoft recently demonstrated the first end-to-end chemistry simulation that combined logical quantum computation with cloud-based high-performance computing (HPC) and AI. This is a practical example of how logical qubits can be applied to real-world challenges, such as developing new materials or finding sustainable energy solutions.

Do Logical Qubits Reduce Computational Power?

While logical qubits offer stability, they do require several physical qubits to form a single reliable unit. This might raise concerns that using logical qubits reduces the overall computational power of the machine — in other words, they might seem to take away from quantum’s computational potential. However, the opposite is true. The reliability and error correction provided by logical qubits enhance the system’s effective computational power, enabling quantum computers to run complex calculations without errors—something that physical qubits alone cannot achieve.

In essence, although more physical qubits are needed to create logical ones, the result is a system that performs better and is far more capable of handling real-world applications. As quantum computers grow in size, logical qubits will be critical if we hope to use the devices for longer, error-free computations.

The Path to Reliable Quantum Computing

Logical qubits represent the future of quantum computing because they provide a pathway to building more reliable machines capable of solving practical problems. While the field of quantum computing is still in its infancy, the ability to create and scale logical qubits signals a turning point in the transition from research to real-world applications.

Want to know more about why logical qubits are important — check out logical qubits in action in this study of the use of quantum techniques in chemistry investigations.