Understanding the Difference Between Physical and Logical Qubits in Quantum Computing

1.    Quantum computing is still in its early stages, but as it advances, one important distinction you'll encounter is between physical qubits and logical qubits. Let's break these terms down simply and see why they're crucial for building reliable quantum computers.

What Are Physical Qubits?

2.    Physical qubits are the actual hardware used to store and manipulate quantum information. These could be atoms, ions, or superconducting circuits, depending on the quantum computing platform. However, these physical qubits are very fragile and prone to errors, caused by environmental noise, imperfections in the hardware, and other disturbances.

What Are Logical Qubits?

3.    Logical qubits are the error-corrected qubits that are stable and reliable enough to be used for quantum computations. They are not a direct representation of a single physical qubit. Instead, logical qubits are encoded across multiple physical qubits using quantum error correction techniques. These techniques help detect and correct errors, ensuring that the quantum computation can continue with high fidelity despite noisy environments.

Why Do We Need Logical Qubits?

4.    The key challenge in quantum computing is that physical qubits are inherently unreliable. To ensure accurate computations, we need logical qubits that are error-resilient. For example, a quantum computer might need 10,000 physical qubits to create 100 logical qubits, because quantum error correction demands several physical qubits to protect each logical qubit from errors.

Quantum Error Correction: What Is It?

5.    Quantum error correction involves encoding quantum information in such a way that errors in physical qubits can be detected and corrected without disrupting the overall computation. Essentially, it’s like having backup systems in place to fix issues when things go wrong.

Some major quantum error correction codes include:

• Shor’s Code – One of the first error-correcting codes, it uses 9 physical qubits to encode 1 logical qubit.

• Steane Code – This code is a 7-qubit code that’s part of the broader class of CSS (Calderbank-Shor-Steane) codes.

• Surface Codes – Widely studied and promising, surface codes can correct errors with relatively fewer physical qubits, making them a candidate for scalable quantum computers.

In a Nutshell

• Physical qubits are the raw units of quantum information, but they are error-prone.

• Logical qubits are the protected, error-corrected qubits used for actual computations.

• Quantum error correction codes like Shor’s Code, Steane Code, and Surface Codes are used to build logical qubits from physical qubits.

6.    As quantum computers scale, the number of physical qubits required will grow significantly to support a much smaller number of logical qubits. For instance, a quantum system might have 10,000 physical qubits but only 100 logical qubits capable of reliable computation. Understanding this difference is crucial to grasping how quantum computers will one day solve complex problems in fields like cryptography, materials science, and artificial intelligence.

7.    In short: more qubits don't always mean more computational power — it’s about how many logical qubits you can reliably create from your physical qubits.