What is the argument that practical quantum computers cannot be built?

[quantumadmin]

The argument that practical quantum computers cannot be built is often based on the significant technical challenges and limitations associated with quantum computing hardware, as well as the fundamental properties of quantum mechanics. While researchers are making rapid progress in the field of quantum computing, there are several key factors that skeptics and critics often highlight:

Decoherence and Noise: Quantum systems are inherently susceptible to environmental noise and decoherence, which can lead to the loss of quantum information and errors in computations. Building and maintaining the necessary level of coherence and isolation for qubits is a daunting challenge.

Error Correction Complexity: Implementing fault-tolerant error correction schemes to combat noise and errors is technically demanding and requires a large number of physical qubits to encode a single logical qubit. The overhead introduced by error correction can outweigh the benefits of quantum speed-up for certain problems.

Quantum Volume and Scalability: The metric of "quantum volume" indicates the computational power of a quantum processor. Achieving high quantum volume is crucial for demonstrating that a quantum computer is capable of outperforming classical computers. As quantum processors scale, maintaining high quantum volume becomes increasingly challenging.

Limited Gate Fidelity: Quantum gates need to be executed with high fidelity to perform accurate computations. Achieving high gate fidelity is a significant engineering challenge, especially as the number of qubits and gates grows.

Physical Constraints: Building and operating qubits with the required properties (long coherence times, controllable interactions, low error rates) is complex and may be subject to physical constraints that limit their performance and scalability.

Quantum Error Suppression: While error correction techniques exist, they typically require many additional qubits and complex operations. Achieving sufficiently low error rates remains a hurdle for large-scale quantum computation.

The No-Cloning Theorem: The no-cloning theorem states that an arbitrary unknown quantum state cannot be perfectly copied. This property limits the ability to redundantly encode and protect quantum information.

Computational Complexity: Even if a practical quantum computer is built, it is not guaranteed to provide exponential speed-up for all problems. Identifying problems where quantum algorithms offer a significant advantage remains an ongoing challenge.

It's important to note that while there are significant challenges, the field of quantum computing is still relatively young, and researchers are actively working to address these issues. Many believe that these challenges can be overcome through continued technological innovation, advancements in error correction techniques, and improved control and manipulation of quantum states.