The bottleneck in building large-scale quantum computers refers to the significant challenges and obstacles that need to be overcome to achieve reliable and scalable quantum computing systems. These challenges include:
Quantum Decoherence: Quantum systems are highly susceptible to noise and decoherence, which refers to the loss of quantum information due to interactions with the environment. Maintaining the fragile quantum states over long periods, known as quantum coherence, is a major challenge. Decoherence limits the time during which quantum operations can be performed, impacting the accuracy and reliability of computations.
Quantum Error Correction: Quantum error correction is crucial to protect quantum information from errors caused by decoherence and other sources of noise. However, implementing effective error correction schemes is challenging due to the need for additional qubits and complex error correction protocols. The overhead of qubits required for error correction can quickly become unmanageable as the system scales up.
Scalability: Building large-scale quantum computers with a significantly high number of qubits is a complex engineering task. Currently, quantum computers are limited to a few hundred qubits, and increasing this number while maintaining low error rates is a significant challenge. Scaling up quantum systems requires addressing issues like qubit connectivity, reducing crosstalk between qubits, and improving the overall system's stability.
Control and Readout: Manipulating and measuring individual qubits accurately is crucial for quantum computation. As the number of qubits increases, the control and readout operations become increasingly complex and prone to errors. Ensuring precise control and reliable readout for large numbers of qubits poses significant challenges.
It is difficult to predict definitively if and when these bottlenecks will be fully resolved. However, researchers and engineers are actively working on addressing these challenges through various approaches, including improved qubit designs, error correction techniques, and better control and measurement methods. As our understanding of quantum systems advances and technology progresses, it is possible that these bottlenecks will be mitigated, leading to the realization of large-scale, fault-tolerant quantum computers.