What are the challenges in building quantum computers with error rates below the fault-tolerance threshold?

Building quantum computers with error rates below the fault-tolerance threshold faces several challenges. One of the main challenges is the presence of noise and errors in quantum systems. Quantum systems are highly sensitive to environmental disturbances, such as temperature fluctuations and electromagnetic radiation, which can introduce errors and decoherence. These errors can disrupt the delicate quantum states required for quantum computation.

Another challenge is the need for qubits with long coherence times. Coherence time refers to the duration for which a qubit can maintain its quantum state before decoherence occurs. To perform complex computations, quantum computers require qubits with coherence times long enough to perform multiple operations. However, maintaining coherence for extended periods is challenging due to interactions with the environment.

Furthermore, scaling up the number of qubits while maintaining low error rates is a significant challenge. Quantum error correction codes can help mitigate errors, but they require additional qubits and operations, increasing the complexity of the system. As the number of qubits increases, so does the likelihood of errors, making it difficult to achieve error rates below the fault-tolerance threshold.

Additionally, controlling and manipulating qubits accurately is a challenge. Quantum systems are highly sensitive, and precise control is necessary to perform operations on individual qubits and entangle them. Achieving this level of control becomes increasingly difficult as the number of qubits grows.

Lastly, the development of suitable error correction techniques is crucial. Error correction is essential to mitigate errors and maintain the integrity of quantum computations. However, designing efficient error correction codes that can handle a large number of qubits and correct errors without introducing additional errors is a complex task.

Overall, building quantum computers with error rates below the fault-tolerance threshold requires addressing challenges related to noise and errors, coherence times, scalability, control and manipulation of qubits, and the development of effective error correction techniques. Overcoming these challenges is crucial for the realization of practical and reliable quantum computers.