What are the main challenges in building practical quantum computers?

Building practical quantum computers faces several major challenges. First, quantum bits (qubits) are extremely fragile and lose their quantum properties through 'decoherence' within microseconds, requiring near-absolute-zero temperatures. Second, error rates remain high—current quantum computers have error rates around 0.1-1%, while useful calculations need rates below 0.001%. Third, scaling is difficult; while we have 100-1000 qubit prototypes, practical applications need millions of qubits. Finally, programming quantum computers requires entirely new algorithms and expertise. Despite these obstacles, companies like IBM and Google continue advancing quantum technology, though widespread adoption for cryptocurrency applications remains years away.