What are some of the challenges in building scalable quantum computers?

Building scalable quantum computers faces several significant technical challenges. The first major hurdle is maintaining qubit stability and minimizing errors. Qubits, the basic units of quantum information, are extremely sensitive to environmental noise like temperature fluctuations or electromagnetic interference. This sensitivity leads to “decoherence,” where qubits lose their quantum state before computations can finish. For example, superconducting qubits—used by companies like IBM and Google—typically maintain coherence for mere microseconds. Even small errors accumulate quickly, and quantum error correction requires a large overhead: thousands of physical qubits might be needed to create a single reliable “logical” qubit. Current systems with hundreds of qubits are far from this threshold, making error management a critical barrier.

A second challenge lies in scaling hardware and control systems. Adding more qubits increases complexity in wiring, cooling, and signal routing. Superconducting qubits, for instance, require precise microwave control pulses and must operate near absolute zero (-273°C), which demands expensive dilution refrigerators. As qubit counts grow, fitting more control lines into limited cooling space becomes impractical. Trapped-ion qubits, an alternative approach, offer longer coherence times but face their own scaling issues: manipulating ions with lasers becomes harder as the number of ions increases. These engineering constraints make it difficult to balance qubit count, control accuracy, and physical footprint.

Finally, developing software and algorithms that work with imperfect hardware complicates scalability. Quantum algorithms like Shor’s factoring algorithm assume error-free qubits, but real-world devices have high error rates. Developers must design “noise-aware” algorithms or hybrid approaches that offload partial computations to classical systems. For example, variational quantum algorithms used in chemistry simulations require iterative optimization between quantum and classical hardware, which introduces latency and communication bottlenecks. Additionally, programming tools and compilers must optimize for qubit connectivity and gate fidelity, which vary across hardware platforms. Until these software and hardware challenges align, achieving practical scalability remains out of reach.