If your quantum circuit outputs keep changing, you're not making a mistake—you're experiencing the fundamental nature of quantum mechanics. Unlike classical circuits that yield deterministic results, quantum measurements collapse superpositions probabilistically. Here's what's really happening under the hood.
The variation stems from three primary sources: quantum randomness, hardware noise, and sampling limitations. When your circuit contains gates that create superposition (like Hadamard gates), the final measurement samples from a probability distribution. Running the circuit 1000 times on ideal hardware would show statistical patterns, but individual shots will differ—this isn't error, it's by design.
Noise compounds the variation. Real quantum devices suffer from decoherence and imperfect gate operations that distort the intended probability distribution. A CNOT gate with 98% fidelity doesn't just fail 2% of the time—it subtly corrupts the entire quantum state. Thermal fluctuations in superconducting qubits or laser instability in trapped ion systems introduce additional randomness between runs.
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Quantum Expert Insights