Quantum computers interact with the environment through decoherence, resulting in information from a computation degrading over time. This introduces errors into a computation. In Chapter 3, Quantum States, Quantum Registers, and Measurement, we discussed ways to quantify this information loss; T₁ helps quantify how quickly the qubits on a given hardware experience energy loss due to environment interaction (energy loss results in a change in frequency, causing decoherence), which can cause a bit flip, and T₂ helps quantifyhow quickly the qubits experience a phase difference due to interaction with the environment, again a cause of decoherence. The bigger T₁ and T₂, the more robust a quantum computation will be to errors.

 IBM QX hardware and other quantum computing companies try to raise T₁ and, T₂ but at the present moment, and for the foreseeable future, T₁ and, T₂ are so low that every practical computation is likely to contain errors. These errors can come in the form...