 While classical information can be copied or cloned, the quantum ‘no cloning’ theorem (Dieks, 1982; Wootters and Zurek, 1982) asserts the impossibility of cloning an unknown quantum state. To see why, consider how we might construct a classical copying device. A NOT gate is a device that takes a bit as input and produces as output either a 1 if the input is 0, or a 0 if the input is 1. In other words, a NOT gate is a 1-bit gate that flips the input bit. A controlled-NOT gate, or CNOT gate, takes two bits as inputs, a control bit and a target bit, and flips the target bit if and only if the control bit is 1, while reproducing the control bit. So there are two inputs, the control and target, and two outputs: the control, and either the target or the flipped target, depending on the value of the control. A CNOT gate functions as a copying device for the control bit if the target bit is set to 0, because the output of the target bit is then a copy of the control bit: the input 00 produces output 00, and the input 10 produces output 11 (here the first bit is the control and the second bit is the target). Insofar as we can think of a measurement as simply a copying operation, a CNOT gate is the paradigm of a classical measuring device. Imagine Alice equipped with such a device, with input and output control and target wires, measuring the properties of an unknown classical world. The input control wire is a probe for the presence or absence of a property, represented by a 1 or a 0. The target wire functions as the pointer, which is initially set to 0. The output of the target is a 1 or a 0, depending on the presence or absence of the property.