that the read/write head of the machine is quantum mechanical with its location given by an internal state of the machine (this head location could be thought of as being specified on a separate tape). But then the second problem arises. Assuming a fixed speed of transmission, the receiver can never be sure when a particular signal, say the seventh signal, arrives. This is because the [*total*]{} length of the signals up to that point (from the first to seventh signals) is a quantum mechanical variable (i.e., it is in a superposition of many possible values). Therefore, Bob generally has a hard time in deciding when would be the correct instant to decode the seventh signal in an instantaneous quantum code.

Let us suppose that the above problem can be solved. For example, Bob may wait “long enough” before performing any measurements. We argue that there remains a third difficulty which is fatal for [*instantaneous*]{} quantum codes—that the head location of the encoder is [*entangled*]{} with the total length of the signals. If the decoder consumes the quantum signal (i.e., performs measurements on the signals) before the encoding is completed, the record of the total length of the signals in 