# Why can a qbit be used as a classical bit if information about the measurement axis is needed?

If Alice wants to send one bit of classical information she can use a qbit. Then Bob needs to know which axis to measure to get the information. This needs an extra agreement between Alice and Bob before starting communication. I can figure out why up or down agreement has no importance here since 0 and 1 are just different and need the same level of agreement (two choices available, one for 0 another for 1).

Using qbits, an extra agreement is needed before sending information. A lamp could be on or off without any need for more agreement (which axis in qbit is an extra agreement).

Has this extra contract any importance?

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In your lamp example, additional information is needed between Alice and Bob for them to agree on how to interpret the lamp. If the lamp is on is it a 1 or a zero? They must agree beforehand. – Brandon Enright May 27 '13 at 1:46
Also, you should re-word your question to avoid the use of "contract" as that makes your question rather unclear. I'd reword it for you but I'm concerned about changing the meaning of your question. – Brandon Enright May 27 '13 at 1:47
@BrandonEnright: That contract (on or off for lamp) also needed for up and down. But extra contract is needed for which axis measurement before such contract. – Xaqron May 27 '13 at 11:36
If you're sending bits over radio waves, you need to agree on a frequency, and on AM or FM modulation. Does this mean that radio communication is somehow less useful than shining a lantern out of the steeple of the Old North Church (which still needed an agreement: "one if by land and two if by sea")? – Peter Shor May 27 '13 at 18:16
@PeterShor: My question is edited. I'm new to QM. In terms of metadata (protocol) qbit is more expensive than bit. This fact signals inefficiency is usage and that was the base of my question. – Xaqron May 28 '13 at 6:47