Bitcoin is not a traditional note based currency but similar to commodities like gold, silver etc , some would call it a cryptocommodity which is indeed intrinsically accountable and can be proven to exist if required.
Other currencies like notes are impossible to trace intrinsically and are essentially contract papers whereas a bitcoin stores the entire transaction history(under the larger bitcoin framework).
Islam requires accountability of the commodity spent and you just cant duplicate currency notes to fulfill demand like how govts do with paper currency. Inbuilt Accountability of bitcoin allows the bitcoin commodity to be traceable and thus cant be duplicated on demand.
Far from haram I would say that bitcoin is the currency best suited according to the laws of Islam which required the currency to have intrinsic value not just a signed piece of paper whose value can be lost due to various circumstances.
The best thing about bitcoin is that it is not controlled by central banks and the bitcoin you possess is the actual "coin" which was virtually mined. So it does have an intrinsic value which is nothing but proportional to the difficulty to mine it. This is similar to how difficult it is to mine real gold.
Also, real currency which Islam allows is deflationary in nature that i.e its value always rises e.g: Gold, the value of gold increases as the resource is limited and is being used up. Whereas in the paper currency its inflationary as more notes a printed to keep up with the demand thus reducing the value per note. The total number of Bitcoins also are limited to a few million hence its value will always rise.
Thus I would even go on to say that using bitcoin is more halal then using currency notes supplied by the reserve banks.
