card, your significant other (not to
mention anyone else who gets access
to your bank statements) will know
exactly how much money you gambled.
What you really want is some kind of
money that you can spend without
leaving a trace and that you can carry as
much of as you want without weighing
down your pockets.
IllustratIon by spooky pooka at Début art
This kind of money would be digital:
you could transmit it and fit as much of
it as you want on some small handheld
computer. It would be self-contained,
so you could pay someone without any
third party being involved. And it would
be cryptographically secure: attackers could never produce a counterfeit
bill that passes as real money even with
extraordinary resources at their disposal.
The reason we do not have this kind
of money today is not for lack of try-
ing. Any digital piece of information
that can be sent over a communication
channel can be copied. This makes
digital money seem impossible: if you
had one hundred dollars on your com-
puter, you could back up your com-
puter, spend the money, restore your
computer from the backup, and spend
your money again.
If you look closely enough, everything
is made out of subatomic particles, and
these particles obey the laws of quantum mechanics. Quantum mechanical
systems store information in a way that
is dramatically different from classical
(that is, non-quantum) systems.
One of the simplest examples of a
quantum system is a single electron.
Electrons spin, and their spin can be
characterized by a three-dimensional
vector.a This vector, like any three-dimensional vector, has three components, Sx, Sy, and Sz. It is possible to do
an experiment to measure the vertical
component Sz of an electron’s spin,
but if you do the experiment, you will
discover something strange: Sz can
only take on two values, + 1 and − 1.
a The vector represents the angular momentum
of the electron, but its physical interpretation
is not important for this discussion.