November 2019

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2Vladimir Slepnev8mo(11/?)
Superdense coding.
Alice is told two classical bits of information and sends a qubit to Bob, who
can then recover the two classical bits. Again, it relies on Alice and Bob
sharing a prepared pair beforehand. It's the opposite of quantum teleportation,
where Alice sends two classical bits and Bob can recover a qubit.
First let's talk about bases. This is the usual basis: |00>, |01>, |10>, |11>.
This is the Bell basis: (|00> + |11>)/√2, (|00> - |11>)/√2, (|10> + |01>)/√2,
(|10> - |01>)/√2. Check for yourself that each two vectors are orthogonal to
each other. To "measure a state in a different basis" means to apply a rotation
from one basis into another, then measure. For example, if you have a state and
you know that it's one of the Bell basis states, you can figure out which one,
by rotating into the usual basis and measuring.
One cool thing about the Bell basis is that you can change any basis vector into
any other basis vector by operations on the first qubit only. For example,
rotating the first qubit by 90 degrees turns (|00> + |11>)/√2 into (|10> -
|01>)√2. Flipping the first qubit gives (|10> + |01>)√2, and so on.
Now superdense coding is easy. Alice and Bob start by sharing two halves of a
Bell state. Then depending on which two classical bits Alice needs to send, she
either leaves the state alone or rotates into one of the other three, by
operating only on her qubit. Then she sends her qubit to Bob, who now has both
qubits and can rotate them into the usual basis and measure them, recovering the
two classical bits.

1[comment deleted]8mo

18mo(12/?)
The uncertainty principle, or something like it.
When you measure a qubit cos(φ)|0> + sin(φ)|1>, the result has variance
(1-cos(4φ))/4. (I'm skipping over the trig calculations here and below.) If you
have a choice between either measuring the qubit, or rotating it by an angle ψ
and then measuring it, then the sum of variances of the two operations is at
least (1-|cos(2ψ)|)/2. In particular, if ψ=π/4, the sum of variances is at least
1/2.
So no matter what state a qubit is in, and how precisely you know its state,
there are two possible measurements such that the sum of variances of their
results is at least 1/2. The reason is that the bases corresponding to these
measurements are at an angle to each other, not aligned. Apparently in the real
world, an object's "position" and "momentum" are two such measurements, so
there's a limit on their joint precision.
You can also carry out one measurement and then the other, but that doesn't help
- after the first measurement you have variance in the second. Moreover, after
the second you have fresh variance in the first. This lets you get an infinite
stream of fair coinflips from a single qubit: start with |0>, rotate by π/4,
measure, rotate by π/4, measure...

18moIt seems useful to consider agents that reason in terms of an unobservable
ontology, and may have uncertainty over what this ontology is. In particular, in
Dialogic RL
[https://www.alignmentforum.org/posts/dPmmuaz9szk26BkmD/vanessa-kosoy-s-shortform#Wi65Ahs9abL63gPSe]
, the user's preferences are probably defined w.r.t. an ontology that is
unobservable by the AI (and probably unobservable by the user too) which the AI
has to learn (and the user is probably uncertain about emself). However,
onotlogies are more naturally thought of as objects in a category than as
elements in a set. The formalization of an "ontology" should probably be a POMDP
or a suitable Bayesian network. A POMDP involves an arbitrary set of states, so
it's not an element in a set, and the class of POMDPs can be naturally made into
a category. Therefore, there is need for defining the notion of a probability
measure over a category. Of course we can avoid this by enumerating the states,
considering the set of all possible POMDPs w.r.t. this enumeration and then
requiring the probability measure to be invariant w.r.t. state relabeling.
However, the category theoretic point of view seems more natural, so it might be
worth fleshing out.
Ordinary probably measures are defined on measurable spaces. So, first we need
to define the analogue of "measurable structure" (σ-algebra) for categories. Fix
a category C. Denote Meas the category of measurable spaces. A measurable
structure on C is then specified by providing a Grothendick fibration
[https://ncatlab.org/nlab/show/Grothendieck+fibration] B:MFC→Meas and an
equivalence E:B−1(pt)→C. Here, B−1(pt) stands for the essential fiber
[https://ncatlab.org/nlab/show/essential+fiber] of B over the one point space pt
∈Meas. The intended interpretation of MFC is, the category of families of
objects in C indexed by measurable spaces. The functor B is supposed to extract
the base (index space) of the family. We impose the following conditions on MFC
and B:
Given