...which sounds a LOT more complex than it is. I don't care about
simulation--there are great Python-based packages for that. I don't even
care about verification or optimization yet; I just want to generate a data
structure that's exportable in, say, EDN that I can write a parser for in
Python.
So... I'm dabbling in quantum computing, and the way these things work,
loosely, is that you have a fairly small address space (Google's
Bristlecone has 72 bits, and that's HUGE), and you basically apply "gates"
(atomic operations) to one, two, or occasionally three bits at a time.
That's basically it--you can have larger operations, but at the end of they
day they get broken down into 1/2/3-qubit gates.
Right now the available kits use python to create these, and it just feels
clunky... not even like writing assembler, but like writing Python code to
write assembler, and there's no separation of data structure from execution
environment. I figure at the end of the day you're just making lists of
operations; if only there was a language suited for list processing! :)
Example: here's Python code for a "bell pair" (when measured, both bits are
either 0 or 1), from ProjectQ, one of the Python libraries:
# The H operator is defined as an object elsewhere
# because ProjectQ mixes the construction of the quantum
# code with its execution:
# create a main compiler engine
eng = MainEngine()
# allocate one qubit
b1 = eng.allocate_qubit()
b2 = eng.allocate_qubit()
# put it in superposition
H | b1
CNOT | (b1, b2)
# measure
Measure | q1
Measure | q2
eng.flush()
# print the result:
print("Measured: {}{}".format(int(q1),int(q2)))
What I'd like to see is something more like
;; declare H to be a 1-bit gate and CNOT to be a 2-bit gate
;; note these don't have to DO anything here as long as they can
;; be interpreted in the Python backends
(defgate H 1)
(defgate CNOT 2)
(defq-fn bell-pair [b1 b2]
(H b1)
(CNOT b1 b2))
(defq-pgm run-bell-pair []
(let [b1 (allocate-bit 1) ;; variable of one qubit
b2 (allocate-bit 1)] ;; same
(bell-pair b1 b2)
(measure b1 b2))
...where gates behave like functions (so you can use partial, map, etc,
like (map (partial CNOT 1) [b1 b2 b3])
And doing say (bell-pair 1 2) would spit out EDN
((:H 1)
(:CNOT 1 2))
and (run-bell-pair) might parse everything, include extra data about the
necessary machine, and reassign variables to allocated bits
{:machine {:bitsize 2} ; calculated because only two bits were allocated in
all the subsidiary code
:code ((:block {:name bell-pair
:code (:H 0) (:CNOT 0 1) (:measure 0 1)}))}
That sort of thing (the ":block" idea is mostly to delineate "where you
are" in the output program, and to group operations for display). Then I
could write a parser in Python for the simulators and such (or rewrite the
Clojure code for Hylang, which is its own exercise, but I could use the
Python libs directly then). There are other things I'd like (allocate
multi-bit variables, and apply gates to individual bits of those, like to
have the idea of reusing bits once they're measured and done with, that
sort of thing) but this is a start.
On the surface, this looks really easy--you're just generating and
traversing data structures--but it's funny how hard I'm finding it because
it's key that you can execute proper computational code within (to do
things like determine angles for parameterized gates, apply gates in
complex ways to sequences, etc).
I created a defgate macro that generated function stubs for gates that
created EDN for gates; that was easy. But properly doing the "defq-fn"
macro is eluding me. I want it to return a list of the gates' data
structures, but if I just eval the given form, I'll only get the last
result and (do...) to get a sequence is tricky because we're mixing gates
and clojure code. I tried a solution where the generated "gate" functions
would call an "emit" function which appended to a module-level variable,
and that soooorta worked, but it's not elegant (and not threadsafe).
Ideally the defq-fn macro would create a function that returns a sequence
of gates, one for every "gate function" executed in the execution of the
function, but no other output (I may be making a specious distinction
between defq-fn and defq-pgm, since I'd like to be able to allocate and
"free" bits within qfns).
Aaaanyway, long story. But what's the best/correct approach here? I'm new
to macros, but game to flail wildly. Ideally any solution would be
sorta-portable to Hylang (I'm doing the same exercise there and running
into even stranger problems as for example "map" just returns a "map
object" rather than simply being lazily evaluated, but I'm trying).
And yeah, for "real" QC applications there's a lot missing here, but one
has to start somewhere :)
Vic
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