State Representation for QVM

[sophiaponte]

This PR decouples the notion of "state representation" from the qvm. The QVM currently supports the two simulation modes of quantum states: pure states, or wavefunctions (amplitudes) and density matrices. Before this PR, density matrix state simulations were restricted to only the DENSITY-QVM. This PR implementsPURE-STATE and DENSITY-MATRIX-STATE classes and modifies the existing qvm classes to be compatible with both state types. In large part, this was done by replacing the AMPLITUDES slot of the different qvms with a STATE slot, and modifying the TRANSITION, MEASURE, and APPLY-STATE functions to specialize on the STATE of the qvm to address both pure state and density matrix states. Additionally, I modified the QVMs to work with user defined superoperators, which are written in to quil programs via the ADD-KRAUS pragma. Previously, only the density-matrix-qvm accepted user defined superoperators.

Since this PR touches a ton of different files, I'll list here the general changes I made to each file.

• state-representation.lisp [created] : This file contains the DENSITY-MATRIX-STATE and PURE-STATE classes.

• qvm.lisp [modified] : I added an abstract class BASE-QVM to serve as the base class for all qvms. The slots of this class include the number of qubits, state, compiled program, and superoperator definitions. I modified the pure-state-qvm to operate on a PURE-STATE state. I also modified the PURE-STATE-QVM to be compatible with user defined superoperator definitions (they are applied stochastically in the same way that "noisy" kraus operators are).

• mixed-state-qvm.lisp [created] : This file implements a qvm that can operate on both pure and mixed quantum states. This qvm has a DENSITY-MATRIX-STATE state. This qvm can also be used with user defined superoperator definitions.

• apply-gate.lisp [modified] : I modified the APPLY-GATE generic method to specialize on the state as well as the gate type. This way, the TRANSITION methods of the different qvm's don't need to do additional logic determine which type of state is being dealt with. The APPLY-NOISE (kraus operator) functions were also changed to specialize on the state.

• transition.lisp [modified] : I made a instr:gate-application TRANSITION method for BASE-QVM. This method is used by pure-state-qvm, mixed-state-qvm, channel-qvm and basic-noise-qvm. The state specialization in apply-gate greatly simplified the transition logic for the different qvms, so they now either just use the base-qvm gate-application transition or implement :before / :after methods.

• measurement.lisp [modified] : I created a measure and measure-all methods for the additional classes, and modified some of the measurement functionality to specialize on state type.

• channel-qvm.lisp + basic-noise-qvm.lisp [modified]: These QVMs were modified to take either PURE-STATEs or DENSITY-MATRIX-STATEs as arguments. They also now accept user defined superoperator definitions.

• density-qvm.lisp [modified] : Made DENSITY-QVM compatible with DENSITY-MATRIX-STATE. The functionality of DENSITY-QVM is replaced by MIXED-STATE-QVM and the noisy qvms.

• noisy-qvm.lisp + depolarizing-noise.lisp [modified] : Made the NOISY-QVM and DEPOLARIZING-QVM compatible with DENSITY-MATRIX-STATE.

[sophiaponte]

what I just changed:

• mostly style stuff
• only create a cflonum-vector for trial-amplitudes if necessary, to avoid extra memory usage.

a few more smol TODOs:

• M-x whitespace and M-x indent everything [DONE]
• I'm a still unsure about what to do w/regards to state allocation for trial-amplitudes on pure states, but this should be a minor change regardless [follow up PR]
• qvm-app-ng related generic functions that @appleby suggested + make-xx-qvms for all the qvms [follow up PR]

[appleby]

qvm-app-ng related generic functions that @appleby suggested

If you haven't already implemented it, maybe hold off on this one. Maybe this should just continue to live in qvm-app-ng. In any case, should be a minimal change and can be done in a follow on PR.

[sophiaponte]

Tests are failing when loading qvm-tests because #:CL-QUIL does not match version 1.13.1, required by #<SYSTEM "qvm-examples"> I am assuming I should change the CL-QUIL version in the Dockerfile, as the failure happens after
$ docker run --rm --entrypoint=make rigetti/qvm:${CI_COMMIT_SHORT_SHA} test-lib DISABLE_SHARED_MEMORY_QVM_TESTS=1 RIGETTI_LISP_LIBRARY_HOME=/src

Update: changing the Dockerfile quilc version to 1.13.1 fixed this.