View source for Quil (instruction set architecture)

'''Quil''' is a [[Quantum computing|quantum]] [[Instruction set architecture|instruction set architecture]] that first introduced a shared quantum/classical memory model. It was introduced by Robert Smith, Michael Curtis, and William Zeng in ''A Practical Quantum Instruction Set Architecture''.<ref name=":0">{{cite arXiv|last1=Smith|first1=Robert S.|last2=Curtis|first2=Michael J.|last3=Zeng|first3=William J.|date=2016-08-10|title=A Practical Quantum Instruction Set Architecture|eprint=1608.03355|class=quant-ph}}</ref>  Many [[Quantum algorithm|quantum algorithm]]s (including [[Quantum teleportation|quantum teleportation]], [[Quantum error correction|quantum error correction]], simulation,<ref>{{Cite journal|last1=McClean|first1=Jarrod R.|last2=Romero|first2=Jonathan|last3=Babbush|first3=Ryan|last4=Aspuru-Guzik|first4=Alán|date=2016-02-04|title=The theory of variational hybrid quantum-classical algorithms|arxiv=1509.04279|journal=New Journal of Physics|volume=18|issue=2|pages=023023|doi=10.1088/1367-2630/18/2/023023|issn=1367-2630|bibcode=2016NJPh...18b3023M|s2cid=92988541}}</ref><ref>{{cite arXiv|last=Rubin|first=Nicholas C.|date=2016-10-21|title=A Hybrid Classical/Quantum Approach for Large-Scale Studies of Quantum Systems with Density Matrix Embedding Theory|eprint=1610.06910|class=quant-ph}}</ref> and optimization algorithms<ref>{{cite arXiv|last1=Farhi|first1=Edward|last2=Goldstone|first2=Jeffrey|last3=Gutmann|first3=Sam|date=2014-11-14|title=A Quantum Approximate Optimization Algorithm|eprint=1411.4028|class=quant-ph}}</ref>) require a shared memory architecture. Quil is being developed for the superconducting quantum processors developed by [[Company:Rigetti Computing|Rigetti Computing]] through the Forest [[Cloud-based quantum computing|quantum programming API]].<ref>{{Cite web|url=https://spectrum.ieee.org/tech-talk/computing/software/rigetti-launches-fullstack-quantum-computing-service-and-quantum-ic-fab|title=Rigetti Launches Full-Stack Quantum Computing Service and Quantum IC Fab|website=IEEE Spectrum: Technology, Engineering, and Science News|date=26 June 2017|language=en|access-date=2017-07-06}}</ref><ref>{{Cite web|url=https://quantumcomputingreport.com/news/rigetti-quietly-releases-beta-of-forest-platform-for-quantum-programming-in-the-cloud/|title=Rigetti Quietly Releases Beta of Forest Platform for Quantum Programming in the Cloud {{!}} Quantum Computing Report|website=quantumcomputingreport.com|date=8 March 2017|language=en-US|access-date=2017-07-06}}</ref>  A [[Python (programming language)|Python]] library called <code>pyQuil</code> was introduced to develop Quil programs with higher level constructs. A Quil [[Front and back ends|backend]] is also supported by other quantum programming environments.<ref>{{Cite web|url=https://ornl-qci.github.io/xacc/accelerators/rigetti|title=XACC Rigetti Accelerator|website=ornl-qci.github.io|access-date=2017-07-06}}</ref><ref>{{Citation|last=Doiron|first=Nick|title=jsquil: Quantum computer instructions for JavaScript developers|date=2017-03-07|url=https://github.com/mapmeld/jsquil|access-date=2017-07-06}}</ref>
== Underlying quantum abstract machine ==
In the paper presented by Smith, Curtis and Zeng, Quil specifies the [[Instruction set architecture|instruction set]] for a Quantum Abstract Machine (QAM,) akin to a Turing machine, yet more practical for accomplishing "real-world" tasks.<ref name=":0" /> The state of the QAM can be represented as a 6-[[Tuple|tuple]] <math>(|\Psi\rangle, C, G, G', P, \kappa)</math> where:

* <math>|\Psi\rangle</math> is the (quantum) state of a fixed but [[Philosophy:Arbitrariness|arbitrary]] number of [[Qubit|qubit]]s <math>N_q</math> indexed using a 0-based indexing.
* <math>C</math> is a classical [[Computer memory|memory]] of a number <math>N_c</math> of classical [[Bit|bit]]s indexed using a 0-based indexing.
* <math>G</math> a fixed but arbitrary list of static gates ([[Quantum logic gate|quantum gates]] that do not depend on parameters, like the Hadamard gate.)
* <math>G'</math> a fixed but arbitrary list of parametric gates (gates that depend on a number of [[Complex number|complex]] parameters like the phase shift gate that requires an angle [[Parameter (computer programming)|parameter]] to be completely defined.)
* <math>P</math> a sequence of Quil instructions to be executed, representing the program. The length of <math>P</math> is denoted by <math>|P|</math>.
* <math>\kappa</math> an integer [[Program counter|program counter]] pointing to the next instruction to be executed. <math>\kappa</math> always starts at 0 (pointing to the <math>0^{th}</math> instruction) and ends at <math>|P|</math> indicating program halting (note that the last instruction has the index <math>|P| - 1</math>.) The program counter is incremented after every instruction, except for special [[Control flow|control flow]] instructions (conditional and unconditional jumps, and the special <code>HALT</code> instruction that halts the program by setting <math>\kappa</math> to <math>|P|</math>.

The [[Semantics (computer science)|semantics]] of the QAM are defined using [[Tensor product|tensor product]]s of [[Hilbert space]]s and the [[Linear map|linear map]]s between them.<ref name=":0" />

== Features ==
Quil has support for defining possibly parametrized gates in matrix form (the language does not include a way to verify that the matrices are [[Unitary matrix|unitary]], which is a necessary condition for the physical realizability of the defined gate) and their application on qubits. The language also supports [[Macro (computer science)|macro]]-like definitions of possibly parametrized [[Quantum circuit|quantum circuit]]s and their expansion, qubit [[Physics:Measurement in quantum mechanics|measurement]] and recording of the outcome in classical memory, synchronization with classical computers with the <code>WAIT</code> instruction which pauses the execution of a Quil program until a classical program has ended its execution, conditional and unconditional [[Branch (computer science)|branching]], pragma support, as well as inclusion of files for use as [[Library (computing)|libraries]] (a standard set of gates is provided as one of the libraries.)

== Rigetti QVM ==
Rigetti Computing developed a Quantum [[Virtual machine|Virtual Machine]] in [[Common Lisp]] that simulates the defined Quantum Abstract Machine on a classical computer and is capable of the [[Parsing|parsing]] and execution of Quil programs with possibly remote execution via HTTP.<ref>{{Citation|title=The @rigetti high-performance quantum virtual machine.: rigetti/qvm|date=2019-04-26|url=https://github.com/rigetti/qvm|publisher=Rigetti Computing|access-date=2019-04-28}}</ref>

== Example ==  
The following example demonstrates the classical control flow needed to do [[Quantum teleportation|quantum teleportation]] of the [[Qubit|qubit]] in [[Processor register|register]] 2 to register 1:<ref>{{cite book |last1=Nielsen |first1=Michael A. |last2=Chuang |first2=Isaac L. |title=Quantum Computation and Quantum Information |date=2000 |publisher=Cambridge University Press |isbn=978-0-521-63503-5 |page=27}}</ref><ref>{{cite web |last1=Computing |first1=Rigetti |title=pyQuil Documentation|date=28 May 2019 |url=https://media.readthedocs.org/pdf/pyquil/latest/pyquil.pdf |website=pyQuil Documentaion |access-date=6 June 2019}}</ref>
<source lang="asm">
# Declare classical memory
DECLARE ro BIT[2]
# Create Bell Pair
H 0
CNOT 0 1
# Teleport
CNOT 2 0
H 2
MEASURE 2 ro[0]
MEASURE 0 ro[1]
# Classically communicate measurements
JUMP-UNLESS @SKIP ro[1]
X 1
LABEL @SKIP
JUMP-UNLESS @END ro[0]
Z 1
LABEL @END
</source>Examples of the implementations of the [[Quantum Fourier transform|quantum fourier transform]] and the variational quantum Eigensolver are given in the paper.

== References == 
{{reflist}}

==External links==
* {{Github|quil-lang/quil}}  GitHub repository

{{Quantum computing}}

[[Category:Quantum computing]]
[[Category:Instruction set architectures]]
[[Category:Quantum programming]]

{{Sourceattribution|Quil (instruction set architecture)}}