apaszke / qvm

The @rigetti high-performance quantum virtual machine.

Rigetti Quantum Virtual Machine

This directory contains two projects. The first, qvm, is a classical implementation of the Quantum Abstract Machine (QAM), called a "Quantum Virtual Machine" (QVM). The second, qvm-app, is the application interface to interacting with the QVM, either directly through the qvm binary or via its server interface.

The definition of the QAM was developed at Rigetti in a paper titled A Practical Quantum Instruction Set Architecture.

QVM, the library

The QVM library is contained within ./src/, and provides the implementation of the Quantum Abstract Machine. It evaluates Quil programs (parsed and compiled by quilc) on a virtual machine that can model various characteristics of (though without needing access to) a true quantum computer.

The library is released under the Apache license 2.0.

Usage

The QVM library is best loaded using Quicklisp. Please read and follow the instructions in lisp-setup.md#install-quicklisp to get Quicklisp installed. Pay particular attention to the section "Telling Quicklisp Where Your Code Is".

Note: qvm is not yet available through Quicklisp's distribution mechanism, and must be installed manually.

Download both this repository and quilc into the ql:*local-project-directories* location. If all is correct, the qvm library can be loaded with

$ sbcl

* (ql:quickload :qvm)
(:QVM)

QVM objects are created with (qvm:make-qvm n) where n is the number of qubits the QVM should support; a program can then be loaded into the QVM object with (qvm:load-program *qvm* *program*) where *qvm* is a QVM object and *program* is a cl-quil:parsed-program object.

Alternatively, the qvm:run-program function will handle QVM object creation. For example,

* (setq *qvm* (qvm:run-program 2 (cl-quil:parse-quil-string "H 0")))

creates a 2-qubit QVM object and on it runs the Quil program H 0.

The qubit amplitudes can be inspected

* (qvm::amplitudes *qvm*)
#(#C(0.7071067811865475d0 0.0d0) #C(0.7071067811865475d0 0.0d0)
  #C(0.0d0 0.0d0) #C(0.0d0 0.0d0))

which shows, as expected, that H 0 has put qubit-0 (the first two complex numbers above) into an equal superposition of states |0> and |1>.

Measurement of a quantum state causes it to collapse into one of its basis states (|0> or |1>). This can be simulated with

* (qvm:measure-all *qvm*)
#<PURE-STATE-QVM {1004039753}>
(0 0)

Inspecting the QVM object's state shows that this effect mutates the information stored on the QVM; i.e. the previous state information is lost

* (qvm::amplitudes *qvm*)
#(#C(1.0d0 0.0d0) #C(0.0d0 0.0d0)
  #C(0.0d0 0.0d0) #C(0.0d0 0.0d0))

Qubit zero's state has collapsed into the state |0>. Repeating this process (from creating the QVM object to measuring qubits) would show that both states would each come up with probability 0.5.

* (loop :with results := (vector 0 0)
        :with program := (cl-quil:parse-quil-string "H 0")
        :repeat 100
        :for (qvm state) := (multiple-value-list (qvm:measure (qvm:run-program 1 program) 0))
        :do (incf (aref results state))
        :finally (return results))
#(54 46)

Examples

The QVM comes with some example code to illustrate usage of the QVM. The example code can be found under ./examples/. To run the example code, first load qvm-examples

* (ql:quickload :qvm-examples)
(:QVM-EXAMPLES)

The function bit-reversal-circuit takes a list of qubit indices and returns a list of instructions that will reverse the qubit amplitudes in "bit-reversal order" (e.g., the coefficient of |1110> gets mapped to |0111>): that the amplitudes get reversed

(qvm-examples:bit-reversal-circuit '(1 2 3 4))
(#<SWAP 1 4> #<SWAP 2 3>)

For a given list of qubit indices, the function qft-circuit returns a Quantum Fourier transform Quil program ready to be passed to quilc for compilation.

* (qvm-examples:qft-circuit '(1 2 3 4))
#<CL-QUIL:PARSED-PROGRAM {10040ABEE3}>

To inspect the object, we can use the cl-quil::print-parsed-program function

* (cl-quil::print-parsed-program (qvm-examples:qft-circuit '(1 2 3 4)))
H 4
CPHASE(pi/2) 3 4
H 3
CPHASE(pi/4) 2 4
CPHASE(pi/2) 2 3
H 2
CPHASE(pi/8) 1 4
CPHASE(pi/4) 1 3
CPHASE(pi/2) 1 2
H 1
SWAP 1 4
SWAP 2 3

QVM, the application

The QVM application is contained with ./app/src/, and provides a stand-alone interface to the QVM library. It can be invoked directly with the binary executable, or alternatively it can provide a server that can be used over the network. Each has their benefits: the former permits a simplified interface using the command-line switches (see output of qvm --help), while the latter allows many remote connections to a single in-memory QVM.

The application is released under the GNU Affero General Public License v3.0.

Usage

To build the QVM application follow instructions in lisp-setup.md. In the top-level directory, run the Makefile with

$ make qvm

This will produce a binary executable qvm in the same directory.

In some situtations, using a large number of qubits may cause heap exhaustion. There are two options to ameliorate this.

The first is to increase the memory available for the QVM, recompile and specify the workspace size (in MB)

$ make QVM_WORKSPACE=4096 qvm

The second is to use a different allocator when running the QVM, by using the --default-allocator argument with "foreign". For example, to run a 30 qubit benchmark on a QVM configured for far less memory, one can do:

$ qvm --default-allocator "foreign" --benchmark 30 -c

This is not the default since this memory is not fully managed by the application.

The QVM application has a few command-line switches used to configure the QVM. To explore those options, see the output of the following command:

$ ./qvm --help

By default, the QVM accepts programs from stdin and writes results to stdout. Log messages are written to stderr.

$ echo 'H 0' | ./qvm
******************************
* Welcome to the Rigetti QVM *
******************************
Copyright (c) 2016-2019 Rigetti Computing.

(Configured with 8192 MiB of workspace and 8 workers.)

<134>1 2019-03-07T22:56:55Z workstation.local qvm 21177 - - Selected simulation method: pure-state
<134>1 2019-03-07T22:56:55Z workstation.local qvm 21177 - - Reading program.
<134>1 2019-03-07T22:56:55Z workstation.local qvm 21177 - - Allocating memory for QVM of 1 qubits.
<134>1 2019-03-07T22:56:55Z workstation.local qvm 21177 - - Allocation completed in 7 ms.
<134>1 2019-03-07T22:56:55Z workstation.local qvm 21177 - - Loading quantum program.
<134>1 2019-03-07T22:56:55Z workstation.local qvm 21177 - - Executing quantum program.
<134>1 2019-03-07T22:56:55Z workstation.local qvm 21177 - - Execution completed in 4 ms.
<134>1 2019-03-07T22:56:55Z workstation.local qvm 21177 - - Printing classical memory and 1-qubit state.
Classical memory (low -> high indexes):
    No memory.
Amplitudes:
    |0>: 0.7071067811865475,                                    P= 50.0%
    |1>: 0.7071067811865475,                                    P= 50.0%

Alternatively the QVM can be started as a server that will accept instructions over a network connection

$ ./qvm -S
******************************
* Welcome to the Rigetti QVM *
******************************
Copyright (c) 2016-2019 Rigetti Computing.

(Configured with 2048 MiB of workspace and 8 workers.)

<134>1 2019-01-28T19:06:07Z workstation.local qvm 3118 - - Selected simulation method: pure-state
<134>1 2019-01-28T19:06:07Z workstation.local qvm 3118 - - Starting server on port 5000.

This is how the pyQuil Python library communicates with a QVM.

Testing

Tests can be run from the Makefile

make test

or from within SBCL

* (asdf:test-system :qvm)

Any contribution to this project should foremost not break any current tests (run tests before making a pull request), and should be accompanied by relevant new tests.

Clearing the Cache

Lisp caches a lot of builds so that not every single file needs to be recompiled. In rare instances, there's confusion and the cache doesn't get properly invalidated. (This can happen when moving files across machines, for example.) Lisp's cache and Quicklisp's system index can be cleaned by doing the following command:

make cleanall

This will delete any built executables as well.

Automated Packaging with Docker

The CI pipeline for qvm produces a Docker image, available at rigetti/qvm.

To get the latest stable version of qvm, run docker pull rigetti/qvm.

Running the QVM with Docker

As outlined above, the QVM supports two modes of operation: stdin and server.

To run the qvm in stdin mode, do the following:

echo "H 0" | docker run --rm -i rigetti/qvm

To run the qvm in server mode, do the following:

docker run --rm -it -p 5000:5000 rigetti/qvm -S

If you would like to change the port of the server to PORT, you can alter the command as follows:

docker run --rm -it -p PORT:PORT rigetti/qvm -S -p PORT

Release Process

1. Update VERSION.txt and dependency versions (if applicable) and push the commit to master.
2. Push a git tag vX.Y.Z that contains the same version number as in VERSION.txt.
3. Verify that the resulting build (triggered by pushing the tag) completes successfully.
4. Publish a release using the tag as the name.
5. Close the milestone associated with this release, and migrate incomplete issues to the next one.