# Changelog¶

## v2.15 (in development)¶

### Improvements and Changes¶

• LaTeX circuit output now ignores RESET instructions by default, rendering instead the (equivalent) program with RESET omitted. (@kilimanjaro, gh-1118).

## v2.14 (November 25, 2019)¶

### Announcements¶

• There is a new QuantumComputer.experiment method for running a collection of quantum programs as defined by a TomographyExperiment. These objects have a main program body and a collection of state preparation and measurement specifications, which capture the structure of many near-term applications and algorithms like the variational quantum eigensolver (VQE). In addition, the TomographyExperiment encodes information about symmetrization, active qubit reset, and the number of shots to perform on the quantum backend (e.g. the QVM or QPU). For more information check out the API documentation sections on the Quantum Computer and on the Experiment Module (@karalekas, gh-1100).

### Improvements and Changes¶

• Type hints have been added to the PauliTerm class (@rht, gh-1075).
• The rigetti/forest Docker image now has less noisy output due to stdout and stderr redirection to log files entrypoint.sh (@karalekas, gh-1105).
• Added a make typecheck target to run mypy over a subset of the pyquil sources, and enabled typechecks in the GitLab CI pipeline (@appleby, gh-1098).
• Added support for the XY (parameterized iSWAP) gate family in Programs and in ISAs (@ecpeterson, gh-1096, gh-1107, gh-1111).
• Removed the tox.ini and readthedocs.yml files (@karalekas, gh-1108).
• Type hints have been added to the PauliSum class (@rht, gh-1104).

### Bugfixes¶

• Fixed a bug in the LaTeX output of controlled unitary operations (@kilimanjaro, gh-1103).
• Fixed an example of using the qc.run method in the docs to correctly declare the size of a memory register (@appleby, gh-1099).
• Specify UTF-8 encoding when opening files that might contain non-ascii characters, such as when reading the pyquil README.md file in setup.py or when serializing / deserializing pyquil.experiment objects to/from JSON (@appleby, gh-1102).

## v2.13 (November 7, 2019)¶

### Announcements¶

• Rather than installing pyQuil from PyPI, conda-forge, or the source directly, users with Docker installed can pull and run the rigetti/forest <https://hub.docker.com/r/rigetti/forest>__ Docker image to quickly get started with compiling and simulating quantum programs! When running the image, a user will be dropped into an ipython REPL that has pyQuil and its requirements preinstalled, along with quilc and qvm servers running in the background (@karalekas, gh-1035, gh-1039).
• Circuit diagram generation has had a makeover! In particular, the pyquil.latex module provides two mechanisms for generating diagrams from pyQuil programs: pyquil.latex.to_latex generates human-readable LaTeX output expressing a Program as a circuit diagram, and pyquil.latex.display renders a Program as an IPython.display.Image for inline viewing in Jupyter Notebooks. Learn more about these features in the new example notebook (@kilimanjaro, gh-1074).

### Improvements and Changes¶

• Added a Makefile with some simple targets for performing common build operations like creating and uploading a package (@karalekas, gh-1032).
• Replaced symmetrization in operator_estimation with functionality contained within QuantumComputer.run_symmetrized_readout (@kylegulshen, gh-1047).
• As part of the CI, we now package and push to TestPyPI on every commit, which de-risks breaking the setup.py and aids with testing (@karalekas, gh-1017).
• We now calculate code coverage as part of the CI pipeline (@karalekas, gh-1052).
• Moved the program generation from measure_observables into its own private function (@kylegulshen, gh-1043).
• All uses of __future__ and six have been dropped (@karalekas, gh-1060).
• The conftest.py has been moved to the project root dir (@karalekas, gh-1064).
• Using protoquil as a positional argument to qc.compile has been deprecated, and it is now a keyword-only argument (@karalekas, gh-1071).
• PauliSum objects are now hashable (@ecpeterson, gh-1073).
• The code in device.py as been reorganized into a new device subdirectory in a completely backwards-compatible fashion (@karalekas, gh-1066, gh-1094).
• PauliTerm and PauliSum now have __repr__ methods (@karalekas, gh-1080).
• The experiment-schema-related code in operator_estimation.py has been moved into a new experiment subdirectory (@karalekas, gh-1084, gh-1094).
• The keyword arguments to measure_observables are now captured as part of the TomographyExperiment class (@karalekas, gh-1090).
• Type hints have been added to the pyquil.gates, pyquil.quilatom, and pyquil.quilbase modules (@appleby, gh-999).
• We now support Python 3.8 and it is tested in the CI (@karalekas, gh-1093).

### Bugfixes¶

• Updated examples/meyer_penny_game.py with the correct path to the Meyer Penny game exercise in docs/source/exercises.rst (@appleby, gh-1045).
• Fixed the Slack Workspace invite link in the README (@amyfbrown, gh-1042).
• QPU.reset() now checks whether pyquil_config.qpu_url exists before updating the endpoint so as not to break custom connections (@kylegulshen, gh-1072).
• Fixed pretty printing of parameter expressions where π is involved (@notmgsk, gh-1076).
• Fixed a regression in PyQVM.execute that prevented it from running programs containing user-defined gates (@appleby, gh-1067).
• Remove some stale code for pulling quilc version info (@notmgsk, gh-1089).

## v2.12 (September 28, 2019)¶

### Announcements¶

• There is now a Contributing Guide for those who would like to participate in the development of pyQuil. Check it out! In addition, pyQuil now has a Bug Report Template, and a Feature Request Template, which contain sections to fill out when filing a bug or suggesting an enhancement (@karalekas, gh-985, gh-986, gh-996).

### Improvements and Changes¶

• The local_qvm context manager has been renamed to local_forest_runtime, which now checks if the designated ports are used before starting qvm/quilc. The original local_qvm has been deprecated (@sauercrowd, gh-976).
• The test suite for pyQuil now runs against both Python 3.6 and 3.7 to ensure compatibility with the two most recent versions of Python (@karalekas, gh-987).
• Add support for the FORKED gate modifier (@kilimanjaro, gh-989).
• Deleted the deprecated modules parameters.py and qpu.py (@karalekas, gh-991).
• The test suite for pyQuil now runs much faster, by setting the default value of the --use-seed option for pytest to True (@karalekas, gh-992).
• Support non-gate instructions (e.g. MEASURE) in to_latex() (@notmgsk, gh-975).
• Test suite has been updated to reduce the use of deprecated features (@kilimanjaro, gh-998, gh-1005).
• Certain tests have been marked as “slow”, and are skipped unless the --runslow option is specified for pytest (@kilimanjaro, gh-1001).
• PauliSum objects can now be constructed from strings via from_compact_str() and PauliTerm.from_compact_str() supports multi-qubit strings (@jlbosse, gh-984).

### Bugfixes¶

• Strength two symmetrization was not correctly producing orthogonal arrays due to erroneous truncation, which has been fixed (@kylegulshen, gh-990).
• The STORE instruction now accepts int or float in addition to MemoryReference as its source argument. As a result, you can now STORE an immediate value into a memory register. Also, the EQ, LT, LE, GT, and GE instructions now all accept float in addition to int or MemoryReference as their third and final argument. As a result, you can now perform classical comparisons against an immediate float value. Finally, the CONVERT instruction now accepts any valid memory reference designator (a MemoryReference, a string, or a tuple of type (str, int)) for both its arguments (@appleby, gh-1010).
• Raise an error if a gate with non-constant parameters is provided to lifted_gate (@notmgsk, gh-1012).

## v2.11 (September 3, 2019)¶

### Announcements¶

• PyQuil’s changelog has been overhauled and rewritten in Markdown instead of RST, and can be found in the top-level directory of the repository as the CHANGELOG.md file (which is the standard for most GitHub repositories). However, during the build process, we use pandoc to convert it back to RST so that it can be included as part of the ReadTheDocs documentation here (@karalekas, gh-945, gh-973).

### Improvements and Changes¶

• Test suite attempts to retry specific tests that fail often. Tests are retried only a single time (@notmgsk, gh-951).
• The QuantumComputer.run_symmetrized_readout() method has been revamped, and now has options for using more advanced forms of readout symmetrization (@joshcombes, gh-919).
• The ProtoQuil restrictions built in to PyQVM have been removed (@ecpeterson, gh-874).
• Add the ability to query for other memory regions after both QPU and QVM runs. This removes a previously unnecessary restriction on the QVM, although ro remains the only QPU-writeable memory region during Quil execution (@ecpeterson, gh-873).
• Now, running QuantumComputer.reset() (and QuantumComputer.compile() when using the QPU) additionally resets the connection information for the underlying QVM/QPU and QVMCompiler/QPUCompiler objects, which should resolve bugs that arise due to stale clients/connections (@karalekas, gh-872).
• In addition to the simultaneous 1Q RB fidelities contained in device specs prior to this release, there are now 1Q RB fidelities for non-simultaneous gate operation. The names of these fields have been changed for clarity, and standard errors for both fidelities have been added as well. Finally, deprecation warnings have been added regarding the fCPHASE and fBellState device spec fields, which are no longer routinely updated and will be removed in release v2.13 (@jvalery2, gh-968).
• The NOTICE has been updated to accurately reflect the third-party software used in pyQuil (@karalekas, gh-979).
• PyQuil now sends “modern” ISA payloads to quilc, which must be of version >= 1.10.0. Check out the details of get_isa for information on how to specify custom payloads (@ecpeterson, gh-961).

### Bugfixes¶

• The MemoryReference warnings have been removed from the unit tests (@maxKenngott, gh-950).
• The merge_programs function now supports merging programs with DefPermutationGate, instead of throwing an error, and avoids redundant readout declaration (@kylegulshen, gh-971).
• Remove unsound logic to fill out non-“ro” memory regions when targeting a QPU (@notmgsk, gh-982).

## v2.10 (July 31, 2019)¶

### Improvements and Changes¶

• Rewrote the README, adding a more in-depth overview of the purpose of pyQuil as a library, as well as two badges – one for PyPI downloads and another for the Forest Slack workspace. Also, included an example section for how to get started with running a simple Bell state program on the QVM (@karalekas, gh-946, gh-949).
• The test suite for pyquil.operator_estimation now has an (optional) faster version that uses fixed random seeds instead of averaging over several experiments. This can be enabled with the --use-seed command line option when running pytest (@msohaibalam, gh-928).
• Deleted the deprecated modules job_results.py and kraus.py (@karalekas, gh-957).
• Updated the examples README. Removed an outdated notebook. Updated remaining notebooks to use MemoryReference, and fix any parts that were broken (@notmgsk, gh-820).
• The AbstractCompiler.quil_to_native_quil() function now accepts a protoquil keyword which tells the compiler to restrict both input and output to protoquil (i.e. Quil code executable on a QPU). Additionally, the compiler will return a metadata dictionary that contains statistics about the compiled program, e.g. its estimated QPU runtime. See the compiler docs for more information (@notmgsk, gh-940).
• Updated the QCS and Slack invite links on the index.rst docs page (@starktech23, gh-965).
• Provided example code for reading out the QPU runtime estimation for a program (@notmgsk, gh-963).

### Bugfixes¶

• unitary_tools.lifted_gate() was not properly handling modifiers such as DAGGER and CONTROLLED (@kylegulshen, gh-931).
• Fixed warnings raised by Sphinx when building the documentation (@appleby, gh-929).

## v2.9.1 (June 28, 2019)¶

### Bugfixes¶

• Relaxed the requirement for a quilc server to exist when users of the QuantumComputer object only want to do simulation work with a QVM or pyQVM backend (@karalekas, gh-934).

## v2.9 (June 25, 2019)¶

### Announcements¶

• PyQuil now has a Pull Request Template, which contains a checklist of things that must be completed (if applicable) before a PR can be merged (@karalekas, gh-921).

### Improvements and Changes¶

• Removed a bunch of logic around creating inverse gates from user-defined gates in Program.dagger() in favor of a simpler call to Gate.dagger() (@notmgsk, gh-887).
• The RESET instruction now works correctly with QubitPlaceholder objects and the address_qubits function (@jclapis, gh-910).
• ReferenceDensitySimulator can now have a state that is persistent between rounds of run or run_and_measure (@joshcombes, gh-920).

### Bugfixes¶

• Small negative probabilities were causing ReferenceDensitySimulator to fail (@joshcombes, gh-908).
• The dagger function was incorrectly dropping gate modifiers like CONTROLLED (@jclapis, gh-914).
• Negative numbers in classical instruction arguments were not being parsed (@notmgsk, gh-917).
• Inline math rendering was not working correctly in intro.rst (@appleby, gh-927).

Thanks to community member @jclapis for the contributions to this release!

## v2.8 (May 20, 2019)¶

### Improvements and Changes¶

• PyQuil now verifies that you are using the correct version of the QVM and quilc (@karalekas, gh-913).
• Added support for defining permutation gates for use with the latest version of quilc (@notmgsk, gh-891).
• The rpcq dependency requirement has been raised to v2.5.1 (@notmgsk, gh-911).
• Added a note about the QVM’s compilation mode to the documentation (@stylewarning, gh-900).
• Some measure_observables params now have the Optional type specification (@msohaibalam, gh-903).

### Bugfixes¶

• Preserve modifiers during address_qubits (@notmgsk, gh-907).

## v2.7.2 (May 3, 2019)¶

### Bugfixes¶

• An additional backwards-incompatible change from gh-870 snuck through 2.7.1, and is addressed in this patch release (@karalekas, gh-901).

## v2.7.1 (April 30, 2019)¶

### Bugfixes¶

• The changes to operator estimation (gh-870, gh-896) were not made in a backwards-compatible fashion, and therefore this patch release aims to remedy that. Going forward, there will be much more stringent requirements around backwards compatibility and deprecation (@karalekas, gh-899).

## v2.7 (April 29, 2019)¶

### Improvements and Changes¶

• Standard deviation -> standard error in operator estimation (@msohaibalam, gh-870).
• Update what pyQuil expects from quilc in terms of rewiring pragmas – they are now comments rather than distinct instructions (@ecpeterson, gh-878).
• Allow users to deprioritize QPU jobs – mostly a Rigetti-internal feature (@jvalery2, gh-877).
• Remove the qubits field from the TomographyExperiment dataclass (@msohaibalam, gh-896).

### Bugfixes¶

• Ensure that shots aren’t lost when passing a Program through address_qubits (@notmgsk, gh-895).
• Fixed the conda install command in the README (@seandiscovery, gh-890).

## v2.6 (March 29, 2019)¶

### Improvements and Changes¶

• Added a CODEOWNERS file for default reviewers (@karalekas, gh-855).
• Bifurcated the QPUCompiler endpoint parameter into two – quilc_endpoint and qpu_compiler_endpoint – to reflect changes in Quantum Cloud Services (@karalekas, gh-856).
• Clarified documentation around the DELAY pragma (@willzeng, gh-862).
• Added information about the local_qvm context manager to the getting started documentation (@willzeng, gh-851).
• Added strict version lower bounds on the rpcq and networkx dependencies (@notmgsk, gh-828).
• A slice of a Program object now returns a Program object (@notmgsk, gh-848).

### Bugfixes¶

• Added a non-None default timeout to the QVMCompiler object and the get_benchmarker function (@karalekas, gh-850, gh-854).
• Fixed the docstring for the apply_clifford_to_pauli function (@kylegulshen, gh-836).
• Allowed the apply_clifford_to_pauli function to now work with the Identity as input (@msohaibalam, gh-849).
• Updated a stale link to the Rigetti Forest Slack workspace (@karalekas, gh-860).
• Fixed a notation typo in the documentation for noise (@willzeng, gh-861).
• An IndexError is now raised when trying to access an out-of-bounds entry in a MemoryReference (@notmgsk, gh-819).
• Added a check to ensure that measure_observables takes as many shots as requested (@marcusps, gh-846).

Special thanks to @willzeng for all the contributions this release!

## v2.5 (March 6, 2019)¶

### Improvements and Changes¶

• PyQuil’s Gate objects now expose .controlled(q) and .dagger() modifiers, which turn a gate respectively into its controlled variant, conditional on the qubit q, or into its inverse.
• The operator estimation suite’s measure_observables method now exposes a readout_symmetrize argument, which helps mitigate a machine’s fidelity asymmetry between recognizing a qubit in the ground state versus the excited state.
• The MEASURE instruction in pyQuil now has a mandatory second argument. Previously, the second argument could be omitted to induce “measurement for effect”, without storing the readout result to a classical register, but users found this to be a common source of accidental error and a generally rude surprise. To ensure the user really intends to measure only for effect, we now require that they supply an explicit None as the second argument.

### Bugfixes¶

• Some stale tests have been brought into the modern era.

## v2.4 (February 14, 2019)¶

### Announcements¶

• The Quil Compiler (quilc) and the Quantum Virtual Machine (QVM), which are part of the Forest SDK, have been open sourced! In addition to downloading the binaries, you can now build these applications locally from source, or run them via the Docker images rigetti/quilc and rigetti/qvm. These Docker images are now used as the services in the GitLab CI build plan YAML (gh-792, gh-794, gh-795).

### Improvements and Changes¶

• The WavefunctionSimulator now supports the use of parametric Quil programs, via the memory_map parameter for its various methods (gh-787).
• Operator estimation data structures introduced in v2.2 have changed. Previously, ExperimentSettings had two members: in_operator and out_operator. The out_operator is unchanged, but in_operator has been renamed to in_state and its data type is now TensorProductState instead of PauliTerm. It was always an abuse of notation to interpret pauli operators as defining initial states. Analogous to the Pauli helper functions sI, sX, sY, and sZ, TensorProductState objects are constructed by multiplying together terms generated by the helper functions plusX, minusX, plusY, minusY, plusZ, and minusZ. This functionality enables process tomography and process DFE (gh-770).
• Operator estimation now offers a “greedy” method for grouping tomography-like experiments that share a natural tensor product basis (ntpb), as an alternative to the clique cover version (gh-754).
• The quilc endpoint for rewriting Quil parameter arithmetic has been changed from resolve_gate_parameter_arithmetic to rewrite_arithmetic (gh-802).
• The difference between ProtoQuil and QPU-supported Quil is now better defined (gh-798).

### Bugfixes¶

• Resolved an issue with post-gate noise in the pyQVM (gh-801).
• A TypeError with a useful error message is now raised when a Program object is run on a QPU-backed QuantumComputer, rather than a confusing AttributeError (gh-799).

## v2.3 (January 28, 2019)¶

PyQuil 2.3 is the latest release of pyQuil, Rigetti’s toolkit for constructing and running quantum programs. A major new feature is the release of a new suite of simulators:

• We’re proud to introduce the first iteration of a Python-based quantum virtual machine (QVM) called PyQVM. This QVM is completely contained within pyQuil and does not need any external dependencies. Try using it with get_qc("9q-square-pyqvm") or explore the pyquil.pyqvm.PyQVM object directly. Under-the-hood, there are three quantum simulator backends:
• ReferenceWavefunctionSimulator uses standard matrix-vector multiplication to evolve a statevector. This includes a suite of tools in pyquil.unitary_tools for dealing with unitary matrices.
• NumpyWavefunctionSimulator uses numpy’s tensordot functionality to efficiently evolve a statevector. For most simulations, performance is quite good.
• ReferenceDensitySimulator uses matrix-matrix multiplication to evolve a density matrix.
• Matrix representations of Quil standard gates are included in pyquil.gate_matrices (gh-552).
• The density simulator has extremely limited support for Kraus-operator based noise models. Let us know if you’re interested in contributing more robust noise-model support.
• This functionality should be considered experimental and may undergo minor API changes.

### Important changes to note¶

• Quil math functions (like COS, SIN, …) used to be ambiguous with respect to case sensitivity. They are now case-sensitive and should be uppercase (gh-774).
• In the next release of pyQuil, communication with quilc will happen exclusively via the rpcq protocol. LocalQVMCompiler and LocalBenchmarkConnection will be removed in favor of a unified QVMCompiler and BenchmarkConnection. This change should be transparent if you use get_qc and get_benchmarker, respectively. In anticipation of this change we recommend that you upgrade your version of quilc to 1.3, released Jan 30, 2019 (gh-730).
• When using a paramaterized gate, the QPU control electronics only allowed multiplying parameters by powers of two. If you only ever multiply a parameter by the same constant, this isn’t too much of a problem because you can fold the multiplicative constant into the definition of the parameter. However, if you are multiplying the same variable (e.g. gamma in QAOA) by different constants (e.g. weighted maxcut edge weights) it doesn’t work. PyQuil will now transparently handle the latter case by expanding to a vector of parameters with the constants folded in, allowing you to multiply variables by whatever you want (gh-707).

### Bug fixes and improvements¶

• The CZ gate fidelity metric available in the Specs object now has its associated standard error, which is accessible from the method Specs.fCZ_std_errs (gh-751).
• Operator estimation code now correctly handles identity terms with coefficients. Previously, it would always estimate these terms as 1.0 (gh-758).
• Operator estimation results include the total number of counts (shots) taken.
• Operator estimation JSON serialization uses utf-8. Please let us know if this causes problems (gh-769).
• The example quantum die program now can roll dice that are not powers of two (gh-749).
• The teleportation and Meyer penny game examples had a syntax error (gh-778, gh-772).
• When running on the QPU, you could get into trouble if the QPU name passed to get_qc did not match the lattice you booked. This is now validated (gh-771).

We extend thanks to community member @estamm12 for their contribution to this release.

## v2.2 (January 4, 2019)¶

PyQuil 2.2 is the latest release of pyQuil, Rigetti’s toolkit for constructing and running quantum programs. Bug fixes and improvements include:

• pauli.is_zero and paulis.is_identity would sometimes return erroneous answers (gh-710).
• Parameter expressions involving addition and subtraction are now converted to Quil with spaces around the operators, e.g. theta + 2 instead of theta+2. This disambiguates subtracting two parameters, e.g. alpha - beta is not one variable named alpha-beta (gh-743).
• T1 is accounted for in T2 noise models (gh-745).
• Documentation improvements (gh-723, gh-719, gh-720, gh-728, gh-732, gh-742).
• Support for PNG generation of circuit diagrams via LaTeX (gh-745).
• We’ve started transitioning to using Gitlab as our continuous integration provider for pyQuil (gh-741, gh-752).

This release includes a new module for facilitating the estimation of quantum observables/operators (gh-682). First-class support for estimating observables should make it easier to express near-term algorithms. This release includes:

• data structures for expressing tomography-like experiments and their results
• grouping of experiment settings that can be simultaneously estimated
• functionality to executing a tomography-like experiment on a quantum computer

Please look forward to more features and polish in future releases. Don’t hesitate to submit feedback or suggestions as GitHub issues.

We extend thanks to community member @petterwittek for their contribution to this release.

Bugfix release 2.2.1 was released January 11 to maintain compatibility with the latest version of the quilc compiler (gh-759).

## v2.1 (November 30, 2018)¶

PyQuil 2.1 is an incremental release of pyQuil, Rigetti’s toolkit for constructing and running quantum programs. Changes include:

• Major documentation improvements.
• QuantumComputer.run() accepts an optional memory_map parameter to facilitate running parametric executables (gh-657).
• QuantumComputer.reset() will reset the state of a QAM to recover from an error condition (gh-703).
• Bug fixes (gh-674, gh-696).
• Quil parser improvements (gh-689, gh-685).
• Optional interleaver argument when generating RB sequences (gh-673).
• Our GitHub organization name has changed from rigetticomputing to rigetti (gh-713).

## v2.0 (November 1, 2018)¶

PyQuil 2.0 is a major release of pyQuil, Rigetti’s toolkit for constructing and running quantum programs. This release contains many major changes including:

1. The introduction of Quantum Cloud Services. Access Rigetti’s QPUs from co-located classical compute resources for minimal latency. The web API for running QVM and QPU jobs has been deprecated and cannot be accessed with pyQuil 2.0
2. Advances in classical control systems and compilation allowing the pre-compilation of parametric binary executables for rapid hybrid algorithm iteration.
3. Changes to Quil—our quantum instruction language—to provide easier ways of interacting with classical memory.

The new QCS access model and features will allow you to execute hybrid quantum algorithms several orders of magnitude (!) faster than the previous web endpoint. However, to fully exploit these speed increases you must update your programs to use the latest pyQuil features and APIs. Please read the documentation on what is New in Forest 2 for a comprehensive migration guide.

An incomplete list of significant changes:

• Python 2 is no longer supported. Please use Python 3.6+
• Parametric gates are now normal functions. You can no longer write RX(pi/2)(0) to get a Quil RX(pi/2) 0 instruction. Just use RX(pi/2, 0).
• Gates support keyword arguments, so you can write RX(angle=pi/2, qubit=0).
• All async methods have been removed from QVMConnection and QVMConnection is deprecated. QPUConnection has been removed in accordance with the QCS access model. Use pyquil.get_qc as the primary means of interacting with the QVM or QPU.
• WavefunctionSimulator allows unfettered access to wavefunction properties and routines. These methods and properties previously lived on QVMConnection and have been deprecated there.
• Classical memory in Quil must be declared with a name and type. Please read New in Forest 2 for more.
• Compilation has changed. There are now different Compiler objects that target either the QPU or QVM. You must explicitly compile your programs to run on a QPU or a realistic QVM.

Version 2.0.1 was released on November 9, 2018 and includes documentation changes only. This release is only available as a git tag. We have not pushed a new package to PyPI.

## v1.9 (June 6, 2018)¶

We’re happy to announce the release of pyQuil 1.9. PyQuil is Rigetti’s toolkit for constructing and running quantum programs. This release is the latest in our series of regular releases, and it’s filled with convenience features, enhancements, bug fixes, and documentation improvements.

Special thanks to community members @sethuiyer, @vtomole, @rht, @akarazeev, @ejdanderson, @markf94, @playadust, and @kadora626 for contributing to this release!

### Qubit placeholders¶

One of the focuses of this release is a re-worked concept of “Qubit Placeholders”. These are logical qubits that can be used to construct programs. Now, a program containing qubit placeholders must be “addressed” prior to running on a QPU or QVM. The addressing stage involves mapping each qubit placeholder to a physical qubit (represented as an integer). For example, if you have a 3 qubit circuit that you want to run on different sections of the Agave chip, you now can prepare one Program and address it to many different subgraphs of the chip topology. Check out the QubitPlaceholder example notebook for more.

To support this idea, we’ve refactored parts of Pyquil to remove the assumption that qubits can be “sorted”. While true for integer qubit labels, this probably isn’t true in general. A notable change can be found in the construction of a PauliSum: now terms will stay in the order they were constructed.

• PauliTerm now remembers the order of its operations. sX(1)*sZ(2) will compile to different Quil code than sZ(2)*sX(1), although the terms will still be equal according to the __eq__ method. During PauliSum combination of like terms, a warning will be emitted if two terms are combined that have different orders of operation.
• PauliTerm.id() takes an optional argument sort_ops which defaults to True for backwards compatibility. However, this function should not be used for comparing term-type like it has been used previously. Use PauliTerm.operations_as_set() instead. In the future, sort_ops will default to False and will eventually be removed.
• Program.alloc() has been deprecated. Please instantiate QubitPlaceholder() directly or request a “register” (list) of n placeholders by using the class constructor QubitPlaceholder.register(n).
• Programs must contain either (1) all instantiated qubits with integer indexes or (2) all placeholder qubits of type QubitPlaceholder. We have found that most users use
1. but (2) will become useful with larger and more diverse devices.
• Programs that contain qubit placeholders must be explicitly addressed prior to execution. Previously, qubits would be assigned “under the hood” to integers 0…N. Now, you must use address_qubits which returns a new program with all qubits indexed depending on the qubit_mapping argument. The original program is unaffected and can be “readdressed” multiple times.
• PauliTerm can now accept QubitPlaceholder in addition to integers.
• QubitPlaceholder is no longer a subclass of Qubit. LabelPlaceholder is no longer a subclass of Label.
• QuilAtom subclasses’ hash functions have changed.

### Randomized benchmarking sequence generation¶

Pyquil now includes support for performing a simple benchmarking routine - randomized benchmarking. There is a new method in the CompilerConnection that will return sequences of pyquil programs, corresponding to elements of the Clifford group. These programs are uniformly randomly sampled, and have the property that they compose to the identity. When concatenated and run as one program, these programs can be used in a procedure called randomized benchmarking to gain insight about the fidelity of operations on a QPU.

In addition, the CompilerConnection has another new method, apply_clifford_to_pauli which conjugates PauliTerms by Program that are composed of Clifford gates. That is to say, given a circuit C, that contains only gates corresponding to elements of the Clifford group, and a tensor product of elements P, from the Pauli group, this method will compute $PCP^{dagger}$ Such a procedure can be used in various ways. An example is predicting the effect a Clifford circuit will have on an input state modeled as a density matrix, which can be written as a sum of Pauli matrices.

### Ease of Use¶

This release includes some quality-of-life improvements such as the ability to initialize programs with generator expressions, sensible defaults for Program.measure_all, and sensible defaults for classical_addresses in run methods.

• Program can be initiated with a generator expression.
• Program.measure_all (with no arguments) will measure all qubits in a program.
• classical_addresses is now optional in QVM and QPU run methods. By default, any classical addresses targeted by MEASURE will be returned.
• QVMConnection.pauli_expectation accepts PauliSum as arguments. This offers a more sensible API compared to QVMConnection.expectation.
• pyQuil will now retry jobs every 10 seconds if the QPU is re-tuning.
• CompilerConnection.compile now takes an optional argument isa that allows per-compilation specification of the target ISA.
• An empty program will trigger an exception if you try to run it.

### Supported versions of Python¶

We strongly support using Python 3 with Pyquil. Although this release works with Python 2, we are dropping official support for this legacy language and moving to community support for Python 2. The next major release of Pyquil will introduce Python 3.5+ only features and will no longer work without modification for Python 2.

### Bug fixes¶

• shift_quantum_gates has been removed. Users who relied on this functionality should use QubitPlaceholder and address_qubits to achieve the same result. Users should also double-check data resulting from use of this function as there were several edge cases which would cause the shift to be applied incorrectly resulting in badly-addressed qubits.
• Slightly perturbed angles when performing RX gates under a Kraus noise model could result in incorrect behavior.
• The quantum die example returned incorrect values when n = 2^m.