Add a backend for IonQ. (#391)

* Add a backend for IonQ.

* Update projectq/setups/ionq.py

Thanks for the correction!

Co-authored-by: Nguyen Damien <ngn.damien@gmail.com>

* Update docstring to match return type.

* Fix copyright header years, imports, docstring.

* Fix comment

* Fix measurement mappings and result parsing.

* Fix some bad logic with result probability mappings.
* Fix some erroneous test cases.
* Fix example code.
* Ensure qubits are "deallocated" and reset after job submit.
* Drop dependency on mappers in favor of API response mappings.
* Add better error handling for API/http errors.
* Fix tests.

* Update example notebook.

* Fix qubit mapping.

* Use a dedicated qubit mapper instead of bashing self.main_engine.
* Update backend, add tests, to reflect new mapper.

* Use response.registers instead of echoing metadata.

* Fix get_probability after register updates

* Fixes after rebase, use metadata to remamp qubit IDs on response

* Lint/CI/Format/Changelog fixes

* Always make sure to give a result, even if it's not likely

* Missed formatter issues

Co-authored-by: Nguyen Damien <ngn.damien@gmail.com>

CHANGELOG.md

@@ -18,6 +18,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Added ``.editorconfig` file
 - Added ``pyproject.toml`` and ``setup.cfg``
 - Added CHANGELOG.md
+- Added backend for IonQ.
 
 ### Deprecated
 

README.rst

@@ -24,7 +24,7 @@ targeting various types of hardware, a high-performance quantum computer
 simulator with emulation capabilities, and various compiler plug-ins.
 This allows users to
 
--  run quantum programs on the IBM Quantum Experience chip, AQT devices or AWS Braket service provided devices
+-  run quantum programs on the IBM Quantum Experience chip, AQT devices, AWS Braket, or IonQ service provided devices
 -  simulate quantum programs on classical computers
 -  emulate quantum programs at a higher level of abstraction (e.g.,
    mimicking the action of large oracles instead of compiling them to
@@ -172,6 +172,37 @@ IonQ from IonQ and the state vector simulator SV1:
        python3 -m pip install -ve .[braket]
 
 
+**Running a quantum program on IonQ devices**
+
+To run a program on the IonQ trapped ion hardware, use the `IonQBackend` and its corresponding setup.
+
+Currently available devices are:
+
+* `ionq_simulator`: A 29-qubit simulator.
+* `ionq_qpu`: A 11-qubit trapped ion system.
+
+.. code-block:: python
+
+    import projectq.setups.ionq
+    from projectq import MainEngine
+    from projectq.backends import IonQBackend
+
+    token = 'MY_TOKEN'
+    device = 'ionq_qpu'
+    backend = IonQBackend(
+        token=token,
+        use_hardware=True,
+        num_runs=1024,
+        verbose=False,
+        device=device,
+    )
+    compiler_engines = projectq.setups.ionq.get_engine_list(
+        token=token,
+        device=device,
+    )
+    eng = MainEngine(backend, engine_list=compiler_engines)
+
+
 **Classically simulate a quantum program**
 
 ProjectQ has a high-performance simulator which allows simulating up to about 30 qubits on a regular laptop. See the `simulator tutorial <https://github.com/ProjectQ-Framework/ProjectQ/blob/feature/update-readme/examples/simulator_tutorial.ipynb>`__ for more information. Using the emulation features of our simulator (fast classical shortcuts), one can easily emulate Shor's algorithm for problem sizes for which a quantum computer would require above 50 qubits, see our `example codes <http://projectq.readthedocs.io/en/latest/examples.html#shor-s-algorithm-for-factoring>`__.