Add information about QuantumComputer (#719)

* Add information about QuantumComputer

* Add info about QPU, QVM, and both generally

docs/source/qvm.rst

@@ -3,25 +3,41 @@
 The Quantum Computer
 ====================
 
+PyQuil is used to build Quil (Quantum Instruction Language) programs and execute them on simulated or real quantum devices. Quil is an opinionated
+quantum instruction language: its basic belief is that in the near term quantum computers will
+operate as coprocessors, working in concert with traditional CPUs. This means that Quil is designed to execute on
+a Quantum Abstract Machine (QAM) that has a shared classical/quantum architecture at its core.
+
+A QAM must, therefore, implement certain abstract methods to manipulate classical and quantum states, such as loading
+programs, writing to shared classical memory, and executing programs.
+
+The program execution itself is sent from pyQuil to quantum computer endpoints, which will be one of two options:
+
+  - A Rigetti Quantum Virtual Machine (QVM)
+  - A Rigetti Quantum Processing Unit (QPU)
+
+Within pyQuil, there is a :py:class:`~pyquil.api.QVM` object and a :py:class:`~pyquil.api.QPU` object which use
+the exposed APIs of the QVM and QPU servers, respectively.
+
+On this page, we'll learn a bit about the :ref:`QVM <qvm_use>` and :ref:`QPU <qpu>`. Then we will
+show you how to use them from pyQuil with a :ref:`quantum_computer`.
+
+For information on constructing quantum programs, please refer back to :ref:`basics`.
+
+.. _qvm_use:
+
 The Quantum Virtual Machine (QVM)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 The Rigetti Quantum Virtual Machine is an implementation of the Quantum Abstract Machine from
 *A Practical Quantum Instruction Set Architecture*. [1]_  It is implemented in ANSI Common LISP and
-executes programs specified in the Quantum Instruction Language (Quil).
-
-Quil is an opinionated quantum instruction language: its basic belief is that in the near term quantum computers will
-operate as coprocessors, working in concert with traditional CPUs.  This means that Quil is designed to execute on
-a Quantum Abstract Machine that has a shared classical/quantum architecture at its core.
+executes programs specified in Quil.
 
 The QVM is a wavefunction simulation of unitary evolution with classical control flow
 and shared quantum classical memory.
 
-.. _qvm_use:
-
-Using the QVM
--------------
-After :ref:`downloading the SDK <sdkinstall>`, the QVM is available on your local machine. You can initialize a local
+The QVM is part of the Forest SDK, and it's available for you to use on your local machine.
+After :ref:`downloading and installing the SDK <sdkinstall>`, you can initialize a local
 QVM server by typing ``qvm -S`` into your terminal. You should see the following message.
 
 .. code:: python
@@ -36,43 +52,242 @@ QVM server by typing ``qvm -S`` into your terminal. You should see the following
 
     [2018-11-06 18:18:18] Starting server on port 5000.
 
-For a detailed description of how to use the ``qvm`` from the command line, see :ref:`The QVM manual page <qvm_man>`.
+By default, the server is started on port 5000 on your local machine. Consequently, the endpoint which
+the pyQuil :py:class:`~pyquil.api.QVM` will default to for the QVM address is ``"http://127.0.0.1:5000"``. When you
+run your program, a pyQuil client will send a Quil program to the QVM server and wait for a response back.
 
-Once the QVM is serving requests, we can run the following pyQuil program to get a ``QuantumComputer`` object which
-will use the QVM.
+It's also possible to use the QVM from the command line. You can write a Quil program in its own file:
+
+.. code:: python
+
+    # example.quil
+
+    DECLARE ro BIT[1]
+    RX(pi/2) 0
+    CZ 0 1
+
+and then execute it with the QVM directly from the command line:
+
+.. code:: python
+
+    $ qvm -e < example.quil
+
+    [2018-11-30 11:13:58] Reading program.
+    [2018-11-30 11:13:58] Allocating memory for QVM of 2 qubits.
+    [2018-11-30 11:13:58] Allocation completed in 4 ms.
+    [2018-11-30 11:13:58] Loading quantum program.
+    [2018-11-30 11:13:58] Executing quantum program.
+    [2018-11-30 11:13:58] Execution completed in 6 ms.
+    [2018-11-30 11:13:58] Printing 2-qubit state.
+    [2018-11-30 11:13:58] Amplitudes:
+    [2018-11-30 11:13:58]   |00>: 0.0, P=  0.0%
+    [2018-11-30 11:13:58]   |01>: 0.0-1.0i, P=100.0%
+    [2018-11-30 11:13:58]   |10>: 0.0, P=  0.0%
+    [2018-11-30 11:13:58]   |11>: 0.0, P=  0.0%
+    [2018-11-30 11:13:58] Classical memory (low -> high indexes):
+    [2018-11-30 11:13:58]     ro:  1 0
+
+For a detailed description of how to use the ``qvm`` from the command line, see :ref:`The QVM manual page <qvm_man>` or
+type ``man qvm`` in your terminal.
+
+We also offer a Wavefunction Simulator (formerly a part of the :py:class:`~pyquil.api.QVM` object),
+which allows users to contruct and inspect wavefunctions of quantum programs. Learn more
+about the Wavefunction Simulator :ref:`here <wavefunction_simulator>`.
+
+.. _qpu:
+
+The Quantum Processing Unit (QPU)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To access a QPU endpoint, you will have to `sign up <https://www.rigetti.com/>`_ for Quantum Cloud Services (QCS).
+Documentation for getting started with your Quantum Machine Image (QMI) is found
+`here <https://www.rigetti.com/qcs/docs/intro-to-qcs>`_. Using QCS, you will ``ssh`` into your QMI, and reserve a
+QPU lattice for a particular time block.
+
+When your reservation begins, you will be authorized to access the QPU. A configuration file will be automatically
+populated for you with the proper QPU endpoint for your reservation. Both your QMI and the QPU are located on premises,
+giving you low latency access to the QPU server. That server accepts jobs in the form of ``BinaryExecutableRequest``s,
+which is precisely what you get back when you compile your program in pyQuil and target the QPU (more on this soon).
+This request contains all the information necessary to run your program on the control rack which sends and receives
+waveforms from the QPU, so that you can receive classified readout results (``0``s and ``1``s).
+
+For information on available lattices, you can check out your dashboard at https://qcs.rigetti.com/dashboard after you've
+been invited to QCS.
+
+
+.. _quantum_computer:
+
+The ``QuantumComputer``
+~~~~~~~~~~~~~~~~~~~~~~~
+
+The :py:class:`~pyquil.api.QuantumComputer` abstraction offered by pyQuil provides an easy access point to the most
+critical objects used in pyQuil for building and executing your quantum programs.
+We will cover the main methods and attributes on this page.
+The `QuantumComputer API Reference <apidocs/quantum_computer.html>`_ provides a reference for all of its methods and
+options.
+
+At a high level, the :py:class:`~pyquil.api.QuantumComputer` wraps around our favorite quantum computing tools:
+
+  - **A quantum abstract machine** ``.qam`` : this is our general purpose quantum computing device,
+    which implements the required abstract methods described :ref:`above <qvm>`. It is implemented as a
+    :py:class:`~pyquil.api.QVM` or :py:class:`~pyquil.api.QPU` object in pyQuil.
+  - **A compiler** ``.compiler`` : this determines how we manipulate the Quil input to something more efficient when possible,
+    and then into a form which our QAM can accept as input.
+  - **A device** ``.device`` : this specifies the topology and Instruction Set Architecture (ISA) of
+    the targeted device by listing the supported 1Q and 2Q gates.
+
+When you instantiate a :py:class:`~pyquil.api.QuantumComputer` instance, these subcomponents will be compatible with
+each other. So, if you get a ``QPU`` implementation for the ``.qam``, you will have a ``QPUCompiler`` for the
+``.compiler``, and your ``.device`` will match the device used by the ``.compiler.``
+
+The :py:class:`~pyquil.api.QuantumComputer` instance makes methods available which are built on the above objects. If
+you need more fine grained controls for your work, you might try exploring what is offered by these objects.
+
+For more information on each of the above, check out the following pages:
+
+ - `Compiler API Reference <apidocs/compilers.html>`_
+ - :ref:`Quil Compiler docs <compiler>`
+ - `Device API Reference <apidocs/devices.html>`_
+ - :ref:`new_topology`
+ - `Quantum abstract machine (QAM) API Reference <apidocs/qam.html>`_
+ - `The Quil Whitepaper <https://arxiv.org/abs/1608.03355>`_ which describes the QAM
+
+Instantiation
+-------------
+
+A decent amount of information needs to be provided to initialize the ``compiler``, ``device``, and ``qam`` attributes,
+much of which is already in your :ref:`config files <_advanced_usage>` (or provided reasonable defaults when running locally).
+Typically, you will want a :py:class:`~pyquil.api.QuantumComputer` which either:
+
+  - pertains to a real, available QPU device
+  - is a QVM but mimics the topology of a QPU
+  - is some generic QVM
+
+All of this can be accomplished with :py:func:`~pyquil.api.get_qc`.
+
+.. code:: python
+
+    def get_qc(name: str, *, as_qvm: bool = None, noisy: bool = None,
+               connection: ForestConnection = None) -> QuantumComputer:
+
+.. code:: python
+
+    from pyquil import get_qc
+
+    # Get a QPU
+    qc = get_qc(QPU_LATTICE_NAME)  # QPU_LATTICE_NAME is just a string naming the device
+
+    # Get a QVM with the same topology as the QPU lattice
+    qc = get_qc(QPU_LATTICE_NAME, as_qvm=True)
+    # or, equivalently
+    qc = get_qc(f"{QPU_LATTICE_NAME}-qvm")
+
+    # A fully connected QVM
+    number_of_qubits = 10
+    qc = get_qc(f"{number_of_qubits}q-qvm")
+
+For now, you will have to join QCS to get ``QPU_LATTICE_NAME`` by running the
+``qcs lattices`` command from your QMI. Access to the QPU is only possible from a QMI, during a booked reservation.
+If this sounds unfamiliar, check out our `documentation for QCS <https://www.rigetti.com/qcs/docs/intro-to-qcs>`_
+and `join the waitlist <https://www.rigetti.com/>`_.
+
+For more information about creating and adding your own noise models, check out :ref:`noise`.
+
+.. note::
+    When connecting to a QVM locally (such as with ``get_qc(..., as_qvm=True)``) you'll have to set up the QVM
+    in :ref:`server mode <server>`.
+
+Methods
+-------
+
+Now that you have your ``qc``, there's a lot you can do with it. Most users will want to use ``compile``, ``run`` or
+``run_and_measure``, and ``qubits`` very regularly. The general flow of use would look like this:
 
 .. code:: python
 
     from pyquil import get_qc, Program
     from pyquil.gates import *
-    qc = get_qc('9q-square-qvm')
 
+    qc = get_qc('9q-square-qvm')            # not general to any number of qubits, 9q-square-qvm is special
 
-One executes quantum programs on the QVM using the ``.run(...)`` method, intended to closely mirror how one will
-execute programs on a real QPU. We also offer a Wavefunction Simulator (formerly a part of the ``QVM`` object),
-which allows users to contruct and inspect wavefunctions of quantum programs. Learn more
-about the Wavefunction Simulator :ref:`here <wavefunction_simulator>`. For information on constructing quantum
-programs, please refer back to :ref:`basics`.
+    qubits = qc.qubits()                    # this information comes from qc.device
+    p = Program()
+    # ... build program, potentially making use of the qubits list
+
+    compiled_program = qc.compile(p)        # this makes multiple calls to qc.compiler
+
+    results = qc.run(compiled_program)      # this makes multiple calls to qc.qam
+
+.. note::
+
+    In addition to a running QVM server, you will need a running ``quilc`` server to compile your program. Setting
+    up both of these is very easy, as explained :ref:`here <server>`.
+
+
+The ``.run_and_measure(...)`` method
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This is the most high level way to run your program. With this method, you are **not** responsible for compiling your program
+before running it, nor do you have to specify any ``MEASURE`` instructions; all qubits will get measured.
+
+.. code:: python
+
+    from pyquil import Program, get_qc
+    from pyquil.gates import X
+
+    qc = get_qc("8q-qvm")
+
+    p = Program(X(0))
+
+    results = qc.run_and_measure(p, trials=5)
+    print(results)
+
+``trials`` specifies how many times to run this program. Let's see our results:
+
+.. parsed-literal::
+
+    {0: array([1, 1, 1, 1, 1]),
+     1: array([0, 0, 0, 0, 0]),
+     2: array([0, 0, 0, 0, 0]),
+     3: array([0, 0, 0, 0, 0]),
+     4: array([0, 0, 0, 0, 0]),
+     5: array([0, 0, 0, 0, 0]),
+     6: array([0, 0, 0, 0, 0]),
+     7: array([0, 0, 0, 0, 0])}
+
+The return value is a dictionary from qubit index to results for all trials.
+Every qubit in the lattice is measured for you, and as expected, qubit 0 has been flipped to the excited state
+for each trial.
 
 The ``.run(...)`` method
-------------------------
+^^^^^^^^^^^^^^^^^^^^^^^^
 
-The ``.run(...)`` method takes in a compiled program. You are responsible for compiling your program before running it.
-Remember to also start up a ``quilc`` compiler server, too, with ``quilc -S``.
+The lower-level ``.run(...)`` method gives you more control over how you want to build and compile your program than
+``.run_and_measure`` does. **You are responsible for compiling your program before running it.**
+The above program would be written in this way to execute with ``run``:
 
 .. code:: python
 
+    from pyquil import Program, get_qc
+    from pyquil.gates import X, MEASURE
+
+    qc = get_qc("8q-qvm")
+
     p = Program()
     ro = p.declare('ro', 'BIT', 1)
     p += X(0)
     p += MEASURE(0, ro[0])
     p += MEASURE(1, ro[1])
     p.wrap_in_numshots_loop(5)
+
     executable = qc.compile(p)
-    results = qc.run(executable)
-    print(results)
+    bitstrings = qc.run(executable)  # .run takes in a compiled program, unlike .run_and_measure
+    print(bitstrings)
 
-The results returned are a *list of lists of integers*. In the above case, that's
+By specifying ``MEASURE`` ourselves, we will only get the results that we are interested in. To be completely equivalent
+to the previous example, we would have to measure all eight qubits.
+
+The results returned is a *list of lists of integers*. In the above case, that's
 
 .. parsed-literal::
 
@@ -86,23 +301,62 @@ we use as our readout register. We see that the result of this program is that t
 the state of qubit 0, which should be ``1`` after an :math:`X`-gate. See :ref:`declaring_memory` and :ref:`measurement`
 for more details about declaring and accessing classical memory regions.
 
-.. [1] https://arxiv.org/abs/1608.03355
+.. tip:: Get the results for qubit 0 with ``numpy.array(bitstrings)[:,0]``.
+
+.. _new_topology:
+
+Providing Your Own Device Topology
+----------------------------------
+
+It is simple to provide your own device topology as long as you can give your qubits each a number,
+and specify which edges exist. Here is an example, using the topology of our 16Q chip (two octagons connected by a square):
+
+.. code:: python
+
+    import networkx as nx
+
+    from pyquil.device import NxDevice, gates_in_isa
+    from pyquil.noise import decoherence_noise_with_asymmetric_ro
+
+    qubits = [0, 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 15, 16, 17]  # qubits are numbered by octagon
+    edges = [(0, 1), (1, 2), (2, 3), (3, 4), (4, 5), (5, 6), (6, 7), (7, 0),  # first octagon
+             (1, 16), (2, 15),  # connections across the square
+             (10, 11), (11, 12), (13, 14), (14, 15), (15, 16), (16, 17), (10, 17)] # second octagon
+
+    # Build the NX graph
+    topo = nx.from_edgelist(edges)
+    # You would uncomment the next line if you have disconnected qubits
+    # topo.add_nodes_from(qubits)
+    device = NxDevice(topo)
+    device.noise_model = decoherence_noise_with_asymmetric_ro(gates_in_isa(device.get_isa()))  # Optional
+
+Now that you have your device, you could set ``qc.device`` and ``qc.compiler.device`` to point to your new device,
+or use it to make new objects.
 
 Simulating the QPU using the QVM
 --------------------------------
 
-The QVM is a powerful tool for testing quantum programs before executing them on the QPU.
+The :py:class:`~pyquil.api.QAM` methods are intended to be used in the same way, whether a QVM or QPU is being targeted.
+Everywhere on this page,
+you can swap out the type of the QAM (QVM <=> QPU) and you will still
+get reasonable results back. As long as the topology of the devices are the same, programs compiled and ran on the QVM
+will be able to run on the QPU and visa-versa. Since :py:class:`~pyquil.api.QuantumComputer` is built on the ``QAM``
+abstract class, its methods will also work for both QAM implementations.
+
+This makes the QVM a powerful tool for testing quantum programs before executing them on the QPU.
 
 .. code:: python
 
-    qc = get_qc("QuantumComputerName")
-    qc = get_qc("QuantumComputerName-qvm")
+    qpu = get_qc(QPU_LATTICE_NAME)
+    qvm = get_qc(QPU_LATTICE_NAME, as_qvm=True)
 
-By simply providing ``-qvm`` in the device name, all programs executed on this QVM will, have the same topology as
-the named QPU. To learn how to add noise models to your virtual ``QuantumComputer`` instance, check out
+By simply providing ``as_qvm=True``, we get a QVM which will have the same topology as
+the named QPU. It's a good idea to run your programs against the QVM before booking QPU time to iron out
+bugs. To learn more about how to add noise models to your virtual ``QuantumComputer`` instance, check out
 :ref:`noise`.
 
-The Quantum Processing Unit
-~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In the next section, we will see how to use the Wavefunction Simulator aspect of the Rigetti QVM to inspect the full
+wavefunction set up by a Quil program.
+
+.. [1] https://arxiv.org/abs/1608.03355
 
-*Coming soon*: Detailed information about how to use :py:func:`~pyquil.get_qc` to target a QPU.