The QVM device

The forest.qvm device provides an interface between PennyLane and the Forest SDK quantum virtual machine or the pyQuil built-in pyQVM. The QVM is used to simulate various quantum abstract machines, ranging from simulations of physical QPUs to completely connected lattices.

Usage

When initializing the forest.qvm device, the following required keyword argument must also be passed:

device (str or networkx.Graph)

The name or topology of the quantum computer to initialize.

  • Nq-qvm: for a fully connected/unrestricted N-qubit QVM.
  • 9q-square-qvm: a \(9\times 9\) lattice.
  • Nq-pyqvm or 9q-square-pyqvm, for the same as the above but run via the built-in pyQuil pyQVM device.
  • Any other supported Rigetti device architecture, for example a QPU lattice such as 'Aspen-8'.
  • Graph topology (as a networkx.Graph object) representing the device architecture.

Note that, unlike forest.wavefunction, you do not pass the number of wires - this is inferred automatically from the requested quantum computer topology.

>>> import pennylane as qml
>>> dev = qml.device('forest.qvm', device='Aspen-8')
>>> dev.num_wires
16

In addition, you may also request a QVM with noise models to better simulate a physical QPU; this is done by passing the keyword argument noisy=True:

>>> dev = qml.device('forest.qvm', device='Aspen-8', noisy=True)

Note that only the default noise models provided by pyQuil are currently supported.

To specify the pyQVM, simply append pyqvm to the end of the device name instead of qvm:

>>> dev = qml.device('forest.qvm', device='4q-pyqvm')

The device can then be used just like other devices for the definition and evaluation of QNodes within PennyLane.

A simple quantum function that returns the expectation value and variance of a measurement and depends on three classical input parameters would look like:

@qml.qnode(dev)
def circuit(x, y, z):
    qml.RZ(z, wires=[0])
    qml.RY(y, wires=[0])
    qml.RX(x, wires=[0])
    qml.CNOT(wires=[0, 1])
    return qml.expval(qml.PauliZ(0)), var(qml.PauliZ(1))

You can then execute the circuit like any other function to get the quantum mechanical expectation value and variance:

>>> circuit(0.2, 0.1, 0.3)
array([0.97517033, 0.04904283])

Measurements and expectations

Since the QVM returns a number of trial measurements of the quantum circuit, the larger the number of ‘trials’ or ‘shots’, the closer PennyLane is able to approximate the expectation value, and as a result the gradient. By default, shots=1024, but this can be increased or decreased as required.

For example, see how increasing the shot count increases the expectation value and corresponding gradient accuracy:

def circuit(x):
    qml.RX(x, wires=[0])
    return qml.expval(qml.PauliZ(0))

dev_exact = qml.device('forest.wavefunction', wires=1)
dev_s1024 = qml.device('forest.qvm', device='1q-qvm')
dev_s100000 = qml.device('forest.qvm', device='1q-qvm', shots=100000)

circuit_exact = qml.QNode(circuit, dev_exact)
circuit_s1024 = qml.QNode(circuit, dev_s1024)
circuit_s100000 = qml.QNode(circuit, dev_s100000)

Printing out the results of the three device expectation values:

>>> circuit_exact(0.8)
0.6967067093471655
>>> circuit_s1024(0.8)
0.689453125
>>> circuit_s100000(0.8)
0.6977

Supported operations

All devices support all PennyLane operations and observables, with he exception of the PennyLane QubitStateVector state preparation operation.

In addition, PennyLane-Forest provides the following PyQuil-specific operations for PennyLane. These are all importable from pennylane_forest.ops.

These operations include:

pennylane_forest.ops.CPHASE(*params[, …]) CHPASE(phi, q, wires) Controlled-phase gate.
pennylane_forest.ops.ISWAP(wires) iSWAP gate.
pennylane_forest.ops.PSWAP(wires) Phase-SWAP gate.

Supported observables

The QVM device supports qml.PauliZ observables values ‘natively’, while also supporting qml.Identity, qml.PauliY, qml.Hadamard, and qml.Hermitian by performing implicit change of basis operations.

Native observables

The QVM currently supports only one measurement, returning 1 if the qubit is measured to be in the state \(|1\rangle\), and 0 if the qubit is measured to be in the state \(|0\rangle\). This is equivalent to measuring in the Pauli-Z basis, with state \(|1\rangle\) corresponding to Pauli-Z eigenvalue \(\lambda=-1\), and likewise state \(|0\rangle\) corresponding to eigenvalue \(\lambda=1\). As a result, we can simply perform a rescaling of the measurement results to get the Pauli-Z expectation value of the \(i\) th wire:

\[\langle Z \rangle_{i} = \frac{1}{N}\sum_{j=1}^N (1-2m_j)\]

where \(N\) is the total number of shots, and \(m_j\) is the \(j\) th measurement of wire \(i\).

Change of measurement basis

For the remaining observables, it is easy to perform a quantum change of basis operation before measurement such that the correct expectation value is performed. For example, say we have a unitary Hermitian observable \(\hat{A}\). Since, by definition, it must have eigenvalues \(\pm 1\), there will always exist a unitary matrix \(U\) such that it satisfies the following similarity transform:

\[\hat{A} = U^\dagger Z U\]

Since \(U\) is unitary, it can be applied to the specified qubit before measurement in the Pauli-Z basis. Below is a table of the various change of basis operations performed implicitly by PennyLane.

Observable Change of basis gate \(U\)
qml.PauliX \(H\)
qml.PauliY \(H S^{-1}=HSZ\)
qml.Hadamard \(R_y(-\pi/4)\)

To see how this affects the resultant Quil program, you may use the program property to print out the Quil program after evaluation on the device.

dev = qml.device('forest.qvm', device='2q-qvm')

@qml.qnode(dev)
def circuit(x):
    qml.RX(x, wires=[0])
    return expval(qml.PauliY(0))
>>> circuit(0.54)
-0.525390625
>>> print(dev.program)
PRAGMA INITIAL_REWIRING "PARTIAL"
RX(0.54000000000000004) 0
Z 0
S 0
H 0
DECLARE ro BIT[1]
MEASURE 0 ro[0]

Note

program will return the last evaluated quantum program performed on the device. If viewing program after evaluating a quantum gradient or performing an optimization, this may not match the user-defined QNode, as PennyLane automatically modifies the QNode to take into account the parameter shift rule, product rule, and chain rule.

Arbitrary Hermitian observables

Arbitrary Hermitian observables, qml.Hermitian, are also supported by the QVM. However, since they are not necessarily unitary (and thus have eigenvalues \(\lambda_i\neq \pm 1\)), we cannot use the similarity transform approach above.

Instead, we can calculate the eigenvectors \(\mathbf{v}_i\) of \(\hat{A}\), and construct our unitary change of basis operation as follows:

\[U=\begin{bmatrix}\mathbf{v}_1 & \mathbf{v}_2 \end{bmatrix}^\dagger.\]

After measuring the qubit state, we can determine the probability \(P_0\) of measuring state \(|0\rangle\) and the probability \(P_1\) of measuring state \(|1\rangle\), and, using the eigenvalues of \(\hat{A}\), recover the expectation value \(\langle\hat{A}\rangle\):

\[\langle\hat{A}\rangle = \lambda_1 P_0 + \lambda_2 P_1\]

This process is done automatically behind the scenes in the QVM device when qml.expval(qml.Hermitian) is returned.

Device options

On initialization, the PennyLane-Forest devices accept additional keyword arguments beyond the PennyLane default device arguments.

forest_url (str)
the Forest URL server. Can also be set by the environment variable FOREST_SERVER_URL, or in the ~/.qcs_config configuration file. Default value is "https://forest-server.qcs.rigetti.com".
qvm_url (str)
the QVM server URL. Can also be set by the environment variable QVM_URL, or in the ~/.forest_config configuration file. Default value is "http://127.0.0.1:5000".
compiler_url (str)
the compiler server URL. Can also be set by the environment variable COMPILER_URL, or in the ~/.forest_config configuration file. Default value is "http://127.0.0.1:6000".

Note

If using the downloadable Forest SDK with the default server configurations for the QVM and the Quil compiler (i.e., you launch them with the commands qvm -S and quilc -R), then you will not need to set these keyword arguments.

Likewise, if you are running PennyLane using the Rigetti Quantum Cloud Service (QCS) on a provided QMI, these environment variables are set automatically and will also not need to be passed in PennyLane.