Lightning Qubit device

The lightning.qubit device uses a custom-built backend to perform fast linear algebra calculations for simulating quantum state-vector evolution.

A lightning.qubit device can be loaded using:

import pennylane as qml
dev = qml.device("lightning.qubit", wires=2)

Check out the Lightning-Qubit installation guide for more information.

Supported operations and observables

Supported operations:

BasisState	Prepares a single computational basis state.
CNOT	The controlled-NOT operator
ControlledPhaseShift	A qubit controlled phase shift.
ControlledQubitUnitary	Apply an arbitrary fixed unitary to wires with control from the control_wires.
CRot	The controlled-Rot operator
CRX	The controlled-RX operator
CRY	The controlled-RY operator
CRZ	The controlled-RZ operator
CSWAP	The controlled-swap operator
CY	The controlled-Y operator
CZ	The controlled-Z operator
DiagonalQubitUnitary	Apply an arbitrary diagonal unitary matrix with a dimension that is a power of two.
DoubleExcitation	Double excitation rotation.
DoubleExcitationMinus	Double excitation rotation with negative phase-shift outside the rotation subspace.
DoubleExcitationPlus	Double excitation rotation with positive phase-shift outside the rotation subspace.
ECR	An echoed RZX(\(\pi/2\)) gate.
Hadamard	The Hadamard operator
Identity	The Identity operator
IsingXX	Ising XX coupling gate
IsingXY	Ising (XX + YY) coupling gate
IsingYY	Ising YY coupling gate
IsingZZ	Ising ZZ coupling gate
ISWAP	The i-swap operator
MultiControlledX	Apply a Pauli X gate controlled on an arbitrary computational basis state.
MultiRZ	Arbitrary multi Z rotation.
OrbitalRotation	Spin-adapted spatial orbital rotation.
PauliX	The Pauli X operator
PauliY	The Pauli Y operator
PauliZ	The Pauli Z operator
PhaseShift	Arbitrary single qubit local phase shift
PSWAP	Phase SWAP gate
QFT	Apply a quantum Fourier transform (QFT).
QubitCarry	Apply the QubitCarry operation to four input wires.
QubitSum	Apply a QubitSum operation on three input wires.
QubitUnitary	Apply an arbitrary unitary matrix with a dimension that is a power of two.
Rot	Arbitrary single qubit rotation
RX	The single qubit X rotation
RY	The single qubit Y rotation
RZ	The single qubit Z rotation
S	The single-qubit phase gate
SingleExcitation	Single excitation rotation.
SingleExcitationMinus	Single excitation rotation with negative phase-shift outside the rotation subspace.
SingleExcitationPlus	Single excitation rotation with positive phase-shift outside the rotation subspace.
SISWAP	The square root of i-swap operator.
SQISW	alias of SISWAP
SWAP	The swap operator
SX	The single-qubit Square-Root X operator.
T	The single-qubit T gate
Toffoli	Toffoli (controlled-controlled-X) gate.

Supported observables:

Identity	The Identity operator
Hadamard	The Hadamard operator
PauliX	The Pauli X operator
PauliY	The Pauli Y operator
PauliZ	The Pauli Z operator
Projector	Observable corresponding to the state projector \(P=\ket{\phi}\bra{\phi}\).
Hermitian	An arbitrary Hermitian observable.
Hamiltonian	alias of LinearCombination
SparseHamiltonian	A Hamiltonian represented directly as a sparse matrix in Compressed Sparse Row (CSR) format.
Exp	A symbolic operator representing the exponential of a operator.
Prod	Symbolic operator representing the product of operators.
SProd	Arithmetic operator representing the scalar product of an operator with the given scalar.
Sum	Symbolic operator representing the sum of operators.

Parallel adjoint differentiation support:

The lightning.qubit device directly supports the adjoint differentiation method, and enables parallelization over the requested observables (Linux/MacOS support only).

To enable parallel differentiation over observables, ensure the OMP_NUM_THREADS environment variable is set before starting your Python session, or if already started, before importing packages:

# Option 1: Before starting Python
export OMP_NUM_THREADS=4
python <your_file>.py

# Option 2: Before importing packages
import os
os.environ["OMP_NUM_THREADS"] = 4
import pennylane as qml

Assuming you request multiple expectation values from a QNode, this should automatically parallelize the computation over the requested number of threads. You should ensure that the number of threads does not exceed the available physical cores on your machine.

If you are computing a large number of expectation values, or if you are using a large number of wires on your device, it may be best to limit the number of expectation value calculations to at-most OMP_NUM_THREADS concurrent executions. This will help save memory, at the cost of additional compute time. To enable this, initialize a lightning.qubit device with the batch_obs=True keyword argument, as:

# Before importing packages
import os
os.environ["OMP_NUM_THREADS"] = 4
import pennylane as qml
dev = qml.device("lightning.qubit", wires=2, batch_obs=True)

Markov Chain Monte Carlo sampling support:

The lightning.qubit device allows users to use the Markov Chain Monte Carlo (MCMC) sampling method to generate approximate samples. To enable the MCMC sampling method for sample generation, initialize a lightning.qubit device with the mcmc=True keyword argument, as:

import pennylane as qml
dev = qml.device("lightning.qubit", wires=2, shots=1000, mcmc=True)

By default, the kernel_name is "Local" and num_burnin is 100. The local kernel conducts a bit-flip local transition between states. The local kernel generates a random qubit site and then generates a random number to determine the new bit at that qubit site.

The lightning.qubit device also supports a "NonZeroRandom" kernel. This kernel randomly transits between states that have nonzero probability. It can be enabled by initializing the device as:

import pennylane as qml
dev = qml.device("lightning.qubit", wires=2, shots=1000, mcmc=True, kernel_name="NonZeroRandom", num_burnin=200)

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