# TensorFlow Quantum design
TensorFlow Quantum (TFQ) is designed for the problems of NISQ-era quantum machine learning. It brings quantum computing primitives---like building quantum circuits---to the TensorFlow ecosystem. Models and operations built with TensorFlow use these primitives to create powerful quantum-classical hybrid systems. Using TFQ, researchers can construct a TensorFlow graph using a quantum dataset, a quantum model, and classical control parameters. These are all represented as tensors in a single computational graph. The outcome of quantum measurements---leading to classical probabilistic events---is obtained by TensorFlow ops. Training is done with the standard [Keras](https://www.tensorflow.org/guide/keras/overview) API. The [`tfq.datasets`](https://www.tensorflow.org/quantum/api_docs/python/tfq/datasets) module allows researchers to experiment with new and interesting quantum datasets. ## Cirq [Cirq](https://github.com/quantumlib/Cirq) is a quantum programming framework from Google. It provides all of the basic operations---such as qubits, gates, circuits, and measurement---to create, modify and invoke quantum circuits on a quantum computer, or a simulated quantum computer. TensorFlow Quantum uses these Cirq primitives to extend TensorFlow for batch computation, model building, and gradient computation. To be effective with TensorFlow Quantum, it's a good idea to be effective with Cirq. ## TensorFlow Quantum primitives TensorFlow Quantum implements the components needed to integrate TensorFlow with quantum computing hardware. To that end, TFQ introduces two datatype primitives: - *Quantum circuit* : This represents [Cirq](https://github.com/quantumlib/Cirq)-defined quantum circuits ([`cirq.Circuit`](https://quantumai.google/reference/python/cirq/Circuit)) within TensorFlow. Create batches of circuits of varying size, similar to batches of different real-valued datapoints. - *Pauli sum* : Represent linear combinations of tensor products of Pauli operators defined in Cirq ([`cirq.PauliSum`](https://quantumai.google/reference/python/cirq/PauliSum)). Like circuits, create batches of operators of varying size. ### Fundamental ops Using the quantum circuit primitives within a [`tf.Tensor`](https://www.tensorflow.org/api_docs/python/tf/Tensor), TensorFlow Quantum implements ops that process these circuits and produce meaningful outputs. The TensorFlow ops are written in optimized C++. These ops sample from circuits, calculate expectation values, and output the state produced by the given circuits. Writing ops that are flexible and performant has some challenges: 1. Circuits are not the same size. For simulated circuits, you are unable to create static operations (like [`tf.matmul`](https://www.tensorflow.org/api_docs/python/tf/linalg/matmul) or [`tf.add`](https://www.tensorflow.org/api_docs/python/tf/math/add)) and then substitute different numbers for circuits of different sizes. These ops must allow for dynamic sizes that the statically sized TensorFlow compute graph doesn't allow. 2. Quantum data can induce a different circuit structure altogether. This is another reason to support dynamic sizes in the TFQ ops. Quantum data can represent a structural change to the underlying quantum state that is represented by modifications to the original circuit. As new datapoints are swapped in and out at runtime, the TensorFlow compute graph can not be modified after it is built, so support for these varying structures is required. 3. `cirq.Circuits` are similar to compute graphs in that they are a series of operations---and some might contain symbols/placeholders. It is important to make this as compatible with TensorFlow as possible. For performance reasons, Eigen (the C++ library used in many TensorFlow ops) is not well suited for quantum circuit simulation. Instead, the circuit simulators used in the [quantum beyond-classical experiment](https://ai.googleblog.com/2019/10/quantum-supremacy-using-programmable.html) are used as verifiers and extended as the foundation of TFQ ops (all written with AVX2 and SSE instructions). Ops with identical functional signatures were created that use a physical quantum computer. Switching between a simulated and physical quantum computer is as easy as changing a single line of code. These ops are located in the [`circuit_execution_ops.py`](https://github.com/tensorflow/quantum/blob/master/tensorflow_quantum/core/ops/circuit_execution_ops.py). ### Layers TensorFlow Quantum layers expose sampling, expectation, and state calculation to developers using the [`tf.keras.layers.Layer`](https://www.tensorflow.org/api_docs/python/tf/keras/Layer) interface. It's convenient to create a circuit layer for classical control parameters or for readout operations. Additionally, you can create a layer with a high degree of complexity supporting batch circuit, batch control parameter value, and perform batch readout operations. See [`tfq.layers.Sample`](https://www.tensorflow.org/quantum/api_docs/python/tfq/layers/Sample) for an example. ### Differentiators Unlike many TensorFlow operations, observables in quantum circuits do not have formulas for gradients that are relatively easy to calculate. This is because a classical computer can only read samples from the circuits that are run on a quantum computer. To solve this problem, the [`tfq.differentiators`](https://www.tensorflow.org/quantum/api_docs/python/tfq/differentiators) module provides several standard differentiation techniques. Users can also define their own method to compute gradients---in both the "real world" setting of sample-based expectation calculation, and the analytic exact world. Methods like finite difference are often the fastest (wall clock time) in an analytic/exact environment. While slower (wall clock time), more practical methods like [parameter shift](https://arxiv.org/abs/1811.11184) or [stochastic methods](https://arxiv.org/abs/1901.05374) are often more effective. A [`tfq.differentiators.Differentiator`](https://www.tensorflow.org/quantum/api_docs/python/tfq/differentiators/Differentiator) is instantiated and attached to an existing op with `generate_differentiable_op`, or passed to the constructor of [`tfq.layers.Expectation`](https://www.tensorflow.org/quantum/api_docs/python/tfq/layers/Expectation) or [`tfq.layers.SampledExpectation`](https://www.tensorflow.org/quantum/api_docs/python/tfq/layers/SampledExpectation). To implement a custom differentiator, inherit from the [`tfq.differentiators.Differentiator`](https://www.tensorflow.org/quantum/api_docs/python/tfq/differentiators/Differentiator) class. To define a gradient operation for sampling or state vector calculation, use [`tf.custom_gradient`](https://www.tensorflow.org/api_docs/python/tf/custom_gradient). ### Datasets As the field of quantum computing grows, more quantum data and model combinations will arise, making structured comparison more difficult. The [`tfq.datasets`](https://www.tensorflow.org/quantum/api_docs/python/tfq/datasets) module is used as the data source for quantum machine learning tasks. It ensures structured comparisons for the model and performance. It is hoped that with large community contributions, the [`tfq.datasets`](https://www.tensorflow.org/quantum/api_docs/python/tfq/datasets) module will grow to enable research that is more transparent and reproducible. Carefully curated problems in: quantum control, fermionic simulation, classification near phase transitions, quantum sensing, etc are all great candidates for addition to [`tfq.datasets`](https://www.tensorflow.org/quantum/api_docs/python/tfq/datasets). To propose a new dataset open a [GitHub issue](https://github.com/tensorflow/quantum/issues).