Cirq was developed by the Google AI Quantum Team, and the public alpha was announced at the International Workshop on Quantum Software and Quantum Machine Learning on July 18, 2018. A demo by QC Ware showed an implementation of QAOA solving an example of the maximum cut problem being solved on a Cirq simulator.
Quantum programs in Cirq are represented by "Circuit" and "Schedule" where "Circuit" represents a Quantum circuit and "Schedule" represents a Quantum circuit with timing information.  The programs can be executed on local simulators. 
The following example shows how to create and measure a Bell state in Cirq.
import cirq # Pick qubits qubit0 = cirq.GridQubit(0, 0) qubit1 = cirq.GridQubit(0, 1) # Create a circuit circuit = cirq.Circuit.from_ops( cirq.H(qubit0), cirq.CNOT(qubit0, qubit1), cirq.measure(qubit0, key='m0'), cirq.measure(qubit1, key='m1') )
Printing the circuit displays it's diagram
print(circuit) # prints # (0, 0): ───H───@───M('m0')─── # │ # (0, 1): ───────X───M('m1')───
Simulating the circuit repeatedly shows that the measurements of the qubits are correlated.
simulator = cirq.Simulator() result = simulator.run(circuit, repetitions=5) print(result) # prints # m0=11010 # m1=11010
OpenFermion-Cirq is a library that compiles quantum simulation algorithms to Cirq. 
- Fingerhuth, Mark; Babej, Tomáš; Wittek, Peter (2018). "Open source software in quantum computing". arXiv:1812.09167.
- Ho, Alan; Bacon, Dave (2018-06-18). "Announcing Cirq: An Open Source Framework for NISQ Algorithms". Google AI Blog. Google AI Quantum Team. Retrieved 2019-03-06.
- "public_demos/max_cut_cirq.py at master · qcware/public_demos · GitHub". web.archive.org. 20 July 2018. Retrieved 29 October 2019.
- "Cirq Circuits". Cirq Github repository. Google AI Quantum Team. 2019-01-09. Retrieved 2019-03-06.
- Google AI Quantum Team (2019-01-09). "Cirq Simulation". Cirq Github repository. quantumlib. Retrieved 2019-03-06.