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Qiskit 0.5 release notes


Terra 0.5


This release brings a number of improvements to Qiskit, both for the user experience and under the hood. Please refer to the full changelog for a detailed description of the changes - the highlights are:

  • new statevector simulators and feature and performance improvements to the existing ones (in particular to the C++ simulator), along with a reorganization of how to work with backends focused on extensibility and flexibility (using aliases and backend providers)
  • reorganization of the asynchronous features, providing a friendlier interface for running jobs asynchronously via Job instances
  • numerous improvements and fixes throughout the Terra as a whole, both for convenience of the users (such as allowing anonymous registers) and for enhanced functionality (such as improved plotting of circuits)

Compatibility Considerations

Please note that several backwards-incompatible changes have been introduced during this release as a result of the ongoing development. While some of these features will continue to be supported during a period of time before being fully deprecated, it is recommended to update your programs in order to prepare for the new versions and take advantage of the new functionality.

QuantumProgram changes

Several methods of the QuantumProgram class are on their way to being deprecated:

  • methods for interacting with the backends and the API:

    The recommended way for opening a connection to the IBM Q API and for using the backends is through the top-level functions directly instead of the QuantumProgram methods. In particular, the qiskit.register() method provides the equivalent of the previous qiskit.QuantumProgram.set_api() call. In a similar vein, there is a new qiskit.available_backends(), qiskit.get_backend() and related functions for querying the available backends directly. For example, the following snippet for version 0.4:

    from qiskit import QuantumProgram
    quantum_program = QuantumProgram()
    quantum_program.set_api(token, url)
    backends = quantum_program.available_backends()

    would be equivalent to the following snippet for version 0.5:

    from qiskit import register, available_backends, get_backend
    register(token, url)
    backends = available_backends()
    backend = get_backend('ibmqx4')
  • methods for compiling and executing programs:

    The top-level functions now also provide equivalents for the qiskit.QuantumProgram.compile() and qiskit.QuantumProgram.execute() methods. For example, the following snippet from version 0.4:

    quantum_program.execute(circuit, args, ...)

    would be equivalent to the following snippet for version 0.5:

    from qiskit import execute
    execute(circuit, args, ...)

In general, from version 0.5 onwards we encourage to try to make use of the individual objects and classes directly instead of relying on QuantumProgram. For example, a QuantumCircuit can be instantiated and constructed by appending QuantumRegister, ClassicalRegister, and gates directly. Please check the update example in the Quickstart section, or the and examples on the main repository.

Backend name changes

In order to provide a more extensible framework for backends, there have been some design changes accordingly:

  • local simulator names

    The names of the local simulators have been homogenized in order to follow the same pattern: PROVIDERNAME_TYPE_simulator_LANGUAGEORPROJECT - for example, the C++ simulator previously named local_qiskit_simulator is now local_qasm_simulator_cpp. An overview of the current simulators:

    • QASM simulator is supposed to be like an experiment. You apply a circuit on some qubits, and observe measurement results - and you repeat for many shots to get a histogram of counts via result.get_counts().
    • Statevector simulator is to get the full statevector (2n2^n amplitudes) after evolving the zero state through the circuit, and can be obtained via result.get_statevector().
    • Unitary simulator is to get the unitary matrix equivalent of the circuit, returned via result.get_unitary().
    • In addition, you can get intermediate states from a simulator by applying a snapshot(slot) instruction at various spots in the circuit. This will save the current state of the simulator in a given slot, which can later be retrieved via result.get_snapshot(slot).
  • backend aliases:

    The SDK now provides an “alias” system that allows for automatically using the most performant simulator of a specific type, if it is available in your system. For example, with the following snippet:

    from qiskit import get_backend
    backend = get_backend('local_statevector_simulator')

    the backend will be the C++ statevector simulator if available, falling back to the Python statevector simulator if not present.

More flexible names and parameters

Several functions of the SDK have been made more flexible and user-friendly:

  • automatic circuit and register names

    qiskit.ClassicalRegister, qiskit.QuantumRegister and qiskit.QuantumCircuit can now be instantiated without explicitly giving them a name - a new autonaming feature will automatically assign them an identifier:

    q = QuantumRegister(2)

    Please note as well that the order of the parameters have been swapped QuantumRegister(size, name).

  • methods accepting names or instances

    In combination with the autonaming changes, several methods such as qiskit.Result.get_data() now accept both names and instances for convenience. For example, when retrieving the results for a job that has a single circuit such as:

    qc = QuantumCircuit(..., name='my_circuit')
    job = execute(qc, ...)
    result = job.result()

    The following calls are equivalent:

    data = result.get_data('my_circuit')
    data = result.get_data(qc)
    data = result.get_data()
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