Changelog

Deprecated

• The gates U and CX are being deprecated in favor of u3 and
  cx. (#2380)
• The gate u0 is being deprecated in favor of using multiple id gates
  to insert delays (#2664)
• The decorator requires_qe_access is being deprecated in favor of
  online_test.
• The as_dict method of Qobj is deprecated in favor of to_dict. (#2556)

Added

• A new pulse instruction, Delay. A Delay occupies a pulse channel for a
  duration of time, blocking other instructions from being inserted in this time. (#2869)
• Ability to check for equality of pulse Schedule and Instruction. (#2873)
• Added tests for gate_map and reference images for testing plot_gate_map
• New CountOpsLongest analysis pass to retrieve the number of operations
  on the longest path of the DAGCircuit. (#2734)
• Added sech and sech_deriv pulses in qiskit.pulse.pulse_lib.
• The option vertical_compression was added to the text drawer and
  to the QuantumCircuit.draw method. The option allows to control
  how much room the text circuit drawing takes.
• The option idle_wires was added to the drawers to control
  if wires without any operation should be included in the drawing. (#2692)
• Introduced a visualization for the Pass Manager. (#2445)
• New pulse schedule method Schedule.filter to filter by instruction
  channel, time, and type. (#2597)
• Decomposition of arbitrary isometries (#2600)
• Decomposition of diagonal gates (#2600)
• Decomposition of multiplexed rotation gates (#2600)
• Decomposition of multiplexed single-qubit unitaries (Option: decompose
  up to a diagonal gate) (#2600)
• ZYZ decomposition for single-qubit unitaries (#2600)
• Gray-Synth and Patel–Markov–Hayes algorithms for synthesis of
  CNOT-Phase and CNOT-only (linear) circuits (#2457)
• Added n-qubit unitaries to BasicAer simulator basis gates (#2342)
• Added a random_circuit function under qiskit.circuit.random
  (#2553)
• Added equiv method to Operator and Statevector classes for
  testing if two objects are equivalent up to global phase (#2910)
• Added output_name as a transpiler parameter to set the name of
  output circuits (#2745)
• Simple expressions of Parameters can now be created via the four basic math
  operations (+,-,*,/). (#2537)
• A ParmeterVector class has been added to ease the construction of circuits
  requiring a large number of parameters. (#2379)
• dag.draw() method to visualize DAGCircuit objects (#3016)

Changed

• Intervals are now defined by start and stop, rather than begin and end.
• TimeslotCollections now are sorted by default and have more efficient merges.
• Pulse samples are now clipped if their norm is between 1 and 1+epsilon.
  Otherwise an error is raised.
• Schedule.instructions now returns with time-ordering.
• More informative errors are now raised if qubit_lo_freq and
  meas_lo_freq are not supplied to assemble_schedules. (#2833)
• pulse.samplers module has now been moved to pulse.pulse_lib.samplers. (#2881)
• The number of memory slots required will now be inferred from the supplied
  schedules if memory_slots is not supplied.
• All circuit drawers now express most commonly used fractions
  of PI (#2808).
• Set default repetition time to be the first available.
• Pulse commands may now start with capitalized letters.
• The pylatexenc and pillow requirements are now optional. These
  are only used by the latex and latex_source circuit
  visualization backends. To continue using them ensure these are
  installed. (#2451)
• When adding a register to a circuit, an error will now be raised if
  a register of the same name is already present. Previously, an error
  would only be raised if the same register was added twice.
• Qubits and classical bits are not represented as a tuples anymore,
  but as instances of Qubit and Clbit respectively. (#2414)
• The ApplyLayout pass is incorporated in all preset pass managers to
  delineate a virtual circuit from a physical circuit (#2672)
• Mapping passes (CXDirection, Swap passes, CheckMap, CheckCnotDirection)
  now operate on a register-less circuit corresponding to
  an already embedded physical circuit. (#2672)
• Replaces LegacySwap by faster, more stable StochasticSwap pass (#2672)
• Uses level 1 by default as transpiler optimization level (#2672)
• Change Snapshot signature to match simulator.snapshot (#2592)
• DAGCircuit.width() formerly returned number of qubits, now returns total
  number of qubits + classical bits (#2564)
• Functions assuming the former semantics of DAGCircuit.width() now call
  DAGCircuit.num_qubits() (#2564)
• DAGCircuit.num_cbits() renamed to DAGCircuit.num_clbits() (#2564)
• Changed definition of Cu3Gate to to equivalent to the canonical
  definition of a controlled U3Gate (#2755)
• coupling_map now required to validate a backend.configuration() (#2836)
• The method QuantumCircuit.count_ops now returns an OrderedDict instead of a dict.
• If layout information is available in
  the circuit, it will be included to the circuit drawing. This can be removed
  using the option with_layout=False in the method
  QuantumCircuit.draw. (#2739)
• Q-sphere visualization is enhanced and corrected (#2932)
• Shorter CH gate definition involving only 1 CX (#2837)
• Now PassManager.draw() (without any argument) will return an in-memory PIL image.

Removed

• The ability to set the Timeslots for a pulse Instruction at initialization.
• The previously deprecated functions
  qiskit.visualization.plot_state and
  qiskit.visualization.iplot_state have been removed. Instead use
  the specific functions for each plot type (#2325).
• International documentation of outdated readme etc (#2302)
• Removed deprecated options in execute, transpile, and assemble.
  Removed deprecated compiler.
• Removed deprecated qcvv in tools. Removed deprecated converters
  qobj_to_circuits and circuits_to_qobj (#2301)
• The previously deprecated qiskit._util module has been removed.
  Use qiskit.util instead. (#2329)
• The logging tools in qiskit.tools.logging are removed. (#2387)
• The qiskit.qiskiterror module has been removed. Please use
  qiskit.exceptions instead. (#2399)
• Removed previously deprecated DAGCircuit methods (#2542)
• Removed CompositeGate class, in favor of adding Instruction
  objects directly (#2543)
• Removed ignore_requires and ignore_preserves options from
  PassManager (#2565).

Fixed

• Fixes a bug where the CmdDef was getting built without buffers on channels.
• Fixed bug in Pulse for multiple parameters being added (#2742)
• Fixed bug in Pulse for CmdDef arguments (#2741)
• Fixed bug in Operator and SuperOp for initializing from circuit
  containing gates without an explicit matrix definition (#2723)
• Possible to decompose SU(4) gate into non-CNOT basis with
  TwoQubitDecomposer
• Fixes a bug that removed id gates from circuit. id gates are
  like a wait command and will never be removed (#2663)
• Fixed bug in CommutationAnalysis pass affecting conditional gates (#2669)
• Fixed bug in measure sampling for BasicAer Qasm simulator if a qubit
  was measured into more than one classical bit (#2735)
• Correctly serialize complex numbers with a nonzero real part
• Fixed bug in measure sampling for BasicAer Qasm simulator if only a
  subset of qubits are measured (#2790)
• Parameter objects can be serialized and communicated between
  sub-processes (#2429)
• Parameterized circuits no longer need to be transpiled individually (#2864)