QPY serialization
qiskit.qpy
QPY is a binary serialization format for QuantumCircuit
and ScheduleBlock
objects that is designed to be cross-platform, Python version agnostic, and backwards compatible moving forward. QPY should be used if you need a mechanism to save or copy between systems a QuantumCircuit
or ScheduleBlock
that preserves the full Qiskit object structure (except for custom attributes defined outside of Qiskit code). This differs from other serialization formats like OpenQASM (2.0 or 3.0) which has a different abstraction model and can result in a loss of information contained in the original circuit (or is unable to represent some aspects of the Qiskit objects) or Python’s pickle which will preserve the Qiskit object exactly but will only work for a single Qiskit version (it is also potentially insecure).
Using QPY
Using QPY is defined to be straightforward and mirror the user API of the serializers in Python’s standard library, pickle
and json
. There are 2 user facing functions: qiskit.qpy.dump()
and qiskit.qpy.load()
which are used to dump QPY data to a file object and load circuits from QPY data in a file object respectively. For example:
from qiskit.circuit import QuantumCircuit
from qiskit import qpy
qc = QuantumCircuit(2, name='Bell', metadata={'test': True})
qc.h(0)
qc.cx(0, 1)
qc.measure_all()
with open('bell.qpy', 'wb') as fd:
qpy.dump(qc, fd)
with open('bell.qpy', 'rb') as fd:
new_qc = qpy.load(fd)[0]
The qiskit.qpy.dump()
function also lets you include multiple circuits in a single QPY file:
with open('twenty_bells.qpy', 'wb') as fd:
qpy.dump([qc] * 20, fd)
and then loading that file will return a list with all the circuits
with open(‘twenty_bells.qpy’, ‘rb’) as fd:
twenty_new_bells = qpy.load(fd)
API documentation
load
qiskit.qpy.load(file_obj, metadata_deserializer=None)
Load a QPY binary file
This function is used to load a serialized QPY Qiskit program file and create QuantumCircuit
objects or ScheduleBlock
objects from its contents. For example:
from qiskit import qpy
with open('bell.qpy', 'rb') as fd:
circuits = qpy.load(fd)
or with a gzip compressed file:
import gzip
from qiskit import qpy
with gzip.open('bell.qpy.gz', 'rb') as fd:
circuits = qpy.load(fd)
which will read the contents of the qpy and return a list of QuantumCircuit
objects or ScheduleBlock
objects from the file.
Parameters
- file_obj (BinaryIO) – A file like object that contains the QPY binary data for a circuit or pulse schedule.
- metadata_deserializer (Type[JSONDecoder] | None) – An optional JSONDecoder class that will be used for the
cls
kwarg on the internaljson.load
call used to deserialize the JSON payload used for the.metadata
attribute for any programs in the QPY file. If this is not specified the circuit metadata will be parsed as JSON with the stdlibjson.load()
function using the defaultJSONDecoder
class.
Returns
The list of Qiskit programs contained in the QPY data. A list is always returned, even if there is only 1 program in the QPY data.
Raises
- QiskitError – if
file_obj
is not a valid QPY file - TypeError – When invalid data type is loaded.
Return type
dump
qiskit.qpy.dump(programs, file_obj, metadata_serializer=None, use_symengine=False)
Write QPY binary data to a file
This function is used to save a circuit to a file for later use or transfer between machines. The QPY format is backwards compatible and can be loaded with future versions of Qiskit.
For example:
from qiskit.circuit import QuantumCircuit
from qiskit import qpy
qc = QuantumCircuit(2, name='Bell', metadata={'test': True})
qc.h(0)
qc.cx(0, 1)
qc.measure_all()
from this you can write the qpy data to a file:
with open('bell.qpy', 'wb') as fd:
qpy.dump(qc, fd)
or a gzip compressed file:
import gzip
with gzip.open('bell.qpy.gz', 'wb') as fd:
qpy.dump(qc, fd)
Which will save the qpy serialized circuit to the provided file.
Parameters
- programs (List[QuantumCircuit |ScheduleBlock] | QuantumCircuit |ScheduleBlock) – QPY supported object(s) to store in the specified file like object. QPY supports
QuantumCircuit
andScheduleBlock
. Different data types must be separately serialized. - file_obj (BinaryIO) – The file like object to write the QPY data too
- metadata_serializer (Type[JSONEncoder] | None) – An optional JSONEncoder class that will be passed the
.metadata
attribute for each program inprograms
and will be used as thecls
kwarg on the json.dump()` call to JSON serialize that dictionary. - use_symengine (bool) – If True, all objects containing symbolic expressions will be serialized using symengine’s native mechanism. This is a faster serialization alternative, but not supported in all platforms. Please check that your target platform is supported by the symengine library before setting this option, as it will be required by qpy to deserialize the payload. For this reason, the option defaults to False.
Raises
These functions will raise a custom subclass of QiskitError
if they encounter problems during serialization or deserialization.
QpyError
QPY Compatibility
The QPY format is designed to be backwards compatible moving forward. This means you should be able to load a QPY with any newer Qiskit version than the one that generated it. However, loading a QPY file with an older Qiskit version is not supported and may not work.
For example, if you generated a QPY file using qiskit-terra 0.18.1 you could load that QPY file with qiskit-terra 0.19.0 and a hypothetical qiskit-terra 0.29.0. However, loading that QPY file with 0.18.0 is not supported and may not work.
QPY Format
The QPY serialization format is a portable cross-platform binary serialization format for QuantumCircuit
objects in Qiskit. The basic file format is as follows:
A QPY file (or memory object) always starts with the following 6 byte UTF8 string: QISKIT
which is immediately followed by the overall file header. The contents of the file header as defined as a C struct are:
struct {
uint8_t qpy_version;
uint8_t qiskit_major_version;
uint8_t qiskit_minor_version;
uint8_t qiskit_patch_version;
uint64_t num_circuits;
}
From V10 on, a new field is added to the file header struct to represent the encoding scheme used for symbolic expressions:
struct {
uint8_t qpy_version;
uint8_t qiskit_major_version;
uint8_t qiskit_minor_version;
uint8_t qiskit_patch_version;
uint64_t num_circuits;
char symbolic_encoding;
}
All values use network byte order [1] (big endian) for cross platform compatibility.
The file header is immediately followed by the circuit payloads. Each individual circuit is composed of the following parts:
HEADER | METADATA | REGISTERS | CUSTOM_DEFINITIONS | INSTRUCTIONS
There is a circuit payload for each circuit (where the total number is dictated by num_circuits
in the file header). There is no padding between the circuits in the data.
Version 10
Version 10 adds support for symengine-native serialization for objects of type ParameterExpression
as well as symbolic expressions in Pulse schedule blocks. Version 10 also adds support for new fields in the TranspileLayout
class added in the Qiskit 0.45.0 release.
The symbolic_encoding field is added to the file header, and a new encoding type char is introduced, mapped to each symbolic library as follows: p
refers to sympy encoding and e
refers to symengine encoding.
FILE_HEADER
The contents of FILE_HEADER after V10 are defined as a C struct as:
struct {
uint8_t qpy_version;
uint8_t qiskit_major_version;
uint8_t qiskit_minor_version;
uint8_t qiskit_patch_version;
uint64_t num_circuits;
char symbolic_encoding;
}
LAYOUT
The LAYOUT
struct is updated to have an additional input_qubit_count
field. With version 10 the LAYOUT
struct is now:
struct {
char exists;
int32_t initial_layout_size;
int32_t input_mapping_size;
int32_t final_layout_size;
uint32_t extra_registers;
int32_t input_qubit_count;
}
The rest of the layout data after the LAYOUT
struct is represented as in previous versions. If input qubit_count
is < 0 that indicates that both _input_qubit_count
and _output_qubit_list
in the TranspileLayout
object are None
.
Version 9
Version 9 adds support for classical Expr
nodes and their associated Type
s.
EXPRESSION
An Expr
node is represented by a stream of variable-width data. A node itself is represented by (in order in the byte stream):
- a one-byte type code discriminator;
- an EXPR_TYPE object;
- a type-code-specific additional payload;
- a type-code-specific number of child EXPRESSION payloads (the number of these is implied by the type code and not explicitly stored).
Each of these are described in the following table:
Qiskit class | Type code | Payload | Children |
---|---|---|---|
Var | x | One EXPR_VAR. | 0 |
Value | v | One EXPR_VALUE. | 0 |
Cast | c | One _Bool that corresponds to the value of implicit . | 1 |
Unary | u | One uint8_t with the same numeric value as the Unary.Op . | 1 |
Binary | b | One uint8_t with the same numeric value as the Binary.Op . | 2 |
EXPR_TYPE
A Type
is encoded by a single-byte ASCII char
that encodes the kind of type, followed by a payload that varies depending on the type. The defined codes are:
Qiskit class | Type code | Payload |
---|---|---|
Bool | b | None. |
Uint | u | One uint32_t width . |
EXPR_VAR
This represents a runtime variable of a Var
node. These are a type code, followed by a type-code-specific payload:
Python class | Type code | Payload |
---|---|---|
Clbit | C | One uint32_t index that is the index of the Clbit in the containing circuit. |
ClassicalRegister | R | One uint16_t reg_name_size , followed by that many bytes of UTF-8 string data of the register name. |
EXPR_VALUE
This represents a literal object in the classical type system, such as an integer. Currently there are very few such literals. These are encoded as a type code, followed by a type-code-specific payload.
Python type | Type code | Payload |
---|---|---|
bool | b | One _Bool value . |
int | i | One uint8_t num_bytes , followed by the integer encoded into that many many bytes (network order) in a two’s complement representation. |
Changes to INSTRUCTION
To support the use of Expr
nodes in the fields IfElseOp.condition
, WhileLoopOp.condition
and SwitchCaseOp.target
, the INSTRUCTION struct is changed in an ABI compatible-manner to its previous definition. The new struct is the C struct:
struct {
uint16_t name_size;
uint16_t label_size;
uint16_t num_parameters;
uint32_t num_qargs;
uint32_t num_cargs;
uint8_t conditional_key;
uint16_t conditional_reg_name_size;
int64_t conditional_value;
uint32_t num_ctrl_qubits;
uint32_t ctrl_state;
}
where the only change is that a uint8_t conditional_key
entry has replaced _Bool has_conditional
. This new conditional_key
takes the following numeric values, with these effects:
Value | Effects |
---|---|
0 | The instruction has its .condition field set to None . The conditional_reg_name_size and conditional_value fields should be ignored. |
1 | The instruction has its .condition field set to a 2-tuple of either a Clbit or a ClassicalRegister , and a integer of value conditional_value . The INSTRUCTION payload, including its trailing data is parsed exactly as it would be in QPY versions less than 8. |
2 | The instruction has its .condition field set to a Expr node. The conditional_reg_name_size and conditional_value fields should be ignored. The data following the struct is followed (as in QPY versions less than 8) by name_size bytes of UTF-8 string data for the class name and label_size bytes of UTF-8 string data for the label (if any). Then, there is one INSTRUCTION_PARAM, which will contain an EXPRESSION. After that, parsing continues with the INSTRUCTION_ARG structs, as in previous versions of QPY. |
Changes to INSTRUCTION_PARAM
A new type code x
is added that defines an EXPRESSION parameter.
Version 8
Version 8 adds support for handling a TranspileLayout
stored in the QuantumCircuit.layout
attribute. In version 8 immediately following the calibrations block at the end of the circuit payload there is now the LAYOUT
struct. This struct outlines the size of the three attributes of a TranspileLayout
class.
LAYOUT
struct {
char exists;
int32_t initial_layout_size;
int32_t input_mapping_size;
int32_t final_layout_size;
uint32_t extra_registers;
}
If any of the signed values are -1
this indicates the corresponding attribute is None
.
Immediately following the LAYOUT
struct there is a REGISTERS struct for extra_registers
(specifically the format introduced in Version 4) standalone register definitions that aren’t present in the circuit. Then there are initial_layout_size
INITIAL_LAYOUT_BIT
structs to define the TranspileLayout.initial_layout
attribute.
INITIAL_LAYOUT_BIT
struct {
int32_t index;
int32_t register_size;
}
Where a value of -1
indicates None
(as in no register is associated with the bit). Following each INITIAL_LAYOUT_BIT
struct is register_size
bytes for a utf8
encoded string for the register name.
Following the initial layout there is input_mapping_size
array of uint32_t
integers representing the positions of the physical bit from the initial layout. This enables constructing a list of virtual bits where the array index is its input mapping position.
Finally, there is an array of final_layout_size
uint32_t
integers. Each element is an index in the circuit’s qubits
attribute which enables building a mapping from qubit starting position to the output position at the end of the circuit.
Version 7
Version 7 adds support for Reference
instruction and serialization of a ScheduleBlock
program while keeping its reference to subroutines:
from qiskit import pulse
from qiskit import qpy
with pulse.build() as schedule:
pulse.reference("cr45p", "q0", "q1")
pulse.reference("x", "q0")
pulse.reference("cr45p", "q0", "q1")
with open('template_ecr.qpy', 'wb') as fd:
qpy.dump(schedule, fd)
The conventional SCHEDULE_BLOCK data model is preserved, but in version 7 it is immediately followed by an extra MAPPING utf8 bytes block representing the data of the referenced subroutines.
New type key character is added to the SCHEDULE_BLOCK_INSTRUCTIONS group for the Reference
instruction.
y
:Reference
instruction
New type key character is added to the SCHEDULE_BLOCK_OPERANDS group for the operands of Reference
instruction, which is a tuple of strings, e.g. (“cr45p”, “q0”, “q1”).
o
: string (operand string)
Note that this is the same encoding with the built-in Python string, however, the standard value encoding in QPY uses s
type character for string data, which conflicts with the SymbolicPulse
in the scope of pulse instruction operands. A special type character o
is reserved for the string data that appears in the pulse instruction operands.
In addition, version 7 adds two new type keys to the INSTRUCTION_PARM struct. "d"
is followed by no data and represents the literal value CASE_DEFAULT
for switch-statement support. "R"
represents a ClassicalRegister
or Clbit
, and is followed by the same format as the description of register or classical bit as used in the first element of the condition of an INSTRUCTION field.
Version 6
Version 6 adds support for ScalableSymbolicPulse
. These objects are saved and read like SymbolicPulse objects, and the class name is added to the data to correctly handle the class selection.
SymbolicPulse block now starts with SYMBOLIC_PULSE_V2 header:
struct {
uint16_t class_name_size;
uint16_t type_size;
uint16_t envelope_size;
uint16_t constraints_size;
uint16_t valid_amp_conditions_size;
_bool amp_limited;
}
The only change compared to Version 5 is the addition of class_name_size. The header is then immediately followed by class_name_size
utf8 bytes with the name of the class. Currently, either SymbolicPulse or ScalableSymbolicPulse are supported. The rest of the data is then identical to Version 5.
Version 5
Version 5 changes from Version 4 by adding support for ScheduleBlock
and changing two payloads the INSTRUCTION metadata payload and the CUSTOM_INSTRUCTION block. These now have new fields to better account for ControlledGate
objects in a circuit. In addition, new payload MAP_ITEM is defined to implement the MAPPING block.
With the support of ScheduleBlock
, now QuantumCircuit
can be serialized together with calibrations
, or Pulse Gates. In QPY version 5 and above, CIRCUIT_CALIBRATIONS payload is packed after the INSTRUCTIONS block.
In QPY version 5 and above,
struct {
char type;
}
immediately follows the file header block to represent the program type stored in the file.
- When
type==c
,QuantumCircuit
payload follows - When
type==s
,ScheduleBlock
payload follows
Different programs cannot be packed together in the same file. You must create different files for different program types. Multiple objects with the same type can be saved in a single file.
SCHEDULE_BLOCK
ScheduleBlock
is first supported in QPY Version 5. This allows users to save pulse programs in the QPY binary format as follows:
from qiskit import pulse, qpy
with pulse.build() as schedule:
pulse.play(pulse.Gaussian(160, 0.1, 40), pulse.DriveChannel(0))
with open('schedule.qpy', 'wb') as fd:
qpy.dump(qc, fd)
with open('schedule.qpy', 'rb') as fd:
new_qc = qpy.load(fd)[0]
Note that circuit and schedule block are serialized and deserialized through the same QPY interface. Input data type is implicitly analyzed and no extra option is required to save the schedule block.
SCHEDULE_BLOCK_HEADER
ScheduleBlock
block starts with the following header:
struct {
uint16_t name_size;
uint64_t metadata_size;
uint16_t num_element;
}
which is immediately followed by name_size
utf8 bytes of schedule name and metadata_size
utf8 bytes of the JSON serialized metadata dictionary attached to the schedule.
SCHEDULE_BLOCK_ALIGNMENTS
Then, alignment context of the schedule block starts with char
representing the supported context type followed by the SEQUENCE block representing the parameters associated with the alignment context AlignmentKind._context_params
. The context type char is mapped to each alignment subclass as follows:
l
:AlignLeft
r
:AlignRight
s
:AlignSequential
e
:AlignEquispaced
Note that AlignFunc
context is not supported because of the callback function stored in the context parameters.
SCHEDULE_BLOCK_INSTRUCTIONS
This alignment block is further followed by num_element
length of block elements which may consist of nested schedule blocks and schedule instructions. Each schedule instruction starts with char
representing the instruction type followed by the SEQUENCE block representing the instruction operands
. Note that the data structure of pulse Instruction
is unified so that instance can be uniquely determined by the class and a tuple of operands. The mapping of type char to the instruction subclass is defined as follows:
a
:Acquire
instructionp
:Play
instructiond
:Delay
instructionf
:SetFrequency
instructiong
:ShiftFrequency
instructionq
:SetPhase
instructionr
:ShiftPhase
instructionb
:RelativeBarrier
instructiont
:TimeBlockade
instructiony
:Reference
instruction (new in version 0.7)
SCHEDULE_BLOCK_OPERANDS
The operands of these instances can be serialized through the standard QPY value serialization mechanism, however there are special object types that only appear in the schedule operands. Since the operands are serialized as SEQUENCE, each element must be packed with the INSTRUCTION_PARAM pack struct, where each payload starts with a header block consists of the char type
and uint64_t size
. Special objects start with the following type key:
c
:Channel
w
:Waveform
s
:SymbolicPulse
o
: string (operand string, new in version 0.7)
CHANNEL
Channel block starts with channel subtype char
that maps an object data to Channel
subclass. Mapping is defined as follows:
d
:DriveChannel
c
:ControlChannel
m
:MeasureChannel
a
:AcquireChannel
e
:MemorySlot
r
:RegisterSlot
The key is immediately followed by the channel index serialized as the INSTRUCTION_PARAM.
Waveform
Waveform block starts with WAVEFORM header:
struct {
double epsilon;
uint32_t data_size;
_bool amp_limited;
}
which is followed by data_size
bytes of complex ndarray
binary generated by numpy.save. This represents the complex IQ data points played on a quantum device. name
is saved after the samples in the INSTRUCTION_PARAM pack struct, which can be string or None
.
SymbolicPulse
SymbolicPulse block starts with SYMBOLIC_PULSE header:
struct {
uint16_t type_size;
uint16_t envelope_size;
uint16_t constraints_size;
uint16_t valid_amp_conditions_size;
_bool amp_limited;
}
which is followed by type_size
utf8 bytes of SymbolicPulse.pulse_type
string that represents a class of waveform, such as “Gaussian” or “GaussianSquare”. Then, envelope_size
, constraints_size
, valid_amp_conditions_size
utf8 bytes of serialized symbolic expressions are generated for SymbolicPulse.envelope
, SymbolicPulse.constraints
, and SymbolicPulse.valid_amp_conditions
, respectively. Since string representation of these expressions are usually lengthy, the expression binary is generated by the python zlib module with data compression.
To uniquely specify a pulse instance, we also need to store the associated parameters, which consist of duration
and the rest of parameters as a dictionary. Dictionary parameters are first dumped in the MAPPING form, and then duration
is dumped with the INSTRUCTION_PARAM pack struct. Lastly, name
is saved also with the INSTRUCTION_PARAM pack struct, which can be string or None
.
MAPPING
The MAPPING is a representation for arbitrary mapping object. This is a fixed length SEQUENCE of key-value pair represented by the MAP_ITEM payload.
A MAP_ITEM starts with a header defined as:
struct {
uint16_t key_size;
char type;
uint16_t size;
}
which is immediately followed by the key_size
utf8 bytes representing the dictionary key in string and size
utf8 bytes of arbitrary object data of QPY serializable type
.
CIRCUIT_CALIBRATIONS
The CIRCUIT_CALIBRATIONS block is a dictionary to define pulse calibrations of the custom instruction set. This block starts with the following CALIBRATION header:
struct {
uint16_t num_cals;
}
which is followed by the num_cals
length of calibration entries, each starts with the CALIBRATION_DEF header:
struct {
uint16_t name_size;
uint16_t num_qubits;
uint16_t num_params;
char type;
}
The calibration definition header is then followed by name_size
utf8 bytes of the gate name, num_qubits
length of integers representing a sequence of qubits, and num_params
length of INSTRUCTION_PARAM payload for parameters associated to the custom instruction. The type
indicates the class of pulse program which is either, in principle, ScheduleBlock
or Schedule
. As of QPY Version 5, only ScheduleBlock
payload is supported. Finally, SCHEDULE_BLOCK payload is packed for each CALIBRATION_DEF entry.
INSTRUCTION
The INSTRUCTION block was modified to add two new fields num_ctrl_qubits
and ctrl_state
which are used to model the ControlledGate.num_ctrl_qubits
and ControlledGate.ctrl_state
attributes. The new payload packed struct format is:
struct {
uint16_t name_size;
uint16_t label_size;
uint16_t num_parameters;
uint32_t num_qargs;
uint32_t num_cargs;
_Bool has_conditional;
uint16_t conditional_reg_name_size;
int64_t conditional_value;
uint32_t num_ctrl_qubits;
uint32_t ctrl_state;
}
The rest of the instruction payload is the same. You can refer to INSTRUCTIONS for the details of the full payload.
CUSTOM_INSTRUCTION
The CUSTOM_INSTRUCTION block in QPY version 5 adds a new field base_gate_size
which is used to define the size of the qiskit.circuit.Instruction
object stored in the ControlledGate.base_gate
attribute for a custom ControlledGate
object. With this change the CUSTOM_INSTRUCTION metadata block becomes:
struct {
uint16_t name_size;
char type;
uint32_t num_qubits;
uint32_t num_clbits;
_Bool custom_definition;
uint64_t size;
uint32_t num_ctrl_qubits;
uint32_t ctrl_state;
uint64_t base_gate_size
}
Immediately following the CUSTOM_INSTRUCTION struct is the utf8 encoded name of size name_size
.
If custom_definition
is True
that means that the immediately following size
bytes contains a QPY circuit data which can be used for the custom definition of that gate. If custom_definition
is False
then the instruction can be considered opaque (ie no definition). The type
field determines what type of object will get created with the custom definition. If it’s 'g'
it will be a Gate
object, 'i'
it will be a Instruction
object.
Following this the next base_gate_size
bytes contain the INSTRUCTION
payload for the ControlledGate.base_gate
.
Additionally an addition value for type
is added 'c'
which is used to indicate the custom instruction is a custom ControlledGate
.
Version 4
Version 4 is identical to Version 3 except that it adds 2 new type strings to the INSTRUCTION_PARAM struct, z
to represent None
(which is encoded as no data), q
to represent a QuantumCircuit
(which is encoded as a QPY circuit), r
to represent a range
of integers (which is encoded as a RANGE), and t
to represent a sequence
(which is encoded as defined by SEQUENCE). Additionally, version 4 changes the type of register index mapping array from uint32_t
to int64_t
. If the values of any of the array elements are negative they represent a register bit that is not present in the circuit.
The REGISTERS header format has also been updated to
struct {
char type;
_Bool standalone;
uint32_t size;
uint16_t name_size;
_bool in_circuit;
}
which just adds the in_circuit
field which represents whether the register is part of the circuit or not.
RANGE
A RANGE is a representation of a range
object. It is defined as:
struct {
int64_t start;
int64_t stop;
int64_t step;
}
SEQUENCE
A SEQUENCE is a representation of an arbitrary sequence object. As sequence are just fixed length containers of arbitrary python objects their QPY can’t fully represent any sequence, but as long as the contents in a sequence are other QPY serializable types for the INSTRUCTION_PARAM payload the sequence
object can be serialized.
A sequence instruction parameter starts with a header defined as:
struct {
uint64_t size;
}
followed by size
elements that are INSTRUCTION_PARAM payloads, where each of these define an element in the sequence. The sequence object will be typecasted into proper type, e.g. tuple
, afterwards.
Version 3
Version 3 of the QPY format is identical to Version 2 except that it defines a struct format to represent a PauliEvolutionGate
natively in QPY. To accomplish this the CUSTOM_DEFINITIONS struct now supports a new type value 'p'
to represent a PauliEvolutionGate
. Enties in the custom instructions tables have unique name generated that start with the string "###PauliEvolutionGate_"
followed by a uuid string. This gate name is reservered in QPY and if you have a custom Instruction
object with a definition set and that name prefix it will error. If it’s of type 'p'
the data payload is defined as follows:
PAULI_EVOLUTION
This represents the high level PauliEvolutionGate
struct {
uint64_t operator_count;
_Bool standalone_op;
char time_type;
uint64_t time_size;
uint64_t synthesis_size;
}
This is immediately followed by operator_count
elements defined by the SPARSE_PAULI_OP_LIST_ELEM payload. Following that we have time_size
bytes representing the time
attribute. If standalone_op
is True
then there must only be a single operator. The encoding of these bytes is determined by the value of time_type
. Possible values of time_type
are 'f'
, 'p'
, and 'e'
. If time_type
is 'f'
it’s a double, 'p'
defines a Parameter
object which is represented by a PARAMETER, e
defines a ParameterExpression
object (that’s not a Parameter
) which is represented by a PARAMETER_EXPR. Following that is synthesis_size
bytes which is a utf8 encoded json payload representing the EvolutionSynthesis
class used by the gate.
SPARSE_PAULI_OP_LIST_ELEM
This represents an instance of PauliSumOp
.
struct {
uint32_t pauli_op_size;
}
which is immediately followed by pauli_op_size
bytes which are .npy format [2] data which represents the SparsePauliOp
.
Version 3 of the QPY format also defines a struct format to represent a ParameterVectorElement
as a distinct subclass from a Parameter
. This adds a new parameter type char 'v'
to represent a ParameterVectorElement
which is now supported as a type string value for an INSTRUCTION_PARAM. The payload for these parameters are defined below as PARAMETER_VECTOR_ELEMENT.
PARAMETER_VECTOR_ELEMENT
A PARAMETER_VECTOR_ELEMENT represents a ParameterVectorElement
object the data for a INSTRUCTION_PARAM. The contents of the PARAMETER_VECTOR_ELEMENT are defined as:
struct {
uint16_t vector_name_size;
uint64_t vector_size;
char uuid[16];
uint64_t index;
}
which is immediately followed by vector_name_size
utf8 bytes representing the parameter’s vector name.
PARAMETER_EXPR
Additionally, since QPY format version v3 distinguishes between a Parameter
and ParameterVectorElement
the payload for a ParameterExpression
needs to be updated to distinguish between the types. The following is the modified payload format which is mostly identical to the format in Version 1 and Version 2 but just modifies the map_elements
struct to include a symbol type field.
A PARAMETER_EXPR represents a ParameterExpression
object that the data for an INSTRUCTION_PARAM. The contents of a PARAMETER_EXPR are defined as:
struct {
uint64_t map_elements;
uint64_t expr_size;
}
Immediately following the header is expr_size
bytes of utf8 data containing the expression string, which is the sympy srepr of the expression for the parameter expression. Following that is a symbol map which contains map_elements
elements with the format
struct {
char symbol_type;
char type;
uint64_t size;
}
The symbol_type
key determines the payload type of the symbol representation for the element. If it’s p
it represents a Parameter
and if it’s v
it represents a ParameterVectorElement
. The map element struct is immediately followed by the symbol map key payload, if symbol_type
is p
then it is followed immediately by a PARAMETER object (both the struct and utf8 name bytes) and if symbol_type
is v
then the struct is imediately followed by PARAMETER_VECTOR_ELEMENT (both the struct and utf8 name bytes). That is followed by size
bytes for the data of the symbol. The data format is dependent on the value of type
. If type
is p
then it represents a Parameter
and size will be 0, the value will just be the same as the key. Similarly if the type
is v
then it represents a ParameterVectorElement
and size will be 0 as the value will just be the same as the key. If type
is f
then it represents a double precision float. If type
is c
it represents a double precision complex, which is represented by the COMPLEX. Finally, if type is i
it represents an integer which is an int64_t
.
Version 2
Version 2 of the QPY format is identical to version 1 except for the HEADER section is slightly different. You can refer to the Version 1 section for the details on the rest of the payload format.
HEADER
The contents of HEADER are defined as a C struct are:
struct {
uint16_t name_size;
char global_phase_type;
uint16_t global_phase_size;
uint32_t num_qubits;
uint32_t num_clbits;
uint64_t metadata_size;
uint32_t num_registers;
uint64_t num_instructions;
}
This is immediately followed by name_size
bytes of utf8 data for the name of the circuit. Following this is immediately global_phase_size
bytes representing the global phase. The content of that data is dictated by the value of global_phase_type
. If it’s 'f'
the data is a float and is the size of a double
. If it’s 'p'
defines a Parameter
object which is represented by a PARAM struct (see below), e
defines a ParameterExpression
object (that’s not a Parameter
) which is represented by a PARAM_EXPR struct (see below).
Version 1
HEADER
The contents of HEADER as defined as a C struct are:
struct {
uint16_t name_size;
double global_phase;
uint32_t num_qubits;
uint32_t num_clbits;
uint64_t metadata_size;
uint32_t num_registers;
uint64_t num_instructions;
}
This is immediately followed by name_size
bytes of utf8 data for the name of the circuit.
METADATA
The METADATA field is a UTF8 encoded JSON string. After reading the HEADER (which is a fixed size at the start of the QPY file) and the name
string you then read the metadata_size
number of bytes and parse the JSON to get the metadata for the circuit.
REGISTERS
The contents of REGISTERS is a number of REGISTER object. If num_registers is > 0 then after reading METADATA you read that number of REGISTER structs defined as:
struct {
char type;
_Bool standalone;
uint32_t size;
uint16_t name_size;
}
type
can be 'q'
or 'c'
.
Immediately following the REGISTER struct is the utf8 encoded register name of size name_size
. After the name
utf8 bytes there is then an array of int64_t values of size size
that contains a map of the register’s index to the circuit’s qubit index. For example, array element 0’s value is the index of the register[0]
’s position in the containing circuit’s qubits list.
Prior to QPY Version 4 the type of array elements was uint32_t. This was changed to enable negative values which represent bits in the array not present in the circuit
The standalone boolean determines whether the register is constructed as a standalone register that was added to the circuit or was created from existing bits. A register is considered standalone if it has bits constructed solely as part of it, for example:
qr = QuantumRegister(2)
qc = QuantumCircuit(qr)
the register qr
would be a standalone register. While something like:
bits = [Qubit(), Qubit()]
qr2 = QuantumRegister(bits=bits)
qc = QuantumCircuit(qr2)
qr2
would have standalone
set to False
.
CUSTOM_DEFINITIONS
This section specifies custom definitions for any of the instructions in the circuit.
CUSTOM_DEFINITION_HEADER contents are defined as:
struct {
uint64_t size;
}
If size is greater than 0 that means the circuit contains custom instruction(s). Each custom instruction is defined with a CUSTOM_INSTRUCTION block defined as:
struct {
uint16_t name_size;
char type;
uint32_t num_qubits;
uint32_t num_clbits;
_Bool custom_definition;
uint64_t size;
}
Immediately following the CUSTOM_INSTRUCTION struct is the utf8 encoded name of size name_size
.
If custom_definition
is True
that means that the immediately following size
bytes contains a QPY circuit data which can be used for the custom definition of that gate. If custom_definition
is False
then the instruction can be considered opaque (ie no definition). The type
field determines what type of object will get created with the custom definition. If it’s 'g'
it will be a Gate
object, 'i'
it will be a Instruction
object.
INSTRUCTIONS
The contents of INSTRUCTIONS is a list of INSTRUCTION metadata objects
struct {
uint16_t name_size;
uint16_t label_size;
uint16_t num_parameters;
uint32_t num_qargs;
uint32_t num_cargs;
_Bool has_conditional;
uint16_t conditional_reg_name_size;
int64_t conditional_value;
}
This metadata object is immediately followed by name_size
bytes of utf8 bytes for the name
. name
here is the Qiskit class name for the Instruction class if it’s defined in Qiskit. Otherwise it falls back to the custom instruction name. Following the name
bytes there are label_size
bytes of utf8 data for the label if one was set on the instruction. Following the label bytes if has_conditional
is True
then there are conditional_reg_name_size
bytes of utf8 data for the name of the conditional register name. In case of single classical bit conditions the register name utf8 data will be prefixed with a null character “x00” and then a utf8 string integer representing the classical bit index in the circuit that the condition is on.
This is immediately followed by the INSTRUCTION_ARG structs for the list of arguments of that instruction. These are in the order of all quantum arguments (there are num_qargs of these) followed by all classical arguments (num_cargs of these).
The contents of each INSTRUCTION_ARG is:
struct {
char type;
uint32_t index;
}
type
can be 'q'
or 'c'
.
After all arguments for an instruction the parameters are specified with num_parameters
INSTRUCTION_PARAM structs.
The contents of each INSTRUCTION_PARAM is:
struct {
char type;
uint64_t size;
}
After each INSTRUCTION_PARAM the next size
bytes are the parameter’s data. The type
field can be 'i'
, 'f'
, 'p'
, 'e'
, 's'
, 'c'
or 'n'
which dictate the format. For 'i'
it’s an integer, 'f'
it’s a double, 's'
if it’s a string (encoded as utf8), 'c'
is a complex and the data is represented by the struct format in the PARAMETER_EXPR section. 'p'
defines a Parameter
object which is represented by a PARAMETER struct, e
defines a ParameterExpression
object (that’s not a Parameter
) which is represented by a PARAMETER_EXPR struct (on QPY format Version 3 the format is tweak slightly see: PARAMETER_EXPR), 'n'
represents an object from numpy (either an ndarray
or a numpy type) which means the data is .npy format [2] data, and in QPY Version 3 'v'
represents a ParameterVectorElement
which is represented by a PARAMETER_VECTOR_ELEMENT struct.
PARAMETER
A PARAMETER represents a Parameter
object the data for a INSTRUCTION_PARAM. The contents of the PARAMETER are defined as:
struct {
uint16_t name_size;
char uuid[16];
}
which is immediately followed by name_size
utf8 bytes representing the parameter name.
PARAMETER_EXPR
A PARAMETER_EXPR represents a ParameterExpression
object that the data for an INSTRUCTION_PARAM. The contents of a PARAMETER_EXPR are defined as:
The PARAMETER_EXPR data starts with a header:
struct {
uint64_t map_elements;
uint64_t expr_size;
}
Immediately following the header is expr_size
bytes of utf8 data containing the expression string, which is the sympy srepr of the expression for the parameter expression. Follwing that is a symbol map which contains map_elements
elements with the format
struct {
char type;
uint64_t size;
}
Which is followed immediately by PARAMETER
object (both the struct and utf8 name bytes) for the symbol map key. That is followed by size
bytes for the data of the symbol. The data format is dependent on the value of type
. If type
is p
then it represents a Parameter
and size will be 0, the value will just be the same as the key. If type
is f
then it represents a double precision float. If type
is c
it represents a double precision complex, which is represented by COMPLEX. Finally, if type is i
it represents an integer which is an int64_t
.
COMPLEX
When representing a double precision complex value in QPY the following struct is used:
struct {
double real;
double imag;
}
this matches the internal C representation of Python’s complex type. [3]
[1]
https://tools.ietf.org/html/rfc1700
https://numpy.org/doc/stable/reference/generated/numpy.lib.format.html
[3]
https://docs.python.org/3/c-api/complex.html#c.Py_complex