Skip to content
This repository has been archived by the owner on Aug 19, 2023. It is now read-only.

Error mitigation RFC #768

Closed
wants to merge 28 commits into from
Closed

Error mitigation RFC #768

Show file tree
Hide file tree
Changes from 10 commits
Commits
Show all changes
28 commits
Select commit Hold shift + click to select a range
2ccc0f0
* Added RFC template.
eggerdj Jan 8, 2020
bd837b6
First draft of error mitigation RFC
eggerdj Jan 8, 2020
c9c2f68
Updated simple error mitigation.
eggerdj Jan 9, 2020
9686cbf
Updated text of more complex schemes
eggerdj Jan 9, 2020
33ab14a
First draft of implementation details
eggerdj Jan 9, 2020
471091d
More implementation details
eggerdj Jan 9, 2020
0d247ff
Tying in command def and scheduling
eggerdj Jan 9, 2020
b2e3912
Implementation details.
eggerdj Jan 9, 2020
63f5cce
First draft complete.
eggerdj Jan 9, 2020
cd0275d
Fixed typo
eggerdj Jan 9, 2020
d027319
Apply suggestions from code review
eggerdj Jan 10, 2020
cfa059f
Update rfcs/0000-error-mitigation.md
eggerdj Jan 10, 2020
3b3b0be
Apply suggestions from code review
eggerdj Jan 10, 2020
f22f363
Apply suggestions from code review
eggerdj Jan 13, 2020
d7c7076
* Added example of how to extend the gateconfig schema.
eggerdj Jan 13, 2020
dcb24f6
Update rfcs/0000-error-mitigation.md
eggerdj Jan 21, 2020
855a3c0
Update rfcs/0000-error-mitigation.md
eggerdj Jan 21, 2020
a2b3dd2
Started to describe the qobj
eggerdj Jan 21, 2020
ab9e936
Qobj and removed scheduler
eggerdj Jan 21, 2020
743ea2e
Outlined changes needed in assemble_circuits
eggerdj Jan 21, 2020
1a66f60
Added QobjExperimentHeader
eggerdj Jan 21, 2020
b72efa8
Added details on qobj.experiments and backend
eggerdj Jan 21, 2020
50bc38b
Added space for figure
eggerdj Jan 21, 2020
c55fbea
Added summary figure.
eggerdj Jan 21, 2020
de27639
Qiskit ignis
eggerdj Jan 21, 2020
f08999d
Changed behaviour of config
eggerdj Jan 22, 2020
2d09e18
Added validation of stretch factors.
eggerdj Jan 22, 2020
e761bd3
Typo
eggerdj Jan 22, 2020
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
166 changes: 166 additions & 0 deletions rfcs/0000-error-mitigation.md
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,166 @@
# RFC Title

| **Status** | **Proposed/Accepted/Deprecated** |
|:------------------|:---------------------------------------------|
| **RFC #** | #### |
| **Authors** | Daniel Egger (deg@zurich.ibm.com) |
| **Deprecates** | RFC that this RFC deprecates |
| **Submitted** | YYYY-MM-DD |
| **Updated** | YYYY-MM-DD |

RFC markdown filename should be of the form `####-rfc-title.md`. Where #### will be set as `max(rfc_####) + 1` after the acceptance of the RFC, but before its merger. If the RFC requires supporting files, a folder may be created with the same name as the RFC, `####-rfc-title`, in which the RFC should reside.

## Summary
Error mitigation can greatly improve results on noisy quantum hardware.
This is done by running a given quantum circuit several times.
In run i of the circuit the duration of all the gates is stretched by a factor c_i to increase the noise in the gates.
An improved result is then obtained by extrapolating to the zero-order noise limit c->0.
The purpose of this RFC is to discuss how to implement error mitigation in Qiskit such that it is usable in applications.

## Motivation
Error mitigation allows users to significantly improve their results.
It is expected that partners and members of the IBM Q network will benefit from this by being able to easily improve the quality of their results.

## User Benefit
The target users of this work are those who run quantum circuits for applications of quantum computing.

## Design Proposal
There are several ways to implement error mitigation.

### Simple error mitigation
The simplest way to implement error mitigation is to run the quantum circuit several times.
In each run the entangling two-qubit gate is replaced several entangling two-qubit gates such that the additional gates have no effect.
For instance, replacing each CNOT gate in a quantum circuit by an odd number of CNOT gates results in the same quantum circuit in the ideal case.
This has, for example, been implemented in https://arxiv.org/abs/1905.02666.
This approach is also valid for other types of two-qubit gates such as the CZ gate.

See https://github.com/Qiskit/qiskit-aqua/pull/683 which aims to implement this error mitigation method.

**The advantage of this method is:**
- Simplicity, it is easy to implement and understand.

**The limitations of this method are**:
- Quantum circuits that are already very deep may not see any gain since replacing each CNOT gate with three CNOT gates may produce circuits which, when executed, result in noise only.
- It does not include single-qubit gates.
- The effective stretch factors to chose from are very limited.
- Some two-qubit gates, such as root-SWAP, need to be applied more than twice to compose to the identity.

### Backend constrained error mitigation
When implementing error mitigation using stretch factors, as is done in https://arxiv.org/abs/1805.04492, new pulses must be defined and calibrated for the different stretch factors c_i.
To implement error mitigation in the manner the backend could have a set of pre-defined calibrated pulses with different stretch factors.
For instance, following Kandala et al., the backend could store calibrated pulses for c=1 (i.e. the pulses used in regular operations), c=1.1, c=1.25, and c=1.5.
At execute time, the user would specify that he wants to run a quantum circuit using error mitigation.
The pulse scheduler would then create four copies of the quantum circuit, each with a different stretch factor supported by the backend.
Alternatively, the user could elect to use only a subset of the calibrate stretch factors.

**The advantages of this method are:**
- The user does not need to know much about error mitigation, a simple flag at execute time would most likely suffice.
- This allows error mitigation to be applied on single-qubit and two-qubit gates.
- Circuits that have many gates may still benefit from error mitigation as stetch factors such as c=1.1, c=1.25, and c=1.5 do not emphasis the noise as much as replacing each two-qubit gate by three two-qubit gates.
- It is fast in that the user does not need to run many quantum circuits.

**The limitations of this method are**:
- The user cannot specify his own stretch factors.
- It increases the amount of gates that the backend needs to calibrate.

### User specified error mitigation
In a more complex implementation the user specifies which stretch factors to use.
This will, therefore, require the user to run calibration procedures to calibrate the gates for each individual stretch factor before the intended quantum circuit can be run with error mitigation.
This solution may be overly complex as the user has to calibrate himself the stretched gates.
The calibration of stretched gates, could be automated to simplify the task for the user, but this would not decrease the run time.
Qiskit-ignis would most likely need to be involved ontop of qiskit-aqua and qiskit-terra.

**The advantages of this method are:**
- It is very flexible.
- This allows error mitigation to be applied on single-qubit and two-qubit gates.
- The stretch factors may be chosen as a function of the depth of the quantum circuit.

**The limitations of this method are**:
- It requires a lot of knowledge from the user.
- It requires that the user run many calibration jobs in addition to his quantum circuit.

## Detailed Design
Here we focus on the implementation details of the Backend constrained error mitigation.
The backend will have a set of calibrated gates with different stretch factors that will be made available to Qiskit through the config file.
eggerdj marked this conversation as resolved.
Show resolved Hide resolved

```
in: config = backend.configuration().to_dict()
eggerdj marked this conversation as resolved.
Show resolved Hide resolved
in: print(config['stretch_factors'])
eggerdj marked this conversation as resolved.
Show resolved Hide resolved
out: [1.0, 1.1, 1.25, 1.5]
eggerdj marked this conversation as resolved.
Show resolved Hide resolved
```

eggerdj marked this conversation as resolved.
Show resolved Hide resolved
To execute a quantum circuit, the user would do
```
execute(cirq, backend, ..., error_mitigation=True)
eggerdj marked this conversation as resolved.
Show resolved Hide resolved
eggerdj marked this conversation as resolved.
Show resolved Hide resolved
```
to use all the stretch factors or
```
execute(cirq, backend, ..., error_mitigation=True, stretch_factors=[1.0, 1.25, 1.5])
eggerdj marked this conversation as resolved.
Show resolved Hide resolved
```
to use only a subset of the stretch factors.
eggerdj marked this conversation as resolved.
Show resolved Hide resolved
The changes needed in Qiskit to implement error mitigation would require a pre-prossessing step in assemble to convert the quantum circuit (or list of quantum circuits) into a list of schedules, using the scheduler, that include the stretched pulses.
eggerdj marked this conversation as resolved.
Show resolved Hide resolved
This also implies that error mitigation will only be a meaningful option for quantum circuits and not for schedules.
For instance, code like the following would be required in `assemble`
```
if error_mitigation and all(isinstance(exp, QuantumCircuit) for exp in experiments):
schedule_config['stretch_factors'] = stretch_factors
error_mitigation_schedules = schedule_circuit_error_mitigation(experiments, schedul_config, method)
eggerdj marked this conversation as resolved.
Show resolved Hide resolved

return assemble_schedules(schedules=error_mitigation_schedules, qobj_id=qobj_id,
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

something funky might happen elsewhere to user code (or internal to qiskit maybe, too) when the number of jobs executed is not equal to the number of circuits given, since this is often assumed

Copy link
Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Do you know where in qiskit this might be an issue?

Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I think this is an example, sort of pseudocode:

job = execute(circuits)  # circuits: List[QuantumCircuit]
result = job.get_results()
result_for_ith_circuit = result.get_counts(i)

I always see something like this on the user side for retrieving their data

Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

specifically, I think the Result object or however results are returned could get hairy. I find the results object already pretty confusing, so I'm not sure what changes would have to be made or what issues to expect

qobj_header=qobj_header, run_config=run_config)
```
The parameters needed for the error mitigation, such as the stretch factors, are included in the `schedule_config`.
The function `schedule_circuit_error_mitigation` creates, for each quantum circuit in `experiments`, several schedules corresponding to different stretch factors in `schedule_config`.
For example
```
schedules = []
stretch_factors = schedule_config['stretch_factors']

for circuit in experiments:
for c in stretch_factors:
schedule_config['stretch_factor'] = c
schedules.append(schedule_circuit(circuit, schedule_config, method))
```
In `schedule_circuit` the translation between gates and pulses is done by the function
```
translate_gates_to_pulse_defs(circuit: QuantumCircuit,
schedule_config: ScheduleConfig) -> List[CircuitPulseDef]
```
located in `scheduler.methods.basic.py`.
This method currently uses the `CmdDef` to relate gates to pulses.
Therefore, the pulses in the `CmdDef` should contain information on the stretch factor they correspond to.
Consider, for example, a CNOT gate
```
Command(name='cx', qubits=[0, 1], sequence=[
PulseQobjInstruction(ch='d0', name='fc', phase=1.5707963267948966, t0=0),
PulseQobjInstruction(ch='u1', name='fc', phase=1.5707963267948966, t0=0),
PulseQobjInstruction(ch='d0', name='Ym_d0_4b7a', t0=0),
PulseQobjInstruction(ch='d1', name='X90p_d1_cfee', t0=0),
PulseQobjInstruction(ch='d1', name='CR90p_d1_4334', t0=160),
PulseQobjInstruction(ch='u0', name='CR90p_u0_0e7b', t0=160),
PulseQobjInstruction(ch='d0', name='Xp_d0_e1b0', t0=672),
PulseQobjInstruction(ch='d1', name='CR90m_d1_f19b', t0=832),
PulseQobjInstruction(ch='u0', name='CR90m_u0_0cdb', t0=832)
])
```
The definition of this gate could be extended to
```
Command(name='cx', qubits=[0, 1], stretch_factor=1.1, sequence=[
eggerdj marked this conversation as resolved.
Show resolved Hide resolved
PulseQobjInstruction(ch='d0', name='fc', phase=1.5707963267948966, t0=0),
...
])
```
to include information on the stretch factor which can be used by `translate_gates_to_pulse_defs` to select the gates with a stretch factor corresponding to `schedule_config['stretch_factor']` (this is a float and not a list).
This would then allow the creation of schedules for error mitigation that can be assembled using `assemble_schedule` and run on the backend.

Here are some additional considerations:
- Currently, the name of a scheduled circuit is the same as the circuit. We will also need to distinguish the schedules with different stretch factors, for instance, by including the stretch factor in the name of the circuit. E.g. `sched = Schedule(name=circuit.name + 'c=%d'.format(schedule_config['stretch_factor']))`.
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Maybe Schedules should also have metadata. I don't really like the idea of using name to hold data that will be used programmatically elsewhere

Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

You are still right that the names will have to be different (because unfortunately name might already be used to grab one schedule's results from a list of a job's results)

Copy link
Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I agree with you on both points. Each schedule will still need a unique name but putting the stretch factor in the schedule name is not particularly appealing. Lets come up with something else.


## Alternative Approaches
See section Simple error mitigation and section User specified error mitigation.

## Questions
- The Backend constrained error mitigation requires extra effort from the backend to calibrate gates with different stretch factors. We need to check that the resulting overhead is acceptable.

## Future Extensions
See section User specified error mitigation.
61 changes: 61 additions & 0 deletions rfcs/0000-template.md
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,61 @@
# RFC Title

| **Status** | **Proposed/Accepted/Deprecated** |
|:------------------|:---------------------------------------------|
| **RFC #** | #### |
| **Authors** | First Author (first.author@ibm.com), ... |
| **Deprecates** | RFC that this RFC deprecates |
| **Submitted** | YYYY-MM-DD |
| **Updated** | YYYY-MM-DD |

RFC markdown filename should be of the form `####-rfc-title.md`. Where #### will be set as `max(rfc_####) + 1` after the acceptance of the RFC, but before its merger. If the RFC requires supporting files, a folder may be created with the same name as the RFC, `####-rfc-title`, in which the RFC should reside.

## Summary
One paragraph explanation of the feature.

## Motivation
- Why are we doing this?
- What will this enable?
- What will be the outcome?
- Who will benefit?

## User Benefit
- Who are the target users of this work?
- How will users or contributors benefit from the work proposed?

## Design Proposal
This is the focus of the document. Explain the proposal from the perspective of
educating another user on the proposed features.

This generally means:
- Introducing new concepts and nomenclature
- Using examples to introduce new features
- Implementation and Migration path with associated concerns
- Communication of features and changes to users

Focus on giving an overview of impact of the proposed changes to the target audience.

Factors to consider:
- Performance
- Dependencies
- Maintenance
- Compatibility

## Detailed Design
Technical reference level design. Elaborate on details such as:
- Implementation procedure
- If spans multiple projects cover these parts individually
- Interaction with other features
- Dissecting corner cases
- Reference definition, eg., formal definitions.

## Alternative Approaches
Discuss other approaches to solving this problem and why these were not selected.

## Questions
Open questions for discussion and an opening for feedback.

## Future Extensions
Consider what extensions might spawn from this RFC. Discuss the roadmap of related projects and how these might interact. This section is also an opening for discussions and a great place to dump ideas.

If you do not have any future extensions in mind, state that you cannot think of anything. This section should not be left blank.