Add guide on operators and observables #3654

rmlarose · 2021-01-08T21:08:30Z

Fixes #3575. (Perhaps #1550 as well.)

As mentioned in the notebook, the intended audience for this guide is someone already familiar with these concepts, just not how they are defined/used in Cirq.

Let me know @balopat how close this is to what you had in mind! Comments and feedback of course welcome.

rmlarose · 2021-01-11T17:03:29Z

Now ready for review.

I learned that PauliSumCollector.collect which uses asyncio doesn't always mix well with notebooks - throws RuntimeError: This event loop is already running or a more mysterious papermill error (KeyError: 'language_info') - but collect_async seems to work fine.

docs/operators_and_observables.ipynb

vtomole · 2021-01-16T20:10:49Z

docs/operators_and_observables.ipynb

+   },
+   "outputs": [],
+   "source": [
+    "qbit = cirq.LineQubit(0)\n",


Nit: change to qubit

Updated here and throughout.

vtomole · 2021-01-16T20:10:55Z

docs/operators_and_observables.ipynb

+   "outputs": [],
+   "source": [
+    "qbit = cirq.LineQubit(0)\n",
+    "unitary_operation = cirq.ops.X.on(qbit)  # cirq.X can also be used for cirq.ops.X\n",


Nit: change to qubit

vtomole · 2021-01-16T20:11:02Z

docs/operators_and_observables.ipynb

+    "id": "6771bf1c439a"
+   },
+   "source": [
+    "Cirq makes a distinction between gates (independent of qubits) and operations (gates acting on qubits). Thus `cirq.X` is a gate where `cirq.X.on(qbit)` is an operation. See the [guide on gates](gates.ipynb) for more details and additional common unitaries defined in Cirq."


Nit: change to qubit

vtomole · 2021-01-16T20:11:09Z

docs/operators_and_observables.ipynb

+   },
+   "outputs": [],
+   "source": [
+    "measurement_operation = measurement.on(qbit)\n",


Nit: change to qubit

vtomole · 2021-01-16T20:11:14Z

docs/operators_and_observables.ipynb

+   "outputs": [],
+   "source": [
+    "circuit = cirq.Circuit(\n",
+    "    cirq.H(qbit),\n",


Nit: change to qubit

vtomole · 2021-01-16T20:11:20Z

docs/operators_and_observables.ipynb

+   "source": [
+    "circuit = cirq.Circuit(\n",
+    "    cirq.H(qbit),\n",
+    "    cirq.depolarize(p=0.01).on(qbit),\n",


Nit: change to qubit

vtomole · 2021-01-16T20:11:25Z

docs/operators_and_observables.ipynb

+    "circuit = cirq.Circuit(\n",
+    "    cirq.H(qbit),\n",
+    "    cirq.depolarize(p=0.01).on(qbit),\n",
+    "    cirq.measure(qbit)\n",


Nit: change to qubit

vtomole · 2021-01-16T20:11:31Z

docs/operators_and_observables.ipynb

+   },
+   "outputs": [],
+   "source": [
+    "qbit = cirq.LineQubit(0)\n",


Nit: change to qubit

vtomole

This is fine to me.

balopat

Thanks for this, it's a great start, good to see a comprehensive overview on this topic! I added some initial comments.

I wonder if we want to talk about projectors (cirq.KET_IMAG.projector()) here too?

balopat · 2021-01-20T02:26:30Z

docs/operators_and_observables.ipynb

+    "    <td rowspan=\"3\">Operators</td>\n",
+    "    <td>Unitary operators</td>\n",
+    "    <td>`cirq.X`</td>\n",
+    "    <td>[Gates](gates.ipynb) and [Custom gates](custom_gates.ipynb)</td>\n",


These links don't seem to work well, because you are mixing HTML with markdown. You could try creating tables with the table syntax.

Changed to markdown. (Only reason for html was rowspan which isn't supported by markdown, but this isn't needed with the new table as below.)

balopat · 2021-01-20T03:03:55Z

docs/operators_and_observables.ipynb

+    "id": "7cabbfa3ba55"
+   },
+   "source": [
+    "Quantum operations (or just *operators*) include unitary gates, measurements, and noisy channels. Every operator in Cirq is an instance of a `cirq.Operation` which supports the Kraus operator representation\n",


Mentioning cirq.Operation is confusing a bit, because gates don't inherit cirq.Operation. It would be more precise to say quantum operators that are acting on a given set of qubits implement cirq.Operation.

Used this wording here and throughout.

balopat · 2021-01-20T03:55:41Z

docs/operators_and_observables.ipynb

+    "    <td></td>\n",
+    "  </tr>\n",
+    "</tbody>\n",
+    "</table>"


I like the Operators vs Observables split.
Item is too generic, mainly because the category of the items is in the first column. We could break it out to two separate tables - that would save a column as well. Actually observables might not even need a table. Also cirq.PauliSum is a generic way to express any operator, hermitian, non-hermitian, anything, not just observables. I feel like we should capture that somehow.

How about something like this:

Operators:

Operator Cirq representation Guides Examples

Unitary operators any class implementing the _unitary_ protocol Protocols, Gates and operations, custom gates cirq.Gate: cirq.X(qubit), cirq.CNOT(q0, q1), cirq.MatrixGate.on(qubit), cirq.Circuit (if it only contains unitary operations)

Measurement cirq.measure and cirq.MeasurementGate Gates and operations cirq.measure(cirq.LineQubit(0))

Quantum channels

Kraus operators (any class implementing the _channel_ protocol)
Pauli basis (cirq.PauliSum)

protocols cirq.DepolarizingChannel(p=0.2)(q0), cirq.X.with_probability(0.5)

Cirq also supports Observables on qubits that can be used to calculate expectation values on a given state.

You can use cirq.PauliString to express them

Or you can use cirq.PauliSum with real coefficients

Thanks! I like this and adopted it almost verbatim with just a few cosmetic changes.

rmlarose · 2021-01-20T13:21:09Z

I wonder if we want to talk about projectors (cirq.KET_IMAG.projector()) here too?

Seems like a good idea - probably in the section on measurements? I'm thinking of a simple example showing measurements in a different basis, after describing cirq.measure_state_vector and cirq.measure_density_matrix which measure in the Z basis. Something like

prob_plus = np.trace(cirq.KET_PLUS.projector() @ rho).real

where rho is the same single-qubit density matrix from the previous example. Or maybe a two-qubit example is better. Is this what you had in mind?

mpharrigan · 2021-01-23T00:32:15Z

How did you fix KeyError: 'language_info' ?

rmlarose · 2021-01-26T21:48:41Z

How did you fix KeyError: 'language_info' ?

Just to document for posterity: Looks like this was unrelated to asyncio as it popped up in #3669, see also #3603 (comment). So here I'm not sure why the error went away.

balopat · 2021-02-03T07:22:31Z

I wonder if we want to talk about projectors (cirq.KET_IMAG.projector()) here too?

Seems like a good idea - probably in the section on measurements? I'm thinking of a simple example showing measurements in a different basis, after describing cirq.measure_state_vector and cirq.measure_density_matrix which measure in the Z basis. Something like
prob_plus = np.trace(cirq.KET_PLUS.projector() @ rho).real
where rho is the same single-qubit density matrix from the previous example. Or maybe a two-qubit example is better. Is this what you had in mind?

Hmm...I was just thinking about a separate section for Projector operators, which are also coming in form of KetBraSum operators soon(ish) #3386. Simulating measurement this way would be an interesting idea too!

I think this all is looking really good now. We can add the projectors as part of a separate issue, after we implemented the KetBraSum features. Then we can expand on pros/cons there - in the end KetBraSum should be a more generic and efficient, sparse representation for projectors.

rmlarose · 2021-02-03T15:18:28Z

We can add the projectors as part of a separate issue, after we implemented the KetBraSum features. Then we can expand on pros/cons there - in the end KetBraSum should be a more generic and efficient, sparse representation for projectors.

Sounds good!

rmlarose added 3 commits January 8, 2021 15:55

Add guide on operators and observables.

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Typos and clarifications.

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rmlarose requested review from cduck, vtomole and a team as code owners January 8, 2021 21:08

google-cla bot added the cla: yes Makes googlebot stop complaining. label Jan 8, 2021

rmlarose added 2 commits January 8, 2021 17:06

Outputs.

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rmlarose changed the title ~~Add guide on operators and observables~~ [WIP] Add guide on operators and observables Jan 8, 2021

rmlarose added 6 commits January 8, 2021 18:57

Merge branch 'master' into opdocs

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Try keeping metadata.

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Merge branch 'opdocs' of https://github.com/rmlarose/Cirq into opdocs

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Verify if asyncio + notebook is the issue.

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Attempted fix.

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Use collect_async.

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rmlarose changed the title ~~[WIP] Add guide on operators and observables~~ Add guide on operators and observables Jan 11, 2021

vtomole reviewed Jan 16, 2021

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vtomole added 2 commits January 16, 2021 16:36

Merge branch 'master' into opdocs

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rmlarose added 4 commits January 20, 2021 06:22

Nits.

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Merge branch 'opdocs' of https://github.com/rmlarose/Cirq into opdocs

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Update table.

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More precise wording around cirq.Operation.

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Attempt to Fix PauliSumCollector.collect_async warning under notebook(short-term solution) #4004

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Add examples that use PauliString on readthedocs #1550

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Add guide on operators and observables #3654

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rmlarose commented Feb 3, 2021

Operator	Cirq representation	Guides	Examples
Unitary operators	any class implementing the `_unitary_` protocol	Protocols, Gates and operations, custom gates	`cirq.Gate`: `cirq.X(qubit)`, `cirq.CNOT(q0, q1)`, `cirq.MatrixGate.on(qubit)`, `cirq.Circuit` (if it only contains unitary operations)
Measurement	`cirq.measure` and `cirq.MeasurementGate`	Gates and operations	`cirq.measure(cirq.LineQubit(0))`
Quantum channels	Kraus operators (any class implementing the `_channel_` protocol) Pauli basis (cirq.PauliSum)	protocols	`cirq.DepolarizingChannel(p=0.2)(q0)`, `cirq.X.with_probability(0.5)`

Add guide on operators and observables #3654

Add guide on operators and observables #3654

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rmlarose commented Jan 20, 2021

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