Editing Openai/6963b524-bf70-8010-b81c-e31f5de96ab9


{{DISPLAYTITLE:Clifford Toffoli Universality}}

''This conversation was exported from ChatGPT''<ref>{{cite web|title=Clifford Toffoli Universality|url=https://chatgpt.com/share/6963b524-bf70-8010-b81c-e31f5de96ab9|publisher=ChatGPT|access-date=2026-01-12}}</ref>

== gpt-5-2 ==
=== User: is clifford+toffoli universal? ===
is clifford+toffoli universal?

=== Assistant: Short answer: No — Clifford + Toffoli is not a universal quantum gate set in the usual sense. ===
Short answer: No — Clifford + Toffoli is ''not'' a universal quantum gate set in the usual sense.

Here’s the vibe breakdown without fluff:

Clifford gates (Hadamard, Phase =S, CNOT, Paulis) + Toffoli gives you all of the stabilizer circuits plus Toffoli’s classical reversible powers. You can do any classical reversible computation (since Toffoli alone is universal for classical logic), and you can do a lot of quantum stuff—but you still live inside the Clifford+classical reversible world. That world is not quantum‐computationally universal because:
* Clifford circuits alone are efficiently classically simulable (Gottesman-Knill theorem).
* Adding Toffoli doesn’t break that simulability in the quantum sense — it just gives richer classical control, not access to arbitrary quantum phases.
* You still can’t generate arbitrary single-qubit rotations or approximate arbitrary unitaries on multiple qubits without a non-Clifford quantum gate like T (π/8).

So Clifford+Toffoli = powerful for reversible classical logic and stabilizer transformations, but not universal for quantum computation.

If you want actual quantum universality, you need something like Clifford+T, Clifford + Toffoli + some non-Clifford single-qubit rotation, or other non-Clifford resources.

In playful terms, you’ve got a toolkit that builds every classical reversible LEGO model and some neat quantum stabilizer sculptures — but you can’t build the full quantum cathedral without those spicy non-Clifford bricks.

=== User: will it atleast run shor? ===
will it atleast run shor?

=== Assistant: Short answer: nope, not real Shor. ===
Short answer: nope, not real Shor. Not even close.

Shor lives and dies by the Quantum Fourier Transform (QFT). And QFT is basically a festival of tiny phase rotations:
R2,R3,R4,…R_2, R_3, R_4, \dotsR2,R3,R4,… with angles like π/2, π/4, π/8, …
Those are not Clifford. Toffoli doesn’t help with phases at all — it’s a beefy classical logic gate in a quantum costume.

Clifford + Toffoli can do:
* all stabilizer stuff (Bell states, GHZ, teleportation, syndrome extraction, etc.)
* any reversible classical computation (modular multiplication, adders, control logic — all good)

But Shor needs:
* modular exponentiation (you can do this with Toffoli-heavy reversible circuits, so that part is fine)
* QFT with non-Clifford phase gates (this is where everything breaks)

Without those non-Clifford rotations, the interference pattern that reveals the period just… never forms. You measure noise, not number theory.

Now, tiny loophole that people sometimes bring up:
You can approximate QFT using Clifford + T (since T is non-Clifford), and Toffoli can be decomposed into Clifford+T anyway. So once you allow T gates, boom, universality unlocked, Shor is back on the menu.

So the scorecard:
* Clifford + Toffoli → ❌ not universal, ❌ can’t run Shor
* Clifford + T → ✅ universal, ✅ can run Shor
* Toffoli alone → king of classical, zero quantum mojo

In other words: you’ve got the calculator and the wiring, but not the phase wizardry. And Shor is basically a phase wizard in a hoodie doing number theory behind the scenes. 🧙‍♂️⚡

This ties straight into your MoE / fast-learning experiments too, by the way: interference is the secret sauce, and interference needs continuous phases, not just discrete flips.

== References ==
<references />


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