Quantum Computer Simulator

Gates (toolbox)

Rotation angle (for Rx / Ry / Rz only)

90°

How gates work: pick a button above (that selects the tool), then click a qubit wire in the circuit diagram to place it. Gates are the operations the quantum computer applies.

Demo circuits

One-click example programs. Great for learning before building your own.

Circuit

Ready. Open Help for a full walkthrough, or load a demo.

Statevector & probabilities

exact H = 0 bits

Exact quantum amplitudes after applying every gate (before random measurement noise). Run recomputes this from the circuit.

Measurement histogram

Shots 512

Ideal = exact Born-rule prediction scaled to N shots. Sample = run the circuit N times with random measurement (like a real device). Click Sample to see statistical noise.

Bloch sphere

x=0 y=0 z=1 |r|=1

One yellow arrow = one qubit’s state. Drag to rotate the view. Try the Bloch tour demo — each qubit points differently.

OpenQASM 2.0

// circuit appears here

Quick tips

Load a Demo first (e.g. Bell pair).
Pick a gate, then click a wire.
Run = exact math. Sample = random shots.
Entangled qubits often look identical on the Bloch sphere (|r|≈0).

What is this?

A simulated quantum computer that lives in your browser. You build a circuit (a sequence of quantum gates on wires called qubits), then the engine computes the exact quantum state and can “measure” it many times.

First 60 seconds

In the left panel, click a Demo circuit such as Bell pair.
Look at the Circuit panel: horizontal lines are qubits; boxes are gates.
Look at Statevector: you should see ~50% on |00⟩ and ~50% on |11⟩ for a Bell pair.
Click Sample — the histogram wiggles a little (random measurement noise).
Click Clear, then try building: select H, click wire q0, select CX, click q0 then q1.

Changing the Qubits dropdown only resizes the register (more wires). It does not run a new algorithm by itself. Use demos or place gates, then Run.

What are the Gate buttons for?

They are a toolbox, not “run this now” buttons. Clicking a gate selects it (highlighted). Then you place it by clicking a qubit wire on the circuit.

Gate	What it does
`H`	Hadamard — creates superposition (\|0⟩ ↔ equal \|0⟩ and \|1⟩)
`X` / `Y` / `Z`	Pauli gates — bit flip (X), phase flips (Z), both (Y)
`S` / `T`	Phase gates (S = 90°, T = 45° phase on \|1⟩)
`Rx` / `Ry` / `Rz`	Rotations — use the angle slider, then place on a wire
`CX`	CNOT — flip target if control is \|1⟩ (creates entanglement with H)
`CZ` / `SWAP` / `CCX`	Controlled-Z, swap two qubits, Toffoli (two controls)
`M`	Measure one qubit (collapses it to 0 or 1 randomly)
`‖`	Barrier — visual separator only (does not change the state)

Multi-qubit placement

CX / CZ: click control qubit, then target qubit.
SWAP: click first qubit, then second.
CCX (Toffoli): click control₁, control₂, then target.

Click the small × on a gate in the circuit to delete it. Use Undo for the last gate.

Qubits dropdown

Sets how many wires the computer has (1–8). More qubits → larger state (2ⁿ amplitudes). Extra unused qubits stay in |0⟩ and do not change the interesting part of the circuit.

Run

Computes the exact quantum state after all gates, from |00…0⟩. Updates the statevector, ideal histogram, Bloch sphere, and QASM text.

If nothing seems to change when you click Run, the circuit did not change — the answer is the same. Try Clear, place an H on q0, then Run. Or click Sample to see random variation.

Sample

Simulates a real experiment: for each of N shots, run the circuit and measure all qubits. Counts fluctuate around the ideal probabilities.

Bell pair → roughly half |00⟩, half |11⟩ (not exactly 256/256 every time).
Grover |11⟩ → almost always |11⟩ (ideal is 100%).
Increase Shots for smoother histograms.

Export QASM downloads the circuit in OpenQASM 2.0 format (usable in other quantum toolchains).

Why do q0, q1, q2… look the same?

Often they really are the same — that is correct physics, not a bug.

Empty circuit / all |0⟩: every qubit points straight up (+Z). Switching q0→q4 looks identical.
Bell or GHZ (entangled): each single qubit is maximally mixed. The Bloch vector shrinks to the center (|r| ≈ 0). Every entangled qubit looks like a tiny/zero arrow at the origin.
To see different arrows: load the Bloch tour demo, or put H only on q0, X only on q1, leave q2 idle — then switch the Bloch dropdown.

The sphere shows one qubit’s reduced state. Entanglement with others makes that qubit look classical/mixed even though the full system is pure.

Drag on the sphere to rotate the camera. Coordinates x, y, z are shown below the canvas.

Statevector panel

Lists every basis state with non-zero amplitude, e.g. |01⟩ 0.707 50%. Labels use little-endian order: rightmost bit is q0. So |01⟩ means q1=0, q0=1.

Entropy H

Shannon entropy of the measurement probabilities (in bits). 0 = definite outcome; 1 bit for a fair coin (like one Hadamard); higher for more spread-out distributions.

Histogram

Bars show how often each bitstring would (ideal) or did (sample) appear. Bitstring order matches the statevector labels.

FAQ

Q: I clicked Run and nothing happened.
A: Run always recomputes the same answer for the same circuit. Watch for the green toast “Exact simulation complete” and a brief highlight on the results panels. Change a gate or use Sample to see a difference.

Q: Gate buttons do nothing.
A: They only select a tool. After selecting, click a horizontal wire in the Circuit panel.

Q: Sample looks the same as ideal.
A: With many shots, noise is small. Try 64 shots, or a Bell pair where 50/50 fluctuates. The histogram title switches to “Sampled (random shots)”.

Q: Is this a real quantum computer?
A: No — it is an exact mathematical simulation on your CPU. Real hardware has noise and limited connectivity; Sample approximates the randomness of measurement only.

Q: Why only up to 8 qubits?
A: State size is 2ⁿ complex numbers. 8 qubits = 256 amplitudes (fine); 20+ needs heavy resources.