013 Two Bits in One Qubit

Simulate superdense coding: Alice sends two classical bits to Bob with one qubit, using an entangled pair they shared earlier, one step at a time. Honest about what it does not do. Simulator only.

You will learn: How entanglement lets one qubit carry two bits of information.

The circuit

Coins A and B start as a linked pair: Spin the coin on A, then Link A to B. Alice holds A and Bob holds B. To send two bits, Alice flips her coin over if the second bit is Tails and shifts its spin if the first bit is Tails (the X and Z gates). She sends A to Bob. Bob links A to B, spins A, and looks at both coins. The circuit is H, CNOT, (X), (Z), CNOT, H, measure both.

What you are seeing

The grid shows what you would get if you looked at both coins at that moment. After Alice writes her bits, the grid barely changes: the message is hidden in how the two coins are linked, mostly in the hidden sign. When Bob links and spins, the hidden message becomes a plain result, and the grid shows 100 of 100 on one outcome. It is the same add or cancel idea as the interference maze.

Honest notes

• It is not free. The linked pair had to be delivered before the message, so two qubits crossed the gap in all. Without a shared pair, one qubit can carry at most one ordinary bit (Holevo's bound).
• Nothing goes faster than light. Bob learns nothing until Alice's coin arrives.
• It is not a practical way to send data. It has been shown in small lab experiments only.
• It pairs with teleportation: teleportation sends one qubit using two ordinary bits, and superdense coding sends two ordinary bits using one qubit.

Run on real hardware: coming soon. The planned provider is IBM Quantum. It is not connected yet. Every result on this page comes from a simulator running in your browser, not from a quantum computer.

How it works: the simulator keeps the exact quantum state of the qubits as a list of complex numbers, applies each gate to it, and draws random samples from the resulting probabilities. The random numbers are ordinary pseudo random numbers from your browser, and a real device would add noise. No QNT, wallet or account is needed.

Sources

• Bennett and Wiesner (1992), Physical Review Letters 69, 2881: Communication via one- and two-particle operators on Einstein-Podolsky-Rosen states
• Wikipedia: Superdense coding

This is an educational simulator, not financial advice. QUANTUM (QNT) is an independent community token on Solana and does not perform quantum computation.

Updated 2026-10-12 | Free, no account, no token needed