SUPERDENSE CODING

4 min readApr 12, 2026

LETS EXPLORE SUPERDENSE CODING!!

Superdense coding is a quantum communication protocol that allows a sender to send two classical bits of information to another user by only utilizing one qubit.First proposed by Charles Bennett and Stephen Wiesner in 1992 and experimentally actualized four years later, in 1996.

Superdense Coding and Quantum Teleportation are closely related, there is a key difference between them. Quantum teleportation is a process by which a user can transmit one qubit using two classical bits whereas Superdense Coding is a process by which a user can transmit two classical bits using one qubit. Basically, Superdense Coding can be thought of as the flipped version of Quantum Teleportation.

Brief idea

In superdense coding, Alice and Bob are the two people used to explain how quantum communication works.

Alice = sender
Bob = receiver

The special thing is that Alice can send 2 classical bits of information by sending only 1 qubit, but this works only because Alice and Bob already share an entangled pair.

They share a special entangled state called a Bell State:

Think of this like two magically connected particles — no matter how far apart they are, their states are linked.

ENCODING CLASSICAL BIT INTO QUBITS

Alice wants to send 2 classical bits to Bob:

00, 01, 10, or 11

Instead of sending 2 bits, she uses quantum operations (gates) on her qubit.

Encoding Table

Each operation transforms the shared entangled state into a different Bell state. So Alice encodes 2 bits of information into 1 qubit by choosing the right gate.

Then she sends that single qubit to Bob.

Circuit Implementation (Step-by-Step)

Now let’s see the work behind the circuit

Step 1: Create Entanglement

Start with two qubits in state |00⟩

Apply Hadamard (H) on first qubit
Apply CNOT

This creates:

∣Φ+⟩

Step 2: Alice Encodes Message

Depending on the message:

Apply I, X, Z, or XZ

Step 3: Alice Sends Qubit

Alice sends her qubit to Bob through a quantum channel.

Step 4: Bob Decodes

Bob now has both qubits.

He applies:

CNOT
Hadamard (H)

Then measures both qubits.

Final Output

Bob gets:

Message successfully received!

Coding it in Qiskit

Now that we have a foundational undertsanding about Superdense Coding, we can utilize Qiskit, an open-source software development kit for working with quantum computers, to test it out. You can access their official tutorial here.