Simple Quantum Email: Super Dense Coding Protocol

Super Dense Coding Protocol

This article demonstrates Super Dense Coding (SDC) in quantum computing. We first use Qiskit’s simulator to test our quantum circuit, and then try it out on a real quantum computer.

Difference between Super Dense Coding and Quantum Teleportation

Quantum teleportation and super dense coding are closely related and might cause confusion if not properly explained or differentiate.

Quantum teleportation is a process by which the state of qubit can be transmitted from one location to another, with the help of classical communication and Bell Pair. In other words, we can say it is a protocol that destroys the quantum state of a qubit in one location and recreates it on a qubit at a distant location, with the help of shared entanglement, while super dense coding is a procedure that allows someone to send two classical bits to another party using just a single qubit of communication

The teleportation protocol is actually a flipped version of the super-dense coding protocol, in the sense that Alice and Bob merely “swap their equipment.”

Super Dense Coding diagram illustration

super dense coding protocol diagram

The process involves 3 steps:

Step 1

The process starts with a third party, who we’ll call (Eve). Two qubits are prepared by Eve in an entangled state. She initially starts the 2 qubits in the basis state |0>. She applies Hadamard gate (H) to the first qubit to create superposition. She then applies CNOT gate (cx) using the first qubit as a control and the second as the target. This is the entangled state we mention earlier.

Outcome States

Eve first applied Hadamard gate. We will ignore the the square root (2) from Hadamard gate matrix for simplicity.

When H gate is applied to first qubit, it enters superposition; thus we get the states 00 + 01, where the second qubit remains 0.

|00> = |00> + |01>

Then Eve applied CNOT gate. The CNOT gate entangled both qubits. That is, it flips the target if the control is 1. Note that the control is on our first qubit.

CNOT (|00> + |10>) = |00> + |11>

Step 2

Eve sent the first qubit to Alice and second qubit to Bob. The goal of the protocol is for Alice to send 2 classical bits of information to Bob using her qubit. But before she does, she needs to apply a set of quantum gates to her qubit depending on the 2 bits of information she wants to send

Encoding Rules for Super Dense Coding (Alice protocol) Quantum gates/rules

00: I                         I(00 + 11) = 00 + 11
01: X X(00 + 11) = 10 + 01
10: Z Z(00 + 11) = 00 - 11
11: ZX ZX(00 + 11) = 10 - 11

Thus if she wants to send a 00, she does nothing to her qubit (apply identity gate). If she sends a 01, then she applies the X gate. Depending on what she wants to send, she applies the appropriate gate. Alice then sends her qubit to Bob for the final step in the process

Step 3

Bob receives Alice’s qubit (first qubit) and uses his qubit to reverse the process Eve created. That is, he is immediately certain which operation he needs to perform in order to restore his state to Alice’s original state. Notice that he does not need to have knowledge of the state in order to restore it — he only needs knowledge of the restoration operation.

Bob applies CNOT gate using the first qubit as control and the second as target. Then he applied Hadamard gate and finally performs a measurement in both qubits to extract the classical bits encoded in Alice’s qubit.

Encoding Rules for Super Dense Coding (Bob protocol)

Alice states     After Bob’s CNOT     After Bob’s (H) gate and meas.
00 + 11 00 + 01 00
10 + 01 10 + 01 01
00 - 11 00 - 11 10
10 - 11 10 - 11 -11

When Bob performs measurements, he ends up with four possible outcomes with probability of 1 each. Note that the last outcome has a negative sign -11, writing it as 11 is also correct because the probability is calculated as the amplitude of squared.

Simulating the Super Dense Coding Protocol

# Importing all necessary libraries for work.from qiskit import *
from qiskit.visualization import plot_histogram
# Create the quantum circuit
q = QuantumRegister(2)
c = ClassicalRegister(2)
qc = QuantumCircuit(q, c)
qc.barrier()
# Next, generate the entangled pair between Alice and Bob (Remember: Hadamard followed by CX generates a Bell pair)
qc.h(q[0])
qc.cx(q[0], q[1])
qc.barrier()
# Next, apply the apply superdense coding protocol. First, we need to prepare the state Alice will like to send
#In this tutorial, Alice is sending 11
qc.z(q[0])
qc.x(q[0])
qc.barrier()
#Apply Bob recovery protocol
qc.cx(q[0], q[1])
qc.h(q[0])
#Take measurement to see which classical bit of information Bob receives
qc.barrier()
qc.measure(q[0], c[0])
qc.measure(q[1], c[1])
#Draw our output
qc.draw(output = "mpl")
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super dense coding circuit. Here, Alice is trying to send 11bits to Bob.

Visualizing our measurements

In [17]:

backend = Aer.get_backend('qasm_simulator')
job_sim = execute(qc, backend, shots=1024)
sim_result = job_sim.result()
measurement_result = sim_result.get_counts(qc)
print(measurement_result)
plot_histogram(measurement_result)
{'11': 1024}
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Using IBM simulator

Super dense coding on a real quantum computer

Let’s see how super dense coding works on real quantum computer.

First, we want to load our account to get the least busy quantum system

from qiskit import IBMQ
from qiskit.providers.ibmq import least_busy
IBMQ.save_account('774cf0100df15d4982f84afe7e4f4f9f3531a08fab92815042916120dbdce6b6d2c6e29ac8740294e77c2a828e83390b170e9649d4a1b94f392365d0c1588f3a')
# Load local account information
IBMQ.load_account()
provider = IBMQ.get_provider('ibm-q')
quantumcomputer = provider.get_backend('ibmq_16_melbourne')
job = execute(qc, backend=quantumcomputer)
#monitoring our jobfrom qiskit.tools.monitor import job_monitor
job_monitor(job)
#plotting our resultresult = job.result()
plot_histogram(result.get_counts(qc))
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Running the same code on IBM real quantum computer.

As we see that there are a few results from the other three states when run in a real quantum computer. These arise due to errors in the gates that were applied.

For security purpose, an eavesdropper can only get access to the qubit Alice sends to Bob. In that case, the eavesdropper cannot tell what information Alice was sending to Bob. However, the eavesdropper can scramble the message by applying a pauli operation on that qubit, but he won’t know what message Bob will receive, but Bob won’t receive what Alice intended.

Some people call me unstable force, while some say I'm quantum dot.

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