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At subatomic level matter behaves very differently from what we have seen in  classical physics and mechanical theories. Before going into quantum communication let us recall what is quantum mechanics is. Quantum mechanics is a field of physics that studies how things behave at an atomic or sub-atomic level.It is the  set of laws that govern very small systems, such as atoms, electrons,  photons and similar particles.

Key principles of quantum mechanics

• Superposition: A quantum system can exist in multiple states simultaneously. For example, a qubit can be both 0 and 1 at the same time until it is measured.
• Entanglement: Two or more quantum particles can become linked in such a way that the state of one instantly affects the state of the others, no matter how far apart they are. This “spooky action at a distance” is a core resource for quantum communication and computing.
• Wave-Particle Duality: Quantum objects like photons and electrons exhibit characteristics of both waves and particles. The behavior observed depends on how the object is measured.
• Heisenberg’s Uncertainty Principle: It’s impossible to simultaneously know both the exact position and momentum of a particle. The more precisely you measure one, the less you know about the other.

These are the fundamental principles that govern the strange and counter-intuitive world of quantum mechanics.

Before diving deep into quantum communication, it is suggested to read our principles of quantum mechanics, Quantum computing and qubits.

• Quantum Computing Key principles explained

What is quantum Communication?

Quantum communication is a new way of transmitting information that uses the strange and powerful rules of quantum mechanics. Present communication systems  use, digital or analog signal to send information through a medium like air, vacuum, copper cables, optical fibres etc.  which uses “bits” (0s and 1s), quantum communication uses qubits (quantum bits). A qubit can be a 0, a 1, or, thanks to a phenomenon called superposition, both at the same time. This is just one of the quantum properties that makes this technology so revolutionary.

Present status of quantum communication

Messages are not directly transmitted in quantum communication in the same way as classical communication. Instead, quantum communication is primarily used to establish an ultra-secure secret key. Once this key is established, the actual message can be encrypted and sent over a conventional, classical channel. This is because quantum communication is currently limited in its ability to transmit large amounts of data quickly over long distances.

The key principles used in  quantum communication :

The Quantum entanglement

At the heart of quantum communication lies quantum entanglement. This is a mind-bending phenomenon where two or more particles become linked in such a way that their properties are forever connected, no matter how far apart they are. If you measure a property of one entangled particle (like its spin), you instantly know the corresponding property of its partner. This is the core resource for quantum networks and is used for secure communication protocols and quantum teleportation—the transfer of a quantum state from one location to another.

Decoherence

Quantum decoherence is the process by which a quantum system loses its “quantumness,” its superposition and entanglement properties, and begins to behave like a classical system.

This property of quantum system is derived from famous  Heisenberg’s Uncertainty Principle. In quantum communications photons are used as qubits to encode data,. A set of photons are entangled in a such a way that altering the property of a photon affects the property of other entangled particles. When somebody from outside tries to measure the property of the photons(using a detector), it causes the photons to lose their quantum state and collapse to their classical state of behaviour. This alerts the entire system and sender and receiver will come to know that there is an attempt to breach and thus preventing attack.

Here is a simple breakdown of how this process works, focusing on quantum teleportation and quantum cryptography(Quantum Key Distribution):

Quantum Teleportation

Quantum teleportation isn’t about beaming objects from one place to another. Instead, it’s a protocol for transferring the quantum state of a particle from a sender (Alice) to a receiver (Bob) without physically moving the particle itself. This is done using a pre-shared entangled pair of particles. Entanglement is a phenomenon where two particles are linked in such a way that measuring the state of one instantly affects the state of the other, no matter how far apart they are.

• Setup: Alice has a particle(a photon) with a specific quantum state (the message), and she and Bob each hold one particle from an entangled pair.
• Process: Alice performs a joint measurement on her “message” particle and her entangled particle. This measurement instantly collapses their states and affects the state of Bob’s entangled particle.
• Classical Communication: Alice then sends Bob the results of her measurement over a classical channel (like a phone or internet line). This information is not the message itself, but rather a set of instructions.
• Reconstruction: Using Alice’s instructions, Bob performs a specific operation on his entangled particle. This corrects its state to perfectly match the original state of Alice’s “message” particle.

While this process transfers information, it’s not a direct, real-time messaging system. The “teleportation” of the quantum state still requires the classical communication of measurement results.

Quantum Key Distribution

The most famous application of quantum communication is Quantum Key Distribution (QKD). QKD is not for transmitting the entire message itself, but for generating and sharing a secret encryption key that is “future-proof” against any attempts at hacking, including from future quantum computers.

Here’s how it works. QKD uses single photons (particles of light) to create and share this key. If anyone tries to “listen in” on the channel, the act of measuring the photon’s quantum state instantly changes it. This change is detected by the sender and receiver, immediately alerting them to the presence of an eavesdropper.

Here’s a breakdown of how a quantum communication setup for QKD works, using the popular BB84 protocol as an example.

The BB84 protocol is a Quantum Key Distribution (QKD) method developed by Charles Bennett and Gilles Brassard in 1984 that uses the polarization states of photons to establish a secure, shared secret key between two parties, Alice and Bob.

The Basic Setup

Imagine two parties, Alice (the sender) and Bob (the receiver), want to establish a secure communication channel. They need two links:

1. A Quantum Channel: This is the medium through which Alice sends single photons to Bob. It can be a fibre-optic cable or free space (like through the air or from a satellite).
2. A Classical Channel: This is a public, traditional communication channel (like a regular internet connection or a phone line) that Alice and Bob use to talk about their measurements after the quantum transmission.

Demonstration of the BB84 Protocol

The BB84 protocol uses the polarization of single photons to encode information. Polarization refers to the direction in which a light wave oscillates. Think of it like a line in a 2D plane.

Light waves, such as those from the sun, are generally unpolarized, meaning their electric field vectors vibrate in many different directions, all perpendicular to the direction the light is traveling. When light is polarized, the electric field oscillations are confined to a single plane. This single-plane vibration is known as the plane of polarization. This restriction can be achieved through methods like filtering, reflection, or scattering.

The protocol uses two sets of polarization bases, each with two possible orientations:

• The Rectilinear Basis (+): Horizontal (0∘) and Vertical (90∘).
• The Diagonal Basis (×): Left-diagonal (45∘) and Right-diagonal (135∘).

Step 1: Alice’s Preparation and Transmission

• Alice generates a random string of classical bits (e.g., 10011010).
• For each bit, she randomly chooses one of the two bases (+ or ×) to encode it as a polarized photon.
  • In the Rectilinear basis (+): A vertical polarization could represent a ‘0’ and a horizontal a ‘1’.
  • In the Diagonal basis (×): A left-diagonal polarization could represent a ‘0’ and a right-diagonal a ‘1’.
• Alice sends each single photon one by one to Bob through the quantum channel

Example of Alice’s Actions:

Alice’s Bit	Alice’s Random Basis	Photon Polarization
1	+ (Rectilinear)	Horizontal
0	× (Diagonal)	Left-diagonal
0	× (Diagonal)	Left-diagonal
1	+ (Rectilinear)	Vertical
1	× (Diagonal)	Right-diagonal
0	+ (Rectilinear)	Horizontal
1	× (Diagonal)	Right-diagonal
0	+ (Rectilinear)	Vertical

Step 2: Bob’s Measurement

• Bob receives the stream of photons.
• For each photon, he randomly chooses a basis (+ or ×) to measure its polarization. He doesn’t know which basis Alice used.
• Based on his measurement, he records a bit value (0 or 1).

Example of Bob’s Actions:

Photon from Alice	Bob’s Random Basis	Bob’s Measurement
Horizontal	+ (Rectilinear)	Horizontal (1)
Left-diagonal	+ (Rectilinear)	Horizontal OR Vertical (random outcome)
Left-diagonal	× (Diagonal)	Left-diagonal (0)
Vertical	× (Diagonal)	Right-diagonal OR Left-diagonal (random outcome)
Right-diagonal	× (Diagonal)	Right-diagonal (1)
Horizontal	+ (Rectilinear)	Horizontal (0)
Right-diagonal	+ (Rectilinear)	Horizontal OR Vertical (random outcome)
Vertical	× (Diagonal)	Right-diagonal OR Left-diagonal (random outcome)

Step 3: Basis Comparison (Key Sifting)

• After all the photons have been sent and measured, Alice and Bob use the public, classical channel to compare their random choices of bases, but they do not reveal the bit values.
• They discard any bits where their bases didn’t match.

Example of Key Sifting:

Alice’s Basis	Bob’s Basis	Do they match?	Result
+	+	Yes	Keep bit
×	+	No	Discard bit
×	×	Yes	Keep bit
+	×	No	Discard bit
×	×	Yes	Keep bit
+	+	Yes	Keep bit
×	+	No	Discard bit
+	×	No	Discard bit

The remaining bits from the matching bases form a preliminary shared key.

Step 4: Eavesdropper Detection and Key Refinement

This is where the “quantum” part provides the security.

• Imagine an eavesdropper, Eve, is trying to intercept the photons.
• When a photon is measured, its quantum state is fundamentally changed (this is a core principle of quantum mechanics, the Heisenberg Uncertainty Principle). Eve would have to guess the correct basis to measure each photon without changing its state, and she would be wrong about half the time.
• Alice and Bob perform a final check. They compare a small, randomly selected portion of their shared key. If there are too many discrepancies (a high quantum bit error rate, or QBER), they know Eve was listening and they discard the entire key.
• If the QBER is acceptably low, they use the remaining bits to create a final, secure cryptographic key through a process called privacy amplification and error correction.

This process ensures that any attempt to eavesdrop introduces detectable errors, and a perfectly secure key can be established. The key can then be used with a standard encryption algorithm to secure a classical message

Quantum Internet

While QKD systems are already commercially available for short distances, the ultimate goal is to create a global quantum internet. This will be a network of interconnected quantum devices (like quantum computers and sensors) that can share information securely over vast distances. To achieve this, scientists are working on a number of challenges:

• Distance Limitations: Quantum signals are fragile and degrade over long distances. Today’s fibre-optic cables can only transmit quantum keys for a few hundred kilometres.
• Quantum Repeaters: Unlike classical communication, you can’t simply amplify a quantum signal without destroying the information it carries. The solution is the development of quantum repeaters, which use entanglement swapping to extend the range of quantum communication.
• Satellite Links: For global communication, satellites are being used to beam quantum signals over long distances, bypassing the signal loss from terrestrial fibre. This is a critical step towards a global quantum network.

Quantum Communication :Applications and Future Impact

The potential applications of quantum communication extend far beyond just secure key exchange.

• Cybersecurity: This is the most immediate and impactful application. Governments, defence agencies, and the financial sector are already exploring and implementing QKD to protect highly sensitive data from future threats posed by quantum computing.
• Scientific Research: The quantum internet will enable scientists to link quantum computers and sensors across the globe, allowing for unprecedented collaboration and discovery in fields like astronomy and particle physics.
• Medical and Financial Industries: Quantum communication can secure everything from patient medical records to online banking transactions, offering a level of data protection that is simply not possible with current technology.

Quantum communication represents more than a routine technological advancement; it signifies a foundational shift in approach. This technology has the potential to transform data security, privacy, and connectivity, paving the way for a new era of inherently secure communications.