Quantum Entanglement

Quantum entanglement is one of the foundational properties of quantum mechanics and quantum technology. It is a phenomenon that explains how two subatomic particles can be intimately linked to each other so that two particles behave like a single unified unit, irrespective of their distance. The Nobel Prize in 2022 for Physics was given for the experimental establishment of the reality of quantum entanglement.

Quantum entanglement properties can be exploited to open up new technological possibilities such as quantum cryptography, superdense coding, and teleportation to solve modern-day problems.

Discovery of Quantum Entanglement Phenomenon

In the early 20th century, Physicists developed the basic concepts behind entanglement once they figured out the mechanics of the quantum world.

• Einstein–Podolsky–Rosen paradox: Albert Einstein, Boris Podolsky and Nathan Rosen (1935), examined the correlationof quantum states that would interact with each other, a phenomenon that came to be later known as “Entanglement”.
  • They discovered that the strongly correlated particles lose their individual quantum states and instead share a single, unified state.
• Schrödinger’s paper: Erwin Schrödinger was the first one to use the word "entanglement".
  • According to him, entanglement is the most significant aspect of quantum mechanics that allows for the transmission of information at speeds faster than light.
  • Albert Einstein termed this entanglement as 'spooky action at a distance'.
• Nobel Prize in Physics 2022: Alain Aspect, John Clauser and Anton Zeilinger were conferred the prize for the experimental establishment of the reality of quantum entanglement.
• The demonstration of the ability to control the teleportation of entangled particles paved the way for the foundation of 21st-century emerging quantum technology.

How does Quantum Entanglement work?

Quantum entanglement is a phenomenon in quantum physics where two or more particles become interconnected in such a way that the state of one particle instantly influences the state of the other, regardless of the distance between them.

• For example, consider a pair of entangled photons with random polarisations.
  • When the polarisation of one photon is measured and found to be vertical, the entanglement ensures that the polarization of the other photon will also be vertical.
• When an operation is performed on one of the entangled particles, there is an instantaneous reaction on the other. Hence, measurements of one state can affect the other.

Methods to Create Entangled Particles

There are many ways of creating pairs of entangled particles, such as photons.

• Special crystals method: A photon with high energy is converted into two photons of lower energy, this is known as "down conversion". This allows large numbers of entangled photon pairs to be produced quickly and easily.
• Cooling of the particles: One method is to cool the particles and place them close enough together so that their quantum states overlap, making it possible for identical particles.
• Nuclear decay: Using the subatomic under the process of nuclear decay, entangled particles can be produced automatically.
• Splitting an individual photon: The entangled pairs of photons can also be produced by splitting a single photon into two, or by mixing two pairs of photons in an optical fibre cable.

Device-Independent Self-Testing (DIST) Method

Due to the fragile nature of entanglement, characteristics of particles during the transit can be lost. Hence, it is extremely important to verify.

• The DIST methodis a novel protocol developed by scientists from S. N. Bose National Centre for Basic Sciences (SNBNCBS), Kolkata.
• This method enables the measurement of the status of entanglement in an unknown quantum state of two particles.
• It is a device-independent method and overcomes the possibility of being hacked or compromised.

Applications of Quantum Entanglement

The predictability of entangled particles (achieved by laws of Quantum) is employed to make technological progress, construct quantum computers, improve measurements, build quantum networks, establish secure quantum encrypted communication etc.

• Quantum Cryptography: The entangled particles are used for secure communication.
  • It prevents the leakages of information by changing particle states if someone tries to intercept them.
  • The entanglement-based quantum distributed keys are being used to improve the security of communications.
  • Improved Microscopy: The entangled photons help in making microscopes sharper and gathering more detailed information.
  • It boosts medical imaging through optical sensing.
  • This helps in various scientific fields, like biophysical characterisation of cells as well as in nanotechnology.
• Quantum Teleportation: Quantum teleportation involves the exchange of quantum information between entangled particles.
  • It helps quantum computers work faster and consume less power.
• Superdense Coding: The basic reason behind superdense coding is to send more information using fewer particles.
  • It makes data transmission more efficient and faster.
• Application in defence: Quantum communications, quantum computing and quantum sensing technologies which are based on entanglement can be applied by military and intelligence agencies.
• Improving GPS positioning: It can improve the high resolution radio frequency detection thereby improving the GPS positioning.
• Entanglement-as-a-service: The importance of quantum entanglement is such that nowadays, there is hot debate about entanglement-as-a-service (EAAS) is going on.
  • By this, a company would provide customers with network access to entangled qubits to be used for secure communications.

Challenges Associated with Quantum Entanglement

Quantum entanglement as a technological resource can not be used directly even though the actual experimental techniques pioneered the entanglement and other properties. It only developed as a foundation for using quantum entanglement as a technological resource.

• Fragility: Entanglement is fragile and is easily lost during the transit of photons through the environment. Hence using them as a resource is extremely challenging.
• Observance: Quantum superposition and entanglement only exist as long as quantum particles are not observed or measured. Observing particles leads to a collapse of the system.
• Practical problems: Researchers have revealed (on simulating a wormhole model) 

some practical limitations like finding the shape of an undiscovered drug, autonomously exploring space or factoring large numbers.

• Vulnerable to physical shock: Researchers are yet to build QCs that completely eliminate these disturbances in systems as Qubits exist in superposition in specific conditions, such as low temperature (~0.01 K), with radiation shielding and protection against physical shock.