Quantum Teleportation, Quantum Decoherence and Entanglement

- Quantum Teleportation for Quantum Internet

Quantum teleportation is a key communication function for the quantum internet, allowing the "transmission" of qubits without the physical transport of the particles storing the qubits. 

Quantum teleportation is facilitated by the action of quantum entanglement, a somewhat counterintuitive physical phenomenon that has no direct counterpart in the classical vocabulary. 

Therefore, the concept of classical communication system models has to be redesigned to take into account the specificity of quantum teleportation. This redesign is a key prerequisite for building any efficient quantum communication protocol.

- Quantum Teleportation

Quantum teleportation is a technique for transferring quantum information from a sender at one location to a receiver some distance away. While teleportation is commonly portrayed in science fiction as a means to transfer physical objects from one location to the next, quantum teleportation only transfers quantum information. 

The sender does not have to know the particular quantum state being transferred. Moreover, the location of the recipient can be unknown, but classical information needs to be sent from sender to receiver to complete the teleportation. Because classical information needs to be sent, teleportation can not occur faster than the speed of light. 

One of the first scientific articles to investigate quantum teleportation is "Teleporting an Unknown Quantum State via Dual Classical and Einstein-Podolsky-Rosen Channels" published by C. H. Bennett, G. Brassard, C. Crépeau, R. Jozsa, A. Peres, and W. K. Wootters in 1993, in which they used dual communication methods to send/receive quantum information. It was experimentally realized in 1997 by two research groups, led by Sandu Popescu and Anton Zeilinger, respectively. 

Experimental determinations of quantum teleportation have been made in information content - including photons, atoms, electrons, and superconducting circuits - as well as distance with 1,400 km (870 mi) being the longest distance of successful teleportation by the group of Jian-Wei Pan using the Micius satellite for space-based quantum teleportation.

- Spooky Action at a Distance

Quantum teleportation — or called “spooky action at a distance” — can transfer information between locations without actually moving the physical matter that holds it. This technology could profoundly change the way data travels from place to place. 

Quantum teleportation not only moves data between quantum computers, but it also does so in such a way that no one can intercept it. 

A quantum computer taps into the strange ways some objects behave if they are very small (like an electron or a particle of light) or very cold (like an exotic metal cooled to nearly absolute zero, or minus 460 degrees Fahrenheit). In these situations, a single object can behave like two separate objects at the same time. 

Traditional computers perform calculations by processing “bits” of information, with each bit holding either a 1 or a 0. By harnessing the strange behavior of quantum mechanics, a quantum bit, or qubit, can store a combination of 1 and 0 — a little like how a spinning coin holds the tantalizing possibility that it will turn up either heads or tails when it finally falls flat on the table. 

This means that two qubits can hold four values at once, three qubits can hold eight, four can hold 16 and so on. As the number of qubits grows, a quantum computer becomes exponentially more powerful.

- Quantum Decoherence

Decoherence can arise from many aspects of the environment: changing magnetic and electric fields, radiation from nearby warm objects, or crosstalk between qubits. It is the job of quantum scientists to address all these potential sources of decoherence. 

Quantum decoherence in physics and quantum computing is the loss of quantum coherence. Quantum coherence means that a single particle or object has a wave function that can split into two separate waves. 

When waves behave together in a coherent manner, this is known as quantum coherence.

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- Quantum Decoherence and Entanglement

Part of the challenge is that a qubit breaks, or “decoheres,” if you read information from it — it becomes an ordinary bit capable of holding only a 0 or a 1 but not both. But by stringing many qubits together and developing ways of guarding against decoherence, scientists hope to build machines that are both powerful and practical. 

But this comes with its own problems. In part because of decoherence, quantum information cannot simply be copied and sent across a traditional network. Quantum teleportation provides an alternative. 

Although it cannot move objects from place to place, it can move information by taking advantage of a quantum property called “entanglement”: A change in the state of one quantum system instantaneously affects the state of another, distant one. 

After entanglement, you can no longer describe these states individually. Fundamentally, it is now one system. 

These entangled systems could be electrons, particles of light or other objects.

- Decoherence is a Problem in Quantum Computing

It appears that quantum computing is becoming more and more advanced. Qubits are getting sharper, gates are getting better, and algorithms are getting more complex. It's clearly only a matter of time before quantum computing becomes a major technology. However, a major hurdle remains that will require enormous effort to overcome: decoherence. 

Quantum computers promise to solve certain types of problems by using quantum principles such as superposition and entanglement, but the use of quantum states also makes quantum computers more error-prone than classical computers. These errors are caused by decoherence, the process by which the environment interacts with the qubits, changing the qubits uncontrollably and causing the loss of information stored by the quantum computer. 

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