## Topics Covered

Introduction

Quantum Computing

Quantum Teleportation

Quantum Cryptography

Single or Entangled Photon Sources

TOPTICA’s Added Value

## Introduction

Shortly after quantum theory was devised and accepted, researchers began to explore and discuss its potential benefits for mankind. Today, the most popular application of quantum theory is in the development quantum computers, which are predicted to reach extraordinary computing speeds.

The ultrafast speeds available to quantum computers will allow data-intensive computations which otherwise would not be possible.

While trapped ions and solid state systems have been applied to take part in initial quantum operations, the quest for true quantum information still continues.

The uncertainty principle forms the basis of quantum computing. It refers to a quantum mechanical property of systems, where the state of one system component is completely dependent on the state of another system component. The well-known example of ‘Schrödinger's cat’ attempts to display how unusual entanglement is when compared to the macroscopic world that we, as humans, experience.

*The theory of entanglement describes how two states (e.g. particles or photons) that are physically seperated are still observed as influencing one another*

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Entanglement raised such significant doubts about quantum theory that even Einstein and his colleagues Rosen and Podolski published an article in 1935 that argued against it. They tried to demonstrate that quantum theory would always be incomplete so another theory, that used different variables, would have to replace it. However, these variables are still not known and are concealed in quantum theory.

Bell (Bell’s theorem) found this paradoxical argument highly inaccurate and demonstrated that quantum mechanics is actually complete. Since he first argued it, Bells theorem has been experimentally substantiated a number of times. Hidden variables are no longer required to fully elucidate quantum behavior.

The basis for quantum teleportation and quantum cryptography is also based on the unusual property of entanglement. In quantum teleportation, a quantum mechanical state is transferred from one device, in one location, to another device at a different location.

In quantum cryptography, data is sent from location to location in a safe and reliable manner. Quantum cryptography depends on the entanglement of photons and is now being commercialized.

## Quantum Computing

Quantum computing is anticipated to enable high-speed operations, simulations and calculations that otherwise could never be reached using traditional computing. For instance, when compared to standard computers, a quantum computer is capable of the factorization of large numbers or the performing of database explorations at a much faster rate.

*In conventional computers each bit can either be 1 or 0. The QuBits in quantum computers can simultaneously be both 1 and 0 meaning complex calculations can be carried out many times faster.*

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Two key principles contribute to the huge calculation power of quantum computers;

- QuBits, a quantum mechanical two state system are used as the basic pieces of data. QuBits can remain as a superposition of the two states, in contrast to a traditional bit which must be either 1 or 0 and not both at the same time.
- The superpositioned state can be acted upon by fundamental calculations using logical operations. This makes it is possible to calculate all the potential realizations of anything between |0> and |1> at the same time, making it easy to realize highly parallel computation.

The basic way computers function is using gate operations, which have been demonstrated to work with both photon-based quantum computers and with trapped ions. A proof of concept for the quantum computed factorization of the number 15 was shown by means of solid state systems (NMR).

## Quantum Teleportation

Quantum teleportation is a process where an object’s quantum mechanical state is completely transferred to another object in a different location. This procedure exploits entanglement's non-locality. The polarization state of one photon can be fully transferred to another photon by performing a series of measurements and entanglement operations on the photons.

Recently, two separate ion traps have been used to demonstrate quantum teleportation between distant QuBits. The quantum logic is closely associated with quantum computing and quantum teleportation. In the field of metrology, this controlled state preparation was applied to develop an advanced atomic clock based on aluminum ions.

## Quantum Cryptography

Elements of quantum behavior such as the back action of measurement procedures on quantum states and entanglement are used by quantum cryptography to achieve safe and reliable communication between senders and receivers.

The process works by both the sender and receiver carrying out measurements on entangled quantum solutions, such as entangled photons, to produce a 'key' for themselves which can be used to decrypyt or encrypt a separate message. This quantum cryptography technique is known as quantum key distribution as both the sender and receiver can apply the same code to decrypt and encrypt the message.

The sender encrypts the message as per the measurement results obtained and sends it via an open channel to the receiver, who can then decrypt the message using the same measurements (which were obtained separately). When a third party tries to find the quantum key it is easily detected. This is because due to the laws of quantum physics, quantum mechanical states are affected by individual measurement. As a result any eavesdropping is easily detected.

*Quantum cryptography wwill facilitate the extremely secure transfer of data. *

*Mclek | Shutterstock*

Due to its high security and strong signal strength quantum cryptography is being heavily researched. It has already been demonstrated that the distribution of a quantum key can be achieved down kilometers of fiber. Currently, satellite-links of entangled photons are being studied with the aim of sending quantum keys between users on a global scale.

A quantum encrypted bank transaction has been carried out to demonstrate that quantum key distribution has real world applications.

## Single or Entangled Photon Sources

Single and entangled photon sources are critical tools for both quantum cryptography and quantum computing. Single photon sources produce single photons at specific times. There are several ways to achieve this, such as using ions or color centers in solids, quantum dot systems, trapped ions or single atoms in optical cavities.

A standard entangled photon source is mainly based on spontaneous parametric downconversion. Within a non-linear optical crystal a blue photon is converted into a pair of red photons. Strong correlations can be observed in the energy, momentum, and polarization of the two photons. Studies are being performed on entangled photon production and attempts are also being made to develop deterministic and efficient sources with the potential for large-scale production.

## How TOPTICA can help with Quantum Experiments

TOPTICA provides lasers for use in quantum data experiments involving trapped atoms or ions. The company’s lasers have been effectively used to logically maneuver, trap, cool, or optically pump atoms and ions.

Care has been taken to ensure that these lasers are designed and adjusted for specific wavelengths so that they can be suitably used for triggering single photon emitters. One way to produce pairs of entangled photons through spontaneous parametric down conversion is to use a basic laser to start the conversion process.

This basic laser must be used at half the wavelength of the photon pair. Entangled photons in the near infrared (at a wavelength around 800 nm) are often produced. To produce an entangled IR photon pair a violet laser (at a wavelength around 400 nm) must be used.

TOPTICA is capable of developing lasers in the UV. In fact, the company was the first to develop diode laser solutions in the UV and is well placed to provide lasers of different power levels and coherence and linewidth properties for both industrial and research applications.

This information has been sourced, reviewed and adapted from materials provided by Toptica Photonics.

For more information on this source, please visit Toptica Photonics.