optical quantum computer
Among the methods for realising quantum computers described in ‘Overview and Reference Information/Reference Books on Quantum Computers’ and ‘Quantum Computers Accelerate Artificial Intelligence’, the optical quantum computer (optical quantum computer) uses photons, which are light particles, to perform calculations, mainly using photons as quantum bits (qubits). It mainly uses photons as qubits, a technology that is expected to enable fast and efficient quantum computation.
Optical quantum computers encode information using photon states (e.g. polarisation state and phase), perform calculations and execute quantum algorithms through operations known as optical quantum gates. Compared to conventional quantum computers that use electrons or superconducting elements, optical quantum computers have Advantages and challenges.
Advantages.
– Faster information transmission: light can travel faster than electricity, enabling high-speed communication, e.g. via optical fibres.
– Heat resistance: optical quantum computers do not require cryogenic environments and can therefore operate more stably than conventional quantum computers.
– Scalability: photons can be easily produced in large quantities, and thus large quanta bits are expected to be generated.
Challenges
– Control of photons: manipulation and control of photons is very difficult, and the generation of errors and the improvement of the accuracy of quantum gates are challenges.
– Quantum error correction: optical quantum computers also require error correction techniques, and high precision quantum gates are required for practical use.
Such optical quantum computers are particularly promising in the following fields
– High-speed computation: they could provide efficient solutions for complex calculations and simulations (e.g. chemical reaction analysis, cryptanalysis, machine learning, etc.).
– Quantum communication: it could be useful for secure quantum communication technologies that utilise the properties of optical quanta.
Research into optical quantum computers is being carried out by various institutions and companies, with technologies utilising silicon photonics, fibre lasers and optical pulses. As the technology matures, it is expected that the computational speed and efficiency will increase and that it will have wide practical applications in the future. Optical quantum computers are still under development, but as a technology that makes use of the characteristics of photons to realise high-speed and high-efficiency quantum computation, it is a technology that has attracted much attention for its future development.
Methods of generating photons in optical quantum computers
One of the key technologies in optical quantum computers is the generation of photons. The main generation methods will be as follows.
1. optical parametric conversion: optical parametric conversion is a method of generating photons using non-linear optical effects, specifically by passing intense light through a non-linear crystal, the energy of the light is converted and a new photon pair is generated. This method is used in optical parametric amplifiers (OPAs) and is particularly important for the generation of optical quantum bits (qubits) in quantum computers and quantum communications.
2. laser-based photon generation: a method that uses a laser to generate intense light from which photons are extracted. Laser light has high coherence (phase consistency), which makes it easy to control quantum states and suitable for photon generation. Techniques are underway to generate single photons at the required timing, particularly using single-photon sources.
3. spontaneous two-photon generation: spontaneous two-photon generation (SPDC: Spontaneous Parametric Down-Conversion) is a method that uses the phenomenon of one high-energy photon being split into two low-energy photons when intense light passes through a non-linear crystal. The two photons produced in this process are often in an entangled state and play an important role in quantum information processing.
4. quantum dots and semiconductor light sources: quantum dots and semiconductor materials are also used to generate single photons. Quantum dots are nanoscale semiconductor structures that emit photons at specific energy levels. This method, combined with silicon photonics technology, has been used as a photon source for quantum computers.
Accuracy and stability are particularly important issues for these methods.
Technology required to control photons
The next technology that will play an important role will be technology to control the generated photons. Compared to electronics and superconducting materials, photons are more difficult to control directly and are thought to be achieved by the following approaches.
1. generation of optical quantum bits (Qubit): In an optical quantum computer, photons take the form of qubits to process information. This is achieved by manipulating the state (polarisation, phase, etc.) of the photon, such as by wavelength conversion and phase control using optical parametric amplifiers (OPAs).
Specifically, a non-linear crystal (e.g. BBO, PPKTP) is irradiated with a high-energy pump light, the pump light splits into two low-energy photons (signal photon and idler photon) in the crystal, and in this process, due to energy conservation and phase matching conditions, the generated photons become quantum entangled (entangled state) and This is then realised as a quantum entangled state.’ Quantum communication, as described in ‘Quantum entanglement and quantum communication technology’, is a method of securely transmitting cryptographic keys using this entanglement.
2. Measuring quantum entanglement: one method for measuring quantum entanglement is the verification of the Bell inequality. This inequality, proposed by John Bell, states the limits of correlations based on classical local realism, and it has been shown theoretically that this inequality is violated in quantum entangled states.
Specifically, for a generated entangled photon pair (quantum entanglement), multiple measurement settings are selected, such as different polarisation angles and spin directions, the photon detection results for each setting are recorded, the correlation is analysed from the obtained data, and the left and right sides of the Bell inequality are calculated using the obtained data to check whether the inequality is broken. The inequality is then checked to see if it is broken.
If the Bell inequality is violated, this suggests the existence of quantum entanglement, and this method has become widely used as a powerful tool for directly verifying the existence of quantum entanglement.
Apart from verifying Bell inequalities, quantum entanglement can also be measured by reconstructing quantum states by tomography, using entanglement witnessing and measuring quantum fidelity.
Their realisation requires the development of high-precision measuring instruments, advances in noise rejection techniques and efficient measurement algorithms for exponentially complex states.
3. photon interactions and integration technology: quantum information processing requires technology to artificially induce interactions between photons. However, photons are physically very fast and photons normally interact very little with other photons. To achieve this, devices utilising non-linear optical effects are important, e.g. integrated circuit technology based on silicon photonics is being investigated to process multiple photons simultaneously.
4. error correction techniques: optical quantum computers are very sensitive to errors, and error correction techniques are essential to accurately preserve the photon state. Photon-based error-correcting codes for quantum error correction have been studied, and techniques for detecting and correcting errors are in progress.
What will be achieved when optical quantum computing is realised?
The realisation of an optical quantum computer would enable very efficient solutions to problems that are difficult or take a huge amount of time to compute with conventional or current quantum computers. They are described below.
1. ultra-fast computational capabilities: optical quantum computers are capable of parallel processing due to light-based computation and are expected to solve problems significantly faster than conventional semiconductor technology. This will enable high-precision simulations of molecular structures and chemical reactions, which will accelerate the development of new medicines and progress in materials science, quantum chemistry simulations, and high-speed solutions to optimisation problems where the problem of finding the optimum solution from a huge number of options (e.g. portfolio optimisation in logistics and finance) is solved at high speed. This will be achieved.
2. improved energy efficiency: the use of light will enable computers with low heat loss and low power consumption. This will enable large-scale calculations to be performed in an energy-efficient manner.
3. building new cryptography and breaking existing cryptography: traditional cryptography techniques such as RSA and elliptic curve cryptography can be broken in a short time by quantum computers, and an optical quantum computer could accelerate this further. It is also expected to contribute to the development of new quantum-resistant cryptography.
4. dramatic improvements in machine learning and AI: Optical quantum computers have an excellent ability to process large amounts of data simultaneously, which could dramatically speed up the training process of machine learning. This will enable real-time AI decision-making and complex data analysis.
5. elucidation of unknown physical phenomena: optical quantum computers will be powerful tools for simulating problems that are currently unsolved in theoretical physics and astrophysics (e.g. the nature of black holes, quantum gravity theory).
6. advances in the natural and life sciences: the use of light will enable more accurate modelling and realistic simulations of complex natural phenomena, such as biological evolution and climate change.
7. innovations in communications technology: optical quantum technology will enable ultra-secure quantum communications, enabling data transmission that cannot be intercepted. This will further enhance the financial, military and personal data protection sectors.
reference book
The following reference books are useful for learning about optical quantum computers.
Fundamentals.
1. “Quantum Computation and Quantum Information”
Author(s): Michael A. Nielsen, Isaac L. Chuang
Description: A classic book that provides a comprehensive overview of quantum computation in general. In addition to qubits, gates and algorithms, it also covers quantum communication and quantum error correction.
2. “Introduction to Optical Quantum Information Processing”
Author(s): Pieter Kok, Brendon W. Lovett
Description: details the fundamentals of quantum information processing using light. Learn how light is used to generate, manipulate and measure qubits.
3. “Quantum Mechanics for Scientists and Engineers”
Author: David A. B. Miller
Description: explains the basics of quantum mechanics for scientists and engineers and is suitable as a foundation before learning about optical quantum computers.
Applications.
4. “Photonic Quantum Technologies”
Author: Mohamed Benyoucef
Description: in-depth look at photonics technology and its application to quantum information. Includes specific examples of physical mechanisms and experimental techniques used in optical quantum computers.
5. “Quantum Optics”
Author(s): Marlan O. Scully, M. Suhail Zubairy
Description: in-depth coverage of the theory of quantum optics required for optical quantum computers. Provides a detailed understanding of photons and quantum entanglement.
6. “Principles of Quantum Computation and Information – Volume II: Basic Tools and Special Topics”
Author(s): Giuliano Benenti, Giulio Casati, Gian Luca Strini
Description: a book dealing with the special topics of optical quantum computing, complementing the knowledge on quantum networks and quantum communication.
Practice and research.
7. “Quantum Computation with Linear Optics”
Author(s): Emanuel Knill, Raymond Laflamme, Gerard J. Milburn
Description: a collection of fundamental papers on quantum computation with linear optics. Recommended for researchers who want to link theory to practice.
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