What Is Quantum Optics?
(Quantum Science and Technology)
Quantum Physicist and Brain Scientist
Visiting Professor of Quantum Physics, California Institute of Technology
IEEE-USA Fellow
Ph.D. & Dr. Kazusho Kamuro
AERI：Artificial EvolutionResearch Institute
Pasadena, California
HP: https://www.aeri-japan.com/
・ Quantum optics is primarily an area of physics which uses a combination of semi-classical physics and quantum mechanics principles to investigate and manipulate how photons of light interact with matter, and the phenomena which can be produced, at the subatomic level. This is how you would explain quantum optics in its broadest sense. However, whilst some of the most prominent applications is lasers and quantum computing, there has been a lot of research into the fundamental principles of how photons behave at this level, and this has helped to realize many different subsets and phenomenon within quantum optics which contribute heavily to the realization of the physical applications.
・ Quantum optics is a field of quantum physics that deals specifically with the interaction of photons with matter. The study of individual photons is crucial to understanding the behavior of electromagnetic waves as a whole.
・ To clarify exactly what this means, the word "quantum" refers to the smallest amount of any physical entity that can interact with another entity. Quantum physics, therefore, deals with the smallest particles; these are incredibly tiny sub-atomic particles which behave in unique ways.
・ The word "optics," in physics, refers to the study of light. Photons are the smallest particles of light (though it is important to know that photons can behave as both particles and waves).
Development of Quantum Optics and the Photon Theory of Light
The theory that light moved in discrete bundles (i.e. photons) was presented in Max Planck's 1900 paper on the ultraviolet catastrophe in black body radiation. In 1905, Einstein expanded on these principles in his explanation of the photoelectric effect to define the photon theory of light.
Quantum physics developed through the first half of the twentieth century largely through work on our understanding of how photons and matter interact and inter-relate. This was viewed, however, as a study of the matter involved more than the light involved.
In 1953, the maser was developed (which emitted coherent microwaves) and in 1960 the laser (which emitted coherent light). As the property of the light involved in these devices became more important, quantum optics began being used as the term for this specialized field of study.
Findings
Quantum optics (and quantum physics as a whole) views electromagnetic radiation as traveling in the form of both a wave and a particle at the same time. This phenomenon is called wave-particle duality.
The most common explanation of how this works is that the photons move in a stream of particles, but the overall behavior of those particles is determined by a quantum wave function that determines the probability of the particles being in a given location at a given time.
Taking findings from quantum electrodynamics (QED), it is also possible to interpret quantum optics in the form of the creation and annihilation of photons, described by field operators. This approach allows the use of certain statistical approaches that are useful in analyzing the behavior of light, although whether it represents what is physically taking place is a matter of some debate (although most people view it as just a useful mathematical model).
Applications
Lasers (and masers) are the most obvious application of quantum optics. Light emitted from these devices is in a coherent state, which means the light closely resembles a classical sinusoidal wave. In this coherent state, the quantum mechanical wave function (and thus the quantum mechanical uncertainty) is distributed equally. The light emitted from a laser is, therefore, highly ordered, and generally limited to essentially the same energy state (and thus the same frequency & wavelength).
Coincidence Correlation
・ Coincidence correlation is an area of quantum optics that is used to see if someone is observing a single quantum system. This is done by assuming that a single system can only emit one photon at a time and observes (via a photodetector) the quantum system as a single photon emitter. If it is found that more than detector observes the source, then the likelihood is that it is not a one photon system and is unlikely to be a single quantum system. It is a fundamental process that enables someone to determine the presence of a single quantum system, i.e. it is a test rather than an application, but it can be used in conjunction with other quantum optic application areas.
One example is with quantum entanglement (detailed more below). Coincidence correlation can be used to prove or disprove the correlations with a quantumly entangled network and will employ a combination of optical polarizers and photodetectors to filter quantum states and determine if there is correspondence at both ends of the entangled pair.
・ Determine the presence of a single quantum system
Coincidence correlation with picosecond timing can be used to determine if one is actually observing a single quantum system in the form of a single photon emitter. Here one employs the knowledge that such a system can only emit one photon at a time. This is because in typical quantum systems such as single molecules or defect centers in diamond there is a characteristic average lifetime of the excited state that must pass before the system can be excited again. If one finds that two detectors observing the source „click“ simultaneously (with statistical significance) then obviously the source cannot be a single photon emitter.
・ In case of experiments dealing with photon entanglement one effectively tries to prove or disprove correlations between measurement outcomes using some kind of correlator.
In the case of experiments with photons one may, for instance, employ polarizers to filter out quantum states of interest and then use photon detectors to determine whether or not they occurred correspondingly at both parts of the entangled pair. Now, given that photon detectors are not 100% efficient (and actually neither is the creation of entangled pairs and their transmission) one typically must repeat the experiment many times in order to arrive at a statistically reliable answer. Since there can also be unwanted photons from background radiation or detector artifacts it is a smart common practice to perform the coincidence correlation with picosecond timing. The correlations can then be determined for narrow time windows where the knowledge of the time the photons travel can be used to eliminate background.
・ In a coincidence correlation set-up, the photons emitted by the systems are split using, e.g. a 50 / 50 beamsplitter or a polarization splitter and send onto two single photon sensitive detectors. The output of these detectors is then fed into a time tagging unit with high temporal resolution that allows not only to detect coincidences in a certain time window but obtain the full second or higher oder correlations.
Quantum Entanglement
A common quantum mechanical state of separated systems
・ Quantum entanglement is a physical phenomenon that occurs when quantum systems such as photons, electrons, atoms or molecules interact and then become separated, so that they subsequently share a common quantum mechanical state. Even when a pair of such entangled particles are far apart, they remain "connected" in the sense that a measurement on one of them instantly reveals the corresponding aspect of the quantum state of its twin partner. These "aspects" of quantum state can be position, momentum, spin, polarization, etc. While it can only be described as a superposition with indefinite value for the entangled pair, the measurement on one of the partners produces a definite value that instantly also determines the corresponding value of the other. The surprising "remote connection" between the partners and their instantaneous action "faster than light" that would seem to contradict relativity has been the reason for intense research efforts, both theoretically and experimentally. In the corresponding experiments, entanglement is proven by correlation of the measurment outcomes on the separated twins.
・ Entangled quantum systems are typically analysed via coincidence correlation methods. For that purpose, the photons emitted by the systems are split using, e.g., a 50 / 50 beamsplitter or a polarization splitter and send onto two single photon sensitive detectors. The output of these detectors is then fed into a time tagging unit with high temporal resolution that allows not only to detect coincidences in a certain time window but obtain the full second or higher oder correlations.
・ Quantum entanglement is a phenomenon that occurs between quantum systems, where the components of each quantum system become one and indescribable from each other, i.e. instead of two separate quantum states, the whole system becomes one quantum network state. The types of components which can experience this phenomenon include electrons, photon, atoms and molecules. This extends to long-range distances, and the measurement of one part of the quantum system enables the properties of the corresponding particle in the quantum system to be revealed.
The different properties that can be revealed at different ends of an entangled network include position, momentum, spin and polarization. In many cases, one of the quantum particles is described as a superimposition with an indefinite value for the entangled particle. However, if one of these particles is measured, it can provide definite value for the corresponding pair. Quantum entanglement is often utilized in quantum computing applications.
Quantum Teleportation
A qubit transmitted from one location to another
・ Quantum teleportation is closely related to entanglement of quantum systems. It may be defined as a process by which a qubit (the basic unit of quantum information) can be transmitted from one location to another, without the qubit actually being transmitted through space. It is useful for quantum information processing and quantum communication. As with entanglement, it is applicable to simple and more complex quantum systems such as atoms and molecules. Recent research demonstrated quantum teleportation between atomic systems over long distances.
・ Quantum teleportation experiments generally have several prerequisites:
1.means of generating an entangled EPR pair of qubits as well as a qubit that is to be teleported
2.a conventional communication channel capable of transmitting two classical bits
3.means of performing a Bell measurement on the EPR pair, and manipulating the quantum state of one of the pair
・ The teleportation success is then typically analysed via coincidence correlation methods. For that purpose, the photons emitted by the systems are split using, e.g., a 50 / 50 beamsplitter or a polarization splitter and send onto two single photon sensitive detectors. The output of these detectors is then fed into a time tagging unit with high temporal resolution to measure the coincidence.
・ Quantum teleportation is another phenomenon that has a lot of use in quantum computing, as well as quantum communications, and is closely related to quantum entanglement. Quantum teleportation is the process by which the information held within a qubit can be transported from one location to another, without the qubit itself being transported.
For those who don’t know, a qubit, otherwise known as a quantum bit, is the building block of many quantum networks, especially in quantum information processing applications and can adopt a 0 value, a 1 value or a superimposed 0 or 1 value. This means that qubits can perform quantum operations in more than one value simultaneously.
Quantum Information Processing
Computing with qubits
・ Quantum Information Processing focuses on information processing and computing based on quantum mechanics. While current digital computers encode data in binary digits (bits), quantum computers aren't limited to two states. They encode information as quantum bits, or qubits, which can exist in superposition. Qubits can be implemented with atoms, ions, photons or electrons and suitable control devices that work together to act as computer memory and a processor. Because a quantum computer can contain these multiple states simultaneously, they provide an inherent parallelism. This will enable them to solve certain problems much faster than any classical computer using the best currently known algorithms, like integer factorization or the simulation of quantum many-body systems. Right now the quantum computer is still in its infancy. First steps on that road are the simplest building blocks such as quantum logic gates and memory based on genuine quantum effects such as superposition and entanglement.
・ There are several methods used in the development of the necessary building blocks of a quantum computer. The study and the functionality test of these blocks is then very often performed via coincidence correlation methods or timing analysis of photon detector signals in order to prove the general working pricingple of the building block under study.
・ Quantum information processing, i.e quantum computing, is a computing process (and memory storage) that relies on qubits as opposed to binary bits. The ability to of qubits to superimpose, compared to binary bits that adopt a 0 or 1 value only, enables simultaneous operations to occur (several quantum systems can be operated in parallel), and in turn allows quantum computers to be much faster than their classical counterparts. The qubits within a quantum computer stores information quantum mechanically by utilizing the ½ spin state of electrons (up and down) and the polarization of photons (horizontal and vertical) within the quantum network. This correlates to a positional arrangement that can be identified when the qubits are entangled, and so long as the computer can control the spin operation and the interactions between electron spins, the readout can measure the single spin states and bulk spin resonance of each quantum network to determine the information contained within.
Quantum Communication
Quantum mechanics guarantee secure communication
・ Quantum communication is a field of applied quantum physics closely related to quantum information processing and quantum teleportation. Its most interesting application is protecting information channels against eavesdropping by means of quantum cryptography. The most well known and developed application of quantum cryptography is quantum key distribution (QKD). QKD describes the use of quantum mechanical effects to perform cryptographic tasks or to break cryptographic systems. The principle of operation of a QKD system is quite straightforward: two parties (Alice and Bob) use single photons that are randomly polarized to states representing ones and zeroes to transmit a series of random number sequences that are used as keys in cryptographic communications. Both stations are linked together with a quantum channel and a classical channel. Alice generates a random stream of qubits that are sent over the quantum channel. Upon reception of the stream Bob and Alice — using the classical channel — perform classical operations to check if an eavesdroper has tried to extract information on the qubits stream. The presence of an eavesdropper is revealed by the imperfect correlation between the two lists of bits obtained after the transmission of qubits between the emitter and the receiver. One important component of virtually all proper encryption schemes is true randomnessm which can elegantly be generated by means of quantum optics.
・ Quantum communication is an area which is closely related to quantum information processing but is more to do with quantum cryptography than it is computing—such as quantum key distribution. Quantum key distribution uses quantum mechanics to perform cryptographic tasks or break encrypted systems. Quantum key distribution works when two people use a communication system that utilizes single photons, which are randomly polarized, to transmit a series of random number sequences. The randomness of the polarization is generated by using quantum optics. These sequences act as the keys in the cryptographic system and the system uses both a classical channel and a quantum channel to connect the communication points.
The qubits are sent over the quantum channel and the classical channel performs classical operations and can be used to see if anyone is trying to hack the system. Because the information is transmitted via the quantum network, and not the classical channel, the classical channel can be hacked but no information will be obtained. However, because the signals under normal conditions are correlated, any correlation imperfections (due to a hack) between the classical network and the quantum network will be detected by the receiver and can be used to determine when a hack has been attempted.
・ In a typical QKD set-up, the photons are generated by a single photon source, encoded into binary values (i.e., representing "0" and "1") and then transmitted to the receiver either via optical fibers or in free space. The receiver then decodes the state of photons and detects them using single photon sensitive detectors and time-tagging electronics. There are several methods for encoding and decoding the photons:
・ via polarization: the binary information "1" or "0" is defined by the polarization of the single photons, e.g., binary "0" correlates with the horizontally polarized photon and binary "1" with vertically polarized photon
・ via the phase, which requires the use of a interferometer system: the phase difference Δφ = φAlice - φBob of the two interferometers is then used for encoding the binary values, e.g., a phase difference Δφ=0 correlates with the binary "0" and the phase difference Δφ=π correlates with the binary "1"
・ via entangled photons, which requires one sender of entangled photon pairs and two receivers (Alice and Bob) each equipped with a polarizer. Alice and Bob set the two angles at their respective polarization rotator randomly. If the angles of Alice and Bob match, both photons behave exactly the same at the beam splitter, i.e., they are either transmitted (binary "1") or reflected (binary "0").
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Prof. PhD.Dr. Kamuro
Quantum Physicist and Brain Scientist involved in Caltech Assosiate Professor and Brain Scientistficial Evolution Research Institute（AERI: https://www.aeri-japan.com/）
IEEE-USA Fellow
email: info@aeri-japan.com
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Keywords Artificial EvolutionResearch Institute：AERI
HP： https://www.aeri-japan.com/
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