QUANTUM PHOTONICS

From Single Photons to Global Scale Quantum Internet (QuINET)

Lasers & Amplifiers Typically Used in Our Experiments

Lasers and Amplifiers

Solid-State Nonlinear Optical Crystals

Photodetectors

Electronics

Data-Acquisition and Data-Analysis

Above Picture: Continuum's High Energy, Injection-Seeded, Repetatively Pulsed, and Frequency Doubled Nd:YAG Laser.

Above Picture: Coherent’s Mira-HP: Kerr-Lens, Mode-Locked, Ti:Sapphire, Ultrafast Laser Oscillator.

Question: What is Laser Physics ?

Answer: Study of light-matter interaction is called laser physics. Where matter is refereed to atom, molecule, or solid.

Question: What is a Laser ?

Answer: Laser physics has led to development lasers, which is a quantum device that works on principle of stimulated emission. Lasers are used for many-many practical applications. Examples of such applications are scientific & engineering research, industry, and medical science. For more detail about lasers and how they works click on the link below.

Click: Laser

Question: What is Nonlinear Optics (NLO) ?

What is Low Field Nonlinear Optics (LFNLO) ?

Answer: During light-matter interaction, when intensity in the light beam is large enough so that its electric field become comparable to the existing atomic field in the medium, the light-matter interaction become nonlinear i.e. polarization induced in the medium scales nonlinearly with the applied electric field. Branch of laser physics, which deals with light-matter interaction in the nonlinear regime, is called nonlinear optics. Nonlinear optical processes are characterized by frequency mixing and are generally practiced with high peak power pulsed lasers. Today, nonlinear optics is the most productive area within the entire field of laser physics ! For more detail about nonlinear optics click on the link below.

Click: NONLINEAR -OPTICS

Typically nonlinear interaction is practiced using high atomic density in the sample and high intensity in the input optical pulses. However, for nonlinear interaction, above requirement of high atomic density and high intensity in the input optical pulses can be reduced by following techniques. (1) Frequency tuning the input optical pulses to the vicinity of the atomic resonance of the sample. (2) Reducing linewidth of atomic resonance of the sample, which can be achieved using cold atoms. (3) Introducing electromagnetically induced transparency (EIT) in the sample. (4) Using slow optical pulses. With combination of above four techniques, nonlinear interaction at very low atomic density approaching single atom limit and very low intensity approaching single photon limit can be achieved. Nonlinear interaction, which can be practiced with few atoms and few photons is referred to as Low Field Nonlinear Optics (LFNLO). In an advanced quantum network (QuNET), some important operations such as generation of photonic quantum entangled states, complete Bell state measurements (BSM), and quantum frequency conversion (QFC) relies on nonlinear interaction at single photon level. Therefore, low field nonlinear optics (LFNLO) is a key for development of the next generation of quantum internet (QuINET). For how-to practice low field nonlinear optics experiments , click on Page: PROJECT-2, which is located on TOP-ROW of this website.

Question: Where is Laser Physics Headed ?

Answer: In the past very impressive progress in laser physics has resulted in following two major achievements, which are

(1) Development of high-power pulsed laser system with power of the order of few Petawatts.

Where. 1 Petawatt = 10^ +15 (ten raise to the power- +15) Watts

= 1 0000 0000 0000 000 Watts.

High-power lasers are used in applications where extremely high optical intensity is required. Examples of such applications is high density physics. For more details about high-power lasers and their applications, click on the link below.

Click: HIGH-POWER-LASER

(2) Development of short pulsed laser systems with pulse width of the order of few Attoseconds.

Where. 1 Attosecond = 10^ -18 (ten raise to the power - 18) Second

= 0. 0000 0000 0000 0000 01 Second

Attosecond lasers are used in applications where extremely high time resolution is required. Examples of such applications are study of electron dynamic in matter and switching of entanglement among quantum patricles. For more details about attosecond lasers and their applications, click on the link below.

Click: ATTOSECOND

Next important application of laser physics is to quantum computing and quantum communication. This application aims to develope the next generation of quantum computer and quantum internet, using photon as a quantum particle.

Question: What is Quantum Photonics ?

What Role does Quantum Photonics Play for Quantum Computation and Quantum Communication?

Answer: Quantum photonics, which is the most recent application of laser physics, use quantum nature of photons such as superposition and entanglement among single photons to built next generation of quantum computers and quantum internet.

In this application single photons are used as quantum particles, and qubit is encoded in the state of polarization of the photon. Major advantages of photonic qubit are following: (1) photonic qubit is negligibly affected by decoherence, which is a major limitation for building quantum computer, (2) photonic qubit function at room temperature and therefore experiments using photonic qubit do not require cryogenic cooling, (3) experimental techniques for generation, manipulation, transmission, and detection of photons are already well developed. Based on above advantages photonic qubit provides very suitable platform for processing quantum information and building next generation of photonic quantum computer. Therefore, quantum photonics is playing vital role for very new and fast-growing field of quantum computation. Quantum photonics along with superconducting circuits are two most competing platforms for building next generation of practical quantum computers. A large-scale quantum computer based on quantum photonics is currently under development by US company: PsiQuantum (Click: https://www.psiquantum.com/). More detail about photonic quantum computing can be found in a published paper: A Manufacturable Platform for Photonic Quantum Computing, Nature 641, page 876 (2025): Download: Photonic Quantum Computing Pdf.

Photonic qubit offers further advantages, which are following: (4) photonic qubit is the best and the fastest carrier of information, (5) photonic qubit is compatible with existing fiber telecommunication technology, and (6) entanglement among photonic qubits can be used to design quantum interconnections (QuIC) among remotely located nodes of a quantum network. Based on above advantages photonic qubit provides the only platform (with NO second option ! ) for building the next generation of global scale quantum internet. Therefore, quantum photonics is playing pivotal role for very new and very rapidly advancing field of quantum communication. Boeing, which is a US company (Click:BOEING), is taking major step to build Global Scale Quantum Internet. For more details Click: Q4-MISSION , Cisco, which is another US company (Click: CISCO) is working on advanced quantum networking. To know how Cisco is accelarating practical quantum computing and quantum networking, Click: QUANTUM-NETWORKING .

Some quantum tricks, which are currently being used to built photonic quantum computer and photonic quantum internet are given below.

Question: What is Quantum Information Science (QIS) ?

How Laser Physics is Related to Quantum Information Science (QIS) ?

Answer: Quantum information science (QIS), which is union of quantum mechanics and information science is very new and very fast-growing field of research. Major goal of quantum information science is to improve performance and security of information systems such as computers and internet. Laser physics provide a suitable platform to meet this goal.

Question: What is Single Photonics ?

Answer: Single Photon, which is often called Planck's quantum particle, is a quantized mode of electromagnetic radiation. Energy of single photon at wavelength 500 nm is equal to 1.42 X 10^-19 (10 raise to the power minus 19) Joule, which is approximately 2.5 eV that is greater than bandgap in silicon and germanium. Therefore, single photon can knock out electron in the semiconductors. Photon has no mass, no charge, and is always in motion. Speed of photon is so fast that photon makes 12 round trips per second across the diameter of the earth, which means that photon can carry quantum information from one end to other end of earth in only 0.04 second !

Single Photonics is a new emerging field of photonics in which light-matter interaxtion can be practiced with single photons. Single photons are likely to replace electrons from existing electrical, electronics, computing, and communication devices.

Nobel Prize

TO: Max Karl Ernst Ludwig Planck

For: Discovery of Energy Quanta

Click: NOBEL-PRIZE-1918

Question: What is Single-Photon Source ?

Answer: A single-photon source emits time spaced train of single photons. An ideal single photon source emits single-photons with probability of 100 % and multi-photons with probability 0 %. However, currently available single-photon sources are NOT ideal. Photons generated by a single-photon sources are indistinguishable and these photons exhibit quantum characteristics such as photon antibunching. Such sources are used for quantum communication, quantum key distribution (QKD), development of quantum network, and photonic quantum computation. For more detail about single-photon sources click on the link below.

Single-Photon Sources and Detectors,

M. D. Eisaman, J. Fan, A. Migdall, and S. V. Polyakov, Rev. Sci. Instrum. 82, 071101 (2011).

Download: SINGLE-PHOTON-SOURCE Pdf

Question: What are Entangled Photons ?

Answer: Individual photons in a light beam, generated by a laser or a light bulb, are independent from one another i.e. such photons are NOT entangled. However, when two (or more than two) photons are interconnected by quantum correlation between them, they are called entangled photons. Entangled photons are NOT independent but are quantum mechanically interconnected. Consider a composite system of two spatially separated photons, which are connected by quantum correlation between them. In this composite system, when first photon is projected into a specific state then the second photon is instantaneously projected into a different state. Projected state of second photon always depends on projected state of first photon, which means that these two projections are not independent but are always quantum correlated, even if two photons are far apart and there is no physical connection between them. In the laboratory entangled photons are generated by spontaneous parametric down-conversion (SPDC) of special purpose high-repetition-rate (10 GHz) ultrafast (femtosecond) laser pulses in second-order non-centrosymmetric solid-state nonlinear optical crystal. Experimental generation of complex entangled state of large number of photons require special purpose photon number resolving (PNR) detectors and multi-channel coincidence counting electronics. Entangled photons are used to establish photonic quantum interconnections (QuIC) among nodes of a quantum network. Therefore, most important application of entangled photons is to design next generation of global scale quantum communication internet. For how-to design quantum network using entangled photons, click on Page: PROJECT-1, which is located on TOP-ROW of this website.

Question: What is Quasi-Phase Matching (QPM) ?

Answer: Entangled photons are generated by spontaneous parametric down-conversion (SPDC) of pump photons into signal and idler photons in a nonlinear crystal. This generation is a second-order nonlunear optical process. Pump and generated photons are at different wavelengths and therefore they experience different refractive indices in the crystal. As a result, during this generation process, after every coherence length, there is a Pi (180 degree) phase slip between driving nonlinear polarization and generated electromagnetic field. This reduce the generation efficiency of entangled photons. However, this phase slip can be compensated by periodically reversing, after every coherence length, the sign of second-order nonlinear susceptibility of the crystal. This tehnique of reversing sign of nonlinear susceptibility of the crystal is called periodic poling and resulting phase matching condition is called quasi-phase matching (QPM). Quasi-phase matching is implemented by periodically reversing domains of ferroelectric crystals such as lithium niobate (LiNbO3). Net benefit of quasi-phase-matching is to restore and enhance generation efficiency of entangled photons. For more detail about quasi-phase-matching, click on the link below.

Quasi-Phase-Matched Second Harmonic Generation: Tuning and Tolerances

Martin M. Fejer, G. A. Magels, Dieter H. Jundt, and Robert L Byer,

IEEE J. Quantum Electron. Vol. 28, pp. 2631 (1992).

Download: QPM Pdf

Question: What is Extended-Phase Matching (EPM) ?

What is Group Velocity Matching (GVM) ?

Answer: Entangled photons are generated by Spontaneous parametric down-conversion (SPDC) of pump photons into signal and idler photons in a nonlinear crystal. Spontaneous parametric down-conversion (SPDC) based on femtosecond pulsed excitation offer advantages of short pulse width and greater pump intensity, but with disadvantage of large pump bandwidth, which due to dispersion in the nonlinear crystal limits the phase matching bandwidth. As a result, the generation efficiency and hence the flux of generated entangled photons is greatly reduced. This problem can be rectified by technique of extended-phase matching (EPM), which is given below.

Group velocity matching (GVM) is a phase matching technique, which satisfy following condition.

(2/Vg)p = (1/Vg)s + (1/Vg)i

Where,

(1/Vg)p: Reciprocal of group velocity of pump pulse in the nonlinear crystal.

(1/Vg)s: Reciprocal of group velocity of signal pulse in the nonlinear crystal.

(1/Vg)i: Reciprocal of group velocity of idler pulse in the nonlinear crystal.

Phase matching technique in which, condition of quasi-phase matching (QPM) and condition of group velocity matching (GVM) are simultaneously satisfied is called extended-phase matching (EPM). Extended-phase matching (EPM) can be experimentally realized by pre-determined parameters of crystal and pump beam such as crystal length, crystal poling period, pump wavelength, and pump polarization. Benefits of extended-phase matching (EPM) are following: (a) large increase of phase matching bandwidth, (b) large increase of interaction length in the crystal, and (c) for Type II down-conversion, wave-packets describing signal and idler photons become indistinguishable. Briefly, extendedd-phase matching do away the effect of dispersion on broadband optical pulse propagating through nonlinear crystal. The net result of extended-phase matching (EPM) is great increase in the flux of entangled photons generated from the crystal, when punped with ultrafast (femtosecond) pulses. For more detail about extended-phase-matching (EPM), click on the link below.

Extended Phase-Matching Conditions for Improved Entanglement Generation,

V. Giovannetti, L. Maccone, Jeffrey H. Shapiro, and F. N. C. Wong,

Phys. Rev. A 66, 043813 (2002),

Download: EPM Pdf

Question: What is Single-Photon Detector ?

Answer: Quantum information science experiments require special purposed photodetectors, which are called single-photon detector. These detectors are so sensitive that they can detect optical radiation with energy as low as energy of a single photon. In these detectors this capability is achieved by providing internal gain to the photo signal by avalanche multiplication process. For more detail about single-photon detectors click on the link below.

Single-Photon Detectors for Optical Quantum Information Applications

R. H. Hadfield, Nature Photonics 3, 696 (2009)

Download: (Single-Photon-Detector) Pdf

Question: What is Photon Number Resolving (PNR) Detectors ?

Answer: Single photon detectors cannot resolve detection of individual photons and therefore cannot count number of photons in the optical radiation incident on the detector. Therefore, single photon detectors give fix (same) output regardless of the number of photons incident on the detector. On the other hands, photon number resolving (PNR) detectors can resolve detection of individual photons and therefore can count the number of photons in the optical radiation incident on the detector. Photon number resolving (PNR) detectors are used with strongly attenuated femtosecond optical pulses, which are required for some advanced quantum information science applications such as generation of multi-photon quantum entangled states, development of long distance all photonic quantum repeaters, development of multi-node quantum network, fusion based photonic quantum computing (FBQC), and low field nonlinear optics (LFNLO) experiments. For more detail about photon number resolving detectors click on the link below.

An Avalanche-Photodiode-Based Photon-Number-Resolving-Detector,

B. E. Kardynal, Z. L. Yuan, and A. J. Shields, Nature Photonics 2, 425 (2008).

Download: PNR-DETECTOR Pdf

Question: What is Quantum Teleportation ?

Answer: Disembodied transport of quantum states from one quantum particle (photon) to a distinct quantum particle (distinct photon), which is achieved with the aid of entanglement, is called quantum teleportation. During teleportation there is no transfer of matter or energy, but what is transferred is quantum state of the particle (photon). During this transfer quantum state does not propagate through any material medium. Teleportation shakes Einstein theory of relativity. Einstein's theory deals with transfer of classical signals, but teleportation deals with transfer of quantum signals. Therefore, Einstein's theory is not suitable to explain speed of teleportation. Quantum teleportation is useful for communication of quantum information among different units of a quantum computer as well as among different nodes of a quantum network. Therefore, quantum teleportation is essential for building next generation of quantum information technology (QIF) such as modular quantum computers and quantum internet. Quantum teleportation is the central pillar for building the next generation of global scale quantum communication internet. For more detail about quantum teleportation, click on the link below.

Progress in Quantum Teleportation,

Xiao-Min Hu et. al., Nature Reviews Physics 5, 339 (2023).

Download: TELEPORTATION Pdf

Question: What is Entanglement Swapping ?

Answer: Consider two independent entangled photon sources, each generating two-qubit entangled state of photons. Bell state measurement (BSM) performed on two photons, one from each source, resulting in switching of entanglement among photons generated from two different sources. Photons that were entangled before Bell state measurement (BSM) become un-entangled and photons that were un-entangled before Bell state measurement (BSM) become entangled. This switching of entanglement among photons generated from different sources is called entanglement swapping. Switching of entanglement among photons is so fast that it occurs in a time scale, which is less than one attosecond i.e. less than 10^-18 (ten raise to the power minus 18) second. Concept of two-qubit entanglement swapping can be extended to multi-qubit entanglement swapping. Entanglement swapping allows entangling photons generated from independent sources. It also allows teleportation of quantum information among photons, which had no pre-shared entanglement. In addition, it allows merging quantum networks with smaller number of nodes to design quantum networks with larger number of nodes. Further, it allows increasing distance among communicating nodes in a quantum network. Therefore, entanglement swapping is very powerful technique for building multi-node long-distance quantum communication network. For more detail about entanglement swapping, click on the link below.

Click: ENTG-SWAP

Question: What is a Quantum Computer (QuCOMP) ?

Answer: Quantum computer, which is the next gift of physics to technology, is computing machine, which fully operate on principles of quantum mechanics. In this machine not only the design of hardware, but also all upper-level functions such as storage, processing, and communication of quantum information are performed quantum mechanically. Quantum computer is a very complex quantum system. Quantum computer is exponentially faster than present-day digital computer. In quantum computers, computation is performed on superposition of quantum states. This provides quantum jump to the power of quantum computer. As a result, a problem that takes thousand of years to solve on present-day digital computer can be solved in fraction of second on a quantum computer. Power of quantum computer is so high that no other available supercomputer can surpass the computing power of quantum computer. Development of quantum computers is currently in research stages and there is a world-wide race to build the next generation of quantum computers. To achieve this goal there are several different approaches. Approaches based on superconducting and photonic qubits are the leading approaches to build real-world practical quantum computers. For more detail about quantum computers, click on the link below.

Click: Quantum-Computing

Click: PSi-Quantum (Use Photonic Qubit)

Click: Google-Quantum-AI (Use Superconducting Qubit)

Click: IBM-Quantum (Use Superconducting Qubit)

Question: What is Quantum Network (QuNET) (Quantum Internet (QuINET)) ?

Answer: Quantum network, which is next gift of physics to technology, consists of nodes, which represent the locations of quantum computers that are inter-connected with high-speed quantum channels created by entanglement. Quantum information is stored and processed at the nodes and is teleported with high-fidelity from any node to any other node in the network. A large-scale quantum network is a very complex entangled quantum system. The core of quantum network are quantum interconnections (QuIC) among nodes of the quantum network. Such interconnections are established by distribution of multi-qubit photonic entanglement to remotely located nodes of the quantum network. Therefore, quantum network can be viewed as dance of entangled photons among nodes of quantum network. Hence, development of quantum netwirk is essentially a photonic problem to solve. In quantum network, generation and processing of quantum entangled states is performed on individual photon-by-photon basis. At nodes of quantum network, multi-photon introduce errors in the quantum gate operation and reduce fidelity of quantum gates. Further, within the quantum network, during the transfer of quantum information any extra photon can leak information to the eavesdropper. Therefore, operation of an advanced quantum network require single photons and any stray second photon wreck the operation of quantum network. Quantum network offers some important advantages over the present-day digital internet, we use these days. For instance, quantum interconnections among the nodes of quantum network results in exponentially larger state space as compared to the state space of equivalent digital internet. This results in enhanced capability of quantum network over the digital internet. Such enhanced capabilities include, (a) massive amount of data transfer, (b) ultra-fast & ultra-secure data transfer. Development of quantum internet is currently in research stages and there is a world-wide race to build the next generation of global scale quantum internet. Building global scale quantum internet require following major developments. (1) Increasing number of communicating nodes, (2) Increasing distances among communicating nodes, (3) Quantum key distribution (QKD) within the network, (4) Interconversion of stationary (matter) and flying (photonic) qubits, (5) Interfacing different components of quantum internet, which operate at different frequencies, (6) Quantum error correction (QEC) within the quantum network. For how-to design an elementry quantum network , click on Page: PROJECT-1, which is located on TOP-ROW of this website. For more detail about quantum nework, click on the link below.

Click: Quantum -Network (Use Photonic Qubit)

Question: Which One is Harder to Build, Quantum Computers or Quantum Internet ?

Answer: Although, building quantum computers and building quantum internet are both hard experimental problems to solve, but they are NOT equally difficult. Building quantum internet is much harder then building of quantum computers. This is explained as follows: power of quantum computers can be increased by increasing number of qubits in the quantum register. Few hundred qubits are sufficient to demonstrate quantum supremacy and build a working practical quantum computers. However, number of nodes required to build global scale quantum internet is equal to the number of users using world-wide-web (www). Therefore, number of nodes required to build global scale quantum internet approach to infinity. Further, building global scale quantum internet require intercontinental distances (i.e. distance greater than 1000 km) among remotely located nodes of the quantum internet. Thus, building global scale quantum internet with infinite number of nodes located at intercontinental distances is much harder then building quantum computers with only few hundard qubits.

Question: What is Quantum Frequency Conversion (QFC) ?

Answer: Quantum frequency conversion is a nonlinear optical process that shift the frequency of single photons while preserving the quantum state of the photon. Quantum frequency conversion is generally implemented using technique of sum frequency generation (SFG), difference frequency generation (DFG), and four-wave mixing (FWM). Quantum frequency conversion is used in a quantum network to interface different components of the network, which operate at different frequencies. For example, superconducting quantum processor operate at microwave frequency (1 mm - 1 meter) while quantum network operates at telecom frequency (1500 nm). Such superconducting quantum processor can be interfaced with telecom quantum network using technique of quantum frequency conversion.