Quantum information (QI) and quantum computation (QC) are arising sciences that establish links between quantummechanical description of nature and information and communication technology (ICT). One of the basic concepts that QI and QC introduce is the “qubits” (quantum bits) which, compared to the twolevel classical bits, can take complex superposition values of 0 and 1. Manipulation of qubits in the space defined by the firm theories of quantum mechanics moves us to a new arena of applications and capabilities for ICT. For example, the factorization problem of a coprime number (the product of two prime numbers) is a problem which is widely known in classical computation to be a complex problem (requires time/resources exponentially proportional to the number of digits). Till the moment, the complexity of this problem has a vital role in classical information encryption.
In 1996, Peter Shor presented quantum algorithm (a sequence of quantum operations) that enables solving the factorization problem of coprime numbers as an easy problem (time/resources having a polynomial proportion to the number of digits). This shows the fascinating capabilities of quantum computation and the real threat that it constitutes to the currently used encryption protocols. Commercially, two generations of the first quantum computer branded DwaveI and DwaveII at a cost 10^7$ have represented the first wave of quantum computing machine. Besides their original manufacturer, upgrades of these models have been conducted as well by scientists belonging to Google, IBM, and NASA. Alternatively, several quantum cryptography protocols have shown unconditional security (independent of the computational power of the intruder) for information transfer between communicating parties. Quantum encryption also has the unique and extraordinary feature that any eavesdropping can be unveiled by the legitimate communicating parties.
One key quantum feature that QI and QC use is “quantum entanglement”: a property which has no counterpart in classical world. It describes a very special behavior for multiqubit system. Initially, each qubit of the entangled set does not have its own value away from the ensemble, until one qubit of the set is measured. This gives instant information about the other qubits of the set, even when the set is distributed over very distant places. Quantum entanglement is of great importance for QI and QC ranging from its fundamental role as a basic multiqubit operation to several applications as quantum teleportation, quantum dense coding, and quantum cryptography. The development of a photonic entanglement source will be shortly of major interest for critical applications and telecommunication links to replace the classical underthreat security protocols.
We aim at building an enhanced ultrabright compact source of hyperentangled photons and then using it in realizing one of the quantum cryptography protocols. The design of this enhanced hyperentanglement source uses novel structures of the generation crystals so as to selfcompensate the group velocity mismatch and/or spatial phase variations (we mean by selfcompensation here the compensation at the generation crystals without any additional optical components). The new method serves the source brightness as the compensation acts all over the output stream and its compactness and cost as there is no need for additional elements. Our numerical analysis has led so far to several structures of the hyperentanglement source capable of generating selfcompensated photon pairs.


