Quantum logic gates are basic blocks to construct quantum computing and quantum information processing systems.
Quantum logic gates based on coupled-resonator optical waveguide (CROW) are analyzed with the advantage of integration
with other devices. Photonic crystal CROW (PhC CROW) structure with different dimension and background medium
exhibits electromagnetically induced transparency (EIT), which is used as single quantum bit (qubit) phase gate. Quantum
Hadamard gate can also be realized by coupling element. Two qubit conditional quantum phase gate is achieved by
concatenating two Hadamard gates and a phase gate doped with dopant. The simplicity and dynamic tunability of the device…
Quantum logic gates are basic blocks to construct quantum computing and quantum information processing systems.
Quantum logic gates based on coupled-resonator optical waveguide (CROW) are analyzed with the advantage of integration
with other devices. Photonic crystal CROW (PhC CROW) structure with different dimension and background medium
exhibits electromagnetically induced transparency (EIT), which is used as single quantum bit (qubit) phase gate. Quantum
Hadamard gate can also be realized by coupling element. Two qubit conditional quantum phase gate is achieved by
concatenating two Hadamard gates and a phase gate doped with dopant. The simplicity and dynamic tunability of the device…
Unlike classical digital computers in which a bit can represent either 1 or 0 at any time, quantum computers use a two-level system, i.e., a qubit, to implement logical operations based on quantum mechanical laws, which can represent both values at once. Owing to the superposition property of qubits, quantum computers have natural parallel processing advantages and thus have potential to exceed the computational efficiency of classical computers for particular tasks. Quantum logic gates are the generalization of classical logic gates in computational networks. It has been proved that two-qubit quantum gates together with one-qubit quantum gates are adequate for constructing networks with any possible quantum computational property. Directional couplers are the most critical elementsfor constructing the quantum gates. In recent years, photonic quantum technologies have emerged as a promising experimental platform for quantum computing. Single photons have robust noise resistance, long coherence time, high transmission speed and great compatibility with other systems. They can be easily manipulated and encoded in any of several degrees of freedom, for example, polarization, path, spatial mode or time bin. Optical waveguide technology enables the realizing of complex optical schemes comprised of many elements with desired scalability, stability and miniaturization. Femtosecond laser direct writing of waveguide has been adopted as a powerful tool for integrated quantum photonics with characteristics of rapidness, cost-effectiveness, mask-less and single-step process. In particular, it has the ability to build arbitrary three-dimensional circuits directly inside bulk materials, which is impossible to achieve with conventional lithography. In this article we review the femtosecond laser writing and quantum characterization of directional coupler and important one-qubit and two-qubit optical quantum logic gates, such as Hadamard gate, Pauli-X gate, controlled-NOT gate, and controlled-Phase gate. The qubits in these gates are usually encoded through optical paths or polarizations of photons. The key to the realization of polarization-encoded one-qubit gates is to achieve flexible wave-plate operations, which is described in detail. Controlled-NOT gate and controlled-phase gate are the most crucial two-qubit gates in the linear optics computation and sometimes they can be converted into each other by adding some one-qubit gates or special superposition states. Many different kinds of waveguide circuits have been used to implement these two-qubit gates. The outlook and challenges for the femtosecond laser writing of three-qubit gates, such as Toffoli gate and Fredkin gate, are briefly introduced.
Abstract: An all optical method has been proposed for quantum Hadamard gate implementation. This quantum gate was realized by using Y-junction beam splitter. Normal modes in waveguides have been used as quantum bit. This all optical gate can be used in quantum computation and communication. The proposed Hadamard gate has potential of being more compact and easily realized compared to other optical implementations. By using planar lightwave circuit in implementation, the width of Y-junction input waveguide, width of each branch, angle of bend and length of bend were obtained 1 micron, 0.75 micron, 45º and 0.75 micron, respectively. By using planar lightwave circuit in implementation, the radius of large air holes and the radius of small holes were obtained 0.2 ands 0.1 micron, respectively. The index of substrate was 1.325. Implementation based on photonic crystal structures and planar lightwave circuit technology was used in integrated optics.
Quantum logic operations are the elemental operations for quantum computing and have been implemented in various time-evolved atomic systems. Here, we present a scheme to simulatively implement the typical quantum NOT- and Hadamard gates operations, alternatively with the coupled three-waveguide structure (rather than the usual atomic levels); two modes in sinuous waveguides encoding the two logic states of a qubit and the mode in the center straight waveguide serve as auxiliary the coupler. Based on the Lewis-Riesenfeld invariant theory, we show that the desired mode conversions for implementing the NOT gate and mode split to realize the Hadamard gate are fast achieved by engineering the coupling parameters between the optical waveguides. The implementations of the logic gates demonstrated here relay on the structure parameters of the waveguides, rather than the durations of time evolutions with the usual driven atomic systems, and thus should be feasible robustly with the current integrated optical technology.
Quantum computers require technologies that offer both sufficient control over coherent quantum phenomena and minimal spurious interactions with the environment. We show, that photons confined to photonic crystals, and in particular to highly efficient waveguides formed from linear chains of defects doped with atoms can generate strong non-linear interactions which allow to implement both single and two qubit quantum gates. The simplicity of the gate switching mechanism, the experimental feasibility of fabricating two dimensional photonic crystal structures and integrability of this device with optoelectronics offers new interesting possibilities for optical quantum information processing networks.
The frequency-sensitive optical response due to two-photon resonance of electromagnetically induced transparency (EIT) in a tunable band structure of an EIT-based layered medium is considered. The unit cells of this periodic layered structure are composed of dielectric (e.g., GaAs) and EIT atomic vapor. The frequency-sensitive behavior of controllable reflectance and transmittance depending on the external control field can be applicable to new device design (e.g., it can serve as the fundamental working mechanism for photonic switches and photonic logic gates). Some two-input logic gates (e.g., OR and NAND gates) are designed based on the present effect of sensitive switching control that results from the two-photon resonance.
One of the most significant challenges in quantum computing is the variability in implementing quantum algorithms across different chips. In this work, we present a Hadamard photonic gate based on the quantum Fourier transform, which superimposes several numbers encoded in the classical domain. We recognize that various mathematical operations - such as addition, subtraction, multiplication, division, and other complex calculations - are typically processed separately for each nth number in classical computers. This approach consumes considerable memory space and results in slower processing speeds as the number of bits increases. With the advancements provided in this work, we can eliminate the need for multiple memory spaces, allowing all numbers to be processed simultaneously in a single computation step. This is a key reason why quantum computers are significantly faster than their classical counterparts. Notably, as the number of bits increases, the number of processes remains consistent, enabling the same operations to be performed without requiring additional memory capacity. We have designed this algorithm using an optical integrated circuit that includes a silicon dioxide waveguide in an air bed, operating at a wavelength of 1.55 µm. Importantly, this circuit can not only be implemented independently on hybrid chips but also integrated into quantum monolithic chips based on Silicon Dioxide-Air optical waveguides as a key component of the overall structure.