Introduction to Quantum Gates : Implementation of Single and Multiple Qubit Gates

A quantum gate or quantum logic gate is an elementary quantum circuit working on a small number of qubits. It means that quantum gates can grasp two primary feature of quantum mechanics that are entirely out of reach for classical gates : superposition and entanglement. In simpler words quantum gates are reversible. In classical computing sets of logic gates are connected to construct digital circuits. Similarly, quantum logic gates operates on input states that are generally in superposition states to compute the output. In this paper the authors will discuss in detail what is single and multiple qubit gates and scope and challenges in quantum gates.

Experimental realization of high-fidelity quantum logic gate between photons

Quantum logic gates with fidelity above fault-tolerant threshold are building blocks for scalable quantum technologies[1,2]. Compared to other types of qubits, photon is one of a kind due to its unparalleled advantages in long-distance quantum information exchange[3-5]. As a result, high-fidelity photonic quantum operations are not only indispensable for photonic quantum computation[6-8] but also critical for quantum network[2,9]. However, two-qubit photonic quantum logic gate with fidelity comparable to that of leading physical systems, i.e. 99.7% for superconducting circuits[10] and 99.9% for trapped ions[11], has not been achieved. A major limitation is the imperfection of single photons[12]. Here, we overcome this limitation by using high-quality single photons generated from Rydberg atoms as qubits for the interference-based gate protocol, and achieve a gate fidelity up to 99.84(3)%. Our work paves the way for scalable photonic quantum applications[13-15] based on near-optimal single-photon qubits and photon-photon gates.

A quantum-logic gate between distant quantum-network modules

The big challenge in quantum computing is to realize scalable multi-qubit systems with cross-talk–free addressability and efficient coupling of arbitrarily selected qubits. Quantum networks promise a solution by integrating smaller qubit modules to a larger computing cluster. Such a distributed architecture, however, requires the capability to execute quantum-logic gates between distant qubits. Here we experimentally realize such a gate over a distance of 60 meters. We employ an ancillary photon that we successively reflect from two remote qubit modules, followed by a heralding photon detection, which triggers a final qubit rotation. We use the gate for remote entanglement creation of all four Bell states. Our nonlocal quantum-logic gate could be extended both to multiple qubits and many modules for a tailor-made multi-qubit computing register.

2D Material-based Quantum Logic Gate Operating Via Self-Organization of Quantum Dots

In the paper, nanotrigger-based electronic device, capable of performing the quantum computation procedure, is described. The device represents a quantum logic gate formed from a two-dimensional material and controlled by a quantum dot. The operation of the quantum dot is analyzed. In the model representation, the transition between two states of the quantum dot, each of which controls the flow of the nanotransistor current (one of the shoulders of the nanotrigger), is equivalent to tunneling through an energy barrier separating the states. Fundamentally important is the fact that in one of these states the quantum dot is diamagnetic, and in the other it is paramagnetic. The paramagnetism of the quantum dot is due to the electronic exchange interaction, characteristic of the systems with unpaired electrons. Thus, the elementary self-organized 2D-material-derived logic gate disclosed in the present work can be employed for design of an electronic reversible unit.In other words, such a unit is able to prepare and to trigger the computation procedure.