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A Universal Gate Set refers to a collection of quantum gates that can be used to approximate any unitary transformation on a quantum computer to an arbitrary degree of accuracy. In classical computing, logical operations can be performed using a set of basic gates like AND, OR, and NOT. Similarly, in quantum computing, a universal gate set allows for the construction of any quantum operation, making it a foundational concept in quantum computation.
The universal gate set is a key concept that bridges the theoretical foundations of quantum computing with its practical realization, making it an essential term for anyone engaged with quantum technologies.
Classical computers famously allow every possible logic circuit to be built using nothing but NAND gates, which are negated AND gates. Instead of outputting TRUE when both inputs are TRUE, it outputs TRUE for all other combinations of inputs. Quantum gates are not quite as simple, since there are single-qubit and multi-qubit gates, but every possible quantum circuit can be coded using a relatively small set of basis gates.
Because of the conceptual overlap between classical logic gates and quantum logic gates, some background on the classical implementation can be found in an Electronics Tutorials article titled “Universal Logic Gates.” Then with an understanding of logic gates, in general, a guest lecture by Professor Tom Wong can provide a comprehensive introduction to the quantum implementation.
The most common universal gate set includes single-qubit rotations and a specific two-qubit entangling gate, such as the Controlled-NOT (CNOT) gate. Single-qubit rotations can be represented by gates like the Pauli-X, Y, and Z gates, and the Hadamard gate. Together with the CNOT gate, these form a complete set that can be used to build any quantum circuit. Some quantum computers might use different gates, but as long as they are universal, they can be used to simulate one another.
The concept of a universal gate set is vital for quantum algorithm design and the practical implementation of quantum computers. It ensures that a quantum computer can perform any computable quantum operation, thus enabling a wide range of applications. However, implementing a universal gate set in physical quantum systems can be challenging. Errors, noise, and the precise control required for specific gates can lead to difficulties in realizing a truly universal set of quantum gates.
To make quantum coding simpler, these basis gates are usually found within larger gate sets that include several high-level gates. These other gates are each composed of multiple basis gates and have to be decomposed into those basis gates before execution. The advantage of using these gates, though, is that repetitive sequences of basis gates can be added to quantum circuits simply by adding the high-level gate that is composed of that sequence of gates.
One of the of challenges in creating a universal set of quantum gates is that not every operation can efficiently by implemented with error-corrected logical qubits. The application of universal gates on logical qubits introduces the concept of magic states, which are pre-prepared quantum states that can be executed together with certain logical gates to implement the logical operations that cannot be efficiently implemented directly.
For a broad range of responses, be sure to read the article “What are the advantages and limitations of using universal quantum gates?” on LinkedIn. This question is broken down into several sections, each of which offers insights from one or more contributors. These responses are ranked by reader reactions, which may help indicate how insightful a particular response might be.
