## 10 Quantum Computing (QC) buzzwords you should know

Following the growing interest of the media and the general public in quantum computing, as well as responds we’ve been getting on our previous publication (Is the Age of Quantum Computing Closer Than Imagined?), we thought it would be useful to offer a basic glossary to help our readers catch up with the QC jargon. Let’s start from the basics, a quantum computer is a machine that uses quantum mechanics to perform calculations. It’s enormous processing power, allowed by its ability to be in a superposition of multiple states and perform tasks using all possible states simultaneously, will result in performance gains in the billion-fold realm and beyond.**Qubits**In a classic computer, a bit is the basic unit of information. In quantum computing, a qubit, or a quantum bit, is the basic unit of quantum information. A qubit is a two-state (or two-level) quantum-mechanical system.**Superposition**In a classical system, a bit would have to be in one state or the other. In quantum computing, qubits can be in a superposition of both states at the same time. Quantum superposition is a fundamental principle in quantum mechanics. Contrary to a classical bit that can only be in the state corresponding to 1 or 0, a qubit may be in a superposition of both states.**Entanglement**Quantum entanglement is a phenomenon where two particles, or qubits, are in a correlated state that cannot be described as a separable combination of states of each qubit alone. For example, two qubits in Bell State are in superposition of 50% being in |00> and 50% in |11>. This state cannot be broken into a well-defined state for the first qubit and another well-defined state for the second qubit. The interaction between the qubits is a fundamental and yet unexplained attribute of quantum mechanics.**Quantum Gate**A generalization of a logic gate: it describes the transformation that one or more qubits will experience after the gate is applied on them, given their initial state.**Gate Fidelity**In quantum computing, a quantum logic gate (or simply quantum gate) is a basic quantum circuit operating on a small number of qubits. They are the building blocks of quantum circuits. The most common quantum logic gates are represented by unitary matrices and operate on spaces of one or two qubits. Quantum computation will require qubit technology based on a scalable platform, and quantum error correction protocols with strict limits on infidelities, or error rates, for one and two-qubit gate operations. Gate fidelities depend on the qubit technology. For example, in a Trapped-Ion technology single-qubit operation fidelity exceeds 99.99% while the best reached dual-qubit operation fidelities are around 99.9%.**Quantum Supremacy**Quantum Supremacy is a term proposed by Professor John Preskill in 2012, describing a point where quantum computers can solve a problem that classical computers can’t. To achieve this goal, one would need to (A) build a quantum computer and (B) find a problem that can be solved with current technology but would take a lot of time and effort with classical algorithms (not necessarily a practical problem).On last October, Google claimed to have reached quantum supremacy and its researchers published their paper on Nature. IBM researchers, however, have responded with a publication claiming that some of the claims are inaccurate. To read more about the skepticism responses to Google’s declaration and the supremacy race, read Professor Gil Kalai’s publication.**NISQ**Noisy Intermediate-Scale Quantum (NISQ) technology. The first quantum computers will be NISQ computers, limited by noise and number of qubits. As noise in quantum gates will limit the number of quantum gates that can be execute reliably — NISQ devices will be useful tools for research towards more accurate quantum gates and less-faulty quantum computing.**Fault Tolerance**Quantum fault-tolerance is avoiding the uncontrollable cascade of errors caused by the interaction of quantum-bits. To achieve fault tolerance, future quantum computers will need to significantly improve the gate fidelity as well as number of qubits, which will allow the use of quantum error correction to mitigate errors. Due to the huge overhead of the quantum error correction protocols, fault-tolerance with a large number of protected (logical) qubits is not expected in the near future.**Coherence**Quantum coherence is rooted in the superposition principle. If an object’s wave-like nature is split in two, then the two waves may coherently interfere with each other in a way that they form a single state that is a superposition of the two states. This idea is at the heart of quantum computing, in which a qubit’s superposition of the 0 and 1 states will result in a speed-up over classical algorithms.**Decoherence time**In real-life, the quantum object is never fully isolated, so the superposition of states stop after a while. The time it takes for the superposition to disappear is called decoherence time. It is crucial for quantum computing and gives valuable information about the interactions between the quantum object and its environment.

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