Where does gate collide?

A Boolean primitive that maps one or more binary inputs to a binary output by a fixed truth table, such as AND, OR, and NOT. Shannon (1938) showed that the behaviour of relay and switching networks could be written in the algebra of Boole, so that an arrangement of switches computes a logical function and any Boolean expression can be realised as a circuit. The referent is an idealised element in a combinational circuit, and its unit is a bit at each terminal. It says nothing about voltage, current, or how the element is built: the same gate is the same whether it is made from relays, vacuum tubes, or transistors. Most such gates are irreversible, because an AND that outputs 0 hides which input was 0 and the mapping cannot be undone. It shares the word with the electrode that carries a physical signal and with the checkpoint in a project plan, but here a gate is a function on truth values.[1]

Unit

A bit at each input and output terminal

Recovery means

Not applicable

Measured by

Truth table, gate count, and propagation delay

A reversible unitary operation acting on the state of one or more qubits, rotating a vector in a complex Hilbert space rather than mapping bits to bits. Deutsch (1985) defined the universal quantum computer and the class of operations it runs; Barenco and colleagues (1995) showed that any such operation decomposes into single-qubit rotations and the two-qubit controlled-NOT, the quantum counterpart of a universal gate set. The referent is a unitary matrix, its unit is a qubit's amplitude and phase, and because unitary maps are invertible every quantum gate has an inverse, so no information is discarded. Unlike a Boolean gate, its inputs can be superpositions and its action is continuous, parameterised by angles rather than a finite truth table. It borrows the word from digital logic, but names an operation that is reversible, analog, and defined on probability amplitudes.[2][3]

Unit

A qubit's amplitude and phase, as a unitary matrix

Recovery means

Not applicable

Measured by

Gate fidelity, the unitary matrix, and gate count in a decomposition

In a field-effect transistor the gate is a physical electrode whose voltage controls the current flowing between the other two terminals, the source and the drain. Hofstein and Heiman (1963) reported the silicon insulated-gate field-effect transistor, in which a metal gate separated from the channel by a thin oxide modulates conduction through the electric field it imposes, without itself drawing current. The referent is a fabricated part of a device, its unit is the gate voltage in volts, and what it names is a control terminal for a flow of charge, not a logical operation. A single logic gate is built from several such transistor gates, so the one word sits at two levels at once: the electrode and the function it helps implement. Here a gate is hardware that opens or pinches off a channel.[4]

Unit

The gate voltage, in volts

Recovery means

Not applicable

Measured by

Gate voltage, threshold voltage, and drain current

In physiology, gating is the opening and closing of an ion channel: the conformational switch by which a membrane protein passes or blocks a flux of ions. Hodgkin and Huxley (1952) modelled the nerve action potential by treating the sodium and potassium conductances as voltage-dependent gates that activate and inactivate over time, and the term now covers channels gated by voltage, by ligands, or by mechanical force. The referent is a protein's physical state, its unit is the probability that the channel is open, and what it measures is a flux of ions across a membrane. Like the transistor's gate it controls a flow rather than computing a value, but the flow is an ionic current through a living cell and the control is a change of molecular shape. Here a gate is a biological valve.[5]

Unit

The probability that the channel is open

Recovery means

Not applicable

Measured by

Open probability, single-channel conductance, and gating current

In project and new-product management, a gate is a scheduled decision point at which a project is reviewed and either allowed to continue, sent back, held, or killed. Cooper (1990) named the stage-gate system, in which development is divided into stages separated by gates where managers weigh the evidence gathered so far against criteria and commit resources to the next stage. The referent is a governance checkpoint, its unit is a go or no-go decision made by people, and what it controls is not a signal or a flux but the flow of a project through an organisation. It shares the metaphor of a gate that opens or closes, but here the thing gated is work and money, the gatekeepers are managers, and the criterion is business judgement rather than a truth table or a voltage. Here a gate is a moment of choice.[6]

Unit

A go or no-go decision at a checkpoint

Recovery means

Not applicable

Measured by

Go and no-go outcomes, criteria scores, and projects killed at each gate