Quadrature Amplitude Modulation. RF digital modulation technology used to pack higher data rates into band-limited but low-noise channels, such as cables. It combines amplitude and phase shifts (PSK and ASK) to encode binary data on a carrier wave. It’s not good for satellite links, which are so close to the margin already they can’t afford to modulate amplitude, and it’s more vulnerable to interference than most schemes.
The modulation schemes are designated 8-QAM, 16-QAM, 32-QAM, etc. The number refers to the 2n possible symbols (states of the signal), where n is the number of bits per symbol. Where a modulation is referred to as, for example, 16-QAM (8/4), the numbers in parentheses refer to the number of possible phase states and amplitude levels employed. QAM uses double-sideband, suppressed-carrier modulation, so the RF bandwidth is twice the baseband. See TCM, constellation.
QoS Class Identifier. A value describing the transmission requirements of network traffic, to allow prioritization of data based on its tolerance for delay or loss.
Quantum Cascade Laser. The lasing medium is semiconductor layers that alternate large and small bandgap energies. A quantum tunneling current emits photons as it crosses the layers. QCLs are currently one of the few ways to generate terahertz (THz) radiation.
Quality factor. The figure of merit for a resonant circuit. At the resonant frequency ω0, Q is the ratio of the voltage across the reactive element to the supply voltage. It’s also described as the ratio of resonant frequency to change in frequency, [ω0 / Δω]. Higher values of Q indicate a sharper resonance peak. See tank circuit.
Quantum Fast Fourier Transform. A version of the fast Fourier transform (FFT) for use by a quantum computer, with the same speed advantage over QFT that the FFT has over the original Fourier algorithm.
Quality of Service. A feature of some data transmission protocols. It assigns higher priority to the data packets of more time-critical applications. IP wasn’t designed to support QoS – but see MPLS.
Quadrature Phase Shift Keying. 4-ary PSK, carrying two bits per symbol, achieving the same data rate and BER as BPSK in half the bandwidth. It starts with two serial data rails, I and Q (in-phase and quadrature). The I rail modulates a cosine wave; a binary 1 produces cos(ωt), and a 0, treated as –1, produces –cos(ωt). This is a phase shift of either 0 or 180° (π radians), which is simply a BPSK signal. Likewise, the Q rail produces BPSK from a sine wave, so it’s 90° out of phase with the I rail. The summation of ±cos(ωt) and ±sin(ωt) yields a cosine wave with one of four possible phase states: ±π/4 and ±3π/4. (An arbitrary change of phase reference makes them 0, ±π/2, and π.)
A trigonometric representation of this summing is (A/sqrt2) cos(ωt) + (A/sqrt2) sin(ωt) = A cos(ωt + θ), with θ being the phase state. Each state represents a bit pair: 00, 01, 11, 10. The signal phase undergoes a discontinuous shift at the symbol boundary, and the modulation envelope momentarily passes through zero if that shift is 180°.
To avoid the latter, π/4-QPSK advances the phase by 45° (π/4) with every symbol, meaning its signal has 8 possible phase states instead of 4. This is NOT the same thing as staggered or offset QPSK (OQPSK).
Quick Response Code. An ISO-standard printed code in the form of a square of black and white pixels, which an optical scanning reader can decipher. They can contain alphanumeric messages, URLs, etc. Resolution ranges from from Version 1’s 21 × 21 pixels to (as of 2020) Version 40’s 177 × 177 pixels. There are many free QR code generator apps available online.
A free electron confined by electric fields within a nanocrystal of semiconductor material smaller than the electron’s de Broglie wavelength, and behaving similarly to a valence electron. Following excitation by light or electric current, it emits light at a precise wavelength determined by the size of the confined area. The most efficient nanocrystals use toxic cadmium, which is regulated by RoHS. Some instead use indium.
Since quantum dots can make more precise and efficient LEDs, they’ve been added to the LCD and OLED screens used in the flat panel display market. A width of about 50 atoms produces red light, and 30-atom width yields green light. A single LCD screen contains billions of such quantum dots. Integrated with the high-efficiency blue LEDs already used for backlighting the screen, they provide the basic toolkit of red-green-blue pixels for rendering images. The result has finer control and displays a greater number of colors, hence more accurate color, with less power than a standard LCD.
Researchers are at work (2023) on displays that rely on electrolumniscent quantum dots alone, rather than using them to supplement LEDs.
A bit in a quantum computing device. A particle (say, an electron) in a steady-state magnetic field can be described as holding one of two quantum states – spin up or spin down – with the axis of spin parallel to the magnetic field. An RF pulse of the correct frequency and duration perpendicular to the field will flip the spin state. A shorter pulse will tip the spin into a superposition of up and down states, allowing simultaneous calculation on both states. Therefore, with m qubits, it’s possible to carry out a single calculation on 2m numbers in parallel.
How it all works is immensely counterintuitive. A quantum algorithm can generate a solution in a fixed number of steps, regardless of how many inputs it receives. This is potentially a huge advantage over traditional computing, in which the number of inputs inherently increases the solution time. It holds out the promise of being able to, for example, factor huge prime numbers very quickly, which would render some widely used encryption schemes suddenly vulnerable.
Quantum states are very fragile and easily disturbed. As of 2023, quantum computing requires operating temperatures very close to absolute zero to minimize thermal noise. Even then it suffers error rates of about 1 per 1000 computations, far too high for practical use.
Quick UDP Internet Connections. A more secure version of TCP created by Google. It incorporates features of UDP and TLS. The next release of HTTP, HTTP/3, will use it instead of TCP.
