Glossary

Common terms are listed by their working definition, the boundaries that are easily confused, and the main places to read about them. The definitions keep only the boundaries most needed when looking things up; for the full physical picture, formulas, orders of magnitude and references, see the corresponding chapter in the main text.

Data, Statistics and Inference

Term	Usage in this book	Main location
Event table	A photon-by-photon record containing at least a time and a detection channel; it can be extended with frequency, polarization, position, quality flags and weight. Do not equate it with an already binned light curve.	Chapters A Guided Tour: What Does a Telescope Actually Record, and Where Is This Book Taking You?, Detectors, Clocks, and Event Tables, Correlators and Event-Table Data Analysis.
Light curve	The projection of an event table onto the time axis, usually discarding channel, polarization and single-event interval information.	Chapters A Guided Tour: What Does a Telescope Actually Record, and Where Is This Book Taking You?, Correlators and Event-Table Data Analysis.
Exposure function	A function describing how the effective observing time, area, efficiency and selection function vary with time or channel.	Chapters Correlators and Event-Table Data Analysis, Observation Design, Error Budget, and Feasibility.
Live time	The effective integration time after removing dead time, bad weather, saturation and quality selection.	Chapters Detectors, Clocks, and Event Tables, Observation Design, Error Budget, and Feasibility.
Poisson process	A counting model in which, given the instantaneous rate, events arrive independently; non-stationary sources require a time-varying rate.	Chapters A Guided Tour: What Does a Telescope Actually Record, and Where Is This Book Taking You?, Correlators and Event-Table Data Analysis, Common Misconceptions.
Cox process	A Poisson process whose rate itself varies randomly; it can produce super-Poisson counts without being new physics.	Chapter Common Misconceptions.
Likelihood	The probability density or probability mass of obtaining the observed data given the model parameters. Do not conflate it with the posterior.	Chapters Correlators and Event-Table Data Analysis, Observation Design, Error Budget, and Feasibility.
Posterior	The parameter distribution after combining the likelihood with the prior. When reporting a posterior, state the prior and the data vector.	Chapters Correlators and Event-Table Data Analysis, Teaching Experiments and Computational Experiments.
Fisher information	A measure of the local sensitivity of the likelihood to the parameters; it is not an automatic substitute for systematic errors.	Chapters A Guided Tour: What Does a Telescope Actually Record, and Where Is This Book Taking You?, Correlators and Event-Table Data Analysis, Quantum Estimation, the Rayleigh Limit, and SPADE Sub-Rayleigh Resolution.
Covariance matrix	The matrix of correlations between the errors of data points or parameters. Multi-baseline $	V
Fano factor	The ratio of count variance to mean, used to diagnose slowly varying sources, dead time and afterpulsing.	Chapter Common Misconceptions.
Trial factor	The correction to global significance caused by multiple testing. Time bins, frequency channels, baselines and the number of targets can all contribute trials.	Chapter Common Misconceptions.
Null test	A check that should remove the astrophysical signal while retaining instrumental or background structure, such as time-shift, off-source, off-band or wrong-polarization tests.	Chapters Correlators and Event-Table Data Analysis, Teaching Experiments and Computational Experiments, Common Misconceptions.

Quantum Optics and Coherence Functions

Term	Usage in this book	Main location
Mode	A distinguishable degree of freedom of the light field, which may be a spatial, temporal, frequency or polarization mode. The number of modes determines whether the bunching contrast is diluted.	Chapters Single-Mode States of Light: Number, Coherent, Thermal, and Squeezed States, The Coherence Functions g^{(1)}, g^{(2)} and the Siegert Relation.
Occupation number	The mean photon number in a single mode; large in the radio, often very small for a single mode in optical astronomy.	Chapters Single-Mode States of Light: Number, Coherent, Thermal, and Squeezed States, The Quantum Language of Astrophysical Radiation Mechanisms.
Coherent state	An ideal light state with a Poisson photon-number distribution and \(g^{(2)}(0)=1\); an astrophysical laser/maser is not necessarily equivalent to a stable coherent state.	Chapters Single-Mode States of Light: Number, Coherent, Thermal, and Squeezed States, Common Misconceptions.
Thermal state	A chaotic light field that exhibits bunching in a single mode; multiple modes and finite response reduce the observed contrast.	Chapters Single-Mode States of Light: Number, Coherent, Thermal, and Squeezed States, The Coherence Functions g^{(1)}, g^{(2)} and the Siegert Relation.
Squeezed state	A light-field state in which one quadrature has noise below vacuum while the other is raised; similar language also appears in cosmological fluctuations.	Chapters Single-Mode States of Light: Number, Coherent, Thermal, and Squeezed States, Quantum Problems in Cosmology.
Density matrix	A state object describing a pure or mixed state; especially natural when an astrophysical light source consists of many indistinguishable components.	Chapters Single-Mode States of Light: Number, Coherent, Thermal, and Squeezed States, Quantum-Network Telescopes.
Decoherence	The loss of usable phase relations due to the environment, averaging or unobserved degrees of freedom; do not simply equate it with “no quantum.”	Chapters Single-Mode States of Light: Number, Coherent, Thermal, and Squeezed States, Quantum Problems in Cosmology.
First-order coherence function	Describes field-amplitude correlations; both amplitude interferometry and complex visibility depend on this quantity.	Chapters The Coherence Functions g^{(1)}, g^{(2)} and the Siegert Relation, Spatial Coherence, van Cittert–Zernike, and Intensity Interferometry.
Second-order coherence function	Describes correlations of intensity or detection events; the central observable of HBT and photon statistics.	Chapters The Coherence Functions g^{(1)}, g^{(2)} and the Siegert Relation, Correlators and Event-Table Data Analysis.
Bunching	The positive second-order correlation excess of thermal or chaotic light near zero delay.	Chapters A Guided Tour: What Does a Telescope Actually Record, and Where Is This Book Taking You?, The Coherence Functions g^{(1)}, g^{(2)} and the Siegert Relation.
Antibunching	The nonclassical photon-statistics signature of \(g^{(2)}(0)<1\); extremely hard to preserve in an astronomical setting.	Chapters The Coherence Functions g^{(1)}, g^{(2)} and the Siegert Relation, Common Misconceptions.
Coherence time	The time width over which the first-order coherence of the light field is significant, usually set by the spectral bandwidth.	Chapters A Guided Tour: What Does a Telescope Actually Record, and Where Is This Book Taking You?, The Coherence Functions g^{(1)}, g^{(2)} and the Siegert Relation.
Siegert relation	The relation that links the second-order correlation to the squared modulus of the first-order coherence under thermal light or a Gaussian field.	Chapters A Guided Tour: What Does a Telescope Actually Record, and Where Is This Book Taking You?, The Coherence Functions g^{(1)}, g^{(2)} and the Siegert Relation, Spatial Coherence, van Cittert–Zernike, and Intensity Interferometry.
Glauber correlation	The quantum-optical correlation function defined via detection operators, the basis of the \(g^{(n)}\) language.	Chapters Single-Mode States of Light: Number, Coherent, Thermal, and Squeezed States, The Coherence Functions g^{(1)}, g^{(2)} and the Siegert Relation.

Interferometry, Imaging and Mode Measurement

Term	Usage in this book	Main location
Complex visibility	A normalized Fourier component of the sky brightness distribution; both amplitude and phase are first-order coherence information.	Chapters A Guided Tour: What Does a Telescope Actually Record, and Where Is This Book Taking You?, Spatial Coherence, van Cittert–Zernike, and Intensity Interferometry.
Baseline	The vector separation between two telescopes; what enters the visibility is \(B_\perp\), the projection onto the sky plane.	Chapter Spatial Coherence, van Cittert–Zernike, and Intensity Interferometry.
\(u,v\) coverage	The sampling of an observation in the spatial-frequency plane; it determines imaging and model degeneracies.	Chapters Spatial Coherence, van Cittert–Zernike, and Intensity Interferometry, Observation Design, Error Budget, and Feasibility.
VCZ theorem	The theorem linking the sky brightness of an incoherent far-field source to the first-order spatial coherence.	Chapter Spatial Coherence, van Cittert–Zernike, and Intensity Interferometry, Appendix Reading Routes and Ranges of Validity for the Core Relations.
Uniform disk	The simplest model of a stellar angular diameter; real stars often need limb-darkening or non-spherical corrections.	Chapters A Guided Tour: What Does a Telescope Actually Record, and Where Is This Book Taking You?, Spatial Coherence, van Cittert–Zernike, and Intensity Interferometry, Stars as Quantum Light Sources.
Limb darkening	The effect whereby the stellar-disk edge is fainter than the center, which changes the visibility curve and the interpretation of the angular diameter.	Chapter Stars as Quantum Light Sources.
Intensity interferometry	Measures $	V
Zero-baseline contrast	The correlation-peak amplitude when not yet suppressed by spatial resolution, often used to calibrate the instrumental dilution factor.	Chapters Spatial Coherence, van Cittert–Zernike, and Intensity Interferometry, Observation Design, Error Budget, and Feasibility.
Missing phase	When only $	V
Phase retrieval	The algorithmic problem of recovering phase from intensity or $	V
SPADE	The measurement idea of projecting the focal-plane field onto spatial modes to estimate sub-Rayleigh separations.	Chapters Quantum Estimation, the Rayleigh Limit, and SPADE Sub-Rayleigh Resolution, Teaching Experiments and Computational Experiments.
Rayleigh curse	The phenomenon of declining Fisher information when direct imaging estimates the separation of a closely spaced pair of sources; not a limit of all measurements.	Chapters Quantum Estimation, the Rayleigh Limit, and SPADE Sub-Rayleigh Resolution, Common Misconceptions.
Quantum Fisher information	The upper limit on parameter information over the allowed set of measurements; an achievable target only when the model and measurement assumptions are satisfied.	Chapter Quantum Estimation, the Rayleigh Limit, and SPADE Sub-Rayleigh Resolution.

Instruments, Detectors and Calibration

Term	Usage in this book	Main location
SPAD	Single-photon avalanche diode, suited to time-tagged photon counting; dead time and afterpulsing must be handled.	Chapter Detectors, Clocks, and Event Tables.
PMT	Photomultiplier tube, often used for fast photon counting and Cherenkov instruments; gain, pulse shape and background must be calibrated.	Chapters Detectors, Clocks, and Event Tables, Observation Design, Error Budget, and Feasibility.
TDC	Time-to-digital converter, which turns a detection pulse into a digital time tag.	Chapters Detectors, Clocks, and Event Tables, Teaching Experiments and Computational Experiments.
Jitter	The timing jitter introduced by the detection and timing chain, which broadens the correlation peak and dilutes contrast.	Chapter Detectors, Clocks, and Event Tables.
Dead time	The interval after an event during which a detector or electronics cannot record a new event.	Chapters Detectors, Clocks, and Event Tables, Common Misconceptions.
Afterpulsing	Delayed spurious events caused by the release of internal detector traps, producing positive correlation at short delays.	Chapters Detectors, Clocks, and Event Tables, Common Misconceptions.
Dark count	The rate of detection events in the absence of astrophysical photons; temperature and bias voltage change it.	Chapter Detectors, Clocks, and Event Tables.
Electronic crosstalk	An electronic pulse in one channel contaminating another channel, which may masquerade as a zero-delay correlation.	Chapters Detectors, Clocks, and Event Tables, Common Misconceptions.
White Rabbit	One of the sub-ns network synchronization technologies; whether it is sufficient depends on the correlation-peak width and the baseline model.	Chapter Detectors, Clocks, and Event Tables.
Spectral channel	A frequency or wavelength label in the event table; a narrow channel increases the coherence time but reduces the photon number per channel.	Chapters Detectors, Clocks, and Event Tables, From White Paper to Research Plan.
Polarization channel	A polarization label in the event table; used to distinguish source physics, scattering, Faraday effects and instrumental polarization.	Chapters Detectors, Clocks, and Event Tables, Dark Matter, Axions, and the Polarization Quantum Channel.
Calibrator star	A reference star used to determine the zero-baseline contrast, the system response or the angular-diameter scale.	Chapters Observation Design, Error Budget, and Feasibility, From White Paper to Research Plan.
Systematic-error floor	The lower error limit below which further integration no longer improves as \(T^{-1/2}\).	Chapters Observation Design, Error Budget, and Feasibility, Common Misconceptions.

Astrophysical Sources and Radiation Mechanisms

Term	Usage in this book	Main location
Brightness temperature	A quantity expressing radiation intensity as an equivalent blackbody temperature; a high brightness temperature often hints at a coherent mechanism or an extremely small angular scale.	Chapter The Quantum Language of Astrophysical Radiation Mechanisms.
Thermal radiation	Radiation controlled by temperature and optical depth; optical stars approximate thermal light but are not perfect blackbodies.	Chapters The Quantum Language of Astrophysical Radiation Mechanisms, Stars as Quantum Light Sources.
Synchrotron radiation	Radiation from relativistic electrons in a magnetic field, often carrying polarization and a nonthermal spectrum.	Chapters The Quantum Language of Astrophysical Radiation Mechanisms, Black Holes, Accretion Disks, and the Photon Ring.
Inverse Compton scattering	The process in which high-energy electrons boost low-energy photons to higher energies.	Chapter The Quantum Language of Astrophysical Radiation Mechanisms.
Maser	A microwave or radio stimulated-emission source, often controlled by velocity coherence and pump fluctuations in astrophysical environments.	Chapters The Quantum Language of Astrophysical Radiation Mechanisms, Common Misconceptions.
Natural laser	A candidate for optical/near-infrared stimulated emission in astrophysical objects, not equivalent to a stable laboratory laser cavity.	Chapters First-Generation Quantum-Astronomy Science Cases, Common Misconceptions.
Broad-line region	The gas region in an AGN that produces broad emission lines, which can be jointly constrained by time delay, spectral lines and angular scale.	Chapter Black Holes, Accretion Disks, and the Photon Ring.
Photon ring	The fine structure formed by correlated light-orbit paths near the strong gravity of a black hole.	Chapter Black Holes, Accretion Disks, and the Photon Ring.
Type Ia supernova	A thermonuclear supernova usable as a distance indicator; intensity-interferometry schemes care about the photospheric angular radius.	Chapters Bursts, Transients, and Multi-Messenger Quantum Astronomy, Teaching Experiments and Computational Experiments.
Kilonova	An r-process transient following a binary-neutron-star merger, for which multi-messenger delay and diffusion time are central quantities.	Chapter Bursts, Transients, and Multi-Messenger Quantum Astronomy.
GRB afterglow	The multiband radiation produced by the interaction of a gamma-ray-burst jet with its environment.	Chapter Bursts, Transients, and Multi-Messenger Quantum Astronomy.
TDE	A transient event following the tidal disruption of a star by a black hole, which can constrain the black-hole mass and the reprocessing photosphere.	Chapter Bursts, Transients, and Multi-Messenger Quantum Astronomy.

Propagation, Cosmology and New Physics

Term	Usage in this book	Main location
DM	Dispersion measure, the frequency-dependent delay caused by the electron column density.	Chapter Propagation Effects: Plasma, Dust, and Gravitational Lensing.
RM	Rotation measure, the amount of Faraday polarization rotation caused by a magnetized plasma.	Chapters Propagation Effects: Plasma, Dust, and Gravitational Lensing, Dark Matter, Axions, and the Polarization Quantum Channel.
Scatter broadening	Multipath propagation broadening a pulse or correlation peak, which can mask the intrinsic time structure.	Chapter Propagation Effects: Plasma, Dust, and Gravitational Lensing.
Extinction	The flux reduction caused by dust absorption and scattering; it affects the photon rate and color.	Chapter Propagation Effects: Plasma, Dust, and Gravitational Lensing.
Lens equation	The relation between source position, image position and deflection angle.	Chapter Propagation Effects: Plasma, Dust, and Gravitational Lensing.
Time-delay distance	The distance combination in strong-lensing time-delay cosmology.	Chapter Propagation Effects: Plasma, Dust, and Gravitational Lensing.
Wave-optics lensing	Lensing phenomena when the wavelength, lens mass and path difference make interference non-negligible.	Chapters Propagation Effects: Plasma, Dust, and Gravitational Lensing, Dark Matter, Axions, and the Polarization Quantum Channel.
Axion birefringence	A candidate effect in which coupling to a light field causes the polarization angle to vary with time or direction.	Chapters Dark Matter, Axions, and the Polarization Quantum Channel, Quantum Problems in Cosmology.
CMB B mode	The curl-type component of CMB polarization, which can arise from gravitational waves, lensing or systematic errors.	Chapter Quantum Problems in Cosmology.
Cosmic variance	The large-scale statistical uncertainty caused by observing only a single universe.	Chapter Quantum Problems in Cosmology.
Standard siren	A source that gives a distance directly from gravitational waves and then, combined with a redshift or electromagnetic counterpart, does cosmology.	Chapter Bursts, Transients, and Multi-Messenger Quantum Astronomy.

Quantum Networks and Project Management

Term	Usage in this book	Main location
Quantum-network telescope	A far-future architecture that uses entanglement, quantum memory, frequency conversion or local mode measurement to assist long-baseline interferometry.	Chapters Quantum-Network Telescopes, From White Paper to Research Plan.
Entanglement-distribution rate	The number of usable entanglement resources the network can provide per second; link loss rapidly suppresses it.	Chapter Quantum-Network Telescopes.
Quantum memory	A device that temporarily stores a quantum state or entanglement resource; the storage time must cover the baseline light travel time and processing latency.	Chapter Quantum-Network Telescopes.
Bell fidelity	The fidelity of the actual entangled state relative to the ideal Bell state, used to gauge the quality of network resources.	Chapter Quantum-Network Telescopes.
Frequency conversion	Converting astronomical light or laboratory quantum resources to a band that can be transmitted, stored or detected.	Chapter Quantum-Network Telescopes.
Readiness	A quantitative judgment of whether the observables, instrument metrics, calibration, code and data products have reached an executable stage.	Chapter From White Paper to Research Plan.
Milestone	A project node carrying observables, target precision, a deadline, null tests and data delivery.	Chapter From White Paper to Research Plan.
Data product	A reusable data deliverable, for example an event table, correlation histogram, $	V
Risk register	A project table that records technical risks, science risks, probabilities, impacts and mitigation plans together.	Chapter From White Paper to Research Plan.
Failure criteria	Abandonment or downgrade conditions defined before the project begins; for example, a target too faint, a systematic floor too high, or a trigger too late.	Chapters Observation Design, Error Budget, and Feasibility, From White Paper to Research Plan.