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Dictionary

Quantum technology and security dictionary.

Items: 64
Title Abbreviation Description Reference
Afterpulsing

Spurious click caused by carriers trapped during a previous avalanche.
Asymmetric Mach-Zehnder interferometer AMZI

Unbalanced interferometer converting phase differences into distinguishable arrival times.
Attack potential

Rating of the expertise, equipment and time an attack requires to succeed.
Attack surface

The set of points where an attacker can try to interact with a system.
Authentication

Confirming a message came from the claimed party, required on the classical channel.
Avalanche photodiode APD

Semiconductor detector where one absorbed photon triggers a measurable current avalanche.
└ Active quenching

Stopping an avalanche with driven electronics that reset the detector quickly.
└ Free-running detector

Detector sensitive continuously rather than only inside timed gates.
└ Gated detector

Detector made sensitive only during short windows synchronised to expected arrivals.
└ Geiger mode

Detector regime above breakdown where a single photon triggers a full avalanche.
└ Linear mode

Detector regime below breakdown where output scales with incident optical power.
└ Negative-feedback avalanche diode NFAD

Avalanche diode with integrated feedback resistance for fast self-quenching.
└ Passive quenching

Stopping an avalanche with a resistor, leaving a slow recharge the attacker can exploit.
└ Single-photon avalanche diode SPAD

Avalanche photodiode biased above breakdown so it responds to individual photons.
Beam splitter

Optic dividing a beam between two paths, the basis of passive basis choice.
Blinding pulse

The bright light that holds a detector insensitive to single photons during an attack.
Common Criteria CC

International standard for specifying and evaluating IT security requirements.
Common Vulnerabilities and Exposures CVE

Public catalogue assigning each disclosed vulnerability a stable identifier.
Continuous Wave CW
Countermeasure

Control that prevents, detects or reduces the impact of an attack.
CV-QKD Continuous-variable QKD — quantum information encoded in field quadratures (Gaussian states).
Defence in depth

Layering independent controls so no single failure exposes the whole system.
Denial of service DoS

Attack that prevents legitimate key exchange rather than trying to learn the key.
Detection efficiency

Probability that a photon reaching a detector produces a recorded click.
Detector dead time

Interval after a click during which a detector cannot register another photon.
Device-Independent QKD DI-QKD

Security argued from observed statistics alone, trusting neither source nor detectors.
Evaluation Assurance Level EAL

Common Criteria rating describing how rigorously a product was evaluated.
Exploit

Concrete technique or code that turns a vulnerability into an actual compromise.
Faraday mirror

Mirror that rotates polarisation on reflection, stabilising two-way plug-and-play systems.
Forward secrecy

Property where compromising a long-term secret does not expose past session keys.
Intensity modulator

Component setting pulse brightness, used for decoy states and signal preparation.
Kill chain

Ordered stages an attacker moves through, used to plan detection and disruption.
Man-in-the-middle attack MITM

Attacker sits between two parties, impersonating each to the other.
MDI-QKD Measurement-device-independent QKD — detector is treated as untrusted (Lo-Curty-Qi 2012).
Mean photon number MPN

Average photons per pulse, the parameter decoy states vary to expose splitting.
One-time pad

Cipher that is unbreakable when the key is random, secret and used exactly once.
Optical isolator

Component passing light one way only, used to block injected probing light.
Optical power monitor

Component measuring incoming light level to reveal bright-light attacks.
Patch

Change issued to remove a vulnerability from a deployed system.
Penetration testing

Authorised simulated attack used to find weaknesses before a real attacker does.
Phase encoding

Carrying bits in the relative optical phase between two pulses.
Phase modulator

Component imposing a chosen optical phase, used to encode or decode bits.
Phase randomisation Source assumption that pulse-to-pulse phases are uniformly random, enforcing Poisson statistics.
Plug-and-play system

Two-way architecture where pulses originate at the receiver and return encoded.
Polarisation encoding

Carrying bits in the polarisation state of each photon.
Post-quantum cryptography PQC

Classical algorithms designed to resist attack by a large quantum computer.
Protection Profile PP

Common Criteria document stating the security requirements for a class of product.
QBER Quantum bit-error rate — fraction of sifted bits that disagree between Alice and Bob.
Quality of Service QoS
Quantum Bit Error Rate QBER
Quantum communication

Transmitting information encoded in quantum states, of which key distribution is one use.
└ Alice
└ Bell inequality

Test whose violation proves correlations cannot be explained by any local hidden variable.
└ Blind quantum computing

Delegating a computation to a server that learns neither input nor algorithm.
└ Bob
└ Channel loss Fibre or free-space attenuation, scaling as 10^(-alpha·L/10) per km.
└ CHSH inequality CHSH

The standard four-correlator Bell test used to certify device-independent security.
└ Classical channel

The authenticated public link carrying sifting, error correction and privacy amplification.
└ Conference key agreement CKA

Establishing one shared key among three or more parties at once.
└ Eve
└ Monogamy of entanglement

Limit on how far entanglement can be shared, bounding what an eavesdropper can hold.
└ No-cloning theorem

Result that an unknown quantum state cannot be copied, the basis of quantum security.
└ Position verification

Proving a party is physically where it claims, using timing and quantum states.
└ Quantum channel

The path carrying quantum signals, assumed fully under an adversary's control.
└ Quantum coin flipping

Two mistrustful parties agreeing a random bit neither can bias alone.
└ Quantum digital signature QDS

Signing scheme whose unforgeability rests on quantum rather than computational assumptions.
└ Quantum fingerprinting

Comparing large inputs by exchanging exponentially smaller quantum states.
└ Quantum Key Distribution QKD
└ B92

Two-state protocol encoding bits in a pair of non-orthogonal states rather than four.
└ BBM92

Entanglement-based counterpart of BB84 in which both parties measure photons of a shared pair.
└ Coherent One-Way COW

Protocol encoding bits in the time position of coherent pulses, with a monitoring line.
└ Continuous-Variable QKD CV-QKD
└ Channel transmittance

Fraction of signal power surviving the channel, estimated during parameter estimation.
└ Coherent state

Minimum-uncertainty state of light produced by a laser, the usual CV-QKD carrier.
└ Covariance matrix

Second-moment description of a Gaussian state, sufficient on its own for security analysis.
└ Direct reconciliation

Correcting errors toward the sender's data, which fails beyond 3 dB channel loss.
└ Electronic noise

Receiver noise of electronic rather than optical origin, added after the signal arrives.
└ EPR state

Strongly correlated two-mode state used in the entanglement-based view of CV protocols.
└ Excess noise

Channel noise above the vacuum level, the quantity that limits continuous-variable key rate.
└ Gaussian channel

Channel that maps every Gaussian state to another Gaussian state.
└ Gaussian state

State fully described by the first and second moments of its quadratures.
└ Heterodyne detection

Measuring both quadratures at once, at the cost of extra vacuum noise.
└ Homodyne detection

Measuring one quadrature of a light field by mixing it with a reference beam.
└ Local oscillator LO

Bright reference beam a continuous-variable receiver mixes with the signal.
└ Low-density parity-check code LDPC

Sparse-graph error-correcting code that performs near the limit at low signal-to-noise.
└ Modulation variance

Spread of the sender's Gaussian modulation, a parameter optimised per channel.
└ Multidimensional reconciliation

Mapping Gaussian data onto a sphere so binary codes can correct it at low signal-to-noise.
└ Noise budget

Accounting of every noise contribution, used to decide which parts are trusted.
└ Phase noise

Random drift between signal and reference phase, which appears in the measurement as excess noise.
└ Phase recovery

Estimating and correcting the signal-to-reference phase from pilot measurements.
└ Pilot tone

Reference signal sent alongside the quantum signal so the receiver can recover phase.
└ Post-selection

Discarding data where the legitimate parties hold no advantage over the attacker.
└ Quadrature

One of two conjugate components of a light field, the variables CV-QKD encodes on.
└ Reverse reconciliation

Correcting errors toward the receiver's data, which removes the 3 dB loss limit.
└ Shot noise

Irreducible fluctuation of a coherent light field, the reference all CV noise is measured against.
└ Shot-noise unit SNU

Normalisation in which the vacuum fluctuation variance equals exactly one.
└ Slice reconciliation

Quantising continuous values into slices that are error-corrected separately.
└ Squeezed state

State whose noise in one quadrature falls below the vacuum level, at the other's expense.
└ Three-decibel loss limit

Range bound on direct reconciliation, lifted by reconciling toward the receiver instead.
└ Trusted noise

Detector noise assumed outside the attacker's reach, which raises the computed key rate.
└ Two-mode squeezed vacuum TMSV

Entangled Gaussian state, the entanglement-based equivalent of Gaussian modulation.
└ Untrusted noise

Noise attributed entirely to the attacker, the conservative assumption for a security proof.
└ Vacuum fluctuations

Quantum noise present even with no light, setting the floor for any measurement.
└ Weak coherent pulse Laser pulse approximating a low-photon-number state, used in real QKD systems.
└ Counterfactual QKD CFQKD

Key exchange in which, ideally, no signal photon ever travels the quantum channel.
└ CV B92

Continuous-variable analogue of the two-state B92 protocol.
└ CV MDI-QKD

Continuous-variable protocol measured at an untrusted central relay.
└ Decoy-state BB84

BB84 run with several pulse intensities so photon-number-splitting is detectable.
└ Differential Phase Shift QKD DPS-QKD
└ Differential phase time shifting DPTS

Protocol combining differential phase encoding with time-shifted pulse positions.
└ Discrete-modulation CV-QKD DM CV-QKD

Continuous-variable scheme sending a small finite set of coherent states.
└ Discrete-Variable QKD DV-QKD
└ Afterpulse probability

Chance that an avalanche leaves trapped carriers that fire a spurious later click.
└ Basis bias

Choosing one basis more often than the other to raise the sifted-key yield.
└ Basis reconciliation

Publicly comparing measurement bases so incompatible results can be discarded.
└ BB84 The original quantum key distribution protocol (Bennett & Brassard 1984), using non-orthogonal bases.
└ Coincidence window

Time interval within which two detections count as one correlated pair.
└ Dark count Detector click in absence of an incoming photon — sets a noise floor.
└ Decoy intensity Lower intensity setting used to certify the single-photon yield in decoy-state QKD.
└ Decoy state Protocol variant using multiple intensities to certify the single-photon contribution.
└ Decoy-state estimation

Using several intensities to bound the single-photon yield and its error rate.
└ Detector efficiency Probability that a photon arriving at the detector produces a click.
└ Detector gate width

Duration of the window in which a gated detector is sensitive.
└ Gain

Probability that a sent pulse produces a detection, per intensity setting.
└ Key sifting ratio

Fraction of detections surviving basis reconciliation.
└ Multi-photon pulse

Pulse containing more than one photon, the opening that photon-number splitting exploits.
└ Photon-number distribution

Statistical spread of photons per pulse, Poissonian for an attenuated laser.
└ Photon-number-splitting attack Multi-photon-pulse attack that lets Eve siphon photons without disturbance.
└ Poissonian source

Attenuated laser whose photon number per pulse follows a Poisson distribution.
└ Sifted key Bits remaining after Alice and Bob discard rounds in which their bases disagreed.
└ Sifting factor Probability that Alice and Bob choose the same basis (q ≈ 1/2 for symmetric BB84).
└ Single-photon source Source emitting exactly one photon per pulse; required for the Shor-Preskill proof.
└ Single-photon yield

Detection probability for the single-photon component, the quantity decoy states bound.
└ Unambiguous State Discrimination USD
└ Visibility

Interference contrast, which sets the achievable error rate in phase-encoded systems.
└ Yield

Probability that an n-photon pulse produces a detection at the receiver.
└ E91

Ekert's entanglement-based protocol that certifies secrecy through violation of a Bell inequality.
└ Entanglement-Based QKD EB-QKD
└ Gaussian-Modulated Coherent State GMCS

Continuous-variable scheme modulating coherent-state amplitude and phase from a Gaussian distribution.
└ Grosshans-Grangier 2002 GG02

Continuous-variable protocol using Gaussian-modulated coherent states and homodyne detection.
└ LM05

Deterministic two-way protocol where the receiver encodes on a returning qubit.
└ Local local oscillator CV-QKD LLO CV-QKD

Continuous-variable scheme generating the reference beam at the receiver, not the sender.
└ MDI three-state protocol

Measurement-device-independent scheme using three rather than four prepared states.
└ Measurement-Device-Independent QKD MDI-QKD
└ PE-BB84

BB84 variant with an explicit parameter-estimation stage built into sifting.
└ Phase-Matching QKD PM-QKD

Twin-field variant matching optical phase at a central node to extend range.
└ Prepare-and-Measure QKD P&M QKD
└ Round-Robin Differential Phase Shift RRDPS

Protocol bounding an eavesdropper's information without monitoring the error rate.
└ SARG04

BB84 variant whose sifting step resists photon-number-splitting on weak coherent pulses.
└ Sending-or-Not-Sending SNS

Twin-field variant tolerant of phase misalignment because bits ride on sending choices.
└ Six-state protocol SSP

BB84 extended to three mutually unbiased bases, tolerating a higher error rate.
└ Squeezed-state CV-QKD

Continuous-variable protocol using squeezed light rather than coherent states.
└ Toshiba T12

Decoy-state BB84 variant with biased basis choice used in commercial systems.
└ Two-way CV-QKD

Continuous-variable scheme in which signals travel to the far party and back.
└ Quantum money

Unforgeable tokens whose security rests on the impossibility of copying quantum states.
└ Quantum Non-Demolition QND
└ Quantum oblivious transfer

Transfer where the sender does not learn which message the receiver obtained.
└ Quantum secret sharing QSS

Splitting a secret among parties so only an authorised subset can reconstruct it.
└ Quantum secure direct communication QSDC

Sending a message itself over the quantum channel rather than first sharing a key.
└ Security proof Terminology specific to QKD security-proof construction and analysis.
└ Asymptotic regime Infinite-data analysis ignoring finite-statistics corrections.
└ Coherent attack Most general attack: Eve coherently entangles a probe with every pulse.
└ Collective attack Eve uses an identical probe for every pulse, but measurements may be joint.
└ Composable security Security definition under which the protocol remains secure when composed with others.
└ Detection loophole

Gap in a Bell test caused by low detector efficiency, which faked violations exploit.
└ Devetak-Winter bound

Key rate as the parties' mutual information minus the attacker's Holevo information.
└ Error correction Public-channel reconciliation that fixes the bit errors counted by the QBER.
└ Fair sampling assumption

Assumption that detected events represent all events, broken by efficiency mismatch.
└ Finite-key analysis Security accounting for statistical fluctuations in a finite-size run.
└ GLLP analysis GLLP

Security treatment separating single-photon from multi-photon contributions in an imperfect source.
└ Hashing inequality Bound used to relate von-Neumann entropy to the smooth min-entropy.
└ Holevo bound

Upper limit on the classical information obtainable from a quantum ensemble.
└ Individual attack Eve attacks each pulse independently with independent measurements.
└ Information-theoretic security

Security holding against any adversary regardless of computing power or time.
└ Leftover hash lemma

Result stating how much uniform key a hash can extract from a partly known string.
└ Linear program Optimisation method used in Curty 2014 to tightly bound the single-photon contribution.
└ Locality loophole

Gap in a Bell test where the measurement events are not space-like separated.
└ Min-entropy

Measure of the guessing probability of a variable, setting how much key can be extracted.
└ Mutual information

Amount of information two correlated data sets share.
└ Optimality of Gaussian attacks

Theorem that the worst case against a Gaussian-modulated protocol is itself Gaussian.
└ Parameter estimation

Using a sampled subset of results to bound the eavesdropper's information.
└ Phase error Error rate inferred for the complementary (test) basis; bounds Eve information.
└ Privacy amplification Final step that compresses a partially-leaked key into a shorter secret one.
└ Purification

Describing a prepare-and-measure protocol by an equivalent entangled source, simplifying its proof.
└ Quantum bit error rate threshold

Error level above which no secret key can be distilled and the session aborts.
└ Reconciliation efficiency

How close error correction comes to the theoretical minimum leakage.
└ Secret key fraction

Portion of raw measurement data that survives distillation to become key.
└ Secret-key rate Asymptotic length of secret key extracted per channel use (or per sifted signal).
└ Security assumption

Condition a proof relies on, which an implementation must actually satisfy.
└ Security parameter

Explicit bound on the probability that the delivered key is not perfectly secret.
└ Sifting

Discarding results where the parties used incompatible measurement settings.
└ Smooth min-entropy

Min-entropy allowing a small failure probability, the quantity finite-key proofs bound.
└ Squashing model Reduction that maps a multi-mode detector to an equivalent single-photon detector.
└ Symplectic eigenvalue

Eigenvalue of a covariance matrix under symplectic transformation, used to compute entropies.
└ Trusted device assumption

Assumption that named hardware behaves as modelled and is outside the attacker's reach.
└ Universal hashing

Family of hash functions whose collision probability is bounded for any input pair.
└ Wegman-Carter authentication

Information-theoretically secure message authentication using a short shared key.
└ Superdense coding

Sending two classical bits in one qubit using pre-shared entanglement.
Quantum hacking

Attacking a quantum cryptosystem through its physical implementation rather than its protocol.
└ Implementation attack

Attack exploiting the gap between an idealised protocol and the hardware realising it.
└ After-gate attack

Sending bright pulses just after a gate closes, when the detector responds classically.
└ Backflash

Light emitted by an avalanching detector that can leak which detector fired.
└ Beam-splitting attack

Tapping a fraction of every pulse from the channel and storing it for later measurement.
└ Bright illumination attack

Detector control achieved by shining bright light into the receiver's optical input.
└ Calibration attack

Manipulating a system's calibration routine so it adopts attacker-favourable settings.
└ Dead time attack

Exploiting the interval after a click when a detector cannot register another photon.
└ Detector blinding

Driving a single-photon detector out of Geiger mode so it stops seeing single photons.
└ Detector control attack

Steering a receiver's detectors so the attacker decides which detector clicks.
└ Detector efficiency mismatch DEM

Exploitable difference in detector sensitivity across time, wavelength or spatial mode.
└ Entangling cloner attack

Canonical collective attack where the attacker taps with a beamsplitter and stores one mode.
└ Faked state

Classical light pulse crafted to force a chosen detection outcome in a blinded detector.
└ Induced-photorefraction attack

Changing a lithium niobate modulator's response with light to bias its encoding.
└ Injection locking attack

Forcing a sender's laser to follow injected light, controlling the emitted state.
└ Laser damage attack

Permanently altering a component with high-power light to create an exploitable weakness.
└ Local oscillator calibration attack

Exploiting a receiver that derives its noise reference from a beam the attacker can alter.
└ Local oscillator intensity attack

Manipulating reference-beam power so the receiver mis-calibrates its shot-noise scale.
└ Memory attack

Exploiting a device that retains state between sessions to leak earlier key material.
└ Phase-remapping attack

Shifting the modulator's timing so the sender's phase states are remapped exploitably.
└ Pulse illumination attack

Blinding with pulsed rather than continuous light, evading average-power monitors.
└ Saturation attack

Overwhelming a detector or monitor so genuine signals stop being registered correctly.
└ Seed light attack

Injecting light into a sender's laser cavity to read or steer its output.
└ Spatial mode mismatch

Efficiency that varies with the incoming beam's spatial mode, opening a side channel.
└ Time-shift attack

Shifting signal arrival time to exploit detectors whose efficiency peaks at different moments.
└ Trojan horse attack

Probing a party's optics with injected light and reading the reflection for settings.
└ Wavelength-dependent attack

Using light outside the design band, where optics and monitors behave differently.
└ Zero-error attack

Detector control precise enough that the legitimate parties see no raised error rate.
└ Intercept-resend attack

Measuring every signal and resending a fresh state, the baseline eavesdropping strategy.
└ Side-channel attack

Attack using unintended physical leakage rather than a flaw in the protocol itself.
Quantum network

Connected set of quantum links sharing entanglement or key across many parties.
└ Availability

Fraction of time a link or service actually delivers key to its consumers.
└ Backbone link

Long-haul connection carrying aggregated traffic between metropolitan networks.
└ Dense wavelength division multiplexing DWDM

Carrying many channels down one fibre, letting quantum and classical traffic share it.
└ End-to-end security

Property where only the two endpoints hold the key, with no trusted intermediary.
└ Entanglement-based network

Network distributing entanglement rather than relaying key through trusted nodes.
└ Entanglement distribution

Delivering entangled pairs to distant parties as the resource for key or teleportation.
└ Entanglement swapping

Joining two entangled pairs so distant particles become entangled without meeting.
└ ETSI GS QKD 004

Standard defining an application interface for key delivery with explicit key streams.
└ ETSI GS QKD 014

Standard defining a REST interface for delivering keys to applications.
└ ETSI ISG QKD

The ETSI industry group that publishes the QKD interface and security standards.
└ Failover

Switching to an alternative path when a link or node stops delivering key.
└ Free-space QKD

Key distribution through air or vacuum rather than optical fibre.
└ ISO/IEC 23837

Standard on security requirements and test methods for QKD components.
└ ITU-T Y.3800

Recommendation describing the overall framework for networks supporting QKD.
└ Key consumption rate

Speed at which applications draw key, which must not exceed the distillation rate.
└ Key management system KMS

System storing, distributing and retiring key material for the applications consuming it.
└ Key buffering

Holding produced key in reserve so demand can exceed instantaneous production.
└ Key consumer

Application that draws key from the management layer and uses it to protect data.
└ Key delivery interface

API through which an application asks the key manager for key material.
└ Key destruction

Irreversibly erasing key material once it is spent or expired.
└ Key exhaustion

Condition where consumers draw key faster than the links can produce it.
└ Key identifier Key ID

Label letting both endpoints request the same key without transmitting it.
└ Key lifecycle

Stages a key passes through from generation to destruction.
└ Key provisioning

Making key material available to the endpoints that will consume it.
└ Key store

Protected storage holding key material until an application requests it.
└ Key synchronisation

Keeping both endpoints' key stores consistent in content and ordering.
└ Key rate budget

Planned allocation of a link's key production across its consuming applications.
└ Key relay

Passing key material hop by hop across trusted nodes to link distant parties.
└ Metropolitan network

Short-range deployment linking sites across a single city.
└ Multi-hop path

Route crossing several links, requiring key relay at each intermediate node.
└ Network controller

Component deciding how key and traffic are routed across a quantum network.
└ Network topology

Arrangement of nodes and links, typically point-to-point, ring, star or mesh.
└ Optical switch

Component routing a quantum signal between paths without measuring it.
└ Path computation

Choosing which route a key request should take across the network.
└ QKD link

A single point-to-point quantum connection between two nodes.
└ QKD node

Site housing QKD equipment that terminates one or more quantum links.
└ Quantum backbone

Core long-distance segment of a quantum network.
└ Quantum key pool

Buffer of already-distilled key material held ready for consuming applications.
└ Quantum memory

Device storing a quantum state long enough to synchronise operations across a network.
└ Quantum repeater

Node extending entanglement across long distances without measuring the transmitted key.
└ Quantum teleportation

Transferring a quantum state using shared entanglement plus classical communication.
└ Satellite QKD

Key distribution using a spacecraft as a relay or source to span continents.
└ Software-defined networking SDN

Centralised control plane that configures links, increasingly used to manage QKD paths.
└ Trusted node

Relay that decrypts and re-encrypts key material, and so must itself be trusted.
Quantum random number generator QRNG

Device deriving unpredictable numbers from a quantum process rather than an algorithm.
Raman scattering

Fibre noise from bright classical channels that can swamp a co-propagating quantum signal.
Red team

Group that attacks a system adversarially to test its defences.
Repeaterless bound PLOB

Limit on secret key rate achievable over a lossy link without repeaters.
Residual risk

Risk that remains after a countermeasure has been applied.
Security certification

Independent confirmation that a product meets a stated security standard.
Signal-to-Noise Ratio SNR
Superconducting nanowire single-photon detector SNSPD

Detector sensing single photons through loss of superconductivity in a cooled nanowire.
Symmetric encryption

Encryption where both parties hold the same secret key, the usual consumer of QKD output.
Threat actor

Person or group conducting attacks, described by capability, resources and intent.
Threat model

Explicit statement of what an attacker can do and what is assumed trustworthy.
Time-bin encoding

Carrying bits in which of several time slots a photon occupies.
Timing jitter

Variation in the delay between photon arrival and the detector's output pulse.
Traffic Light Protocol TLP

Labelling scheme stating how widely a piece of shared information may be redistributed.
Twin-Field QKD TF-QKD

Protocol pairing single photons at a middle node to beat the repeaterless rate bound.
Variable optical attenuator VOA

Adjustable component setting the mean photon number of outgoing pulses.
Vulnerability

Weakness in a system that an attack can exploit to defeat a security goal.
Watchdog detector

Extra monitor at the receiver input that flags unexpected incoming optical power.
Zero-day

Vulnerability being exploited before the vendor has published a fix.