QuantWare’s Quantum Glossary - your curated guide to essential quantum computing terms. Whether you're new to quantum technology or deepening your expertise, this resource is designed to assist your navigation throughout the vast quantum space.

1Q Gate Fidelity in quantum computing refers to the accuracy with which a single-qubit gate operation is performed. It quantifies the probability that the quantum gate will correctly manipulate the qubit’s state as intended without introducing errors. A higher 1Q Gate Fidelity indicates fewer errors, which is crucial for reliable quantum computations, particularly in the context of error correction and overall system performance. Fidelity is typically expressed as a percentage, with values close to 100% indicating near-perfect gate performance.

2Q Gate Fidelity in quantum computing refers to the accuracy of a two-qubit gate operation. It measures the likelihood that a gate operation between two qubits is performed correctly without introducing errors. High 2Q Gate Fidelity is essential for reliable quantum algorithms and for minimising errors in quantum computations. Like 1Q Gate Fidelity, it is typically expressed as a percentage, with values close to 100% indicating that the gate operation is performed with minimal error.

Classical computers are the traditional computing systems that operate based on classical mechanics, where information is processed using bits that exist in one of two states: 0 or 1. Classical computers are the foundation of modern digital technology, used in everything from personal devices to large-scale data centres.

The coherence time, also known as T1, measures the duration a qubit can maintain its quantum state before decohering due to interactions with its environment.

Computational complexity quantifies the computing resources, such as time or memory, required to solve a particular computational problem. This concept helps in categorising problems based on how efficiently they can be solved using quantum algorithms compared to classical algorithms. Computational complexity is often expressed using big O notation to describe the worst-case scenario for scaling as the size of the input increases. Quantum computers can solve certain complex problems more efficiently than classical computers, some of them even exponentially so, such as prime factorisation.

Concerto is QuantWare’s compact packaging and shielding solution. The first Concerto package was launched for Crescendo-S and can house up to six J-TWPAs in a single magnetic shield. Concerto allows Crescendo users to achieve fast, high-fidelity and multiplexed readout on up to six feedlines.

Connectivity is the ability of qubits to interact with each other through quantum gates. Enhanced connectivity allows for more direct interactions between non-neighbouring qubits, without requiring extensive qubit relocations or intermediary operations.

QuantWare’s QPU Contralto offers up to 21 fully connected and 4 isolated qubits. With partial cryogenic pre-characterisation, expert support, double magnetic shielding and high-density cables, Contralto-D is the most reliable quantum processor for industry-scale quantum computers. Whether it's a demonstration of NISQ algorithms or the latest error correction schemes: Contralto-D gives you the power to perform your experiments at scale.

In the context of a quantum computer, control electronics manage the generation, timing, and delivery of signals used to manipulate qubits, perform quantum gates, and read out qubit states. These electronics are crucial for precise control and synchronisation in quantum operations.

Crescendo-E is a traveling wave parametric amplifier (TWPA) offering tailored to enterprise users. It combines the cutting edge performance of Crescendo-S with customisable designs, frequency bands, packaging, and pre-characterisation to meet enterprise-specific system and process needs. Volume-based pricing ensures Crescendo-E is competitive with in-house fabrication and the flexible delivery schedules enable reliable time to market.

Crescendo-S is the best commercially available Josephson travelling wave parametric amplifier (J-TWPA), capable of elevating qubit readout towards the state of the art in quantum computers of all sizes. It features cutting-edge gain and noise performance over a wide readout band to enable highly multiplexed and cost-effective readout. Designed to be flexible, scalable and easy to use, Crescendo is the readout amplifier for your application.

Crosstalk refers to unintended interactions between qubits, or between qubits and control or readout signals, leading to errors in quantum operations. It can degrade the performance of quantum circuits by causing qubit states to change unintentionally.

A cryostat is a device used to maintain extremely low temperatures, close to absolute zero, essential for operating quantum processors by keeping qubits in a stable environment and minimising thermal noise. It is crucial for the reliable functioning of superconducting qubits as well as other quantum technologies.

The DiVincenzo Criteria are a set of five essential requirements proposed by physicist David DiVincenzo that any physical system must meet to be considered a viable quantum computer. These criteria include the ability to scale the system to many qubits, reliably initialise qubits to a known state, maintain qubit coherence long enough for computation, perform a universal set of quantum gates, and accurately measure the quantum states of qubits. These criteria provide a foundational framework for developing practical quantum computing systems.

Entanglement is a quantum phenomenon where two or more particles become correlated in such a way that the state of one particle instantly influences the state of the other, regardless of the distance between them. This interconnectedness is crucial for quantum computing, as it allows qubits to work together in ways that classical bits cannot, enabling the execution of complex quantum algorithms. Entanglement is one of the foundational principles that give quantum computers their power.

An error-corrected qubit, also known as a logical qubit, is a qubit that has been stabilised against errors through quantum error correction techniques. These techniques use multiple physical qubits to encode a single logical qubit, allowing the system to detect and correct errors that occur during quantum computation. This error correction is essential for achieving fault-tolerant quantum computation, enabling reliable and scalable quantum computers that can perform complex calculations without being disrupted by quantum noise and other error sources.

Fault tolerance in quantum computing refers to a system's ability to perform accurate quantum computations even in the presence of errors. This is achieved by using error-correction to detect and correct errors as they occur. Fault-tolerant systems allow for the execution of complex algorithms over extended periods without the accumulation of errors that would otherwise render the computation incorrect.

QuantWare's Foundry Services offer a market-leading quantum fabrication technology to make customer designs a reality. Grow your company with QuantWare’s VIO powered Foundry Services. Whether you develop error protected qubits, novel couplers, or co-designed, algorithm specific chips, our Foundry Services enable you to develop small, then to scale big with our VIO platform.

Gate fidelity, also known as “error rate”, measures the accuracy with which a quantum gate operation is performed, often expressed as a percentage. Higher gate fidelity indicates that the quantum gate closely matches the intended operation, with fewer errors introduced during the process. Low error rates, or high gate fidelity, are crucial for executing quantum algorithms accurately, as errors can accumulate and degrade the overall computation.

Gate speed refers to the duration required to perform a quantum gate operation. Utility scale, Error Corrected quantum computers will require 1) very fast gate speeds, and 2) a very large number of qubits. Because encoding logical qubits via error correction requires operations itself, gate speed becomes very important at utility scale. Without the very fast gates of superconducting qubits, performing an algorithm on logical qubits can take decades!

‘Housing’ refers to the physical packaging that encloses and supports quantum processing units (QPUs). It is designed to protect delicate quantum components from environmental disturbances and to maintain optimal conditions for quantum coherence.

A Josephson Parametric Amplifier (JPA) is a device used in quantum computing for low-noise amplification of weak signals, typically within superconducting qubit systems. It operates by utilising a resonant circuit made from Josephson junctions to amplify signals near the quantum noise limit. JPAs have a narrower bandwidth and lower compression point than their travelling wave counterparts (TWPAs). This limitation makes them less well suited for scalable quantum computing systems.

Measurement is the final step in a quantum computation, where the results are obtained. During measurement, the qubit's superposition state collapses into one of its basis states, typically a 0 or 1. The pre-measurement state of the qubits influences the final outcome. This aspect of quantum computation often causes confusion, leading some to believe that all quantum computations are probabilistic because superpositions collapse probabilistically during measurement. However, every step in a quantum computation before measurement is entirely deterministic.

Noise refers to any unwanted interaction or disturbance that disrupts the quantum state of qubits. This can arise from external environmental factors, such as temperature fluctuations, electromagnetic interference, or imperfections in the quantum system itself. Noise leads to errors in quantum computations by causing qubits to lose coherence or undergo unintended state changes.

Noise temperature measures the noise level within a system, expressed as an equivalent temperature. It quantifies the noise, manifested as random fluctuation in voltage or current, affecting a quantum signal. The lower the noise temperature, the less thermal noise present. Low noise temperature is key in optimising qubit lifetimes and gate and readout fidelities.

A physical superconducting qubit is the electrical circuit on a chip that is used to store quantum information. Qubits are sometimes called ‘physical’ to distinguish them from logical qubits as defined by error correction codes which each comprise a large set of physical qubits.

A PDK is a comprehensive set of design rules and components that allow customers to design quantum processing units (QPUs) to be fabricated by Foundry Services. A PDK provides the necessary guidelines and resources to ensure that quantum components are compatible with specific fabrication technologies, enabling efficient development and production of quantum hardware.

QPU topology refers to the way the qubits in a QPU are arranged and connected to each other. For instance, a QPU can have a square grid of qubits for surface code based error correction as used in Contralto, or a star topology as used in Soprano, with many qubits connected to a centre bus.

Quantum advantage refers to the point where a quantum computer outperforms classical computers in terms of speed or resource efficiency. In other words, solving problems that classical computers can't handle or solving problems more accurately. So far, demonstrations of quantum advantage have been on theoretical problems, showcasing progress but not yet offering economic benefits.

A quantum algorithm is a sequence of quantum operations or instructions designed to solve specific computational problems by exploiting quantum mechanical properties like superposition and entanglement, allowing for faster solutions than classical algorithms for certain tasks.

A quantum bit (qubit) is the basic unit of quantum information in a quantum computer, capable of existing in multiple states simultaneously (superposition), unlike a classical bit that is either 0 or 1. This is one of the properties, along with e.g. entanglement with other qubits, that allows quantum computers to perform non classical calculations.

A quantum circuit is a sequence of quantum logic gates applied to a set of qubits to perform a computation.

In quantum computing, Quantum Efficiency (QE) refers to the quality of a readout chain and its components. During readout, weak signals travel up from the qubits through the quantum computer’s readout chain, passing through cables, a TWPA and other components. Each component adds a bit of noise to the signal and either amplifies or attenuates. The QE of a component is the ratio between the signal-to-noise ratio at its output versus its input. An ideal component that adds no noise has a QE of 100 %. Advanced TWPAs such as Crescendo have QE exceeding 70 %. A high QE is key to achieving good Readout Fidelity.

Quantum Error Correction is a set of methods used in quantum computing to protect quantum information from errors due to decoherence, noise, and other quantum state disturbances. These techniques involve encoding quantum information across multiple physical qubits to create a logical qubit, which can detect and correct errors without directly measuring and disturbing the quantum data, thereby maintaining the integrity of quantum computations.

Quantum Information Science (QIS) is an interdisciplinary field that explores how quantum systems can be harnessed for information processing, communication, and computation. It merges principles from quantum mechanics, computer science, mathematics, physics, and information theory to study and develop technologies like quantum computers, quantum cryptography, and quantum communication networks.

Quantum Logic Gates are operations that change the state of qubits in a quantum computer. They manipulate qubits in superposition and can create entanglement, enabling quantum computation. These gates are the essential building blocks of quantum circuits.

Quantum Mechanics is the area of physics that studies the behaviour and interactions of particles at the smallest scales, such as atoms and subatomic particles. The principles of quantum mechanics are utilised in quantum computing for information processing.

Quantum Open Architecture (QOA) is the approach to building full-stack quantum computers that enables companies to specialise in key components (e.g., quantum processing units) that together constitute increasingly powerful quantum computers. QOA enables customers and system integrators to combine components from multiple vendors and flexibly develop a range of quantum computings optimised for different tasks.

A Quantum Processing Unit (QPU), is an integrated system composed of several key components: a **Qubit Design**, which defines the properties of individual qubits; the **Qubit Chip**, where physical implementation of the qubit design is located and the qubits interact to perform quantum operations; a Scaling Platform that allows the QPU to contain many qubits; and **Shielding, Packaging, **and** Signal Conditioning**, which protect qubits from external interference and ensure precise signal integrity.

‘Quantum Processor’ is another name for Quantum Processing Unit (QPU). See definition above.

A qubit chip is a chip with qubits and supporting circuitry. These chips serve as the foundational building blocks of quantum processors, enabling the physical realisation of quantum circuits and operations.

Qubit count refers to the total number of qubits in a quantum computer, which is one of the metrics that determine its computational power. Utility scale, quantum error corrected quantum computers will require very large qubit counts (>1M) in a system (or ideally, in a single QPU).

Qubit design is the process of defining the properties and drawing the shapes of the electrical circuits that comprise superconducting qubits. Qubits are very sensitive to their environment and their design is key to minimising the influence that their environment has on them.

The lifetime of a qubit, also known as the relaxation time or T1 time, is the duration for which a qubit remains in its excited state before decaying to its ground state.

Readout fidelity is the accuracy with which a qubit's state can be measured. It quantifies the probability that the measurement correctly identifies the qubit's true state. In practice, achieving high readout fidelity often involves optimising the measurement routine and minimising noise in the readout chain. TWPAs are a key component required for optimising readout fidelity.

Most superconducting qubits are exceedingly sensitive to magnetic fields and thermal radiation. Therefore, qubits are typically packaged in protective Housings and Shielding.

QuantWare’S QPU Soprano is designed to reduce the time and costs associated with building your own quantum computer. With better performance, lead times of 90 days or less, expert support and full cryogenic characterisation, Soprano allows you to meet the most stringent deadlines with confidence. It contains fully-controllable superconducting transmon qubits, offering high speed and low error operations. Contact us and start building your own quantum computer now.

State Preparation and Measurement (SPAM) Errors in quantum computing are inaccuracies that arise during the two crucial phases of a quantum operation: initialising qubits to a desired starting state (state preparation) and determining their final state after computation (measurement). These errors can stem from various sources, including imperfect control mechanisms, environmental noise, or limitations in the measurement equipment.

Superconducting qubits are a type of physical qubit where quantum states are represented by the electrical currents in superconducting circuits which act as "artificial atoms" at extremely low temperatures. These qubits are favoured for their fast gate operations and high scalability, which are essential for building large-scale quantum computers.

Superposition is a fundamental principle in quantum computing where a qubit can exist in multiple states simultaneously, rather than being limited to just 0 or 1, as in classical computing.

The largest available quantum processor with 64 fully controllable qubits. Tenor is powered by QuantWare’s VIO platform, enabling scale with reduced crosstalk. Currently available to selected early access partners only.

The transmon is a type of superconducting qubit designed to be insensitive to charge noise, a feature that enhances its coherence time compared to earlier generations of devices. Due to its balance of simplicity, ease of control, and robustness against errors, the transmon is one of the most widely used qubit architectures in modern quantum computing.

A Travelling Wave Parametric Amplifier (TWPA) is an advanced superconducting device used to amplify low-power signals, such as those from qubit readouts, with minimal added noise. Composed of a long chain of Josephson elements, a TWPA uses nonlinear wave mixing to transfer energy from a strong pump tone to the signal, exponentially amplifying it across a wide bandwidth. Unlike resonant amplifiers, TWPAs support broad frequency multiplexing, essential for scaling quantum computers.

A tunable coupler is a device used in quantum circuits to dynamically control the interaction strength between qubits, allowing for adjustable coupling that can be turned on or off as needed. This capability reduces qubit interactions at times when they are undesired.

VIO is QuantWare’s proprietary scaling platform that enables rapid scaling of quantum processors and drastically reduces crosstalk. Current QPUs face a number of scaling bottlenecks which VIO solves by placing the qubit chips in a patented 3D chip architecture. VIO enables the realisation of any qubit design and unlocks QPUs with more than 1 million qubits. This technology powers QuantWare’s Tenor QPU and Foundry Services.

The first generation of VIO. VIO-176 allows the design of QPUs with up to 176 signal lines, giving space for more than 100 qubits into a QPU (depending on the design parameters). This is the same technology that powers QuantWare’s Tenor QPU and Foundry Services.