**Quantum metrology** is the study of making high-resolution and highly sensitive measurements of physical parameters using quantum theory to describe the physical systems,^{ [1] }^{ [2] }^{ [3] }^{ [4] }^{ [5] }^{ [6] } particularly exploiting quantum entanglement and quantum squeezing. This field promises to develop measurement techniques that give better precision than the same measurement performed in a classical framework. Together with quantum hypothesis testing,^{ [7] }^{ [8] } it represents an important theoretical model at the basis of quantum sensing.^{ [9] }^{ [10] }

A basic task of quantum metrology is estimating the parameter of the unitary dynamics

where is the initial state of the system and is the Hamiltonian of the system. is estimated based on measurements on

Typically, the system is composed of many particles, and the Hamiltonian is a sum of single-particle terms

where acts on the kth particle. In this case, there is no interaction between the particles, and we talk about linear interferometers.

The achievable precision is bounded from below by the quantum Cramér-Rao bound as

where is the quantum Fisher information.^{ [1] }^{ [11] }

One example of note is the use of the NOON state in a Mach–Zehnder interferometer to perform accurate phase measurements.^{ [12] } A similar effect can be produced using less exotic states such as squeezed states. In atomic ensembles, spin squeezed states can be used for phase measurements.

An important application of particular note is the detection of gravitational radiation in projects such as LIGO or the Virgo interferometer, where high-precision measurements must be made for the relative distance between two widely-separated masses. However, the measurements described by quantum metrology are currently not used in this setting, being difficult to implement. Furthermore, there are other sources of noise affecting the detection of gravitational waves which must be overcome first. Nevertheless, plans may call for the use of quantum metrology in LIGO.^{ [13] }

A central question of quantum metrology is how the precision, i.e., the variance of the parameter estimation, scales with the number of particles. Classical interferometers cannot overcome the shot-noise limit

where is the number of particles. Quantum metrology can reach the Heisenberg limit given by

However, if uncorrelated local noise is present, then for large particle numbers the scaling of the precision returns to shot-noise scaling ^{ [14] }^{ [15] }

There are strong links between quantum metrology and quantum information science. It has been shown that quantum entanglement is needed to outperform classical interferometry in magnetrometry with a fully polarized ensemble of spins.^{ [16] } It has been proved that a similar relation is generally valid for any linear interferometer, independent of the details of the scheme.^{ [17] } Moreover, higher and higher levels of multipartite entanglement is needed to achieve a better and better accuracy in parameter estimation.^{ [18] }^{ [19] }

**Quantum teleportation** is a technique for transferring quantum information from a sender at one location to a receiver some distance away. While teleportation is commonly portrayed in science fiction as a means to transfer physical objects from one location to the next, quantum teleportation only transfers quantum information. Moreover, the sender may not know the location of the recipient, and does not know which particular quantum state will be transferred.

**Quantum entanglement** is a physical phenomenon that occurs when a pair or group of particles is generated, interact, or share spatial proximity in a way such that the quantum state of each particle of the pair or group cannot be described independently of the state of the others, including when the particles are separated by a large distance. The topic of quantum entanglement is at the heart of the disparity between classical and quantum physics: entanglement is a primary feature of quantum mechanics lacking in classical mechanics.

In quantum physics, a **measurement** is the testing or manipulation of a physical system in order to yield a numerical result. The predictions that quantum physics makes are in general probabilistic. The mathematical tools for making predictions about what measurement outcomes may occur were developed during the 20th century and make use of linear algebra and functional analysis.

In physics, a **squeezed coherent state** is a quantum state that is usually described by two non-commuting observables having continuous spectra of eigenvalues. Examples are position and momentum of a particle, and the (dimension-less) electric field in the amplitude and in the mode of a light wave. The product of the standard deviations of two such operators obeys the uncertainty principle:

In quantum mechanics, **einselections**, short for "environment-induced superselection", is a name coined by Wojciech H. Zurek for a process which is claimed to explain the appearance of wavefunction collapse and the emergence of classical descriptions of reality from quantum descriptions. In this approach, classicality is described as an emergent property induced in open quantum systems by their environments. Due to the interaction with the environment, the vast majority of states in the Hilbert space of a quantum open system become highly unstable due to entangling interaction with the environment, which in effect monitors selected observables of the system. After a decoherence time, which for macroscopic objects is typically many orders of magnitude shorter than any other dynamical timescale, a generic quantum state decays into an uncertain state which can be decomposed into a mixture of simple pointer states. In this way the environment induces effective superselection rules. Thus, einselection precludes stable existence of pure superpositions of pointer states. These 'pointer states' are stable despite environmental interaction. The einselected states lack coherence, and therefore do not exhibit the quantum behaviours of entanglement and superposition.

**Quantum tomography** or **quantum state tomography** is the process by which a quantum state is reconstructed using measurements on an ensemble of identical quantum states. The source of these states may be any device or system which prepares quantum states either consistently into quantum pure states or otherwise into general mixed states. To be able to uniquely identify the state, the measurements must be **tomographically complete**. That is, the measured operators must form an operator basis on the Hilbert space of the system, providing all the information about the state. Such a set of observations is sometimes called a **quorum**.

In quantum mechanics, **separable quantum states** are states without quantum entanglement.

**Squashed entanglement**, also called **CMI entanglement**, is an information theoretic measure of quantum entanglement for a bipartite quantum system. If is the density matrix of a system composed of two subsystems and , then the CMI entanglement of system is defined by

A **NOON state** is a quantum-mechanical many-body entangled state:

**Time-bin encoding** is a technique used in quantum information science to encode a qubit of information on a photon. Quantum information science makes use of qubits as a basic resource similar to bits in classical computing. Qubits are any two-level quantum mechanical system; there are many different physical implementations of qubits, one of which is time-bin encoding.

The **one-way** or **measurement-based quantum computer** (**MBQC**) is a method of quantum computing that first prepares an entangled *resource state*, usually a cluster state or graph state, then performs single qubit measurements on it. It is "one-way" because the resource state is destroyed by the measurements.

The field of quantum sensing deals with the design and engineering of quantum sources and quantum measurements that are able to beat the performance of any classical strategy in a number of technological applications. This can be done with photonic systems or solid state systems.

In the case of systems composed of subsystems, the classification of **quantum-entangled****states** is richer than in the bipartite case. Indeed, in **multipartite entanglement** apart from fully separable states and fully entangled states, there also exists the notion of partially separable states.

In applied mathematics, the **numerical sign problem** is the problem of numerically evaluating the integral of a highly oscillatory function of a large number of variables. Numerical methods fail because of the near-cancellation of the positive and negative contributions to the integral. Each has to be integrated to very high precision in order for their difference to be obtained with useful accuracy.

The **spin stiffness** or **spin rigidity** or **helicity modulus** or the "**superfluid density**" is a constant which represents the change in the ground state energy of a spin system as a result of introducing a slow in plane twist of the spins. The importance of this constant is in its use as an indicator of quantum phase transitions—specifically in models with metal-insulator transitions such as Mott insulators. It is also related to other topological invariants such as the Berry phase and Chern numbers as in the Quantum hall effect.

**Spin squeezing** is a quantum process that decreases the variance of one of the angular momentum components in an ensemble of particles with a spin. The quantum states obtained are called spin squeezed states. Such states can be used for quantum metrology, as they can provide a better precision for estimating a rotation angle than classical interferometers.

**Many body localization** (MBL) is a dynamical phenomenon occurring in isolated many-body quantum systems. It is characterized by the system failing to reach thermal equilibrium, and retaining a memory of its initial condition in local observables for infinite times.

The **quantum Cramér–Rao bound** is the quantum analogue of the classical Cramér–Rao bound. It bounds the achievable precision in parameter estimation with a quantum system:

The **quantum Fisher information** is a central quantity in quantum metrology and is the quantum analogue of the classical Fisher information. The quantum Fisher information of a state with respect to the observable is defined as

The **symmetric logarithmic derivative** is an important quantity in quantum metrology, and is related to the quantum Fisher information.

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