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physics Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...
, a squeezed coherent state is a quantum state that is usually described by two
non-commuting In mathematics, a binary operation is commutative if changing the order of the operands does not change the result. It is a fundamental property of many binary operations, and many mathematical proofs depend on it. Perhaps most familiar as a pr ...
observables In physics, an observable is a physical property or physical quantity that can be measured. In classical mechanics, an observable is a real-valued "function" on the set of all possible system states, e.g., position and momentum. In quantum me ...
having continuous spectra of
eigenvalues In linear algebra, an eigenvector ( ) or characteristic vector is a vector that has its direction unchanged (or reversed) by a given linear transformation. More precisely, an eigenvector \mathbf v of a linear transformation T is scaled by a ...
. Examples are position x and momentum p of a particle, and the (dimension-less) electric field in the amplitude X (phase 0) and in the mode Y (phase 90°) of a light wave (the wave's quadratures). The product of the standard deviations of two such
operator Operator may refer to: Mathematics * A symbol indicating a mathematical operation * Logical operator or logical connective in mathematical logic * Operator (mathematics), mapping that acts on elements of a space to produce elements of another sp ...
s obeys the
uncertainty principle The uncertainty principle, also known as Heisenberg's indeterminacy principle, is a fundamental concept in quantum mechanics. It states that there is a limit to the precision with which certain pairs of physical properties, such as position a ...
: :\Delta x \Delta p \geq \frac2\; and \;\Delta X \Delta Y \geq \frac4 , respectively. Trivial examples, which are in fact not squeezed, are the ground state , 0\rangle of the
quantum harmonic oscillator The quantum harmonic oscillator is the quantum-mechanical analog of the classical harmonic oscillator. Because an arbitrary smooth potential can usually be approximated as a harmonic potential at the vicinity of a stable equilibrium point, ...
and the family of
coherent state In physics, specifically in quantum mechanics, a coherent state is the specific quantum state of the quantum harmonic oscillator, often described as a state that has dynamics most closely resembling the oscillatory behavior of a classical harmo ...
s , \alpha\rangle. These states saturate the uncertainty above and have a symmetric distribution of the operator uncertainties with \Delta x_g = \Delta p_g in "natural oscillator units" and \Delta X_g = \Delta Y_g = 1/2. The term squeezed state is actually used for states with a standard deviation below that of the ground state for one of the operators or for a linear combination of the two. The idea behind this is that the circle denoting the uncertainty of a coherent state in the
quadrature phase In physics and mathematics, the phase (symbol φ or ϕ) of a wave or other periodic function F of some real variable t (such as time) is an angle-like quantity representing the fraction of the cycle covered up to t. It is expressed in such a s ...
space (see right) has been "squeezed" to an
ellipse In mathematics, an ellipse is a plane curve surrounding two focus (geometry), focal points, such that for all points on the curve, the sum of the two distances to the focal points is a constant. It generalizes a circle, which is the special ty ...
of the same area. Note that a squeezed state does not need to saturate the uncertainty principle. Squeezed states of light were first produced in the mid 1980s.R. E. Slusher et al., ''Observation of squeezed states generated by four wave mixing in an optical cavity'', Phys. Rev. Lett. 55 (22), 2409 (1985) At that time, quantum noise squeezing by up to a factor of about 2 (3 dB) in variance was achieved, i.e. \Delta^2 X \approx \Delta^2 X_g/2. As of 2017, a squeeze factor of 31 (15 dB) has been directly observed.


Mathematical definition

The most general
wave function In quantum physics, a wave function (or wavefunction) is a mathematical description of the quantum state of an isolated quantum system. The most common symbols for a wave function are the Greek letters and (lower-case and capital psi (letter) ...
that satisfies the identity above is the squeezed coherent state (we work in units with \hbar=1) :\psi(x) = C\,\exp\left(-\frac + i p_0 x\right) where C,x_0,w_0,p_0 are constants (a normalization constant, the center of the
wavepacket In physics, a wave packet (also known as a wave train or wave group) is a short burst of localized wave action that travels as a unit, outlined by an envelope. A wave packet can be analyzed into, or can be synthesized from, a potentially-infini ...
, its width, and the expectation value of its
momentum In Newtonian mechanics, momentum (: momenta or momentums; more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. ...
). The new feature relative to a
coherent state In physics, specifically in quantum mechanics, a coherent state is the specific quantum state of the quantum harmonic oscillator, often described as a state that has dynamics most closely resembling the oscillatory behavior of a classical harmo ...
is the free value of the width w_0, which is the reason why the state is called "squeezed". The squeezed state above is an
eigenstate In quantum physics, a quantum state is a mathematical entity that embodies the knowledge of a quantum system. Quantum mechanics specifies the construction, evolution, and measurement of a quantum state. The result is a prediction for the system re ...
of a linear operator : \hat x + i\hat p w_0^2 and the corresponding
eigenvalue In linear algebra, an eigenvector ( ) or characteristic vector is a vector that has its direction unchanged (or reversed) by a given linear transformation. More precisely, an eigenvector \mathbf v of a linear transformation T is scaled by a ...
equals x_0+ip_0 w_0^2. In this sense, it is a generalization of the ground state as well as the coherent state.


Operator representation

The general form of a squeezed coherent state for a quantum harmonic oscillator is given by : , \alpha,\zeta\rangle = \hat(\zeta), \alpha\rangle = \hat(\zeta) \hat(\alpha), 0\rangle where , 0\rangle is the
vacuum state In quantum field theory, the quantum vacuum state (also called the quantum vacuum or vacuum state) is the quantum state with the lowest possible energy. Generally, it contains no physical particles. However, the quantum vacuum is not a simple ...
, D(\alpha) is the
displacement operator In the quantum mechanics study of optical phase space, the displacement operator for one mode is the shift operator in quantum optics, :\hat(\alpha)=\exp \left ( \alpha \hat^\dagger - \alpha^\ast \hat \right ) , where \alpha is the amount of disp ...
and S(\zeta) is the
squeeze operator In quantum physics, the squeeze operator for a single mode of the electromagnetic field is :\hat(z) = \exp \left ( (z^* \hat^2 - z \hat^) \right ) , \qquad z = r \, e^ where the operators inside the exponential are the ladder operators. It is a ...
, given by :\hat(\alpha)=\exp (\alpha \hat a^\dagger - \alpha^* \hat a)\qquad \text\qquad \hat(\zeta)=\exp\bigg frac (\zeta^* \hat a^2-\zeta \hat a^)\bigg/math> where \hat a and \hat a^\dagger are annihilation and creation operators, respectively. For a
quantum harmonic oscillator The quantum harmonic oscillator is the quantum-mechanical analog of the classical harmonic oscillator. Because an arbitrary smooth potential can usually be approximated as a harmonic potential at the vicinity of a stable equilibrium point, ...
of angular frequency \omega, these operators are given by :\hat a^\dagger = \sqrt\left(x-\frac\right)\qquad \text \qquad \hat a = \sqrt\left(x+\frac\right) For a real \zeta, (note that \zeta = r e^, where ''r'' is squeezing parameter), the uncertainty in x and p are given by :(\Delta x)^2=\frac\mathrm^ \qquad\text\qquad (\Delta p)^2=\frac\mathrm^ Therefore, a squeezed coherent state saturates the
Heisenberg uncertainty principle The uncertainty principle, also known as Heisenberg's indeterminacy principle, is a fundamental concept in quantum mechanics. It states that there is a limit to the precision with which certain pairs of physical properties, such as position a ...
\Delta x\Delta p=\frac, with reduced uncertainty in one of its quadrature components and increased uncertainty in the other. Some expectation values for squeezed coherent states are : \langle\alpha,\zeta , \hat a , \alpha,\zeta\rangle = \alpha \cosh(r) - \alpha^e^ \sinh(r) : \langle\alpha,\zeta , ^2 , \alpha,\zeta\rangle = \alpha ^ \cosh^(r) +^e^ \sinh^(r) - (1+2^)e^ \cosh (r) \sinh (r) : \langle\alpha,\zeta , ^\hat , \alpha,\zeta\rangle = , \alpha, ^2 \cosh^(r) + (1+^)\sinh^2 (r) - (^2 e^ + ^2 e^)\cosh (r) \sinh (r) The general form of a displaced squeezed state for a quantum harmonic oscillator is given by : , \zeta,\alpha\rangle = \hat(\alpha), \zeta\rangle = \hat(\alpha) \hat(\zeta), 0\rangle Some expectation values for displaced squeezed state are : \langle\zeta,\alpha , \hat a , \zeta,\alpha\rangle = \alpha : \langle\zeta,\alpha , ^2 , \zeta,\alpha\rangle = \alpha ^ - e^ \cosh (r) \sinh (r) : \langle\zeta,\alpha , ^\hat , \zeta,\alpha\rangle = , \alpha, ^2 + \sinh^2 (r) Since \hat(\zeta) and \hat(\alpha) do not commute with each other, : \hat(\zeta) \hat(\alpha) \neq \hat(\alpha) \hat(\zeta) : , \alpha, \zeta \rangle \neq , \zeta, \alpha \rangle where \hat(\alpha)\hat(\zeta) =\hat(\zeta)\hat^(\zeta)\hat(\alpha)\hat(\zeta)= \hat(\zeta)\hat(\gamma), with \gamma=\alpha\cosh r + \alpha^* e^ \sinh r


Examples

Depending on the phase angle at which the state's width is reduced, one can distinguish amplitude-squeezed, phase-squeezed, and general quadrature-squeezed states. If the squeezing operator is applied directly to the vacuum, rather than to a coherent state, the result is called the squeezed vacuum. The figures below give a nice visual demonstration of the close connection between squeezed states and
Heisenberg Werner Karl Heisenberg (; ; 5 December 1901 – 1 February 1976) was a German theoretical physicist, one of the main pioneers of the theory of quantum mechanics and a principal scientist in the German nuclear program during World War II. He pub ...
's
uncertainty relation The uncertainty principle, also known as Heisenberg's indeterminacy principle, is a fundamental concept in quantum mechanics. It states that there is a limit to the precision with which certain pairs of physical properties, such as position a ...
: Diminishing the quantum noise at a specific quadrature (phase) of the wave has as a direct consequence an enhancement of the noise of the
complementary Complement may refer to: The arts * Complement (music), an interval that, when added to another, spans an octave ** Aggregate complementation, the separation of pitch-class collections into complementary sets * Complementary color, in the visu ...
quadrature, that is, the field phase shifted by \pi/2. As can be seen in the illustrations, in contrast to a
coherent state In physics, specifically in quantum mechanics, a coherent state is the specific quantum state of the quantum harmonic oscillator, often described as a state that has dynamics most closely resembling the oscillatory behavior of a classical harmo ...
, the
quantum noise Quantum noise is noise arising from the indeterminate state of matter in accordance with fundamental principles of quantum mechanics, specifically the uncertainty principle and via zero-point energy fluctuations. Quantum noise is due to the appa ...
for a squeezed state is no longer independent of the phase of the
light wave In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength, ra ...
. A characteristic broadening and narrowing of the noise during one oscillation period can be observed. The probability distribution of a squeezed state is defined as the norm squared of the wave function mentioned in the last paragraph. It corresponds to the square of the electric (and magnetic) field strength of a classical light wave. The moving wave packets display an oscillatory motion combined with the widening and narrowing of their distribution: the "breathing" of the wave packet. For an amplitude-squeezed state, the most narrow distribution of the wave packet is reached at the field maximum, resulting in an amplitude that is defined more precisely than the one of a coherent state. For a phase-squeezed state, the most narrow distribution is reached at field zero, resulting in an average phase value that is better defined than the one of a coherent state. In phase space, quantum mechanical uncertainties can be depicted by the
Wigner quasi-probability distribution The Wigner quasiprobability distribution (also called the Wigner function or the Wigner–Ville distribution, after Eugene Wigner and Jean-André Ville) is a quasiprobability distribution. It was introduced by Eugene Wigner in 1932 to study q ...
. The intensity of the light wave, its coherent excitation, is given by the displacement of the Wigner distribution from the origin. A change in the phase of the squeezed quadrature results in a rotation of the distribution.


Photon number distributions and phase distributions

The squeezing angle, that is the phase with minimum quantum noise, has a large influence on the
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...
number distribution of the light wave and its
phase Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform *Phase space, a mathematica ...
distribution as well. For amplitude squeezed light the photon number distribution is usually narrower than the one of a coherent state of the same amplitude resulting in
sub-Poissonian In mathematics, a super-Poissonian distribution is a probability distribution that has a larger variance than a Poisson distribution with the same mean. Conversely, a sub-Poissonian distribution has a smaller variance. An example of super-Poissonia ...
light, whereas its phase distribution is wider. The opposite is true for the phase-squeezed light, which displays a large intensity (photon number) noise but a narrow phase distribution. Nevertheless, the statistics of amplitude squeezed light was not observed directly with photon number resolving detector due to experimental difficulty. For the squeezed vacuum state the photon number distribution displays odd-even-oscillations. This can be explained by the mathematical form of the
squeezing operator In quantum physics, the squeeze operator for a single mode of the electromagnetic field is :\hat(z) = \exp \left ( (z^* \hat^2 - z \hat^) \right ) , \qquad z = r \, e^ where the operators inside the exponential are the ladder operators. It is a ...
, that resembles the operator for two-photon generation and annihilation processes. Photons in a squeezed vacuum state are more likely to appear in pairs.


Classification


Based on the number of modes

Squeezed states of light are broadly classified into single-mode squeezed states and two-mode squeezed states, depending on the number of
modes Mode ( meaning "manner, tune, measure, due measure, rhythm, melody") may refer to: Arts and entertainment * MO''D''E (magazine), a defunct U.S. women's fashion magazine * ''Mode'' magazine, a fictional fashion magazine which is the setting fo ...
of the
electromagnetic field An electromagnetic field (also EM field) is a physical field, varying in space and time, that represents the electric and magnetic influences generated by and acting upon electric charges. The field at any point in space and time can be regarde ...
involved in the process. Recent studies have looked into multimode squeezed states showing quantum correlations among more than two modes as well.


Single-mode squeezed states

Single-mode squeezed states, as the name suggests, consists of a single mode of the electromagnetic field whose one quadrature has fluctuations below the shot noise level and the orthogonal quadrature has excess noise. Specifically, a single-mode squeezed ''vacuum'' (SMSV) state can be mathematically represented as, : , \text\rangle = S(\zeta), 0\rangle where the squeezing operator S is the same as introduced in the section on operator representations
above Above may refer to: *Above (artist) Tavar Zawacki (b. 1981, California) is a Polish, Portuguese - American abstract artist and internationally recognized visual artist based in Berlin, Germany. From 1996 to 2016, he created work under the ...
. In the photon number basis, writing \zeta = r e^ this can be expanded as, : , \text\rangle = \frac \sum_^\infty (- e^ \tanh r)^n \frac , 2n\rangle which explicitly shows that the pure SMSV consists entirely of even-photon
Fock state In quantum mechanics, a Fock state or number state is a quantum state that is an element of a Fock space with a well-defined number of particles (or quanta). These states are named after the Soviet physicist Vladimir Fock. Fock states play an im ...
superpositions. Single mode squeezed states are typically generated by degenerate parametric oscillation in an optical parametric oscillator, or using four-wave mixing.


Two-mode squeezed states

Two-mode squeezing involves two modes of the electromagnetic field which exhibit quantum noise reduction below the shot noise level in a linear combination of the quadratures of the two fields. For example, the field produced by a nondegenerate parametric oscillator above threshold shows squeezing in the amplitude difference quadrature. The first experimental demonstration of two-mode squeezing in optics was by Heidmann ''et al.''. More recently, two-mode squeezing was generated on-chip using a four-wave mixing OPO above threshold. Two-mode squeezing is often seen as a precursor to continuous-variable entanglement, and hence a demonstration of the Einstein-Podolsky-Rosen paradox in its original formulation in terms of continuous position and momentum observables. A two-mode squeezed vacuum (TMSV) state can be mathematically represented as, : , \text\rangle = S_2(\zeta), 0,0\rangle = \exp(\zeta^* \hat a \hat b - \zeta \hat a^\dagger \hat b^\dagger) , 0,0\rangle , and, writing down \zeta = r e^, in the photon number basis as, : , \text\rangle = \frac \sum_^\infty (-e^\tanh r)^n , nn\rangle If the individual modes of a TMSV are considered separately (i.e., , nn\rangle=, n\rangle_1 , n\rangle_2), then tracing over or absorbing one of the modes leaves the remaining mode in a thermal state :\begin\rho_1 &= \mathrm_2 optical parametric oscillator An optical parametric oscillator (OPO) is a parametric oscillator that oscillates at optical frequencies. It converts an input laser wave (called "pump") with frequency \omega_p into two output waves of lower frequency (\omega_s, \omega_i) by mea ...
operated below threshold produces squeezed vacuum, whereas the same OPO operated above threshold produces bright squeezed light. Bright squeezed light can be advantageous for certain quantum information processing applications as it obviates the need of sending
local oscillator In electronics, the term local oscillator (LO) refers to an electronic oscillator when used in conjunction with a Frequency mixer, mixer to change the frequency of a signal. This frequency conversion process, also called Heterodyne, heterodyning ...
to provide a phase reference, whereas squeezed vacuum is considered more suitable for quantum enhanced sensing applications. The AdLIGO and
GEO600 GEO600 is a gravitational wave detector located near Sarstedt, a town to the south of Hanover, Germany. It is designed and operated by scientists from the Max Planck Institute for Gravitational Physics, Max Planck Institute of Quantum Optics a ...
gravitational wave detectors use squeezed vacuum to achieve enhanced sensitivity beyond the standard quantum limit.


Atomic spin squeezing

For squeezing of two-level neutral atom ensembles it is useful to consider the atoms as spin-1/2 particles with corresponding
angular momentum operator In quantum mechanics, the angular momentum operator is one of several related operators analogous to classical angular momentum. The angular momentum operator plays a central role in the theory of atomic and molecular physics and other quantum pro ...
s defined as :J_v=\sum_^N j_v^ where v= and j_v^ is the single-spin operator in the v-direction. Here J_z will correspond to the population difference in the two level system, i.e. for an equal superposition of the up and down state J_z=0. The J_xJ_y plane represents the phase difference between the two states. This is also known as the
Bloch sphere In quantum mechanics and computing, the Bloch sphere is a geometrical representation of the pure state space of a two-level quantum mechanical system ( qubit), named after the physicist Felix Bloch. Mathematically each quantum mechanical syst ...
picture. We can then define uncertainty relations such as \Delta J_z \cdot \Delta J_y \geq \left, \Delta J_x\/2. For a coherent (unentangled) state, \Delta J_z=\Delta J_y=\sqrt/2. Squeezing is here considered the redistribution of uncertainty from one variable (typically J_z) to another (typically J_y). If we consider a state pointing in the J_x direction, we can define the Wineland criterion for squeezing, or the metrological enhancement of the squeezed state as :\chi^2=\left(\frac\frac\right)^2. This criterion has two factors, the first factor is the spin noise reduction, i.e. how much the quantum noise in J_z is reduced relative to the coherent (unentangled) state. The second factor is how much the coherence (the length of the Bloch vector, \left, J_x\) is reduced due to the squeezing procedure. Together these quantities tell you how much metrological enhancement the squeezing procedure gives. Here, metrological enhancement is the reduction in averaging time or atom number needed to make a measurement of a specific uncertainty. 20 dB of metrological enhancement means the same precision measurement can be made with 100 times fewer atoms or 100 times shorter averaging time.


Relation with the concept of quantum phase space

The concept of Quantum Phase Space (QPS) extends the notion of
phase space The phase space of a physical system is the set of all possible physical states of the system when described by a given parameterization. Each possible state corresponds uniquely to a point in the phase space. For mechanical systems, the p ...
from classical to
quantum physics Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Addison-Wesley, 1989, It is ...
by taking into account the
uncertainty principle The uncertainty principle, also known as Heisenberg's indeterminacy principle, is a fundamental concept in quantum mechanics. It states that there is a limit to the precision with which certain pairs of physical properties, such as position a ...
. The definition of the QPS is based on the introduction of joint momentum-coordinate quantum states denoted , \langle z \rangle\rangle which can be considered as some kind of squeezed coherent states. The expression of the wavefunction corresponding to a state , \langle z \rangle\rangle in coordinate representation isR.T. Ranaivoson et al : "''Invariant quadratic operators associated with linear canonical transformations and their eigenstates''"
J. Phys. Commun. 6 095010
arXiv:2008.10602 uant-ph (2022)
\varphi(x)=\langle x, \langle z\rangle\rangle =\frac in which : * \hbar is the reduced
Planck constant The Planck constant, or Planck's constant, denoted by h, is a fundamental physical constant of foundational importance in quantum mechanics: a photon's energy is equal to its frequency multiplied by the Planck constant, and the wavelength of a ...
* x and p are respectively the eigenvalues (possible values) of the coordinate operator X and the momentum operator P * \langle x\rangle , \langle p\rangle, A and B are respectively the mean values  and statistical variances of the coordinate and momentum corresponding to the quantum state , \langle z \rangle\rangle itself \begin \langle x\rangle = \langle \langle z \rangle, X, \langle z\rangle\rangle \\ \langle p\rangle = \langle \langle z \rangle, P, \langle z \rangle \rangle \\ A=\langle \langle z\rangle, (X-\langle x\rangle)^2, \langle z \rangle \rangle \\ B =\langle \langle z \rangle, (P-\langle p\rangle)^2, \langle z \rangle\rangle \end A state , \langle z \rangle\rangle saturates the uncertainty relation i.e. one has the following relation \sqrt \sqrt=\frac It can be shown that a state , \langle z \rangle\rangle is an eigenstate of the operator Z=P - \fracBX . The corresponding eigenvalue equation is Z, \langle z \rangle\rangle =\langle z \rangle, \langle z \rangle\rangle with \langle z \rangle = \langle p \rangle - \frac B\langle x\rangle It was also shown that the multidimensional generalization of the states  , \langle z \rangle\rangle are the basic quantum states which corresponds to wavefunctions that are covariants under the action of the
group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic iden ...
formed by multidimensional Linear Canonical Transformations. The quantum phase space (QPS) is defined as the set \ of all possible values of \langle z\rangle , or equivalently as the set \ of possible values of the pair (\langle p \rangle ,\langle x\rangle),  for a given value of the momentum statistical variance B. It follows from this definition that the structure of the quantum phase space depends explicitly on the value of the momentum statistical variance. It is this explicit dependence that makes this definition naturally compatible with the uncertainty principle. It can also be remarked here that, at thermodynamic equilibrium, the momentum statistical variance B can be related to thermodynamics parameters like temperature, pressure and volume shape and size.R. H. M. Ravelonjato et al (2023) "''Quantum and Relativistic corrections to Maxwell-Boltzmann ideal gas model from a Quantum Phase Space approach''":
''Found Phys'' 53, 88
arXiv:2302.13973 ond-mat.stat-mech/ref> At the classical limit, when the momentum and coordinate statistical variances are taken to be equal to zero (ignoring the uncertainty principle), the quantum phase space as defined previously is reduced to the classical phase space. There are more generalized squeezed coherent states, denoted , n,\langle z \rangle\rangle with n a positive integer, that are related to the concept of QPS and which do not saturate the uncertainty relation for n>0. These states can be deduced from to the states , \langle z \rangle\rangle using the following relation , n,\langle z \rangle\rangle =\frac fracn , \langle z \rangle\rangle The coordinate and momentum statistical variances, denoted respectively A_n and B_n, corresponding to a state , n,\langle z \rangle\rangle are \beginA_n =\langle n, \langle z \rangle, (X-\langle x \rangle)^2, n, \langle z \rangle\rangle =(2n+1)A \\B_n =\langle n, \langle z \rangle, (P-\langle p\rangle)^2, n,\langle z \rangle \rangle=(2n+1)B \end We then have the following relation \sqrt\sqrt=(2n+1)\frac\geq \frac This relation shows that a state , n,\langle z \rangle\rangle does not saturate the uncertainty relation for n>0 as said before.


Experimental realizations

There has been a whole variety of successful demonstrations of squeezed states. The first demonstrations were experiments with light fields using
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...
s and
non-linear optics In mathematics and science, a nonlinear system (or a non-linear system) is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathem ...
(see
optical parametric oscillator An optical parametric oscillator (OPO) is a parametric oscillator that oscillates at optical frequencies. It converts an input laser wave (called "pump") with frequency \omega_p into two output waves of lower frequency (\omega_s, \omega_i) by mea ...
). This is achieved by a simple process of four-wave mixing with a \chi^ crystal; similarly travelling wave phase-sensitive amplifiers generate spatially multimode quadrature-squeezed states of light when the \chi^ crystal is pumped in absence of any signal.
Sub-Poissonian In mathematics, a super-Poissonian distribution is a probability distribution that has a larger variance than a Poisson distribution with the same mean. Conversely, a sub-Poissonian distribution has a smaller variance. An example of super-Poissonia ...
current sources driving semiconductor laser diodes have led to amplitude squeezed light. Squeezed states have also been realized via motional states of an
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
in a trap,
phonon A phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, specifically in solids and some liquids. In the context of optically trapped objects, the quantized vibration mode can be defined a ...
states in
crystal lattice In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystal, crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that ...
s, and spin states in neutral
atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
ensembles. Much progress has been made on the creation and observation of spin squeezed states in ensembles of neutral atoms and ions, which can be used to enhancement measurements of time, accelerations, fields, and the current state of the art for measurement enhancement is 20 dB. Generation of spin squeezed states have been demonstrated using both coherent evolution of a coherent spin state and projective, coherence-preserving measurements. Even macroscopic oscillators were driven into classical motional states that were very similar to squeezed coherent states. Current state of the art in noise suppression, for laser radiation using squeezed light, amounts to 15 dB (as of 2016), which broke the previous record of 12.7 dB (2010).


Applications

Squeezed states of the light field can be used to enhance precision measurements. For example, phase-squeezed light can improve the phase read out of interferometric measurements (see for example
gravitational wave Gravitational waves are oscillations of the gravitational field that Wave propagation, travel through space at the speed of light; they are generated by the relative motion of gravity, gravitating masses. They were proposed by Oliver Heaviside i ...
s). Amplitude-squeezed light can improve the readout of very weak spectroscopic signals. Spin squeezed states of atoms can be used to improve the precision of
atomic clock An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...
s. This is an important problem in atomic clocks and other sensors that use small ensembles of cold atoms where the quantum projection noise represents a fundamental limitation to the precision of the sensor. Various squeezed coherent states, generalized to the case of many
degrees of freedom In many scientific fields, the degrees of freedom of a system is the number of parameters of the system that may vary independently. For example, a point in the plane has two degrees of freedom for translation: its two coordinates; a non-infinite ...
, are used in various calculations in
quantum field theory In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines Field theory (physics), field theory and the principle of relativity with ideas behind quantum mechanics. QFT is used in particle physics to construct phy ...
, for example
Unruh effect The Unruh effect (also known as the Fulling–Davies–Unruh effect) is a theoretical prediction in quantum field theory that an observer who is uniformly accelerating through empty space will perceive a thermal bath. This means that even in the ...
and
Hawking radiation Hawking radiation is black-body radiation released outside a black hole's event horizon due to quantum effects according to a model developed by Stephen Hawking in 1974. The radiation was not predicted by previous models which assumed that onc ...
, and generally, particle production in curved backgrounds and
Bogoliubov transformation In theoretical physics, the Bogoliubov transformation, also known as the Bogoliubov–Valatin transformation, was independently developed in 1958 by Nikolay Bogolyubov and John George Valatin for finding solutions of BCS theory in a homogeneous s ...
s. Recently, the use of squeezed states for
quantum information processing Quantum information science is a field that combines the principles of quantum mechanics with information theory to study the processing, analysis, and transmission of information. It covers both theoretical and experimental aspects of quantum phys ...
in the continuous variables (CV) regime has been increasing rapidly. Continuous variable quantum optics uses squeezing of light as an essential resource to realize CV protocols for quantum communication, unconditional quantum teleportation and one-way quantum computing. This is in contrast to quantum information processing with single photons or photon pairs as qubits. CV quantum information processing relies heavily on the fact that squeezing is intimately related to quantum entanglement, as the quadratures of a squeezed state exhibit sub-shot-noise quantum correlations.


See also

*
Negative energy Negative energy is a concept used in physics to explain the nature of certain fields, including the gravitational field and various quantum field effects. Gravitational energy Gravitational energy, or gravitational potential energy, is the po ...
*
Nonclassical light In optics, nonclassical light is light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as h ...
*
Optical phase space In quantum optics, an optical phase space is a phase space in which all quantum states of an optical system are described. Each point in the optical phase space corresponds to a unique state of an ''optical system''. For any such system, a plot of ...
*
Quantum optics Quantum optics is a branch of atomic, molecular, and optical physics and quantum chemistry that studies the behavior of photons (individual quanta of light). It includes the study of the particle-like properties of photons and their interaction ...
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Squeeze operator In quantum physics, the squeeze operator for a single mode of the electromagnetic field is :\hat(z) = \exp \left ( (z^* \hat^2 - z \hat^) \right ) , \qquad z = r \, e^ where the operators inside the exponential are the ladder operators. It is a ...


References


External links


Tutorial about quantum optics of the light field
{{DEFAULTSORT:Squeezed Coherent State Quantum optics Quantum states \mathrm \rangle \langle \mathrm , \ &= \frac \sum_^\infty \tanh^(r) , n \rangle \langle n, , \end with an effective average number of photons \widetilde = \sinh^2(r).


Based on the presence of a mean field

Squeezed states of light can be divided into squeezed vacuum and bright squeezed light, depending on the absence or presence of a non-zero mean field (also called a carrier), respectively. An
optical parametric oscillator An optical parametric oscillator (OPO) is a parametric oscillator that oscillates at optical frequencies. It converts an input laser wave (called "pump") with frequency \omega_p into two output waves of lower frequency (\omega_s, \omega_i) by mea ...
operated below threshold produces squeezed vacuum, whereas the same OPO operated above threshold produces bright squeezed light. Bright squeezed light can be advantageous for certain quantum information processing applications as it obviates the need of sending
local oscillator In electronics, the term local oscillator (LO) refers to an electronic oscillator when used in conjunction with a Frequency mixer, mixer to change the frequency of a signal. This frequency conversion process, also called Heterodyne, heterodyning ...
to provide a phase reference, whereas squeezed vacuum is considered more suitable for quantum enhanced sensing applications. The AdLIGO and
GEO600 GEO600 is a gravitational wave detector located near Sarstedt, a town to the south of Hanover, Germany. It is designed and operated by scientists from the Max Planck Institute for Gravitational Physics, Max Planck Institute of Quantum Optics a ...
gravitational wave detectors use squeezed vacuum to achieve enhanced sensitivity beyond the standard quantum limit.


Atomic spin squeezing

For squeezing of two-level neutral atom ensembles it is useful to consider the atoms as spin-1/2 particles with corresponding
angular momentum operator In quantum mechanics, the angular momentum operator is one of several related operators analogous to classical angular momentum. The angular momentum operator plays a central role in the theory of atomic and molecular physics and other quantum pro ...
s defined as :J_v=\sum_^N j_v^ where v= and j_v^ is the single-spin operator in the v-direction. Here J_z will correspond to the population difference in the two level system, i.e. for an equal superposition of the up and down state J_z=0. The J_xJ_y plane represents the phase difference between the two states. This is also known as the
Bloch sphere In quantum mechanics and computing, the Bloch sphere is a geometrical representation of the pure state space of a two-level quantum mechanical system ( qubit), named after the physicist Felix Bloch. Mathematically each quantum mechanical syst ...
picture. We can then define uncertainty relations such as \Delta J_z \cdot \Delta J_y \geq \left, \Delta J_x\/2. For a coherent (unentangled) state, \Delta J_z=\Delta J_y=\sqrt/2. Squeezing is here considered the redistribution of uncertainty from one variable (typically J_z) to another (typically J_y). If we consider a state pointing in the J_x direction, we can define the Wineland criterion for squeezing, or the metrological enhancement of the squeezed state as :\chi^2=\left(\frac\frac\right)^2. This criterion has two factors, the first factor is the spin noise reduction, i.e. how much the quantum noise in J_z is reduced relative to the coherent (unentangled) state. The second factor is how much the coherence (the length of the Bloch vector, \left, J_x\) is reduced due to the squeezing procedure. Together these quantities tell you how much metrological enhancement the squeezing procedure gives. Here, metrological enhancement is the reduction in averaging time or atom number needed to make a measurement of a specific uncertainty. 20 dB of metrological enhancement means the same precision measurement can be made with 100 times fewer atoms or 100 times shorter averaging time.


Relation with the concept of quantum phase space

The concept of Quantum Phase Space (QPS) extends the notion of
phase space The phase space of a physical system is the set of all possible physical states of the system when described by a given parameterization. Each possible state corresponds uniquely to a point in the phase space. For mechanical systems, the p ...
from classical to
quantum physics Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Addison-Wesley, 1989, It is ...
by taking into account the
uncertainty principle The uncertainty principle, also known as Heisenberg's indeterminacy principle, is a fundamental concept in quantum mechanics. It states that there is a limit to the precision with which certain pairs of physical properties, such as position a ...
. The definition of the QPS is based on the introduction of joint momentum-coordinate quantum states denoted , \langle z \rangle\rangle which can be considered as some kind of squeezed coherent states. The expression of the wavefunction corresponding to a state , \langle z \rangle\rangle in coordinate representation isR.T. Ranaivoson et al : "''Invariant quadratic operators associated with linear canonical transformations and their eigenstates''"
J. Phys. Commun. 6 095010
arXiv:2008.10602 uant-ph (2022)
\varphi(x)=\langle x, \langle z\rangle\rangle =\frac in which : * \hbar is the reduced
Planck constant The Planck constant, or Planck's constant, denoted by h, is a fundamental physical constant of foundational importance in quantum mechanics: a photon's energy is equal to its frequency multiplied by the Planck constant, and the wavelength of a ...
* x and p are respectively the eigenvalues (possible values) of the coordinate operator X and the momentum operator P * \langle x\rangle , \langle p\rangle, A and B are respectively the mean values  and statistical variances of the coordinate and momentum corresponding to the quantum state , \langle z \rangle\rangle itself \begin \langle x\rangle = \langle \langle z \rangle, X, \langle z\rangle\rangle \\ \langle p\rangle = \langle \langle z \rangle, P, \langle z \rangle \rangle \\ A=\langle \langle z\rangle, (X-\langle x\rangle)^2, \langle z \rangle \rangle \\ B =\langle \langle z \rangle, (P-\langle p\rangle)^2, \langle z \rangle\rangle \end A state , \langle z \rangle\rangle saturates the uncertainty relation i.e. one has the following relation \sqrt \sqrt=\frac It can be shown that a state , \langle z \rangle\rangle is an eigenstate of the operator Z=P - \fracBX . The corresponding eigenvalue equation is Z, \langle z \rangle\rangle =\langle z \rangle, \langle z \rangle\rangle with \langle z \rangle = \langle p \rangle - \frac B\langle x\rangle It was also shown that the multidimensional generalization of the states  , \langle z \rangle\rangle are the basic quantum states which corresponds to wavefunctions that are covariants under the action of the
group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic iden ...
formed by multidimensional Linear Canonical Transformations. The quantum phase space (QPS) is defined as the set \ of all possible values of \langle z\rangle , or equivalently as the set \ of possible values of the pair (\langle p \rangle ,\langle x\rangle),  for a given value of the momentum statistical variance B. It follows from this definition that the structure of the quantum phase space depends explicitly on the value of the momentum statistical variance. It is this explicit dependence that makes this definition naturally compatible with the uncertainty principle. It can also be remarked here that, at thermodynamic equilibrium, the momentum statistical variance B can be related to thermodynamics parameters like temperature, pressure and volume shape and size.R. H. M. Ravelonjato et al (2023) "''Quantum and Relativistic corrections to Maxwell-Boltzmann ideal gas model from a Quantum Phase Space approach''":
''Found Phys'' 53, 88
arXiv:2302.13973 ond-mat.stat-mech/ref> At the classical limit, when the momentum and coordinate statistical variances are taken to be equal to zero (ignoring the uncertainty principle), the quantum phase space as defined previously is reduced to the classical phase space. There are more generalized squeezed coherent states, denoted , n,\langle z \rangle\rangle with n a positive integer, that are related to the concept of QPS and which do not saturate the uncertainty relation for n>0. These states can be deduced from to the states , \langle z \rangle\rangle using the following relation , n,\langle z \rangle\rangle =\frac fracn , \langle z \rangle\rangle The coordinate and momentum statistical variances, denoted respectively A_n and B_n, corresponding to a state , n,\langle z \rangle\rangle are \beginA_n =\langle n, \langle z \rangle, (X-\langle x \rangle)^2, n, \langle z \rangle\rangle =(2n+1)A \\B_n =\langle n, \langle z \rangle, (P-\langle p\rangle)^2, n,\langle z \rangle \rangle=(2n+1)B \end We then have the following relation \sqrt\sqrt=(2n+1)\frac\geq \frac This relation shows that a state , n,\langle z \rangle\rangle does not saturate the uncertainty relation for n>0 as said before.


Experimental realizations

There has been a whole variety of successful demonstrations of squeezed states. The first demonstrations were experiments with light fields using
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...
s and
non-linear optics In mathematics and science, a nonlinear system (or a non-linear system) is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathem ...
(see
optical parametric oscillator An optical parametric oscillator (OPO) is a parametric oscillator that oscillates at optical frequencies. It converts an input laser wave (called "pump") with frequency \omega_p into two output waves of lower frequency (\omega_s, \omega_i) by mea ...
). This is achieved by a simple process of four-wave mixing with a \chi^ crystal; similarly travelling wave phase-sensitive amplifiers generate spatially multimode quadrature-squeezed states of light when the \chi^ crystal is pumped in absence of any signal.
Sub-Poissonian In mathematics, a super-Poissonian distribution is a probability distribution that has a larger variance than a Poisson distribution with the same mean. Conversely, a sub-Poissonian distribution has a smaller variance. An example of super-Poissonia ...
current sources driving semiconductor laser diodes have led to amplitude squeezed light. Squeezed states have also been realized via motional states of an
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
in a trap,
phonon A phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, specifically in solids and some liquids. In the context of optically trapped objects, the quantized vibration mode can be defined a ...
states in
crystal lattice In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystal, crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that ...
s, and spin states in neutral
atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
ensembles. Much progress has been made on the creation and observation of spin squeezed states in ensembles of neutral atoms and ions, which can be used to enhancement measurements of time, accelerations, fields, and the current state of the art for measurement enhancement is 20 dB. Generation of spin squeezed states have been demonstrated using both coherent evolution of a coherent spin state and projective, coherence-preserving measurements. Even macroscopic oscillators were driven into classical motional states that were very similar to squeezed coherent states. Current state of the art in noise suppression, for laser radiation using squeezed light, amounts to 15 dB (as of 2016), which broke the previous record of 12.7 dB (2010).


Applications

Squeezed states of the light field can be used to enhance precision measurements. For example, phase-squeezed light can improve the phase read out of interferometric measurements (see for example
gravitational wave Gravitational waves are oscillations of the gravitational field that Wave propagation, travel through space at the speed of light; they are generated by the relative motion of gravity, gravitating masses. They were proposed by Oliver Heaviside i ...
s). Amplitude-squeezed light can improve the readout of very weak spectroscopic signals. Spin squeezed states of atoms can be used to improve the precision of
atomic clock An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...
s. This is an important problem in atomic clocks and other sensors that use small ensembles of cold atoms where the quantum projection noise represents a fundamental limitation to the precision of the sensor. Various squeezed coherent states, generalized to the case of many
degrees of freedom In many scientific fields, the degrees of freedom of a system is the number of parameters of the system that may vary independently. For example, a point in the plane has two degrees of freedom for translation: its two coordinates; a non-infinite ...
, are used in various calculations in
quantum field theory In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines Field theory (physics), field theory and the principle of relativity with ideas behind quantum mechanics. QFT is used in particle physics to construct phy ...
, for example
Unruh effect The Unruh effect (also known as the Fulling–Davies–Unruh effect) is a theoretical prediction in quantum field theory that an observer who is uniformly accelerating through empty space will perceive a thermal bath. This means that even in the ...
and
Hawking radiation Hawking radiation is black-body radiation released outside a black hole's event horizon due to quantum effects according to a model developed by Stephen Hawking in 1974. The radiation was not predicted by previous models which assumed that onc ...
, and generally, particle production in curved backgrounds and
Bogoliubov transformation In theoretical physics, the Bogoliubov transformation, also known as the Bogoliubov–Valatin transformation, was independently developed in 1958 by Nikolay Bogolyubov and John George Valatin for finding solutions of BCS theory in a homogeneous s ...
s. Recently, the use of squeezed states for
quantum information processing Quantum information science is a field that combines the principles of quantum mechanics with information theory to study the processing, analysis, and transmission of information. It covers both theoretical and experimental aspects of quantum phys ...
in the continuous variables (CV) regime has been increasing rapidly. Continuous variable quantum optics uses squeezing of light as an essential resource to realize CV protocols for quantum communication, unconditional quantum teleportation and one-way quantum computing. This is in contrast to quantum information processing with single photons or photon pairs as qubits. CV quantum information processing relies heavily on the fact that squeezing is intimately related to quantum entanglement, as the quadratures of a squeezed state exhibit sub-shot-noise quantum correlations.


See also

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Negative energy Negative energy is a concept used in physics to explain the nature of certain fields, including the gravitational field and various quantum field effects. Gravitational energy Gravitational energy, or gravitational potential energy, is the po ...
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Nonclassical light In optics, nonclassical light is light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as h ...
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Optical phase space In quantum optics, an optical phase space is a phase space in which all quantum states of an optical system are described. Each point in the optical phase space corresponds to a unique state of an ''optical system''. For any such system, a plot of ...
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Quantum optics Quantum optics is a branch of atomic, molecular, and optical physics and quantum chemistry that studies the behavior of photons (individual quanta of light). It includes the study of the particle-like properties of photons and their interaction ...
*
Squeeze operator In quantum physics, the squeeze operator for a single mode of the electromagnetic field is :\hat(z) = \exp \left ( (z^* \hat^2 - z \hat^) \right ) , \qquad z = r \, e^ where the operators inside the exponential are the ladder operators. It is a ...


References


External links


Tutorial about quantum optics of the light field
{{DEFAULTSORT:Squeezed Coherent State Quantum optics Quantum states