Coherent state - Citizendia

In quantum mechanics a coherent state is a specific kind of quantum state of the quantum harmonic oscillator whose dynamics most closely resemble the oscillating behaviour of a classical harmonic oscillator system. Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons The quantum harmonic oscillator is the quantum mechanical analogue of the classical harmonic oscillator. It was the first example of quantum dynamics when Erwin Schrödinger derived it in 1926 while searching for solutions of the Schrödinger equation that satisfy the correspondence principle. Year 1926 ( MCMXXVI) was a Common year starting on Friday (link will display the full calendar of the Gregorian calendar. In Physics, especially Quantum mechanics, the Schrödinger equation is an equation that describes how the Quantum state of a Physical system This article discusses quantum theory For other uses see Correspondence principle (disambiguation. The quantum harmonic oscillator and hence, the coherent state, arise in the quantum theory of a wide range of physical systems. For instance, a coherent state describes the oscillating motion of the particle in a quadratic potential well. In the quantum theory of light (quantum electrodynamics) and other bosonic quantum field theories they were introduced by the work of Roy J. Glauber in 1963. Quantum electrodynamics ( QED) is a relativistic Quantum field theory of Electrodynamics. In Particle physics, bosons are particles which obey Bose-Einstein statistics; they are named after Satyendra Nath Bose and Albert Einstein In quantum field theory (QFT the forces between particles are mediated by other particles Roy Jay Glauber (born 1 September 1925) is an American theoretical Physicist. Here the coherent state of a field describes an oscillating field, the closest quantum state to a classical sinusoidal wave such as a continuous laser wave.

Figure 1: The electric field, measured by optical homodyne detection, as a function of phase for three coherent states emitted by a Nd:YAG laser. Homodyne detection is a method of detecting Frequency-modulated Radiation by non-linear mixing with radiation of a reference frequency, the same principle The amount of quantum noise in the electric field is completely independent of the phase. As the field strength, i. e. the oscillation amplitude α of the coherent state is increased, the quantum noise or uncertainty is constant at 1/2, and so becomes less and less significant. In the limit of large field the state becomes a good approximation of a noiseless stable classical wave. The average photon numbers of the three states from top to bottom are <n>=4. 2, 25. 2, 924. 5 (source: link 1 and ref. 2)

Figure 2: The oscillating wave packet corresponding to the second coherent state depicted in Figure 1. In physics a wave packet is an envelope or packet containing an arbitrary number of wave forms At each phase of the light field, the distribution is a Gaussian of constant width. Carl Friedrich Gauss (1777 &ndash 1855 is the Eponym of all of the topics listed below

Figure 3: Wigner function of the coherent state depicted in Figure 2. See also Wigner distribution, a disambiguation page The Wigner quasi-probability distribution (also called the Wigner function or the The distribution is centered on state's amplitude α and is symmetric around this point. The ripples are due to experimental errors.

1 Coherent states in quantum optics
2 Quantum mechanical definition
3 Mathematical characteristics
4 Coherent states of Bose–Einstein condensates
5 Coherent electron states in superconductivity
6 Generalizations
7 See also
8 External links
9 References

Coherent states in quantum optics

In quantum mechanics a coherent state is a specific kind of quantum state, applicable to the quantum harmonic oscillator, the electromagetic field, etc. Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons The quantum harmonic oscillator is the quantum mechanical analogue of the classical harmonic oscillator. ^[1] that describe a maximal kind of coherence and a classical kind of behavior. Erwin Schrödinger derived it as a mimimum uncertainty Gaussian wavepacket in 1926 while searching for solutions of the Schrödinger equation that satisfy the correspondence principle ^[2]. Year 1926 ( MCMXXVI) was a Common year starting on Friday (link will display the full calendar of the Gregorian calendar. This article discusses quantum theory For other uses see Correspondence principle (disambiguation. It is a minimum uncertainty state, with the single free parameter chosen to make the relative dispersion (standard deviation divided by the mean) equal for position and momentum, each being equally small at high energy. Further, while the expectation value of the Heisenberg equations of motion are zero for all energy eigenstates of the system, in a coherent state the expectation values of the equations of motion are precisely the classical equations of motion, and have small dispersion at high energy. (High energy is guaranteed when mean oscillatory amplitude and momentum have small classical values. ) The quantum linear harmonic oscillator and hence, the coherent state, arise in the quantum theory of a wide range of physical systems. They are found in the quantum theory of light (quantum electrodynamics) and other bosonic quantum field theories. Quantum electrodynamics ( QED) is a relativistic Quantum field theory of Electrodynamics. In Particle physics, bosons are particles which obey Bose-Einstein statistics; they are named after Satyendra Nath Bose and Albert Einstein In quantum field theory (QFT the forces between particles are mediated by other particles

While minimum uncertainty Gaussian wave-packets were well-known, they did not attract much attention until Roy J. Glauber, in 1963, provided a complete quantum-theoretic description of coherence in the electromagnetic field ^[3]. Roy Jay Glauber (born 1 September 1925) is an American theoretical Physicist. Glauber was prompted to do this to provide a description of the Hanbury-Brown & Twiss experiment that generated very wide baseline (hundreds or thousands of miles) interference patterns that could be used to determine stellar diameters. This opened the door to a much more comprehensive understanding of coherence. (For more, see Quantum mechanical description. )

In classical optics light is thought of as electromagnetic waves radiating from a source. Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. Often, coherent laser light is thought of as light that is emitted by many such sources that are in phase. Actually, the picture of one photon being in-phase with another is not valid in quantum theory. Laser radiation is produced in a resonant cavity where the resonant frequency of the cavity is the same as the frequency associated with the atomic transitions providing energy flow into the field. As energy in the resonant mode builds up, the probability for stimulated emission, in that mode only, increases. That is a positive feedback loop in which the amplitude in the resonant mode increases exponentially until some non-linear effects limit it. As a counter-example, a light bulb radiates light into a continuum of modes, and there is nothing that selects any one mode over the other. The incandescent light bulb, incandescent lamp or incandescent light globe is a source of electric Light that works by Incandescence, (a general The emission process is highly random in space and time (see thermal light). Thermal radiation is Electromagnetic radiation emitted from the surface of an object which is due to the object's Temperature. In a laser, however, light is emitted into a resonant mode, and that mode is highly coherent. A laser is a device that emits Light ( Electromagnetic radiation) through a process called Stimulated emission. Thus, laser light is idealized as a coherent state. (Classically we describe such a state by an electric field oscillating as a stable wave. See Fig. 1)

The energy eigenstates of the linear harmonic oscillator (e. g. , masses on springs, lattice vibrations in a solid, or oscillations in the electromagnetic field) are fixed-number quantum states. The Fock state (e. A Fock state, in Quantum mechanics, is any state of the Fock space with a well-defined number of particles in each state g. a single photon) is the most particle-like state; it has a fixed number of particles, and phase is indeterminate. A coherent state distributes its quantum-mechanical uncertainty equally between the canonically conjugate coordinates, position and momentum, and the relative uncertainty in phase [defined heuristically] and amplitude are roughly equal -- and small at high amplitude.

Quantum mechanical definition

Mathematically, the coherent state $|\alpha\rangle$ is defined to be the 'right' eigenstate of the annihilation operator $\hat a$ . In Physics, an annihilation operator is an Operator that lowers the number of particles in a given state by one Formally, this reads:

$\hat{a}|\alpha\rangle=\alpha|\alpha\rangle$

Since $\hat a$ is not hermitian, $α$ is complex, and can be represented as

$\alpha = |\alpha|e^{i\theta}~~~,$

where $~\theta~$ is real number. A number of Mathematical entities are named Hermitian, after the Mathematician Charles Hermite: Hermitian adjoint Here $~|\alpha|~$ and $~\theta~$ are called the amplitude and phase of the state.

Physically, this formula means that a coherent state is left unchanged by the detection (or annihilation) of a particle. The eigenstate of the annihilation operator has a Poissonian number distribution (as shown below). In Probability theory and Statistics, the Poisson distribution is a Discrete probability distribution that expresses the probability of a number of events A Poisson distribution is a necessary and sufficient condition that all detections are statistically independent. Compare this to a single-particle state (Fock state): once one particle is detected, there is zero probability of detecting another. A Fock state, in Quantum mechanics, is any state of the Fock space with a well-defined number of particles in each state

The derivation of this will make use of dimensionless quadratures, X and P. These quadratures are related to the position and momentum of the mass in a spring and mass oscillator:

$\mathbf{P}=\sqrt{\frac{1}{2\hbar m\omega }}\ \mathbf{p}\text{,}\quad \mathbf{X}=\sqrt{\frac{m\omega }{2\hbar }}\ \mathbf{x}\text{,}\quad \quad \text{where }\omega \equiv \sqrt{k/m}$ ,

For an optical field,

$~E_{\rm R} = \left(\frac{\hbar\omega}{2\epsilon_0 V} \right)^{1/2} \cos(\theta) X~$ and $~E_{\rm I} = \left(\frac{\hbar\omega}{2\epsilon_0 V}\right)^{1/2} \sin(\theta) X~$

are the real and imaginary components of the mode of the electric field. The optical field is a term used in physics and vector calculus to designate the electric field shown as E in the Electromagnetic wave equation which can be derived

With these quadratures, the Hamiltonian of either system becomes

$\mathbf{H}=\hbar \omega \left( \mathbf{P}^{2}+\mathbf{X}^{2} \right)\text{, with }\left[ \mathbf{X},\mathbf{P} \right]\equiv \mathbf{XP}-\mathbf{PX}=i/2$

Erwin Schrödinger was searching for the most classical-like states when he first introduced minimum uncertainty Gaussian wave-packets. The quantum state of the harmonic oscillator that minimizes the uncertainty relation with uncertainty equally distributed in both X and P quadratures satisfies the equation

$\left( \mathbf{P}-<\mathbf{P}> \right)\,|\alpha \rangle \ =i\left( \mathbf{X}-<\mathbf{X}> \right)\,|\alpha \rangle \text{,}\quad \text{or}\quad \left( \mathbf{P}-i\mathbf{X} \right)\,\left| \alpha \right\rangle \ =\ \left\langle \mathbf{P}-i\mathbf{X} \right\rangle \,\left| \alpha \right\rangle$ . In Quantum physics, a quantum state is a mathematical object that fully describes a quantum system. In Quantum physics, the Heisenberg uncertainty principle states that locating a particle in a small region of space makes the Momentum of the particle uncertain

It is an eigenstate of the operator (P - iX). (If the uncertainty is not balanced between X and P, the state is now called a squeezed coherent state. In Physics, a squeezed coherent state is any state of the Quantum mechanical Hilbert space such that the Uncertainty principle is saturated )

Schrodinger found minimum uncertainty states for the linear harmonic oscillator to be the eigenstates of (X - iP), and using the notation for multi-photon states, Glauber found the state of complete coherence to all orders in the electromagnetic field to be the right eigenstate of the annihilation operator -- formally, in a mathematical sense, the same state. The name "coherent state" took hold after Glauber's work.

The coherent state's location in the complex plane (phase space) is centered at the position and momentum of a classical oscillator of the same phase θ and amplitude (or the same complex electric field value for an electromagnetic wave). In Mathematics and Physics, a phase space, introduced by Willard Gibbs in 1901 is a Space in which all possible states of a System As shown in Figure 2, the uncertainty, equally spread in all directions, is represented by a disk with diameter 1/2. As the phase increases the coherent state circles the origin and the disk neither distorts nor spreads. This is the most similar a quantum state can be to a single point in phase space.

Since the uncertainty (and hence measurement noise) stays constant at 1/2 as the amplitude of the oscillation increases, the state behaves more and more like a sinusoidal wave, as shown in Figure 1. And, since the vacuum state $|0\rangle$ is just the coherent state with $α = 0$ , all coherent states have the same uncertainty as the vacuum. Therefore one can interpret the quantum noise of a coherent state as being due to the vacuum fluctuations.

(It should be noted that the notation $~|\alpha\rangle~$ does not refer to a Fock state. A Fock state, in Quantum mechanics, is any state of the Fock space with a well-defined number of particles in each state For example, at $α = 1$ , one should not mistake $~|1\rangle~$ as a single-photon Fock state -- it represents a Poisson distribution of fixed number states with a mean photon number of unity. )

The formal solution of the eigenvalue equation is the vacuum state displaced to a location $α$ in phase space, i. e. , is the displacement operator $D (α)$ operating on the vacuum:

$|\alpha\rangle=e^{\alpha \hat a^\dagger - \alpha^*\hat a}|0\rangle = D(\alpha)|0\rangle$

This can be easily seen, as can virtually all results involving coherent states, using the representation of the coherent state in the basis of Fock states:

$|\alpha\rangle=e^{-{|\alpha|^2\over2}}\sum_{n=0}^{\infty}{\alpha^n\over\sqrt{n!}}|n\rangle$ .

where $|n\rangle$ are energy (number) eigenvectors of the Hamiltonian. This is a Poissonian distribution. In Probability theory and Statistics, the Poisson distribution is a Discrete probability distribution that expresses the probability of a number of events The probability of detecting $~n~$ photons is:

$P(n)=e^{-\langle n \rangle}\frac{\langle n \rangle^n}{n!}$

Similarly, the average photon number in a coherent state is $~\langle n \rangle =\langle \hat a^\dagger \hat a \rangle =|\alpha|^2~$ and the variance is $~(\Delta n)^2={\rm Var}\left(\hat a^\dagger \hat a\right)= |\alpha|^2~$ .

Figure 4: The probability of detecting n photons, the photon number distribution, of the coherent state in Figure 3. As is necessary for a Poissonian distribution the mean photon number is equal to the variance of the photon number distribution. In Probability theory and Statistics, the Poisson distribution is a Discrete probability distribution that expresses the probability of a number of events In Probability theory and Statistics, the variance of a Random variable, Probability distribution, or sample is one measure of Bars refer to theory, dots to experimental values.

Figure 5: Phase space plot of a coherent state. This shows that the uncertainty in a coherent state is equally distributed in all directions. The horizontal and vertical axes are the X and P quadratures, respectively (see text). The red dots on the x-axis trace out the boundaries of the quantum noise in Figure 1.

In the limit of large α these detection statistics are equivalent to that of a classical stable wave for all (large) values of $~\alpha~$ . These results apply to detection results at a single detector and thus relate to first order coherence (see degree of coherence). In Optics, Correlation functions are used to characterize the statistical and coherence properties of an electromagnetic field However, for measurements correlating detections at multiple detectors, higher-order coherence is involved (e. g. , intensity correlations, second order coherence, at two detectors). Glauber's definition of quantum coherence involves nth-order correlation functions (n-th order coherence) for all n. The perfect coherent state has all n-orders of correlation equal to 1 (coherent). It is perfectly coherent to all orders.

Roy J. Glauber's work was prompted by the results of Hanbury-Brown and Twiss that produced long-range (hundreds or thousands of miles) first-order interference patterns through the use intensity fluctuations (lack of second order coherence), with narrow band filters (partial first order coherence) at each detector. Roy Jay Glauber (born 1 September 1925) is an American theoretical Physicist. (One can imagine, over very short durations, a near-instantaneous interference pattern from the two detectors, due to the narrow band filters, that dances around randomly due to the shifting relative phase difference. With a coincidence counter, the dancing interference pattern would be stronger at times of increased intensity [common to both beams], and that pattern would be stronger than the background noise. ) Almost all of optics had been concerned with first order coherence. The Hanbury-Brown and Twiss results prompted Glauber to look at higher order coherence, and he came up with a complete quantum-theoretic description of coherence to all orders in the electromagnetic field (and a quantum-theoretic description of signal-plus-noise). He coined the term "coherent state" and showed that they are produced when a classical electrical current interacts with the electromagnetic field. ^[4]

At $~\alpha \gg 1~$ , from Figure 5, simple geometry gives $\Delta\theta|\alpha|=\frac{1}{2}$ From this we can see that there is a tradeoff between number uncertainty and phase uncertainty $\Delta\theta~\Delta n=1/2$ , which sometimes can be interpreted as the number-phase uncertainty relation. This is not a formal uncertainty relation: there is no uniquely defined phase operator in quantum mechanics ^[5] ^[6]

Mathematical characteristics

The coherent state does not display all the nice mathematical features of a Fock state; for instance two different coherent states are not orthogonal:

$\langle\beta|\alpha\rangle=e^{-{1\over2}(|\beta|^2+|\alpha|^2-2\beta^*\alpha)}\neq\delta(\alpha-\beta)$

so that if the oscillator is in the quantum state |α> it is also with nonzero probability in the other quantum state |β> (but the farther apart the states are situated in phase space, the lower the probability is). A Fock state, in Quantum mechanics, is any state of the Fock space with a well-defined number of particles in each state However, since they obey a closure relation, any state can be decomposed on the set of coherent states. They hence form an overcomplete basis in which one can diagonally decompose any state. This is the premise for the Sudarshan-Glauber P representation. The Glauber-Sudarshan P-representation is a way of writing down the state of any type of Light using the Coherent states as a basis This closure relation can be expressed by the resolution of the identity:

$\frac{1}{\pi} \int |\alpha\rangle\langle\alpha| d^2\alpha = 1$ .

Another difficulty is that a^† has no eigenket (and a has no eigenbra). The following formal equality is the closest substitute and turns out to be very useful for technical computations:

$a^{\dagger}|\alpha\rangle=\left({\partial\over\partial\alpha}+{\alpha^*\over 2}\right)|\alpha\rangle$

The last state is known as Agarwal state denoted as $|\alpha,1\rangle.$ Agarwal states for order $n$ can be expressed as $|\alpha,n\rangle=\hat{a}^{\dagger~n}|\alpha\rangle$

Coherent states of Bose–Einstein condensates

A Bose–Einstein condensate (BEC) is a collection of boson atoms that are all in the same quantum state. A Bose–Einstein condensate (BEC is a State of matter of Bosons confined in an external Potential and cooled to Temperatures very near to In a thermodynamic system, the ground state becomes macroscopically occupied below a critical temperature — roughly when the thermal de Broglie wavelength is longer than the interatomic spacing. Superfluidity in liquid helium-4 is believed to be associated with the Einstein-Bose condensation in the ideal gas. But, ⁴He has strong interactions, and the liquid structure factor (a 2nd-order statistic) plays an important role. The use of a coherent state to represent the superfluid component of ⁴He provided a good estimate of the condensate / non-condensate fractions in superfluidity ^[7], consistent with results of slow neutron scattering. Most of the special superfluid properties follow directly from the use of a coherent state to represent the superfluid component — that acts as a macroscopically occupied single-body state with well-defined amplitude and phase over the entire volume. (The superfluid component of ⁴He goes from zero at the transition temperature to 100% at absolute zero. But the condensate fraction is less than 20%^[8] at absolute zero temperature. )
Early in the study of superfluidity, Penrose and Onsager ^[9] proposed a metric ("order parameter") for superfluidity. It was represented by a macroscopic factored component (a macroscopic eigenvalue) in the first-order reduced density matrix. Later, C. N. Yang ^[10] proposed a more generalized measure of macroscopic quantum coherence, called "Off-Diagonal Long-Range Order" (ODLRO), that included fermion as well as boson systems. ODLRO exists whenever there is a macroscopically large factored component (eigenvalue) in a reduced density matrix of any order. Superfluidity corresponds to a large factored component in the first-order reduced density matrix. (And, all higher order reduced density matrices behaved similarly. ) Superconductivity involves a large factored component in the 2nd-order ("Cooper electron-pair") reduced density matrix. In Condensed matter physics, a Cooper pair is the name given to electrons that are bound together at low temperatures in a certain manner first described in 1956 by
The orders of reduced density matrices used to describe macroscopic quantum coherence in superfluids are formally the same as the correlation functions used to describe orders of coherence in radiation. Both are examples of macroscopic quantum coherence. The macroscopically large coherent component, plus noise, in the electromagnetic field, as given by Glauber's description of signal-plus-noise, is formally the same as the macroscopically large superfluid component plus normal fluid component in the two-fluid model of superfluidity.
It should be noted that every-day electromagnetic radiation, such as radio and TV waves, is also an example of near coherent states (macroscopic quantum coherence). That should "give one pause" regarding the conventional demarcation between quantum and classical.

Coherent electron states in superconductivity

Electrons are Fermions, but when they pair up into Cooper pairs they act as bosons, and so can collectively form a coherent state at low temperatures. In Condensed matter physics, a Cooper pair is the name given to electrons that are bound together at low temperatures in a certain manner first described in 1956 by Such coherent states are part of the explanation of effects such as the Quantum Hall effect in low-temperature superconducting semiconductors. The quantum Hall effect (or integer quantum Hall effect) is a quantum-mechanical version of the Hall effect, observed in two-dimensional electron systems

Generalizations

In quantum field theory and string theory, a generalization of coherent states to the case of infinitely many degrees of freedom is used to define a vacuum state with a different vacuum expectation value from the original vacuum. In quantum field theory (QFT the forces between particles are mediated by other particles String theory is a still-developing scientific approach to Theoretical physics, whose original building blocks are one-dimensional extended objects called strings For information on degrees of freedom in other sciences see Degrees of freedom. In Quantum field theory, the vacuum state (also called the vacuum) is the Quantum state with the lowest possible Energy. In Quantum field theory the vacuum expectation value (also called condensate) of an operator is its average Expected value in the vacuum

In one-dimensional many-body quantum systems with fermionic degrees of freedom, low energy excited states can be approximated as coherent states of a bosonic field operator that creates particle-hole excitations. This approach is called bosonization.

External links

References

^ J. In quantum field theory (QFT the forces between particles are mediated by other particles Quantum optics is a field of research in Physics, dealing with the application of Quantum mechanics to phenomena involving Light and its interactions The electromagnetic field is a physical field produced by electrically charged objects. In Optics, Correlation functions are used to characterize the statistical and coherence properties of an electromagnetic field In Physics, the term quantum amplifier may refer to any device which use some non-classical (quantum effects for amplification of signal R. Klauder and B. Skagerstam, "Coherent States", World Scientific Publishing Co. Pte. Ltd. , Singapore, 1985.
^ E. Schrödinger, Naturwissenschaften 14 (1926) 664.
^ R. J. Glauber, Phys. Rev. 131 (1963) 2766.
^ Erroneous reference to "entangled states" removed. Superpostion and entanglement are not the same thing.
^ Susskind and Glowgower, Phys. Letters (1963)
^ Carruthers and Nieto, Rev. Mod. Phys. (1968)
^ F. W. Cummings
^ need actual number
^ Phys. Rev. 104, 576 (1956)
^ Rev. Mod Phys. 34, 694 (1962)

Loudon, Rodney, The Quantum Theory of Light (Oxford University Press, 2000), [ISBN 0-19-850177-3]
G. Breitenbach, S. Schiller, and J. Mlynek, "Measurement of the quantum states of squeezed light", Nature, 387, 471 (1997)

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