Hari Prakash

Density operator for unpolarized radiation

Abstract The form of density operator for unpolarized radiation is obtained by a simple method.

Simultaneous squeezing of two orthogonal spin components

Squeezing of spin is usually discussed for x and y components (Sx and Sy), considering the commutation relation [Sx,Sy] = i SZ for the triad (Sx, Sy, SZ) of three mutually perpendicular spin components. We generalize and discuss squeezing of the spin component Sθ = Sxcosθ+Sysinθ considering all possible such triads, which include Sθ. For an explicit example involving the interaction of an assembly of two excited two-level atoms with single-mode coherent radiation, we find that Sθ and Sθ+π/2 can be squeezed simultaneously.

Squeezing in superposed coherent states

We study squeezing in the most general case of superposition of two coherent states by considering 〈ψ|(ΔXθ)2|ψ〉 where Xθ=X1cosθ+X2sinθ,X1+iX2=a is annihilation operator, θ is real, |ψ〉=Z1|α〉+Z2|β〉, |α〉 and |β〉 are coherent states and Z1,Z2,α,β are complex numbers. We find the absolute minimum value 0.11077 for infinite combinations with α−β=1.59912exp[±i(π/2)+iθ], Z1/Z2=exp(α∗β−αβ∗) with arbitrary values of α+β and θ. For this minimum value of 〈ψ|(ΔX0)2|ψ〉, the expectation value of photon number can vary from the minimum value 0.36084 (for α+β=0) to infinity. We note that the variation of 〈ψ|(ΔXθ)2|ψ〉 near the absolute minimum is less flat when the expectation value of photon number is larger. Thus squeezing can be observed at large intensities also, but settings of the parameters become more demanding.

Operator for Optical Polarization

For monochromatic light propagating along z -direction, we define a polarization operator which is product of inverse annihilation operator for the x -linearly polarized mode and the annihilation o...

An example of enhancement of a non-classical feature of a light beam by mixing with another classical light beam using a beam splitter

We present here an example where a non-classical feature of a light beam is enhanced simply by mixing with another classical coherent light beam using a beam splitter. This non-classical feature is amplitude-squared squeezing of a Gaussian light beam expressed by a negative value of or a negative value of the normalized quantity which can become more negative on enhancement. Here, , where and are the density, annihilation and creation operators, respectively.

Teleportation of a two-mode entangled coherent state encoded with two-qubit information

We propose a scheme to teleport a two-mode entangled coherent state encoded with two-qubit information, which is better than the two schemes recently proposed by Liao and Kuang (2007 J. Phys. B: At. Mol. Opt. Phys. 40 1183) and by Phien and Nguyen (2008 Phys. Lett. A 372 2825) in that our scheme gives higher value of minimum assured fidelity and minimum average fidelity without using any nonlinear interactions. For involved coherent states | ± α, minimum average fidelity in our case is ≥0.99 for |α| ≥ 1.6 (i.e. |α|2 ≥ 2.6), while previously proposed schemes referred above report the same for |α| ≥ 5 (i.e. |α|2 ≥ 25). Since it is very challenging to produce superposed coherent states of high coherent amplitude (|α|), our teleportation scheme is at the reach of modern technology.

SWAPPING BETWEEN TWO PAIRS OF NONORTHOGONAL ENTANGLED COHERENT STATES

We propose a scheme of swapping between two pairs of nonorthogonal entangled coherent states using a beam splitter, two phase shifters and photon counting measurements. We find that an almost perfect swapping is possible for an appreciable mean number of photons.

ORDINARY SQUEEZING AND AMPLITUDE-SQUARED SQUEEZING OF A SINGLE MODE COHERENT RADIATION IN INTERACTION WITH TWO TWO-LEVEL ATOMS

In the present paper, we consider the interaction of a single mode radiation initially in a coherent state with an assembly of two two-level atoms in some different states using the Hamiltonian, H = ω(a+a + Sz)+g(a S+ + a+ S-) in the natural units, where a+ and a are the creation and annihilation operators for radiation, Sz, S± are the collective Dicke operators, g is the coupling constant, ω is the energy of the photons and energy difference between the two atomic levels. We solve it exactly. We study ordinary squeezing and amplitude-squared squeezing of radiation for the general operators, and Yθ=1/2(a2e-iθ+a+2eiθ), when atoms are fully excited, super-radiant or in the ground state. We get a large ordinary squeezing (nearly 88% squeezing) for the super-radiant atomic state, at a time, which is 0.451 of the first revival time, and this should be easily observable. We also get a large amplitude-squared squeezing (nearly 20%) at the time, which is about 0.033 of the first revival time. Variation of variances near their minima with coupling time gt, square root of mean photon number |α| and phase are also discussed.

Change in coherence properties and degree of polarization of light propagating in a lossless isotropic nonlinear Kerr medium

The change in nature of light in propagation through a lossless isotropic nonlinear Kerr medium is studied. Considering a monochromatic unidirectional light beam, we see that coherent light remains coherent but the polarization state changes in general, and completely unpolarized light retains its nature in all respects. For a light beam having a coherent signal and unpolarized noise, a change in polarization state of the signal and depolarization occurs in general. For circular and plane polarized signals, the polarization states of the signal do not change and depolarization is absent for the former and maximum for the latter. Numerical calculations for a practical case indicate that these effects are observable.

Equivalence of second-order sub-Poissonian statistics and fourth-order squeezing for intense light

We study the simultaneous occurrence of second-order sub-Poissonian photon statistics and fourth-order squeezing for the operator Xθ = X1cosθ+X2sinθ, in the displaced state , where the Hermitian operators X1,2 are defined by X1+i X2 = a, the annihilation operator, D(α) = exp(αa+−α*a) is the displacement operator, α = |α|ei θ and is the state of the weak optical field. We show that the two phenomena occur simultaneously when the amplitude |α| of the displacement is very large as compared to the amplitude of the weak optical field. We also propose a balanced homodyne method for detection of fourth-order squeezed light in the same fashion as that for normal squeezing.