Number Of Thermal Photons Research Articles

Temperature-independent Casimir-Polder forces in arbitrary geometries

We show that the Casimir-Polder potential of a particle in an energy eigenstate at nonretarded distance from a well-conducting body of arbitrary shape is independent of the environment temperature. This is true even when the thermal photon numbers at the relevant atomic transition energies are large. A compact expression is obtained for the temperature-independent potential, which can greatly simplify calculations in nontrivial geometries for experimentally relevant systems such as Rydberg atoms and polar molecules. We give criteria for the validity of our temperature-independent result. They are illustrated by numerical studies of a particle near a gold sphere or inside a gold cylindrical cavity.

Application of Channel Capacity Quantization in Optical Communication

quantization of channel capacity has significant value in information theory, and its concept is described in this paper. It is proved that channel capacity of quantum and classical communication theory has similar properties according to related definition. Channel capacity of optical communication is analyzed and studied. Then, the channel capacities with different parameters are calculated and exhibited. The relationship among the channel capacity C, average thermal photon numbers 〈M〉and average signal photon numbers K are displayed. As a result, by choosing the parameters reasonably, the signal communication efficiency is improved.

Entropy squeezing of a Jaynes–Cummings atom with the Glauber–Lachs state

In this paper, we investigate the entropy squeezing of a two-level atom interacting with a quantum field prepared initially in the Glauber–Lachs state by using the Jaynes–Cummings model. We explore the effects of increasing the intensity of the thermal field over a fixed intensity of the coherent field on the entropy squeezing of the atom. We show that increasing the number of thermal photons in the coherent field has a destructive effect on the entropy squeezing of the atom.

Quantum heat engine power can be increased by noise-induced coherence

Laser and photocell quantum heat engines (QHEs) are powered by thermal light and governed by the laws of quantum thermodynamics. To appreciate the deep connection between quantum mechanics and thermodynamics we need only recall that in 1901 Planck introduced the quantum of action to calculate the entropy of thermal light, and in 1905 Einstein's studies of the entropy of thermal light led him to introduce the photon. Then in 1917, he discovered stimulated emission by using detailed balance arguments. Half a century later, Scovil and Schulz-DuBois applied detailed balance ideas to show that maser photons were produced with Carnot quantum efficiency (see Fig. 1A). Furthermore, Shockley and Quiesser invoked detailed balance to obtain the efficiency of a photocell illuminated by "hot" thermal light (see Fig. 2A). To understand this detailed balance limit, we note that in the QHE, the incident light excites electrons, which can then deliver useful work to a load. However, the efficiency is limited by radiative recombination in which the excited electrons are returned to the ground state. But it has been proven that radiatively induced quantum coherence can break detailed balance and yield lasing without inversion. Here we show that noise-induced coherence enables us to break detailed balance and get more power out of a laser or photocell QHE. Surprisingly, this coherence can be induced by the same noisy (thermal) emission and absorption processes that drive the QHE (see Fig. 3A). Furthermore, this noise-induced coherence can be robust against environmental decoherence.Fig. 1.(A) Schematic of a laser pumped by hot photons at temperature T(h) (energy source, blue) and by cold photons at temperature T(c) (entropy sink, red). The laser emits photons (green) such that at threshold the laser photon energy and pump photon energy is related by Carnot efficiency (4). (B) Schematic of atoms inside the cavity. Lower level b is coupled to the excited states a and β. The laser power is governed by the average number of hot and cold thermal photons, and . (C) Same as B but lower b level is replaced by two states b(1) and b(2), which can double the power when there is coherence between the levels.Fig. 2.(A) Schematic of a photocell consisting of quantum dots sandwiched between p and n doped semiconductors. Open circuit voltage and solar photon energy ℏν(h) are related by the Carnot efficiency factor where T(c) is the ambient and T(h) is the solar temperature. (B) Schematic of a quantum dot solar cell in which state b is coupled to a via, e.g., solar radiation and coupled to the valence band reservoir state β via optical phonons. The electrons in conduction band reservoir state α pass to state β via an external circuit, which contains the load. (C) Same as B but lower level b is replaced by two states b(1) and b(2), and when coherently prepared can double the output power.Fig. 3.(A) Photocell current j = Γρ(αα) (laser photon flux P(l)/ℏ(ν(l))) (in arbitrary units) generated by the photovoltaic cell QHE (laser QHE) of Fig. 1C (Fig. 2C) as a function of maximum work (in electron volts) done by electron (laser photon) E(α) - E(β) + kT(c) log(ρ(αα)/ρ(ββ)) with full (red line), partial (brown line), and no quantum interference (blue line). (B) Power of a photocell of Fig. 2C as a function of voltage for different decoherence rates , 100γ(1c). Upper curve indicates power acquired from the sun.

ENTANGLING TWO MULTIATOM CLUSTERS VIA A SINGLE-MODE THERMAL FIELD

We consider a system consisting of two multiple two-level-atom clusters simultaneously interacting with a single-mode cavity field. Entanglement of the two clusters is investigated by use of the negativity measure. We find that the entanglement can be not only induced by a thermal field but also enhanced by increasing the mean thermal photon number.

NONCLASSICALITY AND DECOHERENCE OF PHOTON-ADDED THERMAL STATE

Using the normally ordered form of thermal state characteristic of average photon number nc, we introduce the photon-added thermal state (PATS) and investigate its statistical properties, such as Mandel's Q-parameter, photon number distribution (PND), and Wigner function (WF). We then study its decoherence in a thermal environment with average thermal photon number [Formula: see text] and dissipative coefficient κ by deriving analytical expressions of the WF. The nonclassicality is discussed in terms of the negativity of the WF. It is found that the WF is always positive when [Formula: see text] for any number PATS. The expression for time evolution of the PND and the tomogram of PATS are also derived analytically, which are related to hypergeometric function and single variable Hermite polynomials.

Cavity QED with an ultracold ensemble on a chip: Prospects for strong magnetic coupling at finite temperatures

We study the nonlinear dynamics of an ensemble of cold trapped atoms with a hyperfine transition magnetically coupled to a resonant microwave cavity mode. Despite the minute single atom coupling one obtains strong coupling between collective hyperfine qubits and microwave photons enabling coherent transfer of an excitation between the long lived atomic qubit state and the mode. Evidence of strong coupling can be obtained from the cavity transmission spectrum even at finite thermal photon number. The system makes it possible to study further prominent collective phenomena such as superradiant decay of an inverted ensemble or the building of a narrowband stripline micromaser locked to an atomic hyperfine transition.

Temperature-Independent Casimir-Polder Forces Despite Large Thermal Photon Numbers

We demonstrate that Casimir-Polder potentials can be entirely independent of temperature even when allowing for the relevant thermal photon numbers to become large. This statement holds for potentials that are due to low-energy transitions of a molecule placed near a plane metal surface whose plasma frequency is much larger than any atomic resonance frequencies. For a molecule in an energy eigenstate, the temperature independence is a consequence of strong cancellations between nonresonant potential components and those due to evanescent waves. For a molecule with a single dominant transition in a thermal state, upward and downward transitions combine to form a temperature-independent potential. The results are contrasted with the case of an atom whose potential exhibits a regime of linear temperature dependence. Contact with the Casimir force between a weakly dielectric and a metallic plate is made.

How colors influence numbers: Photon statistics of parametric down-conversion

Parametric downconversion (PDC) is a technique of ubiquitous experimental significance in the production of non-classical, photon-number correlated twin beams. Standard theory of PDC as a two-mode squeezing process predicts and homodyne measurements observe a thermal photon number distribution per beam. Recent experiments have obtained conflicting distributions. In this paper, we explain the observation by an a-priori theoretical model solely based on directly accessible physical quantities. We compare our predictions with experimental data and find excellent agreement.

Monitoring the quantum-classical transition in thermally seeded parametric down-conversion by intensity measurements

We address the pair of conjugated field modes obtained from parametric-downconversion as a convenient system to analyze the quantum-classical transition in the continuous variable regime. We explicitly evaluate intensity correlations, negativity and entanglement for the system in a thermal state and show that a hierarchy of nonclassicality thresholds naturally emerges in terms of thermal and downconversion photon number. We show that the transition from quantum to classical regime may be tuned by controlling the intensities of the seeds and detected by intensity measurements. Besides, we show that the thresholds are not affected by losses, which only modify the amount of nonclassicality. The multimode case is also analyzed in some detail.

Nonclassicality of photon-subtracted and photon-added-then-subtracted Gaussian states

By the use of nonclassical depth as a criterion for nonclassicality, we show that states generated by first adding (subtracting) multiphotons to an arbitrary state and then subtracting (adding) multiphotons from the resulting state is certainly nonclassical if the addition number of photons is equal to or larger than the subtraction number. The explicit expressions of nonclassical depth are found for both the single-photon-subtracted Gaussian state and the photon-added-then-subtracted thermal state. Based on the expressions, we show that the nonclassicality of the initial Gaussian state can be enhanced by the photon subtraction process. We also notice that the photon-added-then-subtracted thermal state is nonclassical if the initial thermal mean photon number is not zero, and the nonclassicality increases monotonously as the initial thermal mean photon number increases.

ENHANCING STATIONARY ENTANGLEMENT OF TWO QUBITS OR QUTRITS BY COLLECTIVELY INTERACTING WITH A COMMON THERMAL RESERVOIR

The stationary entanglement and Bell violation of two qubits collectively interacting with a common thermal reservoir is investigated, and the analytical relations among average thermal photon number of the reservoir, entanglement and Bell violation of two qubits are given in detail. The initial fraction of Bell singlet state determines whether the common thermal reservoir can enhance the entanglement of two qubits or not in this case. Moreover, the stationary maximally entangled mixed state can emerge in the collective decay of two qubits in a thermal reservoir at zero-temperature only if the fraction of Bell singlet state in the initial state is not smaller than 2/3. Furthermore, it is shown that the collective decay can induce the entanglement of two qutrits initially in the maximally mixed state, and induce distillable entanglement from bound entangled states.

Statistical properties of photon-subtracted squeezed vacuum in thermal environment

We investigate statistical properties of photon-subtracted squeezed vacuum (PSSV) in a thermal environment with average thermal photon number n¯ and dissipative coefficient κ by deriving the analytical expressions of the density operator and the Wigner function (WF). The normalization of m-PSSV is proved to be the m-order Lagendre polynomials. Time evolution of the photocount distribution is also related to Lagendre polynomials. The tomogram of PSSV is calculated by using the intermediate coordinate-momentum representation. Especially, the nonclassicality is discussed by virtue of the negativity of WF. It is found that the WF is always negative for all squeezing parameters λ if κt<½ ln(2n¯+2)/(2n¯+1) for the single-PSSV.

Entropy squeezing of a superconducting charge qubit in a single-mode thermal cavity field with intrinsic decoherence

By considering the intrinsic decoherence, we investigate the entropy squeezing in the system of a superconducting charge qubit resonantly coupled to a single-mode thermal field. We find that the entropy squeezing is sensitive to the thermal mean photon numbers and occurs only when the junction is initially prepared in its excited state and the cavity field in a weakly thermal state. It is also shown that the intrinsic decoherence has strongly destruction to entropy squeezing.

Probabilistic purification of noisy coherent states

(pp609-623) A basic feasible probabilistic purification of unknown noisy coherent states, outgoing from different state preparations with unknown mean number of thermal photons, is proposed. The scheme is based only on a linear-optical network with an avalanche photo-diode or heterodyne (homodyne) detection used to post-select a successful processing. The suggested probabilistic method can produce an output state with a lower noise than both quantum deterministic and classical probabilistic purification method. The purification applied in the state preparation can increase classical capacity of communication and security of quantum key distribution.

Performance enhancement of a decoy-state quantum key distribution using a conditionally prepared down-conversion source in the Poisson distribution

We study the behavior of decoy-state quantum key distribution systems using a conditionally prepared down-conversion source, analyzing such a source running in a mode of operation with either a thermal photon number distribution or a Poisson photon number distribution. By comparing both modes of operations to the three-intensity proposal of Wang et al., to the one-intensity proposal of Adachi et al., and to the currently demonstrated faint-laser pulse system, we demonstrate that a down-conversion source in a Poissonian mode of operation can largely enhance the performance of decoy-state quantum key distribution systems. This makes the down-conversion-based decoy-state system an interesting alternative to current technological implementations based on faint-laser pulses.

Superbroadcasting of harmonic oscillators mixed states

We consider the problem of broadcasting quantum information encoded in the displacement parameter for an harmonic oscillator, from N to M > N copies of a thermal state. We show the Weyl-Heisenberg covariant broadcasting map that optimally reduces the thermal photon number, and we prove that it minimizes the noise in conjugate quadratures at the output for general input states. We find that from two input copies broadcasting is feasible, with the possibility of simultaneous purification (superbroadcasting).

Cloning of Gaussian states by linear optics

We analyze in details a scheme for cloning of Gaussian states based on linear optical components and homodyne detection recently demonstrated by U. L. Andersen et al. [PRL 94 240503 (2005)]. The input-output fidelity is evaluated for a generic (pure or mixed) Gaussian state taking into account the effect of non-unit quantum efficiency and unbalanced mode-mixing. In addition, since in most quantum information protocols the covariance matrix of the set of input states is not perfectly known, we evaluate the average cloning fidelity for classes of Gaussian states with the degree of squeezing and the number of thermal photons being only partially known.

Coherent Control of Quantum Correlations in Atomic Systems

A new mechanism of development of quantum correlations due to the combined effects of a common heat bath and a resonant classical electromagnetic field is analyzed for a system of two two-level atoms coupled by dipole-dipole interaction. Conditions are found under which arbitrary steady-state entanglement can be created in the absence of measurement in an entangled basis. The analysis of dynamics of entanglement is focused on the effects of dipole-dipole interaction, mean number of thermal photons, degree of squeezing of thermal radiation, classical field strength, and presence of additional decay channels.

Addendum: Extracting an entangled state ofn−tqubits from ann-qubit entangled state after errors attsites

We present a detailed study of the method recently proposed by Yang and Gea-Banacloche [Phys. Rev. A 64, 032309 (2000)] to protect a maximally entangled state of n atomic qubits against spontaneous-emission-induced decoherence. We compare this to an approach based on nonradiative states, in the case where the number of thermal photons in the environment is nonzero.