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Holger F. Hofmann

Publications and source records attributed to Holger F. Hofmann.

At least 163 records · Page 9Linked to original sources
arXiv2000

Fidelity and information in the quantum teleportation of continuous variables

Ideally, quantum teleportation should transfer a quantum state without distortion and without providing any information about that state. However, quantum teleportation of continuous electromagnetic field variables introduces additional noise, limiting the fidelity of the quantum state transfer. In this article, the operator describing the quantum state transfer is derived. The transfer operator modifies the probability amplitudes of the quantum state in a shifted photon number base by enhancing low photon numbers and suppressing high photon numbers. This modification of the statistical weight corresponds to a measurement of finite resolution performed on the original quantum state. The limited fidelity of quantum teleportation is thus shown to be a direct consequence of the information obtained in the measurement.

quant-ph↗
arXiv2000

Nonclassical correlations between photon number and quadrature components of the light field

Finite resolution quantum nondemolition (QND) measurements allow a determination of light field properties while preserving some of the original quantum coherence of the input state. It is thus possible to measure correlations between the photon number and a quadrature component of the same light field mode. Nonclassical features emerge as photon number quantization is resolved. In particular, a strong anti-correlation of quantization and coherence is observable in QND measurements of photon number, and a correlation between measurement induced quantum jumps and quadrature component measurement results is obtained in QND measurements of quadrature fluctuations in the photon vacuum. Such nonclassical correlations represent fundamental quantum properties of the light field and may provide new insights into the nature of quantization itself.

quant-ph↗
arXiv2000

Nonclassical correlations of photon number and field components in the vacuum state

It is shown that the quantum jumps in the photon number n from zero to one or more photons induced by backaction evasion quantum nondemolition measurements of a quadrature component x of the vacuum light field state are strongly correlated with the quadrature component measurement results. This correlation corresponds to the operator expectation value which is equal to one fourth for the vacuum even though the photon number eigenvalue is zero. Quantum nondemolition measurements of a quadrature component can thus provide experimental evidence of the nonclassical operator ordering dependence of the correlations between photon number and field components in the vacuum state.

quant-ph↗
arXiv2000

Information and noise in quantum measurement

Even though measurement results obtained in the real world are generally both noisy and continuous, quantum measurement theory tends to emphasize the ideal limit of perfect precision and quantized measurement results. In this article, a more general concept of noisy measurements is applied to investigate the role of quantum noise in the measurement process. In particular, it is shown that the effects of quantum noise can be separated from the effects of information obtained in the measurement. However, quantum noise is required to ``cover up'' negative probabilities arising as the quantum limit is approached. These negative probabilities represent fundamental quantum mechanical correlations between the measured variable and the variables affected by quantum noise.

quant-ph↗
arXiv2000

Correlations of measurement information and noise in quantum measurements with finite resolution

The original purpose of measurements is to provide us with information about a previously unknown physical property of the system observed. In the Hilbert space formalism of quantum mechanics, this physical meaning of measurement information is not immediately apparent. In order to study the relationship between the Hilbert space coherence of the quantum state and the measurement information obtained in the laboratory, we introduce a generalized measurement postulate for finite resolution measurements. With this measurement model, correlations between non-commuting observables can be investigated. These experimentally accessible correlations reveal nonclassical features in their dependence on the operator ordering, reflecting the particular measurement context by which they are determined.

quant-ph↗
arXiv1999

Nonclassical correlations of phase noise and photon number in quantum nondemolition measurements

The continuous transition from a low resolution quantum nondemolition measurement of light field intensity to a precise measurement of photon number is described using a generalized measurement postulate. In the intermediate regime, quantization appears as a weak modulation of measurement probability. In this regime, the measurement result is strongly correlated with the amount of phase decoherence introduced by the measurement interaction. In particular, the accidental observation of half integer photon numbers preserves phase coherence in the light field, while the accidental observation of quantized values increases decoherence. The quantum mechanical nature of this correlation is discussed and the implications for the general interpretation of quantization are considered.

quant-ph↗
arXiv1999

Thermal photon statistics in laser light above threshold

We show that the reduction in photon number fluctuations at laser threshold often cited as a fundamental laser property does not occur in typical semiconductor lasers such as the ones commonly used in modern technological applications today. Indeed, such lasers may still exhibit thermal intensity fluctuations far above threshold. We therefore conclude that sub-thermal intensity fluctuations cannot be regarded as a necessary property of laser light. Rather, one should distinguish betwen a thermal laser regime and a sub-thermal laser regime.

quant-ph↗
arXiv1998

Quantum Maxwell-Bloch equations for spontaneous emission in optical semiconductor devices

We present quantum Maxwell-Bloch equations (QMBE) for spatially inhomogeneous optical semiconductor devices taking into account the quantum noise effects which cause spontaneous emission and amplified spontaneous emission. Analytical expressions derived from the QMBE are presented for the spontaneous emission factor beta and the far field pattern of amplified spontaneous emission in broad area quantum well lasers.

quant-ph↗
arXiv1998

Quantum Maxwell-Bloch equations for spatially inhomogeneous semiconductor lasers

We present quantum Maxwell-Bloch equations (QMBE) for spatially inhomogeneous semiconductor laser devices. The QMBE are derived from fully quantum mechanical operator dynamics describing the interaction of the light field with the quantum states of the electrons and the holes near the band gap. By taking into account field-field correlations and field-dipole correlations, the QMBE include quantum noise effects which cause spontaneous emission and amplified spontaneous emission. In particular, the source of spontaneous emission is obtained by factorizing the dipole-dipole correlations into a product of electron and hole densities. The QMBE are formulated for general devices, for edge emitting lasers and for vertical cavity surface emitting lasers, providing a starting point for the detailed analysis of spatial coherence in the near field and far field patterns of such laser diodes. Analytical expressions are given for the spectra of gain and spontaneous emission described by the QMBE. These results are applied to the case of a broad area laser, for which the frequency and carrier density dependent spontaneous emission factor beta and the evolution of the far field pattern near threshold are derived.

physics.optics↗
arXiv1998

Spatial holeburning effects in the amplified spontaneous emission spectra of the non-lasing supermode in semiconductor laser arrays

The amplified spontaneous emission spectrum of the light field in the non-lasing supermode of two coupled semiconductor lasers is analyzed using linearized Langevin equations. It is shown that the interference betweeen the laser mode and the fluctuating light field in the non-lasing mode causes spatial holeburning. This effect introduces a phase sensitive coupling between the laser field and the fluctuations in the non-lasing mode. For high laser fields, this coupling splits the spectrum of the non-lasing mode into a triplet consisting of two relaxation oscillation sidebands which are in phase with the laser light and a center line at the lasing frequency with a phase shift of pi half relative to the laser light. As the laser intensity is increased close to threshold, the spectrum shows a continuous transition from the single amplified spontaneous emission line at the frequency of the laser mode to the triplet structure. An analytical expression for this transition is derived and typical features are discussed.

physics.optics↗
arXiv1998

Quantum control of atomic systems by time-resolved homodyne detection of spontaneous emission

We describe the light-matter interaction of a single two level atom with the electromagnetic vacuum in terms of field and dipole variables by considering homodyne detection of the emitted fields. Spontaneous emission is then observed as a continuous fluctuating force acting on the atomic dipole. The effect of this force may be compensated and even reversed by feedback.

quant-ph↗
arXiv1998

Quantum control of atomic systems by time resolved homodyne detection and feedback

We investigate the possibilities of preserving and manipulating the coherence of atomic two-level systems by ideal projective homodyne detection and feedback. For this purpose, the photon emission process is described on time scales much shorter than the lifetime of the excited state using a model based on Wigner-Weisskopf theory. The backaction of this emission process is analytically described as a quantum diffusion of the Bloch vector. It is shown that the evolution of the atomic wavefunction can be controlled completely using the results of homodyne detection. This allows the stabilization of a known quantum state or the creation of coherent states by a feedback mechanism. However, the feedback mechanism can never compensate the dissipative effects of quantum fluctuations even though the coherent state of the system is known at all times.

quant-ph↗
arXiv1998

Quantum control by compensation of quantum fluctuations

We show that the influence of quantum fluctuations in the electromagnetic field vacuum on a two level atom can be measured and consequently compensated by balanced homodyne detection and a coherent feedback field. This compensation suppresses the decoherence associated with spontaneous emission for a specific state of the atomic system allowing complete control of the coherent state of the system.

quant-ph↗
arXiv1997

Polarization fluctuations in vertical cavity surface emitting lasers: a key to the mechanism behind polarization stability

We investigate the effects of the electron-hole spin dynamics on the polarization fluctuations in the light emitted from a vertical cavity surface emitting laser (VCSEL). The Langevin equations are derived based on a rate equation model including birefringence, dichroism, and two carrier density pools seperately coupled to right and left circular polarization. The results show that the carrier dynamics phase lock the polarization fluctuations to the laser mode. This is clearly seen in the difference between fluctuations in ellipticity and fluctuations in polarization direction. Seperate measurements of the polarization fluctuations in ellipticity and in polarization direction can therefore provide quantitative information on the non-linear contribution of the carrier dynamics to polarization stability in VCSELs.

physics.optics↗
arXiv1997

Spontaneous emission spectrum of the non-lasing supermodes in semiconductor laser arrays

It is shown that the interference between spontaneous emission into the non-lasing supermode and the laser field of a semiconductor laser array causes spatial holeburning which couples the dynamics of the spontaneous emission with the laser field. In particular, phase locking between the spontaneous emission and the lasing mode leads to the formation of a spectral triplet composed of in-phase relaxation oscillation sidebands and an out-of-phase line at the lasing frequency.

physics.optics↗
arXiv1997

Decoherence in localized photon emission

The spatiotemporal dynamics of photon emission into a non-local farfield channel and two local nearfield channels from a pair of coupled two level systems is analysed using a model for emission based on Wigner-Weisskopf theory. The local quantum beats can be observed in the two nearfield channels. However, the presence of the farfield causes decoherence in the quantum beats even if only photon emissions into the near field channels are considered.

quant-ph↗
arXiv1997

A dynamical model of the chemical bond

A new approach to chemical bonding is introduced in order to provide an improved understanding of the connection between basic quantum mechanics and the covalent pair bond. It's focus is on the fact that the energy of the bond is largely given by the kinetic energy of the electrons, while the Coulomb forces are only comparable to the kinetic energy terms close to the atomic nuclei, where they define the shape and the size of the atomic orbitals. If atomic orbitals are used as a starting point, the kinetic energy operator is sufficient to determine the energy of the chemical bond. The simple mathematical structure of this operator allows a calculation of bond energy as a function of the distance between the atoms. For Gaussian wavefunctions, it is possible to calculate this bonding potential analytically, determining the bond length, the bond energy and the elasticity of the bond from only a single parameter, the width of the atomic wavefunction. It is shown that the results correspond surprisingly well with experimental values for diatomic molecules.

physics.chem-ph↗