Quantum Vacuum Field Research Articles

An observer's perspective of the Unruh and Hawking effects—Using coherent signals to extract information from a black hole

The Unruh effect is one of the first calculations that explain what one would see when transiting between an inertial reference frame in its quantum field vacuum state and a non-inertial (specifically, uniformly accelerating) reference frame. The inertial reference frame's vacuum state would not correspond to the vacuum state of the non-inertial frame. The observer in the non-inertial frame would see radiation, with a corresponding Bose distribution and a temperature proportional to the acceleration. In this paper, I compute the response of this non-inertial observer to a single frequency mode in the inertial frame and deduce that, indeed, the cumulative distribution (over the observer's proper time) of frequencies observed by the accelerating observer would be the Bose distribution with a temperature proportional to the acceleration. The conclusion is that the Unruh effect (and the related Hawking effect) is generic, in that it would appear with any incoming incoherent state and the Bose distribution is obtained as a consequence of the non-inertial frame's motion, rather than some special property of the quantum vacuum. As a consequence of the analysis of a coherent set of signals, I show how to extract information from the spectrum that an accelerated observer would see (as well as from the radiation from a black hole).

Quantum vacuum, rotation, and nonlinear fields

In this paper, we extend previous results on the quantum vacuum or Casimir energy, for a noninteracting rotating system and for an interacting nonrotating system, to the case where both rotation and interactions are present. Concretely, we first reconsider the noninteracting rotating case of a scalar field theory and propose an alternative and simpler method to compute the Casimir energy based on a replica trick and the Coleman-Weinberg effective potential. We then consider the simultaneous effect of rotation and interactions, including an explicit breaking of rotational symmetry. To study this problem, we develop a numerical implementation of zeta-function regularization. Our work recovers previous results as limiting cases and shows that the simultaneous inclusion of rotation and interactions produces nontrivial changes in the quantum vacuum energy. Besides expected changes (where, as the size of the ring increases for fixed interaction strength, the angular momentum grows with the angular velocity), we notice that the way rotation combines with the coupling constant amplifies the intensity of the interaction strength. Interestingly, we also observe a departure from the typical massless behavior where the Casimir energy is proportional to the inverse size of the ring.

Shot-noise-suppressing quantum nonlinearity in the vacuum-field-induced photon-photon interaction

The shot noise in coherent control of a quantum system sets an ultimate precision limit in measurement and information processing. It is caused by the fundamental quantum fluctuation of the photon number of a coherent control field and thus is inevitable. By strongly coupling a two-level system to a quantum vacuum field, we propose an effective $N$-type four-level system. In such a configuration, the coupling of a conventional $N$-type system to a coherent control field is replaced by the quantum vacuum field. As a result, we can achieve a shot-noise-suppressing quantum nonlinearity, due to the vanishing fluctuation of photon number of the quantum vacuum field. The resulting photon blockade effect is only subject to the contribution from the probe field. Thus, it becomes shot-noise suppressing. Our work shows an advantage of a quantum vacuum protocol over a coherent-field protocol in manipulation of quantum systems and presents an opportunity to exploit the quantum vacuum field as a powerful tool in quantum information processing.

New Insights into the Lamb Shift: The Spectral Density of the Shift

In an atom, the interaction of a bound electron with the vacuum fluctuations of the electromagnetic field leads to complex shifts in the energy levels of the electron, with the real part of the shift corresponding to a shift in the energy level and the imaginary part to the width of the energy level. The most celebrated radiative shift is the Lamb shift between the 2s1/2 and the 2p1/2 levels of the hydrogen atom. The measurement of this shift in 1947 by Willis Lamb Jr. proved that the prediction by Dirac theory that the energy levels were degenerate was incorrect. Hans Bethe’s non-relativistic calculation of the shift using second-order perturbation theory demonstrated the renormalization process required to deal with the divergences plaguing the existing theories and led to the understanding that it was essential for theory to include interactions with the zero-point quantum vacuum field. This was the birth of modern quantum electrodynamics (QED). Numerous calculations of the Lamb shift followed including relativistic and covariant calculations, all of which contain a nonrelativistic contribution equal to that computed by Bethe. The semi-quantitative models for the radiative shift of Welton and Power, which were developed in an effort to demonstrate physical mechanisms by which vacuum fluctuations lead to the shift, are also considered here. This paper describes a calculation of the shift using a group theoretical approach which gives the shift as an integral over frequency of a function, which is called the “spectral density of the shift.“ The energy shift computed by group theory is equivalent to that derived by Bethe yet, unlike in other calculations of the non-relativistic radiative shift, no sum over a complete set of states is required. The spectral density, which is obtained by a relatively simple computation, reveals how different frequencies of vacuum fluctuations contribute to the total energy shift. The analysis shows, for example, that half the radiative shift for the ground state 1S level in H comes from virtual photon energies below 9700 eV, and that the expressions of Power and Welton have the correct high-frequency behavior, but not the correct low-frequency behavior, although they do give approximately the correct value for the total shift.

Resonant metasurfaces for generating complex quantum states.

Quantum state engineering, the cornerstone of quantum photonic technologies, mainly relies on spontaneous parametric downconversion and four-wave mixing, where one or two pump photons spontaneously decay into a photon pair. Both of these nonlinear effects require momentum conservation for the participating photons, which strongly limits the versatility of the resulting quantum states. Nonlinear metasurfaces have subwavelength thickness and allow the relaxation of this constraint; when combined with resonances, they greatly expand the possibilities of quantum state engineering. Here, we generated entangled photons via spontaneous parametric downconversion in semiconductor metasurfaces with high-quality factor, quasi-bound state in the continuum resonances. By enhancing the quantum vacuum field, our metasurfaces boost the emission of nondegenerate entangled photons within multiple narrow resonance bands and over a wide spectral range. A single resonance or several resonances in the same sample, pumped at multiple wavelengths, can generate multifrequency quantum states, including cluster states. These features reveal metasurfaces as versatile sources of complex states for quantum information.

Detection of quantum-vacuum field correlations outside the light cone

According to quantum field theory, empty space—the ground state with all real excitations removed—is not empty, but filled with quantum-vacuum fluctuations. Their presence can manifest itself through phenomena such as the Casimir force, spontaneous emission, or dispersion forces. These fluctuating fields possess correlations between space-time points outside the light cone, i.e. points causally disconnected according to special relativity. As a consequence, two initially uncorrelated quantum objects in empty space which are located in causally disconnected space-time regions, and therefore unable to exchange information, can become correlated. Here, we have experimentally demonstrated the existence of correlations of the vacuum fields for non-causally connected space-time points by using electro-optic sampling. This result is obtained by detecting vacuum-induced correlations between two 195 fs laser pulses separated by a time of flight of 470 fs. This work marks a first step in analyzing the space-time structure of vacuum correlations in quantum field theory.

Energy Basic State Field of the Universe (I)

This paper proposes the hypothesis that cosmic vacuum is full of energy basic state field (EBSF), and expounds its physical connotation based on cosmological constant of general relativity and Dirac’s negative energy sea. Cosmic vacuum is a special kind of physical object with complexity that can be characterized by quantum super-fluidity; it forms Dark energy in Universe. The rationality and correctness of this hypothesis are demonstrated through the analysis in terms of energy basic distribution on the background of cosmic scale and energy scale, quantum vacuum field, the evolution of EBSF state into static quantum’s state (particles or quasiparticle) and so on. Also it estimates the vacuum energy value in the energy basic state field to the same order of energy as the energy value for driving the accelerated expansion of the universe.

Enhanced Electro-optic Sampling with Quantum Probes.

Employing electro-optic sampling (EOS) with ultrashort probe pulses, recent experiments showed direct measurements of quantum vacuum fields and their correlations on subcycle timescales. Here, we propose a quantum-enhanced EOS where bright photon-number entangled twin beams are used to derive conditioned nonclassical probes. In the case of the quantum vacuum, this leads to a sixfold improvement in the signal-to-noise ratio over the classically probed EOS. In addition, engineering of the conditioning protocol yields a reliable way to extract higher-order moments of the quantum noise distribution and robust discrimination of the input quantum states, for instance, a vacuum and a few-photon cat state. These improvements open a viable route toward robust tomography of quantum fields in space-time, an equivalent of homodyne detection in energy-momentum space, and the possibility of precise experiments in real-space quantum electrodynamics.

Quantum Vacuum Gravitation Matter-Antimatter Antigravity

Without stating any assumptions or making postulates we show that the electromagnetic quantum vacuum plays a primary role in quantum electrodynamics, particle physics, gravitation and cosmology. Photons are local oscillations of the electromagnetic quantum vacuum field guided by a non-local vector potential wave function. The electron-positron elementary charge emerges naturally from the vacuum field and is related to the photon vector potential. We establish the masse-charge equivalence relation showing that the masses of all particles (leptons, mesons, baryons) and antiparticles have electromagnetic origin. In addition, we deduce that the gravitational constant G is an intrinsic property of the electromagnetic quantum vacuum putting in evidence the electromagnetic nature of gravity. We show that Newton’s gravitational law is equivalent to Coulomb’s electrostatic law. Furthermore, we draw that G is the same for matter and antimatter but gravitational forces could be repulsive between particles and antiparticles because their masses bear naturally opposite signs. The electromagnetic quantum vacuum field may be the natural link between particle physics, quantum electrodynamics, gravitation and cosmology constituting a basic step towards a unified field theory.

A Comparative Investigation of the Intrinsic Mobility of the Natural Body in the Mulla Sadra’s Philosophy with the Continuous Evolution of Quantum “Particles”

In quantum theory, objects (particles) are intrinsically evolving and changing based on the Uncertainty Principle, and the particle state is described by a time-dependent wave packet such that the wave packet corresponding to even a free particle is successively evolvable; a quantum particle has an indefinite and transformative state. In the relativistic Quantum Fields Theory, instead of the concepts of particle and wave packet, a continuous object named “field” with permanent fluctuation in its ground state (the quantum vacuum) is considered as the original concept. In Mulla Sadra’s philosophy, based on the Substantial Motion Theory, the subject of continuous evolution and change in the natural body appears as the intrinsic mobility of the natural body. In this paper, we are going to present a comparative investigation of this topic in the two areas of study mentioned. Initially, we present some necessary topics in physics including the motion of fundamental particles, the motion of quantum objects (quantum non-localizability), Quantum Fields Theory, quantum vacuum, and the creation and annihilation of particles in the Quantum Fields Theory; then, we consider some necessary topics in some fields of study in philosophy about the natural body and the nature and the identity of physical bodies including the ideas and the special opinion of Mulla Sadra. Particularly and with more detail, we pay attention to the Sadrian philosophy about the evolution and intrinsic mobility of the natural body based on his famous Substantial Motion Theory. With a comparative study, and by considering Sadrian philosophy as the fundamental philosophical basis for natural sciences, particularly physics, we try to compare the intrinsic motion of natural bodies in Mulla Sadra’s philosophy with the intrinsic evolution of quantum objects in modern physics. Our research method is based on and uses standard resources including books and research papers about Mulla Sadra’s thoughts and theories and the internationally well-known books and papers on standard quantum physics and quantum fields theory. The conclusive remarks are as follows: The transformation of quantum particles or the creation and annihilation of particles in the theory of quantum fields agrees with this point that the intrinsic mobility of the natural body in Mulla Sadra’s philosophy doesn’t reason on the philosophical existence and/or loss of existence. That a quantum particle doesn’t have a constant status in two consecutive instants is in agreement with the Sadrian Substantial Motion Theory that explains ‘motion’ in the ‘bed’ of time. Finally, we should mention that although the intrinsic motion of the natural body in Mulla Sadra’s philosophy is based on its completion while the creation and annihilation of particles in Quantum Fields Theory may be better compared with the emergence and extinction well known in peripatetic philosophy, by considering the background quantum vacuum field as the main basis of the so-called “particles” which is continuously in evolution and permanent fluctuation and is the basis for the creation and annihilation of particles, the comparison between the modern Quantum Fields Theory and Mulla Sadra’s philosophy is done well.

Motion induced by asymmetric excitation of the quantum vacuum

During the last fifty-one years, the effect of excitation of the quantum vacuum field induced by its coupling with a moving object has been systematically studied. Here, we propose and investigate a somewhat inverted setting: an object, initially at rest, whose motion becomes induced by an excitation of the quantum vacuum caused by the object itself. In the present model, this excitation occurs asymmetrically on different sides of the object by a variation in time of one of its characteristic parameters, which couple it with the quantum vacuum field.

Local Model of Entangled Photon Experiments Compatible with Quantum Predictions Based on the Reality of the Vacuum Fields

Arguments are provided for the reality of the quantum vacuum fields. A polarization correlation experiment with two maximally entangled photons created by spontaneous parametric down-conversion is studied in the Weyl-Wigner formalism, that reproduces the quantum predictions. An interpretation is proposed in terms of stochastic processes assuming that the quantum vacuum fields are real. This proves that local realism is compatible with a violation of Bell inequalities, thus rebutting the claim that it has been refuted by experiments. Entanglement appears as a correlation between fluctuations of a signal field and vacuum fields. Key words; local realism, Bell inequalities, entangled photons, Weyl-Wigner, loopholes,vacuum fields

Entangling Superconducting Qubits through an Analogue Wormhole

We propose an experimental setup to test the effect of curved spacetime upon the extraction of entanglement from the quantum field vacuum to a pair of two-level systems. We consider two superconducting qubits coupled to a dc-SQUID array embedded into an open microwave transmission line, where an external bias can emulate a spacetime containing a traversable wormhole. We find that the amount of vacuum entanglement that can be extracted by the qubits depends on the wormhole parameters. At some distances qubits which would be in a separable state in flat spacetime would become entangled due to the analogue wormhole background.

Local Realistic Interpretation of Entangled Photon Pairs in the Weyl-Wigner Formalism

A polarization correlation experiment with two maximally entangled photons created by spontaneous parametric down-conversion is studied in the Weyl-Wigner formalism, that reproduces the quantum predictions. An interpretation is proposed in terms of stochastic processes assuming that the quantum vacuum fields are real. This proves that local realism is compatible with the violation of Bell inequalities, thus rebutting the claim that local realism has been refuted by entangled photon experiments. Entanglement appears as a correlation between fluctuations of a signal field and vacuum fields.

Quantum correlation between optical and terahertz photons generated under multimode spontaneous parametric down-conversion

We study conditions for the generation of strongly correlated photons of two extremely different frequency ranges, optical and terahertz, via the spontaneous parametric down-conversion effect in a nonlinear medium. Analytical expressions for the correlation function are derived using equations of the generalized Kirchhoff law for the second-order field moments with account of both classical thermal and quantum vacuum field fluctuations, possible inherent absorption of the medium at terahertz frequencies, and angular divergences of signal and idler radiation caused by a transverse limited spatial profile of the Gaussian pump beam. Associated with the current absence of terahertz photon counting detectors and coincidence schemes, the possible experimental limitations in measuring the second-order optical-terahertz field correlations are studied. Numerical calculations of noncollinear eee-geometry parametric down-conversion in a $\mathrm{Mg}:\mathrm{LiNb}{\mathrm{O}}_{3}$ crystal demonstrate angular characteristics, particular features of the temperature and spectral behavior of the second-order quantum correlation function of optical-terahertz biphotons.

Radiation properties of an oscillating atom in the presence of external fields

In the first part of the present work, the correction to the photon emission rate of an oscillating two-level atom in the presence of an electromagnetic quantum vacuum field has been investigated for two different configurations: (i) an atom is trapped in the vicinity of a perfect conductor and (ii) an atom is trapped between two perfect conductors. In the second part, the correction to the decay rate of an initially excited oscillating two-level atom due to the presence of a perfectly conducting surface is found.

Axion–photon mixing in quantum field theory and vacuum energy

We analyze axion–photon mixing in the framework of quantum field theory. The condensate structure of the vacuum for mixed fields induces corrections to the oscillation formulae and leads to non-zero energy of the vacuum for the component of the photon mixed with the axion. This energy generates a new effect of the vacuum polarization and it has the state equation of the cosmological constant, w=−1. This result holds for any homogeneous and isotropic curved space–time, as well as for diagonal metrics. Numerical estimates of the corrections to the oscillation formulae are presented by considering the intensity of the magnetic field available in the laboratory. Moreover, we estimate the vacuum energy density induced by axion–photon mixing in the Minkowski space–time. A value compatible with that of the energy density of the universe can be obtained for axions with a mass of (10−3–10−2) eV in the presence of the strong magnetic fields that characterize astrophysical objects such as pulsars or neutron stars. In addition, a value of the energy density less than that of the Casimir effect is obtained for magnetic fields used in experiments such as PVLAS. The vacuum polarization induced by this energy could be detected in next experiments and it might provide an indirect proof of the existence of the axion–photon mixing.The quantum field theory effects presented in this work may lead to new methods for studying axion-like particles.

Speeding up the Universe using dust with pressure

We revise the cosmological standard model presuming that matter, i.e. baryons and cold dark matter, exhibits a non-vanishing pressure mimicking the cosmological constant effects. In particular, we propose a scalar field Lagrangian $\mathcal L_1$ for matter with the introduction of a Lagrange multiplier as constraint. We also add a symmetry breaking effective potential accounting for the classical cosmological constant problem, by adding a second Lagrangian $\mathcal{L}_2$. Investigating the Noether current due to the shift symmetry on the scalar field, $\varphi\rightarrow\varphi+c^0$, we show that $\mathcal{L}_1$ turns out to be independent from the scalar field $\varphi$. Further we find that a positive Helmotz free-energy naturally leads to a negative pressure without introducing by hand any dark energy term. To face out the fine-tuning problem, we investigate two phases: before and after transition due to the symmetry breaking. We propose that during transition dark matter cancels out the quantum field vacuum energy effects. This process leads to a negative and constant pressure whose magnitude is determined by baryons only. The numerical bounds over the pressure and matter densities are in agreement with current observations, alleviating the coincidence problem. Finally assuming a thermal equilibrium between the bath and our effective fluid, we estimate the mass of the dark matter candidate. Our numerical outcomes seem to be compatible with recent predictions on WIMP masses, for fixed spin and temperature. In particular, we predict possible candidates whose masses span in the range $0.5-1.7$ TeV.

Quantum optical measurement with tripartite entangled photons generated by triple parametric down-conversion.

Parametric down-conversion is a second-order nonlinear optical process annihilating a pump photon and creating a pair of photons in the signal and idler modes. Then, by using two parametric down-converters and introducing a path indistinguishability for the two generated idler modes, a quantum coherence between two conjugate signal beams can be induced. Such a double spontaneous or stimulated parametric down-conversion scheme has been used to demonstrate quantum spectroscopy and imaging with undetected idler photons via measuring one-photon interference between their correlated signal beams. Recently, we considered another quantum optical measurement scheme utilizing W-type tripartite entangled signal photons that can be generated by employing three spontaneous parametric down-conversion crystals and by inducing coherences or path-indistinguishabilities between their correlated idler beams and between quantum vacuum fields. Here, we consider an extended triple stimulated parametric down-conversion scheme for quantum optical measurement of sample properties with undetected idler and photons. Noting the real effect of vacuum field indistinguishability on the fringe visibility as well as the role of zero-point field energy in the interferometry, we show that this scheme is an ideal and efficient way to create a coherent state of W-type entangled signal photons. We anticipate that this scheme would be of critical use in further developing quantum optical measurements in spectroscopy and microscopy with undetected photons.

Entangled scent of a charge

We argue that the ground state of a field theory, in the presence of charged particles, becomes an entangled state involving an infinity of soft photons. The quantum field vacuum is altered by the passage of a uniformly moving charge, leaving in its wake a different dressed ground state. In this sense a charged particle leaves its electromagnetic scent even after passing by. Unlike in classical electrodynamics the effect of the charge remains even at infinite time. The calculation is done in detail for the ground state of a spacetime wedge, although the results are more general. This agrees in spirit with recent results over the infrared aspects of field theory, although the technical details are different. These considerations open the possibility that the information carried by quantum fields, being nonlocal, does not disappear beyond the horizon of black holes.