Showing papers in "Journal of Modern Optics in 2012"
TL;DR: In this article, the authors proposed a new optical bistable device (OBD), which is constructed by connecting two symmetrical fiber Bragg gratings with a ytterbium-doped fiber to form a nonlinear Fabry-Perot cavity.
Abstract: We propose a new optical bistable device (OBD), which is constructed by connecting two symmetrical fiber Bragg gratings with a ytterbium-doped fiber to form a nonlinear Fabry–Perot cavity. The principle of this new OBD is described using the transfer-matrix method, and the two groups of transmitted and reflected optical bistability loops under different parameters are investigated symmetrically. Compared with single fiber Bragg grating switching, whose switching power is greater than 2 kW, this new device has evident merits in reducing the switching power to less than 45 mW.
125 citations
TL;DR: In this article, a red-enhanced SPAD (RE-SPAD) was proposed to improve the photon detection efficiency (PDE) in the red and near-infrared range while maintaining a good timing resolution.
Abstract: In this paper we present a new technology for the fabrication of Single Photon Avalanche Diodes (SPADs) aimed at combining the advantages of thin and thick SPADs. The new detector is manufactured in a thick epitaxial layer designed to improve the Photon Detection Efficiency (PDE) in the red and near-infrared range while maintaining a good timing resolution. Experimental characterization of the new red-enhanced SPAD (RE-SPAD) confirmed a significant improvement of the PDE compared with thin SPADs; for example the PDE at a wavelength of 800 nm has increased from 15% to about 40%. Nevertheless the temporal resolution is still good, with a timing jitter of about 90 ps FWHM. In the same operating conditions the dark count rate is comparable with the one attainable with a thin SPAD (e.g. less than 25 cps for a 50 µm diameter device cooled down to −5°C). Moreover, being planar, the new technology is compatible with the fabrication of arrays of detectors.
84 citations
TL;DR: In this article, the authors show that single-photon sources are not helpful for point-to-point quantum key distribution because faint laser pulses do the job comfortably, and they show how single photon sources could become the seed of a revolution in the framework of quantum communication, making the security of QKD device-independent or extending quantum communication over many hundreds of kilometers.
Abstract: In a long-held preconception, photons play a central role in present-day quantum technologies. But what are sources producing photons one by one good for precisely? Well, in opposition to what many suggest, we show that single-photon sources are not helpful for point to point quantum key distribution because faint laser pulses do the job comfortably. However, there is no doubt about the usefulness of sources producing single photons for future quantum technologies. In particular, we show how single-photon sources could become the seed of a revolution in the framework of quantum communication, making the security of quantum key distribution device-independent or extending quantum communication over many hundreds of kilometers. Hopefully, these promising applications will provide a guideline for researchers to develop more and more efficient sources, producing narrowband, pure and indistinguishable photons at appropriate wavelengths.
79 citations
TL;DR: In this article, afterpulsing behavior in InGaAs/InP single photon avalanche diodes (SPADs) was analyzed for gating frequencies between 10 and 50 MHz.
Abstract: The characterization and analysis of afterpulsing behavior in InGaAs/InP single photon avalanche diodes (SPADs) is reported for gating frequencies between 10 and 50 MHz. Gating in this frequency range was accomplished using a matched delay line technique to achieve parasitic transient cancellation, and FPGA-based data acquisition firmware was implemented to provide an efficient, flexible multiple-gate sequencing methodology for obtaining the dependence of afterpulse probability P ap on hold-off time T ho. We show that the detrapping times extracted from the canonical exponential fitting of P ap(T ho) have no physical significance, and we propose an alternative description of the measured data, which is accurately fit with the simple power law behavior P ap ∝ with α ∼ 1.2 ± 0.2. We discuss the physical implications of this functional form, including what it may indicate about trap defect distributions and other possible origins of this power law behavior.
66 citations
TL;DR: In this article, an active hold-o circuit was proposed to reduce the afterpulsing of the avalanche photodiode and improve the performance of the detector, which achieved a free-running operation with a 10% detection accuracy.
Abstract: InGaAs/InP-based semiconductor avalanche photodiode are usually employed for single-photon counting at telecom wavelength. However they are aected by afterpulsing which limits the diode performance. Recently, Princeton Lightwave has commercialised a diode integrating monolithically a feedback resistor. This solution eectively quenches the avalanche and drastically reduces afterpulsing. Here, we report the development and characterization of a detector module based on this diode, implementing an active hold-o circuit which further reduces the afterpulsing and notably improves the detector performances. We demonstrate free-running operation with 600 Hz dark count rate at 10% detection eciency. We also improved the standard double-window technique for the afterpulsing characterization. Our algorithm implemented by a FPGA allows to put the APD in a welldened initial condition and to measure the impact of the higher order afterpulses.
63 citations
TL;DR: In this article, the authors give a brief overview of recent progress in the characterization and parametrization of density matrices of finite dimensional systems, and discuss in some detail the Bloch-vector and Jarlskog-parameterization.
Abstract: This article gives a brief overview of some recent progress in the characterization and parametrization of density matrices of finite dimensional systems. We discuss in some detail the Bloch-vector and Jarlskog parametrizations and mention briefly the coset parametrization. As applications of the Bloch parametrization we discuss the trace invariants for the case of time dependent Hamiltonians and in some detail the dynamics of three-level systems. Furthermore, the Bloch vector of two-qubit systems as well as the use of the polarization operator basis is indicated. As the main application of the Jarlskog parametrization we construct density matrices for composite systems. In addition, some recent related articles are mentioned without further discussion.
54 citations
TL;DR: In this paper, the authors provide an overview of the key theoretical features of Bose-Einstein condensates in cold atomic gases at near zero temperature in the situation where all the bosons occupy at most two single particle states or modes.
Abstract: This topical review provides an overview of the key theoretical features of Bose–Einstein condensates (BECs) in cold atomic gases at near zero temperature in the situation where all the bosons occupy at most two single particle states or modes. This situation applies to single-component BECs in double well trap potentials and to two-component BEC in single well trap potentials, such as occur when BEC are used in interferometry experiments. The Hamiltonian is introduced in terms of field operators and mode expansions are restricted to a total of two modes. Spin operators and their eigenstates are introduced as the fundamental basis states for describing the two-mode N boson quantum system. The spin states have a macroscopic angular momentum quantum number of N/2 and the magnetic quantum number k specifies the relative number of bosons in the two modes. The treatment presented involves an extensive use of angular momentum theory, including unitary rotation operators. Important states of the two-mode system ...
50 citations
TL;DR: In this paper, a technique based on the optofluidic method was proposed to design a photonic crystal fiber (PCF) experiencing small dispersion over a broad range of wavelengths.
Abstract: We present a technique based on the optofluidic method to design a photonic crystal fiber (PCF) experiencing small dispersion over a broad range of wavelengths. Without nano-scale variation in the air-hole diameter or the lattice constant of Λ, or even changing the shape of the air holes, this approach allows us to control the dispersion of the fundamental mode in a PCF simply by choosing a suitable refractive index of the liquid to infiltrate into the air holes of the PCF. Moreover, one can design a different PCF such as a dispersion flattened fiber (DFF), dispersion shifted fiber (DSF), by utilizing fluids of various refractive indices.
45 citations
TL;DR: The analytical results reveal that the modified-AND detection offer best bit-error rate (BER) performance and enables MQC code to support higher transmission rate up to 1.25 Gb/s compared to conventional AND detection.
Abstract: The major drawback of incoherent spectral-amplitude coding optical code-division multiple-access (SAC-OCDMA) systems is their inherent intensity noise originating due to the incoherency of the broadband light sources. In this paper, we propose a developed detection technique named the modified-AND subtraction detection for incoherent SAC-OCDMA systems. This detection technique is based upon decreasing the received signal strength during the decoding process by dividing the spectrum of the utilized code sequence. The proposed technique is capable of mitigating the intensity noise effect, as well as suppressing the multiple-access interference impact. Based on modified quadratic congruence (MQC) code, the analytical results reveal that the modified-AND detection offer best bit-error rate (BER) performance and enables MQC code to support higher transmission rate up to 1.25 Gb/s compared to conventional AND detection. Furthermore, we ascertained that the proposed technique enhances the system performance usin...
44 citations
TL;DR: In this article, the rotation rates of the generated phase masks of two annular rings were measured for different orders and for various values of the difference between the wave-vectors of the superimposing beams, and were shown to be in good agreement with that predicted theoretically.
Abstract: Experimental measurements are reported of the rotation rates of superpositions of higher-order Bessel beams. Digitally generated phase masks of two annular rings, were imprinted on a spatial light modulator and used to obtain superpositions of higher-order Bessel beams of the same order but of opposite topological charge. Such a superposition field carries on average zero orbital angular momentum, yet exhibits a rotation in the intensity pattern: the resultant field rotates at a constant rate about the optical axis as it propagates. The rotation rates of the generated fields were measured for different orders and for various values of the difference between the wave-vectors of the superimposing beams, and are shown to be in good agreement with that predicted theoretically.
44 citations
TL;DR: In this paper, a qualitative, real-time backlighting positioning sensor for the alignment of an optical beam to a minutely deviated diffraction order axis was presented to increase the power coupling efficiency into a multimode fiber in selective launches.
Abstract: This paper presents a qualitative, real-time backlighting positioning sensor for the alignment of an optical beam to a minutely deviated diffraction order axis to increase the power coupling efficiency into a multimode fiber in selective launches. Results show that the technique facilitates the alignment of the lenses to the first diffraction order axis and improves the power coupling efficiency into a multimode fiber.
TL;DR: By using the subsidiary ordinary differential equation method, many explicit Jacobian elliptic periodic solutions of the cubic-quintic nonlinear optical transmission equation with higher-order dispersion nonlinear terms and self-steepening term are obtained as mentioned in this paper.
Abstract: By using the subsidiary ordinary differential equation method, many explicit Jacobian elliptic periodic solutions of the cubic-quintic nonlinear optical transmission equation with higher-order dispersion nonlinear terms and self-steepening term are obtained. The results are discussed.
TL;DR: By using three mutually perpendicular standing-wave fields, a scheme for 3D subwavelength atom localization in a five-level M-type atomic system based on the electromagnetically induced transparency, position probability distribution of the atom in 3D space was determined via measuring the probe absorption which is proportional to the filter function as discussed by the authors.
Abstract: By using three mutually perpendicular standing-wave fields, we propose a scheme for three-dimensional (3D) subwavelength atom localization in a five-level M-type atomic system Based on the electromagnetically induced transparency, position probability distribution of the atom in 3D space could be determined via measuring the probe absorption which is proportional to the filter function It is shown that patterns of 3D atom localization depends sensitively on the coupling schemes of the standing-wave fields When the standing-wave fields couple three different transitions, the same atom localization patterns are formed in the eight subspaces of the 3D space While all the standing-wave fields are applied on one transition, we can realize different atom localization patterns in the eight subspaces From the view of the xy plane, various symmetric or asymmetric atom localization patterns could be formed at different z positions by adjusting the parameters of the laser fields
TL;DR: In this paper, the authors give a comprehensive account of various quantum properties of X states, such as entanglement, negativity, quantum discord and other related quantities, and discuss the transformations that preserve their structure both in terms of continuous time evolution and discrete quantum processes.
Abstract: X states are a broad class of two-qubit density matrices that generalize many states of interest in the literature. In this work, we give a comprehensive account of various quantum properties of these states, such as entanglement, negativity, quantum discord and other related quantities. Moreover, we discuss the transformations that preserve their structure both in terms of continuous time evolution and discrete quantum processes.
TL;DR: In this paper, the authors demonstrate a new, compact design for binary spatial amplitude modulation in a mode-selective transmitter in a multimode fiber, adapted from microscopy, and show that it is possible to retrieve the original continuous-amplitude transverse modal electric field from the binary amplitude-modulated image using a single lens.
Abstract: The demand for portable real-time optical applications such as medical optical sensors and optical transceivers instigate the need for compact optical designs. The aim of this paper is to demonstrate a new, compact design for binary spatial amplitude modulation in a mode-selective transmitter in a multimode fiber, adapted from microscopy. Results show that it is possible to retrieve the original continuous-amplitude transverse modal electric field from the binary amplitude-modulated image using a single lens. The retrieved image is in good agreement with the original image.
TL;DR: This work considers a free space optical system with a multiple receivers' scheme and diversity in space that operates under weak to strong atmospheric turbulence conditions modeled by the gamma–gamma distribution.
Abstract: Terrestrial free space optical telecommunication systems offer license-free very high bandwidth access characteristics with significantly low installation and operational cost. On the other hand, the performance of such a system depends strongly on the weather conditions in the area between the transmitter and the receiver due to the fact that the transmission media is the atmosphere. A very significant phenomenon which decreases a FSO links' performance is atmospheric turbulence. In order to counterbalance this reduction, in this work, we consider a free space optical system with a multiple receivers' scheme and diversity in space that operates under weak to strong atmospheric turbulence conditions modeled by the gamma–gamma distribution. We derive closed form mathematical expressions for the estimation of the outage probability and the average bit error rate of the multiple receivers' system. Additionally, we compare the performance capabilities of the single point to point link with those of the spatia...
TL;DR: In this paper, an InGaAs single-photon avalanche diode (SPAD) operating in the biasing and sensing regime of periodic gating techniques was investigated experimentally.
Abstract: Afterpulsing was investigated experimentally in an InGaAs single-photon avalanche diode (SPAD) operating in the biasing and sensing regime of periodic-gating techniques. These techniques support single-photon counting at rates in the 100 MHz range with low afterpulse probability and are characterized by sub-nanosecond active gates that limit total avalanche-charge flows to the 100 fC range or less. We achieved comparable gating and sensing performance with a system using non-periodic gates and were able to make traditional double-pulse afterpulse measurements from 4.8 ns to 2 µs in this new low-avalanche-current regime. With 0.50 ns gate duration and a detection efficiency of 0.15 at 1310 nm the per-gate afterpulse probability at 4.8 ns is 0.008, while with a 1.5 ns gate it is almost two orders of magnitude higher. We provide a quantitative connection between afterpulse probability and total avalanche charge, and between the performance observed in traditional gating techniques for InGaAs SPADs and those ...
TL;DR: This review concerns itself with the latter, giving an overview of error correction and self-correction, and how they may be used to achieve fault-tolerant quantum computation.
Abstract: Quantum states are inherently fragile, making their storage a major concern for many practical applications and experimental tests of quantum mechanics. The field of quantum memories is concerned with how this storage may be achieved, covering everything from the physical systems best suited to the task to the abstract methods that may be used to increase performance. This review concerns itself with the latter, giving an overview of error correction and self-correction, and how they may be used to achieve fault-tolerant quantum computation. The planar code is presented as a concrete example, both as a quantum memory and as a framework for quantum computation.
TL;DR: In this paper, a white LED combining a blue LED with the blends of nano-YAG phosphors and orange-and red-emission quantum dots (QDs) with a weight ratio of 1:1:1 was obtained.
Abstract: To improve the poor color rendering index (CRI) of YAG:Ce-based white light-emitting diode (LED) due to the lack of red spectral component, core/shell/shell CdSe/CdS/ZnS quantum dots (QDs) were synthesized and blended into nano-YAG:Ce3+ phosphors. Prominent spectral evolution has been achieved by increasing the content of QDs. A white LED combining a blue LED with the blends of nano-YAG phosphors and orange- and red-emission QDs with a weight ratio of 1:1:1 was obtained. This kind of white LED showed excellent white light with luminescent efficiency, color coordinates, CRI and correlated color temperature (CCT) of 82.5 lm/W, (0.3264, 0.3255), 91 and 4580 K, respectively.
TL;DR: In this paper, a review of studies of the high-order harmonic generation of laser radiation in laser-produced plasma revealed recent developments in this field, including two-colour pumps, generation of extremely broadened harmonics, further developments in harmonic generation in clusters (fullerenes, carbon nanotubes), destructive interference of harmonics from different emitters, new approaches in resonance-induced enhancement of harmonicics, applications of high pulse repetition rate lasers for the enhancement of average power of generating harmonics and observation of quantum path signatures, etc.
Abstract: A review of studies of the high-order harmonic generation of laser radiation in laser-produced plasma revealed recent developments in this field. These include new approaches in application of two-colour pumps, generation of extremely broadened harmonics, further developments in harmonic generation in clusters (fullerenes, carbon nanotubes), destructive interference of harmonics from different emitters, new approaches in resonance-induced enhancement of harmonics, applications of high pulse repetition rate lasers for the enhancement of average power of generating harmonics and observation of quantum path signatures, etc. We show that this method of frequency conversion of laser radiation towards the extreme ultraviolet range became mature during multiple sets of studies carried out in many laboratories worldwide and demonstrated new approaches in the generation of strong coherent short-wavelength radiation for various applications.
TL;DR: In this article, the entanglement dynamics of discrete time quantum walks acting on bounded finite-sized graphs were studied and it was shown that, depending on system parameters, the dynamics may be monotonic, oscillatory but highly regular, or quasi-periodic.
Abstract: We study the entanglement dynamics of discrete time quantum walks acting on bounded finite sized graphs. We demonstrate that, depending on system parameters, the dynamics may be monotonic, oscillatory but highly regular, or quasi-periodic. While the dynamics of the system are not chaotic since the system comprises linear evolution, the dynamics often exhibit some features similar to chaos such as high sensitivity to the system's parameters, irregularity and infinite periodicity. Our observations are of interest for entanglement generation, which is one primary use for the quantum walk formalism. Furthermore, we show that the systems we model can easily be mapped to optical beamsplitter networks, rendering experimental observation of quasi-periodic dynamics within reach.
TL;DR: In this paper, the reflection and transmission coefficients of Bragg gratings were analyzed analytically and numerically for a single grating and also for a cavity formed by two gratings, and it was shown that the reflection coefficients depend on the orientation of the polarization vector of light with respect to the holes.
Abstract: We study nanofibers with Bragg gratings from equidistant holes. We calculate analytically and numerically the reflection and transmission coefficients for a single grating and also for a cavity formed by two gratings. We show that the reflection and transmission coefficients of the gratings substantially depend on the number of holes, the hole length, the hole depth, the grating period, and the light wavelength. We find that the reflection and transmission coefficients of the gratings depend on the orientation of the polarization vector of light with respect to the holes. Such a dependence is a result of the fact that the cross-section of the gratings is not cylindrically symmetric.
TL;DR: In this paper, the electromagnetic field scattered by a rough surface of a semi-infinite body is computed up to the second order of a perturbation scheme with the surface roughness as a parameter.
Abstract: The electromagnetic field scattered by a rough surface of a semi-infinite body is computed up to the second order of a perturbation scheme with the surface roughness as a perturbation parameter. The calculations are based on the equation of motion of the polarization within the Lorentz–Drude (plasma) model of polarizable, non-magnetic, homogeneous matter. The surface roughness contributes both to the main (specularly) reflected and refracted fields and diffuse scattering, or gives rise to secondary (second-order) diffraction peaks for a regular grating. The calculations are performed both for the s- and p-waves. Two-dimensional modes, resonant at certain frequencies, are identified, confined to and propagating only on the surface, as a consequence of the surface roughness.
TL;DR: In this paper, a fully parallel eight-channel module based on a monolithic single-photon avalanche diode (SPAD) array with great temporal resolution, high photon detection efficiency and low dark count rate has been designed and fabricated.
Abstract: Multi-dimensional time correlated single-photon counting has reached a prominent position among analytical techniques employed in the medical and biological fields. The development of instruments able to perform temporal and spectral fluorescence analysis (sFLIM) at the same time is limited by the performance of single-photon detectors, since currently available arrays cannot simultaneously satisfy all the requirements. To face this rising quest, a fully-parallel eight-channel module, based on a monolithic single-photon avalanche diode (SPAD) array with great temporal resolution, high photon detection efficiency and low dark count rate, has been designed and fabricated. The system relies on a novel architecture of the single pixel, based on the integration of the timing pick-up circuit next to the photodetector, making the negative effects of electrical and optical crosstalk on photon timing performance negligible. To this end, the custom technological process used to fabricate the SPAD has been modified,...
TL;DR: In this article, an analytical expression for an optical soliton is obtained with the aid of He's semi-inverse variational principle in the presence of third and fourth-order dispersion as well as inter-modal dispersion.
Abstract: In this paper an analytical expression for an optical soliton is obtained with the aid of He's semi-inverse variational principle in the presence of third- and fourth-order dispersion as well as inter-modal dispersion. Three laws of nonlinear media are considered in this paper: the Kerr law, the power law and the log law.
TL;DR: The development of ultrafast and intense laser sources has paved the way to the study of laser-matter interaction in the strong-field regime, where the amplitude of the laser field becomes comparable to the Coulomb field seen by electrons in the proximity of their parent ion as mentioned in this paper.
Abstract: The development of ultrafast and intense laser sources has paved the way to the study of laser–matter interaction in the strong-field regime, where the amplitude of the laser field becomes comparable to the Coulomb field seen by electrons in the proximity of their parent ion. This realm embraces numerous phenomena, such as high-order harmonic generation, tunnel photoionization, laser-induced electron diffraction and holography, ponderomotive electron streaking, and so forth. For a long time, the Ti:Sapphire laser has been the sole workhorse of strong-field scientists; the recent development of intense and tunable ultrafast sources operating in the mid-infrared added an extra dimension to strong-field investigations, with valuable benefits in the understanding of several phenomena and in the development of high-photon-energy coherent sources. In this work we review the recent achievements in strong-field science which were allowed by the exploitation of mid-infrared ultrafast lasers.
TL;DR: Correcting peripheral higher order aberrations with adaptive optics can improve low contrast resolution, provided refractive errors are corrected and the system runs in continuous closed loop.
Abstract: Understanding peripheral optical errors and their impact on vision is important for various applications, e.g. research on myopia development and optical correction of patients with central visual field loss. In this study, we investigated whether correction of higher order aberrations with adaptive optics (AO) improve resolution beyond what is achieved with best peripheral refractive correction. A laboratory AO system was constructed for correcting peripheral aberrations. The peripheral low contrast grating resolution acuity in the 20 � nasal visual field of the right eye was evaluated for 12 subjects using three types of correction: refractive correction of sphere and cylinder, static closed loop AO correction and continuous closed loop AO correction. Running AO in continuous closed loop improved acuity compared to refractive correction for most subjects (maximum benefit 0.15 logMAR). The visual improvement from aberration correction was highly correlated with the subject’s initial amount of higher order aberrations (p ¼ 0.001, R 2 ¼ 0.72). There was, however, no acuity improvement from static AO correction. In conclusion, correction of peripheral higher order aberrations can improve low contrast resolution, provided refractive errors are corrected and the system runs in continuous closed loop.
TL;DR: In this paper, the authors investigated the case of transmission-type one-dimensional magnetophotonic crystals (MPCs) in order to achieve simultaneous high transmittance and large Faraday rotation utilizing few magnetic layers.
Abstract: We have investigated the case of transmission-type one-dimensional magnetophotonic crystals (MPCs) in order to achieve simultaneous high transmittance and large Faraday rotation utilizing few magnetic layers. In a MPC that includes two Ce:YIG magnetic layers, we have achieved a Faraday rotation as large as 85.82° and a transmittance of 95.66%. In addition, another structure in the form of a triple-cavity MPC with a transmittance of 100% and a Faraday rotation as huge as 87.72° has been achieved through precise selection of layer thicknesses and positions. Both of these high-performance structures are very compact and thin, which makes them excellent candidates for application in integrated MO devices.
TL;DR: In this article, the suitability of using spherical metal and dielectric nanoparticles on the top of a silicon solar cell has been investigated, and an enhancement index factor (EIF) for each wavelength of light and an averaged EIF for the AM 1.5 solar spectrum, weighted by the photon flux, has been introduced.
Abstract: The suitability of using spherical metal and dielectric nanoparticles on the top of a silicon solar cell has been investigated. An enhancement index factor (EIF) for each wavelength of light and an averaged EIF for the AM 1.5 solar spectrum, weighted by the photon flux, has been introduced. These factors estimate the effect of the nanoparticles in improving the performance of the solar cells, considering the absorption loss due to joule heating, fraction of radiation scattered into the substrate and the front scattered radiation pattern. A systematic comparison between silver and dielectric nanoparticles (silica, silicon nitride, titanium dioxide) shows that titanium dioxide and silicon nitride nano particles of sizes ≥100 nm exhibit larger enhancements compared to that of silver nanoparticles of similar sizes. Further, as the dielectric constant of the dielectric nanoparticles increases, the optimal particle size corresponding to maximal enhancement shifts towards lower value. At optimal particle sizes, ...
TL;DR: A modification of electrodynamics is proposed, motivated by previously unremarked paradoxes that can occur in the standard formulation, and it is shown by specific examples that gauge transformations exist that radically alter the nature of a problem, even while maintaining the values of many measurable quantities.
Abstract: A modification of electrodynamics is proposed, motivated by previously unremarked paradoxes that can occur in the standard formulation. It is shown by specific examples that gauge transformations exist that radically alter the nature of a problem, even while maintaining the values of many measurable quantities. In one example, a system with energy conservation is transformed to a system where energy is not conserved. The second example possesses a ponderomotive potential in one gauge, but this important measurable quantity does not appear in the gauge-transformed system. A resolution of the paradoxes comes from noting that the change in total action arising from the interaction term in the Lagrangian density cannot always be neglected, contrary to the usual assumption. The problem arises from the information lost by employing an adiabatic cutoff of the field. This is not necessary. Its replacement by a requirement that the total action should not change with a gauge transformation amounts to a supplementary condition for gauge invariance that can be employed to preserve the physical character of the problem. It is shown that the adiabatic cutoff procedure can also be eliminated in the construction of quantum transition amplitudes, thus retaining consistency between the way in which asymptotic conditions are applied in electrodynamics and in quantum mechanics. The ‘gauge-invariant electrodynamics’ of Schwinger is shown to depend on an ansatz equivalent to the condition found here for maintenance of the ponderomotive potential in a gauge transformation. Among the altered viewpoints required by the modified electrodynamics, in addition to the rejection of the adiabatic cutoff, is the recognition that the electric and magnetic fields do not completely determine a physical problem, and that the electromagnetic potentials supply additional information that is required for completeness of electrodynamics.