Low Flux State Research Articles

Influence of cavity decay on unconventional geometric quantum phase gates with superconducting quantum-interference-device qubits inside a cavity

We propose a potential scheme for carrying out two-qubit unconventional geometric logic gates on two identical superconducting quantum-interference-device (SQUID) qubits coupled to a single-mode microwave field. The geometric logic gate operation is performed in two lower flux states, and the excited state |2⟩ does not participate in the procedure. The SQUIDs undergo no transitions during gate operation. Thus, the decoherence due to energy spontaneous emission based on the levels of SQUIDs is suppressed. We present the two-qubit unconventional geometric logic gates in both an ideal cavity and a real cavity with decay. Discussions about the fidelity and the success probability of the proposed scheme as well as the experimental feasibility are given in detail.

XMM-Newton Observations of NGC 4051: Temporal Flux and Spectral Variability during Transition to the Faintest Phase in NGC 4051

Abstract We report on the results from temporal and spectral analyses of NGC 4051 observed with XMM-Newton. The X-ray variabilities in the higher-flux states are well explained by the combination of a variable, direct power law and a temporally stable, absorbed component. In low-flux states, however, spectral variations are mainly due to flux changes of the absorbed component. Furthermore, when the source enters into the lowest-flux state, the absorbed component no longer appears. One promising interpretation is that part of the central emission region is covered by a temporally stable material, while the size of the emission region varies so as to produce a flux variation predominantly in direct power-law component. Once the emission region becomes smaller than the size of the absorber, however, the absorbed flux also begins to decrease. Finally, when the emission region is reduced sufficiently, both the direct and absorbed components disappear (i.e., “switched off” state). In this state, we found the signature of thermal emission, whose temperature of 0.8keV is in agreement with the average value obtained from statistical analyses of Seyfert 2 galaxies with starburst activity. This could suggest that NGC 4051 possesses a nuclear starburst region.

Quantum gate operations in the decoherence-free subspace of superconducting quantum-interference devices

We propose a scheme for implementing single-qubit and nontrivial two-qubit gate operations in a system with superconducting quantum interference devices (SQUIDs) via Raman transitions in circuit QED. The gate operations only involve the two lower flux states of the SQUID, and hence the effect of the excited state can be neglected. By encoding quantum information into the decoherence free subspace of the SQUID system, the evolution of the system results in fault-tolerant quantum gates. The effect of decoherence caused by the decay of the transmission line resonator is investigated.

XMM‐NewtonObservations of the Narrow‐Line Seyfert 1 Galaxy Mrk 335 in a Historical Low X‐Ray Flux State

We report the discovery of strong soft X-ray emission lines and a hard continuum above 2 keV in the narrow-line Seyfert 1 galaxy Mrk 335 during an extremely low X-ray flux state. Mrk 335 was observed for 22 ks by XMM-Newton in 2007 July as a Target of Opportunity to examine it in its X-ray low flux state, which was discovered with Swift. Long-term light curves suggest that this is the lowest flux state this AGN has ever been seen in. However, Mrk 335 is still sufficiently bright that its X-ray properties can be studied in detail. The X-ray continuum spectrum is very complex and requires several components to model. Statistically, partial covering and blurred reflection models work well. We confirm the presence of a strong narrow Fe line at 6.4 keV. High-resolution spectroscopy with the XMM-Newton RGS reveals strong, soft X-ray emission lines not detected in previous, higher signal-to-noise ratio, XMM-Newton observations, such as highly ionized Fe lines, O VII, and Ne IX and Mg XI lines. The optical/UV fluxes are similar to those previously measured with Swift. Optical spectroscopy taken in 2007 September does not show any changes to optical spectra obtained 8 years earlier.

One-Step Realization of SWAP Gate with Superconducting Quantum-Interference Devices and Atoms in Cavity QED

We put forward a simple scheme for one-step realization of a two-qubit SWAP gate with SQUIDs (super-conducting quantum-interference devices) in cavity QED via Raman transition. In this scheme, the cavity field is only virtually excited and thus the cavity decay is suppressed. The SWAP gate is realized by using only two lower flux states of the SQUID system and the excited state would not be excited. Therefore, the effect of decoherence caused from the levels of the SQUID system is possibly minimized. The scheme can also be used to implement the SWAP gate with atoms.

Unconventional Geometric Phase Gate with Superconducting Quantum Interference Device Qubits in Cavity QED

In the system with superconducting quantum interference devices (SQUID) in cavity, a scheme for constructing two-qubit quantum phase gate via a conventional geometric phase-shift is proposed by using a quantized cavity field and classical microwave pulses. In this scheme, the gate operation is realized in the subspace spanned by the two lower flux states of the SQUID system and the population operator of the excited state has no effect on it. Thus the effect of decoherence caused from the levels of the SQUID system is possible to minimize. Under cavity decay, our strictly numerical simulation shows that it is also possible to realize the unconventional geometric phase gate. The experimental feasibility is discussed in detail.

The Seyfert 1 Galaxy Mrk 335 at a very low flux state: mapping the soft X-ray photoionised gas

Context. This paper reports on an XMM-Newton observation of the Seyfert 1 Galaxy Mrk 335 performed as a target of opportunity when the source was in an unusually low flux state.

Preparation of Entanglement and Schrödinger Cat States of Superconducting Quantum Interference Devices Qubits via Coupling to a Microwave Cavity

An alternative scheme is proposed for the generation of n-qubit W states of superconducting quantum interference devices (SQUID) in cavity QED. In this scheme, Raman coupling of two lower flux states of SQUID system is achieved via a microwave pulse and the cavity mode. Conditioned on no photon leakage from the cavity, the n-qubit W state can be generated whether the effective coupling parameters of the SQUID to cavity mode and classical microwave fields are the same or different. Our strictly numerical simulations of the time evolution of the system including decay show that the success probability of our scheme is almost unity and the interaction time is on the order of 10−9 s. The scheme can also be used to generate the Schrödinger cat states of multi-SQUID.

EFFICIENT SCHEME FOR EPR STATE AND CLUSTER STATES GENERATION WITH SQUID QUBITS VIA RAMAN TRANSITION

In this paper, a theoretical scheme is proposed to generate the EPR state of two SQUID qubits and create the multipartite cluster states of many SQUID qubits in cavity via Raman transition. We also show how to transfer quantum information from one SQUID qubit to another directly. In this scheme, the cavity field is only virtually excited and thus the cavity decay is suppressed. The quantum information processing and cluster states generation are realized by using only two lower flux states of the SQUID system and the excited state would not be excited. Therefore, the effect of decoherence caused from the levels of the SQUID system is possibly minimized.

Evidence of a Parsec-Scale X-Ray Jet from the Accreting Neutron Star Circinus X-1

We analyzed the zero-order image of a 50 ks Chandra gratings observation of Circinus X-1, taken in 2005 during the source's low-flux state. Circinus X-1 is an accreting neutron star that exhibits ultrarelativistic arcsecond-scale radio jets and diffuse arcminute-scale radio jets and lobes. The image shows a clear excess along the general direction of the northwestern counterjet, coincident with the radio emission, suggesting that it originates either in the jet itself or in the shock that the jet is driving into its environment. This makes Circinus X-1 the first neutron star for which an extended X-ray jet has been detected. The kinetic jet power that we infer is significantly larger than the minimum power required for the jet to inflate the large-scale radio nebula.

Observation of Very High Energy γ‐Rays from the AGN 1ES 2344+514 in a Low Emission State with the MAGIC Telescope

The MAGIC collaboration has observed very high energy gamma ray emission from the AGN 1ES 2344+514. A gamma-ray signal corresponding to an 11 sigma excess and an integral flux of (2.38+-0.30(stat)+-0.70(syst))*10^-11 cm^-2 s^-1 above 200 GeV has been obtained from 23.1 hours of data taking between 2005 August 3 and 2006 January 1. The data confirm the previously detected gamma-ray emission from this object during a flare seen by the Whipple collaboration in 1995 and the evidence (below 5 sigma significance level) from long-term observations conducted by the Whipple and HEGRA groups. The MAGIC observations show a relatively steep differential photon spectrum that can be described by a power law with a photon index of alpha=-2.95+-0.12(stat)+-0.2(syst) between 140 GeV and 5.4 TeV. The observations reveal a low flux state, about six times below the 1995 flare seen by Whipple and comparable with the previous Whipple and HEGRA long term measurements. During the MAGIC observations no significant time variability has been observed.

Unconventional Geometric Phase-Shift Gates Based onSuperconducting Quantum Interference Devices Coupledto a Single-Mode Cavity

We present a scheme to realize geometric phase-shift gate for twosuperconducting quantum interference device (SQUID) qubits coupled to asingle-mode microwave field. The geometric phase-shift gate operation isperformed in two lower flux states, and the excited state |2⟩ wouldnot participate in the procedure. The SQUIDs undergo no transitions duringthe gate operation. Thus, the docoherence due to energy spontaneous emissionbased on the levels of SQUIDs are suppressed. The gate is insensitive to thecavity decay throughout the operation since the cavity mode is displacedalong a circle in the phase space, acquiring a phase conditional upon thetwo lower flux states of the SQUID qubits, and the cavity mode is still inthe original vacuum state. Based on the SQUID qubits interacting with thecavity mode, our proposed approach may open promising prospects for quantumlogic in SQUID-system.

Quantum controlled phase gate and cluster states generation via two superconducting quantum interference devices in a cavity

A scheme for implementing 2-qubit quantum controlled phase gate (QCPG) is proposed with two superconducting quantum interference devices (SQUIDs) in a cavity. The gate operations are realized within the two lower flux states of the SQUIDs by using a quantized cavity field and classical microwave pulses. Our scheme is achieved without any type of measurement, does not use the cavity mode as the data bus and only requires a very short resonant interaction of the SQUID-cavity system. As an application of the QCPG operation, we also propose a scheme for generating the cluster states of many SQUIDs.

Investigating the nature of narrow-line Seyfert 1 galaxies with high-energy spectral complexity

(abridged) With the commissioning of XMM came the discovery of 2.5-10 keV spectral complexity in some narrow-line Seyfert 1 galaxies (NLS1). In this work, we define two samples of NLS1: a complex sample whose members exhibit high-energy complexity (C-sample), and a general sample of NLS1 whose 2.5-10 keV spectra do not strongly deviate from a simple power law (S-sample). Considering historical light curves of each object we find that the C-sample is representative of NLS1 in a low X-ray flux state, whereas the members of the S-sample appear to be in a typical flux state. Moreover, from measurements of alpha_ox with contemporaneous UV/X-ray data, we find that the C-sample of NLS1 appear X-ray weaker at the time of the observation. For two NLS1 in the C-sample multi-epoch measurements of alpha_ox are available and suggest that alpha_ox approaches more normal values as the complexity between 2.5-10 keV diminishes. This implies that a source could transit from one sample to the other as its X-ray flux varies. Secondly, there are indications that the C-sample sources, on average, exhibit stronger optical FeII emission, with the three most extreme (FeII/H_beta > 1.8) FeII emitters all displaying complexity in the 2.5-10 keV band. However, it is not clear if the possible connection between FeII strength and spectral complexity is due to the FeII producing mechanism or because strong FeII emitters may exhibit the greatest variability and consequently more likely to be caught in an extreme (low) flux state. Based on the current analysis it we can not straightforwardly dismiss absorption or reflection as the cause of the X-ray complexity.

XMM-Newton study of the complex and variable spectrum of NGC 4051

We study the X-ray spectral variability of the narrow line Seyfert 1 galaxy NGC 4051 as observed during two XMM–Newton observations. To gain insight on the general behaviour, we first apply model-independent techniques such as rms spectra and flux–flux plots. We then perform time-resolved spectral analysis by splitting the observations into 68 spectra (2 ks each). The data show evidence for a neutral and constant-reflection component and for constant emission from photoionized gas, which are included in all spectral models. The nuclear emission can be modelled both in terms of a ‘standard model’[pivoting power-law plus a blackbody (BB) component for the soft excess] and of a two-component one (power law plus ionized reflection from the accretion disc). Both the models reproduce the source spectral variability and cannot be distinguished on a statistical ground. The distinction has thus to be made on a physical basis. The standard model results indicate that the soft excess does not follow the standard BB law (LBB∝T4), despite a variation in luminosity by about one order of magnitude. The resulting temperature is consistent with being constant and has the same value as observed in the PG quasars. Moreover, although the spectral slope is correlated with flux, which is consistent with spectral pivoting, the hardest photon indices are so flat (Γ∼ 1.3–1.4) as to require rather unusual scenarios. Furthermore, the very low flux states exhibit an inverted Γ–flux behaviour which disagrees with a simple pivoting interpretation. These problems can be solved in terms of the two-component model in which the soft excess is not thermal, but due to the ionized reflection component. In this context, the power law has a constant slope (about 2.2) and the slope–flux correlation is explained in terms of the relative contribution of the power-law and reflection components which also explains the shape of the flux–flux plot relationship. The variability of the reflection component from the inner disc closely follows the predictions of the light-bending model, suggesting that most of the primary nuclear emission is produced in the very innermost regions, only a few gravitational radii (rg) from the central black hole.

Simultaneous Ultraviolet and X‐Ray Observations of Seyfert Galaxy NGC 4151. I. Physical Conditions in the X‐Ray Absorbers

We present a detailed analysis of the intrinsic X-ray absorption in the Seyfert 1 galaxy NGC 4151 using Chandra High Energy Transmission Grating Spectrometer data obtained in 2002 May as part of a program that included simultaneous ultraviolet (UV) spectra using the Hubble Space Telescope Space Telescope Imaging Spectrograph and the Far Ultraviolet Spectrographic Explorer. Previous studies, most recently using Advanced Satellite for Cosmology and Astrophysics (ASCA) spectra, revealed a large (>1022 cm-2) column of intervening gas, which has varied both in ionization state and total column density. NGC 4151 was in a relatively low flux state during the observations reported here (~25% of its historic maximum), although roughly 2.5 times as bright in the 2-10 keV band as during a Chandra observation in 2000. At both epochs, the soft X-ray band was dominated by emission lines, which show no discernible variation in flux between the two observations. The 2002 Chandra data show the presence of a very highly ionized absorber, in the form of H-like and He-like Mg, Si, and S lines, as well as lower ionization gas via the presence of inner-shell absorption lines from lower ionization species of these elements. The latter accounts for both the bulk of the soft X-ray absorption and the high covering factor UV absorption lines of O VI, C IV, and N V with outflow velocities ≈500 km s-1. The presence of high-ionization gas, which is not easily detected at low resolution (e.g., with ASCA), appears common among Seyfert galaxies. Since this gas is too highly ionized to be radiatively accelerated in sources such as NGC 4151, which is radiating at a small fraction of its Eddington Luminosity, it may be key to understanding the dynamics of mass outflow. We find that the deeper broadband absorption detected in the 2000 Chandra data is the result of both (1) lower ionization of the intervening gas due to the lower ionizing flux and (2) a factor of ~3 higher column density of the lower ionization component. To account for this bulk motion, we estimate that this component must have a velocity 1250 km s-1 transverse to our line of sight. This is consistent with the rotational velocity of gas arising from the putative accretion disk. While both thermal wind and magnetohydrodynamic models predict large nonradial motions, we suggest that the latter mechanism is more consistent with the results of the photoionization models of the absorbers

X-rays from the radio-quiet quasar PG 1407+265: relativistic jet or accretion disc emission?

We present two XMM-Newton observations of the luminous (L_x > 10^46 erg/s), radio-quiet quasar, PG 1407+265, separated by eleven months. The data indicate two distinct states: a highly variable, bright state (first epoch); and a quiescent, low-flux one (second epoch). During the low-flux state the spectrum is consistent with a single, unabsorbed power law. However, during the brighter state a highly variable, steep component is statistically required. Contemporaneous UV data from the Optical Monitor allow an estimate of the optical-to-X-ray spectral index (alpha_ox), which appears typical of radio-quiet quasars during the low-flux state, but extremely flat during the high-flux state. The XMM-Newton data can be described as originating from a combination of jet and accretion disc processes, in which the (relativistic) X-ray jet only works intermittently. The scenario could help describe some of the complexities seen in the broadband spectral energy distribution of PG 1407+265, such as weak high-ionisation emission lines, strong Fe II, unbeamed continuum, and the weak radio emission relative to the optical.

AChandraHETGS Spectral Study of the Iron K Bandpass in MCG −6‐30‐15: A Narrow View of the Broad Iron Line

We present a high-resolution X-ray spectrum of the iron K bandpass in MCG -6-30-15 based on a 522 ks observation with Chandra's High Energy Transmission Grating Spectrometer (HETGS). The Chandra spectrum is consistent with the presence of a relativistically broadened, highly redshifted iron Kα emission line with a profile similar to previous observations. A number of narrow features are detected above 2 keV, including a narrow Fe Kα emission line and narrow absorption lines from H- and He-like Fe, H-like S, and H-like Si. This absorption is well described by a photoionized plasma with a column density log NH = 23.2 and an ionization parameter log ξ = 3.6, assuming the iron abundance has the solar value and a velocity dispersion parameter b = 100 km s-1. Applying this absorption model to a high-fidelity XMM-Newton EPIC pn spectrum, we find that a broad iron line is still required with emission extending to within 1.9rg of the black hole. If the iron line comes from an accretion disk truncated at the innermost stable circular orbit, this indicates that the black hole must be spinning rapidly with a > 0.95. Ionized absorption models attempting to explain the 3-6 keV spectral curvature without strong gravity predict absorption lines in the 6.4-6.6 keV range that are inconsistent with the Chandra spectrum. The H- and He-like iron absorption lines in the Chandra spectrum are blueshifted by 2.0 × 103 km s-1 compared to the source frame and may originate in a high-velocity, high-ionization component of the warm absorber outflow. This high-ionization component may dominate the energy budget of the outflow and account for a significant fraction of the outflowing mass. Detailed modeling of the warm absorber below 2 keV will be addressed in a later paper, but our results are robust to the broader details of the warm absorber behavior. The difference spectrum between the high- and low-flux states is well described by a power law, in agreement with previous studies.

X-ray reflection in the Seyfert galaxy 1H 0419−577 revealing strong relativistic effects in the vicinity of a Kerr black hole

We report results obtained from six XMM-Newton observations of the Seyfert galaxy 1H 0419-577. The source was observed in a wide range of different flux levels, allowing its long-term spectral variability to be studied in detail, as already reported recently by Pounds et al. Here we show that the X-ray spectrum is well described by a simple two-component model comprising a power law with constant spectral shape and variable normalization, and a much more constant ionized reflection component from the inner accretion disc, which carries the signature of strong relativistic effects. One of the observations was performed when the source was in a particularly low-flux state in which the X-ray spectrum is rather peculiar and exhibits a very flat hard spectrum (with spectral index close to 1 in the 2-10 keV band), with broad residuals below 6.6 keV (rest frame) and a steep soft excess below about 1 keV. We interpret the spectrum as being reflection-dominated by X-ray reprocessed emission from the inner accretion disc. The primary continuum, which illuminates the disc, is almost completely unobserved, possibly because of strong light bending towards the central supermassive black hole. The ionized reflection model simultaneously accounts for the broad hard residuals and hard flat spectrum and for the soft excess. The same model provides an excellent description of the data at all the other flux levels, the most important difference being a variation in the power-law normalization. Our spectral decomposition and interpretation of the spectral variability implies that most of the X-ray emission in this source originates from within a few gravitational radii from the central black hole and requires that the compact object is almost maximally spinning.

Quantum logic gate operation and entanglement with superconducting quantum interference devices in a cavity via a Raman transition

In the system with superconducting quantum interference devices (SQUIDs) in cavity, the quantum logic gates operation and entanglement can be achieved by using a quantized cavity field and classical microwave pluses, via Raman transition. In this scheme, no transfer of quantum information between the SQUIDs and cavity is required, the cavity field is only virtually excited and thus the cavity decay is suppressed during the gate operation and entanglement generations. The gate operation and entanglement generations are realized by using only the two lower flux states of the SQUID system and the excited state would not be excited. Therefore, the effect of docoherence based on the levels of the SQUID system is possible to minimize.