Decay processes in the presence of thin superconducting films

Rekdal, Per Kristian; Skagerstam, Bo-Sture

doi:10.1103/physreva.75.022904

Cited by 6 publications

(7 citation statements)

References 25 publications

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“…With the parameters of Refs. [24,26,27], i.e. S 2 = 1/8, g S ≈ 2 and ω A /2π = 560 kHz, we have that b A ≈ 3 · 10 32 .…”

Section: Final Remarksmentioning

confidence: 79%

“…Refs. [23][24][25][26][27]), which also has been observed in the laboratory [28][29][30][31][32]. A natural question to be considered could then be to what extent deviations from an exponential decay of atoms or molecules can be observed, a subject which has been addressed in great detail in the literature (see e.g.…”

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confidence: 79%

“…Refs. [23][24][25][26][27]) and one could then perhaps expect to encounter deviations from exponential decay. With the parameters of Refs.…”

Section: Final Remarksmentioning

confidence: 99%

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Memory effects in spontaneous emission processes

et al. 2013

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We consider a quantum-mechanical analysis of spontaneous emission in terms of an effective twolevel system with a vacuum decay rate Γ0 and transition angular frequency ωA. Our analysis is in principle exact, even though presented as a numerical solution of the time-evolution including memory effects. The results so obtained are confronted with previous discussions in the literature. In terms of the dimensionless lifetime τ = tΓ0 of spontaneous emission, we obtain deviations from exponential decay of the form O(1/τ ) for the decay amplitude as well as the previously obtained asymptotic behaviors of the form O(1/τ 2 ) or O(1/τ ln 2 τ ) for τ ≫ 1. The actual asymptotic behavior depends on the adopted regularization procedure as well as on the physical parameters at hand. We show that for any reasonable range of τ and for a sufficiently large value of the required angular frequency cut-off ωc of the electro-magnetic fluctuations, i.e. ωc ≫ ωA, one obtains either a O(1/τ ) or a O(1/τ 2 ) dependence. In the presence of physical boundaries, which can change the decay rate with many orders of magnitude, the conclusions remains the same after a suitable rescaling of parameters.

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“…With the parameters of Refs. [24,26,27], i.e. S 2 = 1/8, g S ≈ 2 and ω A /2π = 560 kHz, we have that b A ≈ 3 · 10 32 .…”

Section: Final Remarksmentioning

confidence: 79%

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confidence: 79%

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Memory effects in spontaneous emission processes

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“…At a trap height less than 10 ÷ 20 µm, thermal magnetic noise exceeds all other harmful influences on the atom cloud (technical noise due to the current supply instability, residual gas collisions, stray magnetic fields) and provides the dominant limit for the lifetime [1,3]. In the last few years, the application of superconducting materials to atom chips has been widely discussed as a perspective to extend the lifetime of cold atoms [7,11,12,13,14]. A recent theoretical estimate [13] of the magnetic noise caused by a superconductor in the Meissner state showed that the lifetime of atoms trapped above a superconducting layer would be, at least, six orders of magnitude longer than above a normal metal in the same geometry.…”

Section: Introductionmentioning

confidence: 99%

Superconducting atom chips: advantages and challenges

et al. 2008

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Superconductors are considered in view of applications to atom chip devices. The main features of magnetic traps based on superconducting wires in the Meissner and mixed states are discussed. The former state may mainly be interesting for improved atom optics, while in the latter, cold atoms may provide a probe of superconductor phenomena. The properties of a magnetic side guide based on a single superconducting strip wire placed in an external magnetic field are calculated analytically and numerically. In the mixed state of type II superconductors, inhomogeneous trapped magnetic flux, relaxation processes and noise caused by vortex motion are posing specific challenges for atom trapping. PACS: 37.10.Gh Atom traps and guides 85.25.Am Superconducting device characterization, design, and modeling Progress towards this goal demands the control and reduction of magnetic noise produced by the metallic components of the atom chip. Randomly fluctuating magnetic fields are generated by thermal current noise in the conducting chip elements and reduce the number of trapped atoms (losses), increase their temperature (heating) and lead to a phase uncertainty in the atom's state (decoherence) -see, for example, [1, 2, 3] and references therein. Theoretical analysis of the magnetic noise generated by a normal metal [4,5,6,7] predicts a fast reduction of the lifetime with the decrease of the distance z t between the trapped atom and the metal surface (trap height); this is in excellent agreement with lifetime measurements [8,9,10]. At a trap height less than 10 ÷ 20 µm, thermal magnetic noise exceeds all other harmful influences on the atom cloud (technical noise due to the current supply instability, residual gas collisions, stray magnetic fields) and provides the dominant limit for the lifetime [1,3]. In the last few years, the application of superconducting materials to atom chips has been widely discussed as a perspective to extend the lifetime of cold atoms [7,11,12,13,14]. A recent theoretical estimate [13] of the magnetic noise caused by a superconductor in the Meissner state showed that the lifetime of atoms trapped above a superconducting layer would be, at least, six orders of magnitude longer than above a normal metal in the same geometry. The analysis presented in [14] predicts an atom lifetime of 5000 s at a trap height of 1 µm. For comparison, at the same height in a normal metal trap the lifetime is less than 0.1 s [9]. At larger heights, the lifetime in a superconducting niobium chip can be much larger (up to 10 11 s at temperature T = 4.2 K).Two first realizations of atom chips with superconducting elements have been reported in Refs. [15] and [16]. In both setups, the trapped atoms were 87 Rb. In the experiment by Nirrengarten et al. [15], the current-carrying wires (in "U" and "Z" shape) were made of niobium and operated at about 4.2 K. The obtained atom spin relaxation time was estimated as 115 s. This value is comparable to the best one achieved for atoms trapped near normal-metal wires [17]. In the second ...

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“…According to a theorem of Anderson [27,28], the presence of non-magnetic impurities, which we only consider in the present paper, will not modify the superconducting energy gap as given by Eq. (17). The complex conductivity will, however, in general be modified due to the presence of such impurities.…”

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Photon emission near superconducting bodies

Skagerstam

Rekdal

2007

Phys. Rev. A

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In the present paper we study the spontaneous photon emission due to a magnetic spin-flip transition of a two-level atom in the vicinity of a dielectric body like a normal conducting metal or a superconductor. For temperatures below the transition temperature Tc of a superconductor, the corresponding spin-flip lifetime is boosted by several orders of magnitude as compared to the case of a normal conducting body. Numerical results of an exact formulation are also compared to a previously derived approximative analytical expression for the spin-flip lifetime and we find an excellent agreement. We present results on how the spin-flip lifetime depends on the temperature T of a superconducting body as well as its thickness H. Finally, we study how non-magnetic impurities as well as possible Eliashberg strong-coupling effects influence the spin-flip rate. It is found that non-magnetic impurities as well as strong-coupling effects have no dramatic impact on the spin-flip lifetime.It is well-known that the rate of spontaneous emission of atoms will be modified due to the presence of a dielectric body [1]. In current investigations of atom microtraps this issue is of fundamental importance since such decay processes have a direct bearing on the stability of e.g. atom chips.In magnetic microtrap experiments, cold atoms are trapped due to the presence of magnetic field gradients created e.g. by current carrying wires [2]. Such microscopic traps provide a powerful tool for the control and manipulation of cold neutral atoms over micrometer distances [3]. Unfortunately, this proximity of the cold atoms to a dielectric body introduces additional decay channels. Most importantly, Johnson-noise currents in the material give rise to electromagnetic field fluctuations. For dielectric bodies at room temperature made of normal conducting metals, these fluctuations may be strong enough to deplete the quantum state of the atom and, hence, expel the atom from the magnetic microtrap [4]. Reducing this disturbance from the surface is therefore strongly desired. In order to achieve this, the use of superconducting dielectric bodies instead of normal conducting metals has been proposed [5]. Some experimental work in this context has been done as well, e.g. by Nirrengarten et al. [6], where cold atoms were trapped near a superconducting surface.In the present article we will consider the spin-flip rate when the electrodynamic properties of the superconducting body are described in terms of either a simple two-fluid model or in terms of the detailed microscopic Mattis-Bardeen [7] and Abrikosov-Gor'kov-Khalatnikov [8] theory of weak-coupling BCS superconductors. In addition, we will also study how non-magnetic impurities, as well as strong coupling effects according to the lowfrequency limit of the Eliashberg theory [9], will affect the spontaneous emission rate.Following Ref.[10] we consider an atom in an initial state |i and trapped at position r A = (0, 0, z) in vacuum near a dielectric body. The rate Γ B of spontaneous and thermally stimul...

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Decay processes in the presence of thin superconducting films

Cited by 6 publications

References 25 publications

Memory effects in spontaneous emission processes

Memory effects in spontaneous emission processes

Superconducting atom chips: advantages and challenges

Photon emission near superconducting bodies

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