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The parallelism between diffraction and interference in optics and quantum interference in Josephson junctions is discussed and studied in details. The interdisciplinary character of the present work is highlighted through specific examples. The Fraunhofer-like pattern of the maximum Josephson current in a single Josephson junction and the periodic field dependence of the critical current in two-junction and in multi-junction quantum interferometers is analyzed and discussed in comparison with the homologous classical optical phenomena.

A Josephson junction (JJ) is a device consisting of two weakly coupled superconductors [

where I is the current flowing through the junction (I_{J} being the maximum value that can flow in the zero-voltage state), ħ = h/2π, h being Planck’s constant, and V is the voltage across the two superconductors. The above equations are named after b. d. Josephson, who received the Nobel Prize in 1973 for having predicted, through Equations (1a) and (1b), the so called d. c. and a. c. Josephson effects [_{J} represents the maximum value of I flowing in the junction in the zero-voltage state. In the a. c. Josephson effect, the voltage across the JJ is kept at a fixed non-zero value V_{0}. Integrating both sides of Equation (1b) we obtain_{0} is the constant of integration. Therefore the current I is seen to oscillate at a frequency_{c} which depended on the particular substance considered [

When Josephson junctions are in the presence of an external magnetic field, interesting phenomena, recalling diffraction and interference in optics [_{J} vs. H curves [_{c} which can be injected in a parallel connection of two JJs (a two-junction quantum interferometer) shows a magnetic field dependence qualitatively similar to the interference pattern seen in the Young’s two-slit experiment [_{c} vs. H curves are similar to those observed in the optical interference with N slits.

In the present work we shall therefore take a close look at these surprising parallelisms. In optics, of course, light itself provides the necessary oscillatory behavior, giving rise to interference phenomena. On the other hand, in superconducting systems the wavelike source is given by the macroscopic wave function describing the quantum state of each superconducting element in the JJ. On the basis of this analogy, in the following section we briefly review fluxoid quantization in a superconducting ring containing a Josepshon junction. In the third section the behavior of a single JJ in the presence of a magnetic field and the Fraunhofer-like pattern in the I_{J} vs. H curves are studied. In the fourth section two-junction quantum interferometers are seen to give I_{c} vs. H curves similar to the interference pattern seen in the Young’s two-slit experiment. In the fifth section quantum interference in a multi-junction quantum interferometer is considered in various examples. Conclusions are drawn in the last section.

In discussing magnetic properties of Josephson junction devices, it is convenient to give a first brief look at flux and fluxoid quantization in multiply connected superconducting systems, by defining the current density J_{S} of super-electrons flowing in a superconductor S. As in any other quantum system described by a wave function Ψ, the particle current density J in a superconducting system can be derived by considering Schroedinger equation for a free particle of mass m and the continuity equation, respectively reported below:

By expanding the time derivative in Equation (2b) and by considering Equation (2a), the expression of the supercurrent J_{S} can be found to be

where the vector potential A has been introduced by means of the minimal substitution _{s} and of the superconducting phase θ [

In this way, the supercurrent J_{S} becomes:

Considering now a multiply connected superconductor (a superconducting ring at the absolute temperature T below the critical temperature T_{c}) in the presence of a magnetic field H, along a path C well inside the superconductor we can consider J_{S} = 0, so that:

where

The quantized values of the trapped flux in a field cooling experiment (i.e., in a situation in which the superconductor temperature T is lowered from T > T_{c} to T < T_{c} in the presence of a magnetic field H) was given in terms of the applied field intensity H by Goodman and Deaver in 1970 [_{ex}). The function Ω is such that, when applied to a real number x, gives the closest integer to x. This function can be easily interpreted by

considering the minima of the normalized magnetic energy_{1} being

the inductance coefficient pertaining to the superconducting ring, _{0} is the magnetic permeability of vacuum and S_{h} is the area of the inner hole of the superconducting structure in which a magnetic field h is present. In fact, by fixing the value of the applied field (which, for a fixed area S_{h}, determines the value flux number n_{ex}), the system arranges itself in the quantized flux state with n trapped fluxons inside the hole that minimizes the magnetic energy ε_{M}.

Let us now consider a superconducting ring interrupted by a Josephson junction, as shown in

By following the path C as prescribed by the right-hand screw rule, the first integral can be calculated in terms of the superconducting phase difference θ_{2} − θ_{1} across the JJ and the second can be opportunely labeled as in

we can rewrite Equation (8) as follows:

Equation (10) is similar to the flux quantization relation (7). However, one can immediately notice that the magnetic flux linked to path C is not quantized when the ring is interrupted by a JJ. Nevertheless, we can notice that the quantity

From Equation (10) we can also argue that magnetic flux Φ linked to a superconducting ring and the gauge- invariant superconducting phase difference ϕ across a Josephson junction interrupting the same ring are two intimately related quantities.

We have seen that the gauge-invariant superconducting phase difference ϕ across a Josephson junction interrupting a superconducting ring is related to the magnetic flux trapped inside the same ring. When considering an isolated extended Josephson junction in the presence of a magnetic field, we may notice that a similar relation exists between ϕ and the flux linked to the barrier. This property leads us to the first type of quantum interference phenomenon: the Fraunhofer-like pattern in the maximum Josephson current I_{0} vs. H curves.

By referring to _{1}A_{2}B_{2}B_{1} is _{1} and l_{2} inside S_{1} and S_{2},

In the limit of

where _{0} is a constant to be determined. By assuming a uni- form current density J flowing in the JJ as shown in

where_{J} flowing in the device can be found by maximizing the expression for I in Equation (13) with respect to ϕ_{0}.

One thus finds:

A graph of the above Fraunhofer-like function I_{J}/I_{0} is reported in _{J}/Φ_{0}, as shown in the reported figure.

In the present section we describe the similarity between the I_{c} vs. H curves for a two-junction quantum interferometer in the presence of a magnetic field H and the interference pattern seen in the Young’s two-slit experiment. Let us then consider the two-junction quantum interferometer schematically represented in _{B} is seen to split in two branch currents, I_{1} and I_{2}. A magnetic field H is applied perpendicularly to the plane of the quantum interferometer. We may start our analysis by writing the fluxoid quantization condition for the system, so that:

where ϕ_{1} and ϕ_{2} are the gauge-invariant superconducting phase differences across JJ1 and JJ2, respectively. The sign for the superconducting phase difference across JJ2 is negative, given that the oriented path around the superconducting loop crossing this junction opposes the assumed positive direction of the current I_{2}.

We may also write the electrodynamic equation defining the flux Φ inside the loop as the sum of the induced flux and the applied flux_{0} being the area of the loop. We may therefore set:

where L is the self-inductance coefficient pertaining to a single branch. Notice that the magnetic flux induced by I_{1} and I_{2} are of opposite signs. Having defined these constraints, we may write down the dynamical equation for each Josephson junction in the loop. By adopting the Resistively Shunted Junction (RSJ) model [

where the two junctions are assumed to be equal, so that they possess the same resistive parameter R and the same maximum Josephson current I_{J}, and where k = 1, 2. Notice that the terms in Equation (17) obey a current conservation relation, when we schematize the JJ by a resistive branch in parallel with an ideal Josephson element carrying a current I_{J}sinϕ_{k}. By now introducing the normalized quantities

By summing and subtracting homologous sides of the above equations, and by defining the new variables ϕ and ψ implicitly as _{ }and

where _{J}_{0} across the two identical JJs is equal to dϕ/dτ. The simplest approach to the solution of the above dynamical equations is to assume that the normalized applied flux is equal to the flux number, so that ψ = ψ_{ex} and only the first of the two above equations is needed in this approximation, namely:

In the zero-voltage state (dϕ/dτ = 0) we notice that the maximum bias current that can be injected in the system has to satisfy the following relation:

In this way, we have:

In _{c} vs. ψ_{ex} quantum interference curves are shown along with the optical figures obtained in a two-slit Young’s experiment [

In the present section we consider the dynamic equation of the multi-junction quantum interferometer. As in the previous section, we see that quantum interference observed in these systems can be related to classical optical phenomena, namely, the interference pattern given by an N slit grating. Let us start by considering the parallel connection of N + 1 Josephson junctions (N ³ 2) as in

where_{k}’s are integers, and

Let us now write the electrodynamic equation defining the flux Φ_{k} inside each loop _{ex}, so that

where the magnetic field H is applied in a direction perpendicular to the plane of the figure and pointing upward with respect to this same plane. The applied flux is thus written as_{0} being the area of each loop. By again adopting the RSJ model [_{J}. In order to simplify our problem, we take all integers n_{k} in Equation (23) equal to zero, and make the hypothesis of zero-inductance loops (L = 0), so that Φ_{k} = Φ_{ex}. Moreover, by considering all JJs to be in the zero-voltage state and by taking Φ_{k} = Φ_{ex}, we may write, for all JJs in the network,

By now recalling the partial sum of a geometric series, we have:

In order to find the value of the maximum current I_{c}, which can be injected in the system without causing phase slips in the JJs, we need to find the value of ϕ_{0} which maximizes the value of I_{B}. Therefore, we finally write:

Up to this point we have not made any assumption on the dependence of I_{J} from the applied flux. Therefore, by recalling Equation (14), we may think that also the pre-factor of the oscillating term depends on the applied field amplitude H, so that

where, as specified in Section 3, I_{0} is the maximum Josephson current and_{J} varies very slowly with H.

In this case, then, we can consider I_{J} constant in Equation (28). In Figures 7(a)-(c) we show the normalized interference pattern in (27) for N = 2, 3, and 4, respectively, for a constant value of I_{J}, along with the correspondingly parallel expressions derived for an interference pattern from a grating with M slits normalized to M, namely:

for M = 3, 4 and 5, observing that the number of JJs in the array is N + 1. We perform the normalization in Equation (29) in order to get the same maximum value of M as in Equation (27), when setting M = N + 1. We notice that the positions of the zeros of both full and dashed curves in Figures 7(a)-(c) are given by specific requirements for_{1} = 1/3, x_{2} = 2/3. For M = 4 (_{1} = 1/4, x_{2} = 1/2, x_{3} = 3/4. Finally, for M = 5 (_{1} = 1/5, x_{2} = 2/5, x_{3} = 3/5, x_{4} = 4/5. In this way, these results can be easily generalized for any M. We may finally notice that the number of secondary maxima inside two successive principal maxima reaching the height M in the curves are in number equal to M − 2.

The parallelism between classical interference phenomena in optics and quantum interference patterns observed in superconducting devices containing Josephson junctions is studied. It is inferred that an irradiated single slit and a single Josephson junction in a magnetic field show similar behavior. In fact, the former optical system presents a Fraunhofer pattern of the light intensity when observed on a distant screen. On the other hand, the Fraunhofer-like pattern of the maximum Josephson current can be detected in the Josephson device. Similarly, in a two-junction quantum interferometer, one may notice a behavior of the critical current I_{c} as a function of the applied magnetic flux Φ_{ex} analogous to the intensity pattern in a two-slit Young’s experiment. Finally, when a multi-junction quantum interferometer containing M JJs is considered, a I_{c} vs. Φ_{ex} curve similar to the light interference pattern given by an M slit grating. In the latter case we may argue that, even though the functions defining the two interference patterns are formally different, the overall qualitative behaviour is similar. In fact,

when we consider the positions of the zeros and the number of lobes in between the principal maxima in the I_{c} vs. Φ_{ex} curves of a multi-junction quantum interferometer containing M JJs and the light interference pattern given by an M-slit grating, we notice a perfect correspondence between these parallel features. Apart from the interdisciplinary aspects of the present work, it is important to consider the nature of the parallelism between the classical and the superconducting quantum phenomena. In fact, while the wave-like nature of light gives rise to interference and diffraction in optics, the oscillating character of the macroscopic wave function in superconducting devices is responsible for quantum interference in Josephson junctions. Therefore, because of the common undulatory nature of optics and quantum dynamics, we may argue that classical interference can be related, in a non-strict sense, to quantum interference in Josephson junction devices.

The author thanks A. Giordano for having critically read the manuscript.