A Josephson diode (JD) is a special type of Josephson junction (JJ), which conducts (super)current in one direction better that in the opposite direction. In other words it has asymmetric current-voltage characteristic. Since Josephson diode is a superconducting device, the asymmetry of the supercurrent transport is the main focus of attention. Opposite to conventional Josephson junctions, the critical (maximum) supercurrents and for opposite bias directions are different by absolute values (). In the presence of such a non-reciprocity, the bias currents of any magnitude in the range between and can flow without resistance in only one direction.
This asymmetry, characterized by the ratio of critical currents , is the main figure of merit of Josephson diodes and is the subject of new developments and optimizations. The Josephson diode effect can occur, e.g., in superconducting devices where time reversal symmetry and inversion symmetry are broken. [1] [2]
Josephson diodes can be subdivided into two categories, those requiring an external (magnetic) field to be asymmetric and those not requiring an external magnetic field --- the so-called “field-free” Josephson diodes (more attractive for applications). In 2021, the Josephson diode was realized in the absence of applied magnetic field in a non-centrosymmetric material, [3] followed shortly by the first realization of the zero-field Josephson diode in an inversion-symmetric device. [4]
Since decades the physicists tried to construct Josephson junction devices with asymmetric critical currents. This didn't involve new physical principles or advanced (quantum) material engineering, but rather electrical engineering tricks like combining several JJs in a special way (e.g. asymmetric 3JJ SQUID) or specially designed long JJs or JJ arrays, where Josephson vortex transport was asymmetric in opposite directions. After all, if one does not look inside the device, but treats such a device as a black box with two electrodes, its current-voltage characteristic is asymmetric with . Such devices were especially popular in the context of Josephson ratchets -- devices used to rectify random or deterministic signals with zero time-average. These devices can be subdivided into several classes:
Starting from 2020 one observes a new wave of interest to the systems with non-reciprocal supercurrent transport based on novel materials and physical principles.
In-depth review of recent developments. [36]
The superconducting diode effect is an example of nonreciprocal superconductivity, where a material is superconducting in one direction and resistive in the other. This leads to half-wave rectification when a square wave AC-current is applied. In 2020, this effect was demonstrated in an artificial [Nb/V/Ta]n superlattice. [27] The phenomenon in the Josephson diode is believed to originate from asymmetric Josephson tunneling. [3] Unlike conventional semiconducting junction diodes, the superconducting diode effect can be realized in Josephson junctions as well as junction-free bulk superconductors. [36]
Currently, the precise mechanism behind the Josephson diode effect is not fully understood. However, some theories have emerged that are now under theoretical investigation. There are two types of Josephson diodes, relating to which symmetries are being broken. The inversion breaking Josephson diode, and the Josephson diode breaking inversion breaking and time-reversal. The minimal symmetry breaking requirement for forming the Josephson diode is inversion symmetry breaking, and is required to obtain nonreciprocal transport. [37] One proposed mechanism originates from finite momentum Cooper pairs. [1] [2] It may also be possible that the superconducting diode effect in the JD originates from self-field effects, but this still has to be rigorously studied. [38] [39]
Depending on the potential application different parameters of the Josephson diodes, from operation temperature to their size can be important. However, the most important parameter is the asymmetry of critical currents (also called non-reciprocity). It can be defined as dimensionless ratio of larger to smaller critical currents
to be always positive and lay in the range from 1 (symmetric JJ) to (infinitely asymmetric one). Instead, some researchers use the so-called efficiency, defined as
It lays in the range from 0 (symmetric system) to 1 (infinitely asymmetric system). [lower-alpha 2] Among other things the efficiency shows the theoretical limit for thermodynamic efficiency (ratio of output to input power) that can be reached by the diode during rectification.
Intuitively it is clear that the larger the asymmetry is, the better the diode performs. A quantitative analysis [14] [40] showed that a large asymmetry allows one to achieve rectification in a wide range of driving current amplitudes, a large counter current (corresponding to a heavy load), against which rectification is still possible, and a large thermodynamic efficiency (ratio of output dc to input ac power).
Thus, to create a practically relevant diode one should design a system with high asymmetry. The asymmetry ratios (efficiency) for different implementations of Josephson diodes are summarized in the table below.
Size. Previously demonstrated Josephson diodes were rather large (see the table), which hampers their integration into micro- or nano-electronic superconducting circuits or stacking. Novel devices based on heterostructures can potentially have 100nm-scale dimensions, which is difficult to achieve using previous approaches with long JJs, fluxons, etc.
Voltage. Important parameter of any nano-rectifier is the maximum dc voltage produced. See the table for comparison.
Operating temperature. Ideally one would like to operate the diode in wide temperature range. Obviously, an upper limit in operation temperature is given by the transition temperature of the superconducting material(s) used to fabricate the Josephson diodes. In the table below we quote the operating temperature for which the other parameters such as asymmetry are quoted. Many novel diodes, unfortunately, operate below 4.2K.
Reference | type | [lower-greek 1] | area() [lower-greek 2] | (K) [lower-greek 3] | ||
---|---|---|---|---|---|---|
Carapella (2001) [11] | ALJJ [lower-greek 4] | 1.2 | 9% | 20 | 44500 | 6.5 |
Beck (2005) [13] | ALJJ [lower-greek 4] Nb-AlO-Nb | 2.2 | 38% | 20 | 5700 | 6 |
Wang (2009) [15] | ASILJJ [lower-greek 5] BSCCO | 2.8 | 47% | 100 | 800 | 4.2 |
Knufinke (2012) [14] | ALJJ [lower-greek 4] Nb-AlO-Nb | 1.6 | 23% | 40 | 4900 | 4.2 |
Sterck (2005,2009) [8] [9] | 3JJ-SQUID Nb-AlO-Nb | 2.5 | 43% | 25 | 1125 | 4.2 |
Paolucci (2023) [10] | 2JJ SQUID | 3 | 50% | 8 | 72 | 0.4 |
Menditto (2016) [24] | -JJ Nb-AlO-NbCu-Nb | 2.5 | 43% | 150 | 2000 | 1.7 |
Golod (2022) [25] | inline Nb JJ | 4(10) | 60%(82%) | 8 | 7.2 | 7 |
Schmid (2024) [26] | inline YBCO JJ | 7 | 75% | 212 | 1 | 4.2 |
Wu (2022) [3] | NbSe2-Nb3Br8-NbSe2 | 1.07 | 3.4% | 1600 [lower-greek 6] | 3.7 | 0.02 |
Jeon (2022) [34] | Nb-Pt+YIG-Nb | 2.07 | 35% | - | ~4 | 2 |
Pal (2022) [35] | Nb-Ti-NiTe2-Ti-Nb | 2.3 | 40% | 8 [lower-greek 6] | ~3 | 3.8 |
Baumgartner (2022) [33] | Al-2DEG-Al | 2 | 30% | - | 7 | 0.1 |
Ghosh (2024) [41] | twisted BSCCO flakes | 4 | 60% | 25 [lower-greek 6] | 100 | 80 |
In the philosophy of thermal and statistical physics, the Brownian ratchet or Feynman–Smoluchowski ratchet is an apparent perpetual motion machine of the second kind, first analysed in 1912 as a thought experiment by Polish physicist Marian Smoluchowski. It was popularised by American Nobel laureate physicist Richard Feynman in a physics lecture at the California Institute of Technology on May 11, 1962, during his Messenger Lectures series The Character of Physical Law in Cornell University in 1964 and in his text The Feynman Lectures on Physics as an illustration of the laws of thermodynamics. The simple machine, consisting of a tiny paddle wheel and a ratchet, appears to be an example of a Maxwell's demon, able to extract mechanical work from random fluctuations (heat) in a system at thermal equilibrium, in violation of the second law of thermodynamics. Detailed analysis by Feynman and others showed why it cannot actually do this.
In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. The effect is named after the British physicist Brian Josephson, who predicted in 1962 the mathematical relationships for the current and voltage across the weak link. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mechanics are observable at ordinary, rather than atomic, scale. The Josephson effect has many practical applications because it exhibits a precise relationship between different physical measures, such as voltage and frequency, facilitating highly accurate measurements.
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In quantum computing, and more specifically in superconducting quantum computing, a transmon is a type of superconducting charge qubit designed to have reduced sensitivity to charge noise. The transmon was developed by Robert J. Schoelkopf, Michel Devoret, Steven M. Girvin, and their colleagues at Yale University in 2007. Its name is an abbreviation of the term transmission line shunted plasma oscillation qubit; one which consists of a Cooper-pair box "where the two superconductors are also [capacitively] shunted in order to decrease the sensitivity to charge noise, while maintaining a sufficient anharmonicity for selective qubit control".
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Alexandre Bouzdine (Buzdin) (in Russian - Александр Иванович Буздин; born March 16, 1954) is a French and Russian theoretical physicist in the field of superconductivity and condensed matter physics. He was awarded the Holweck Medal in physics in 2013 and obtained the Gay-Lussac Humboldt Prize in 2019 for his theoretical contributions in the field of coexistence between superconductivity and magnetism.
Alexander Avraamovitch Golubov is a doctor of physical and mathematical sciences, associate professor at the University of Twente (Netherlands). He specializes in condensed matter physics with the focus on theory of electronic transport in superconducting devices. He made key contributions to theory of Josephson effect in novel superconducting materials and hybrid structures, and to theory of multiband superconductivity.