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Beilstein Journal of Nanotechnology

Coherent amplification of radiation from two phase-locked Josephson junction arrays

Mikhail A. Galin, V. M. Krasnov, I. A. Shereshevsky, Nadezhda K. Vdovicheva, Vladislav V. Kurin

Published Dec 6, 2022• 5 citations
Publisher

Abstract

We analyze experimentally and theoretically mutual phase locking and electromagnetic interaction between two linear arrays with a large number of Josephson junctions. Arrays with different separation, either on the same chip or on two separate substrates are studied. We observe a large coherent gain, up to a factor of three, of emitted power from two simultaneously biased arrays, compared to the sum of powers from two individually biased arrays. The phenomenon is attributed to the phase locking of junctions in different arrays via a common electromagnetic field. Remarkably, the gain can exceed the factor of two expected for a simple constructive interference of two oscillators. The larger gain is explained by an additional consequence of mutual interaction between two large arrays. Mutual phase locking of large arrays does not only result in constructive interference outside the arrays, but also improved synchronization of junctions inside each array. Our conclusion is supported by numerical modelling.

DOI
10.3762/bjnano.13.119

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We analyze experimentally and theoretically mutual phase locking and electromagnetic interaction between two linear arrays with a large number of Josephson junctions. Arrays with different separation, either on the same chip or on two separate substrates are studied. We observe a large coherent gain, up to a factor of three, of emitted power from two simultaneously biased arrays, compared to the sum of powers from two individually biased arrays. The phenomenon is attributed to the phase locking of junctions in different arrays via a common electromagnetic field. Remarkably, the gain can exceed the factor of two expected for a simple constructive interference of two oscillators. The larger gain is explained by an additional consequence of mutual interaction between two large arrays. Mutual phase locking of large arrays does not only result in constructive interference outside the arrays, but also improved synchronization of junctions inside each array. Our conclusion is supported by numerical modelling.
We performed experimental investigations of the interaction between separate discrete niobium Josephson junction (JJ) arrays. It was found out that after tuning the arrays to the equal Josephson frequency the amplification of power is registered. The arrays are straight strip lines with 332 JJs oriented in parallel and located on common silicon substrate as well as on different substrates configured in a stack. The effect of amplification is observed at frequencies of up to 204 GHz while the gain relatively to the sum of power from alone radiated arrays can be as large as 3. These results indicate the mutual synchronization of niobium JJ arrays that would forces the submillimeter generation technique based on large JJ arrays with superwavelength dimensions.
We have studied a new type of Josephson junction amplifier which is based on a two-junction array; the junctions are mutually phase locked at the Josephson self-oscillation frequency. With this frequency at 82 GHz, the voltages of the junctions remain equal (locked) for a bias current range as large as 60% of the critical current. Over a much smaller bias range, with an applied signal frequency of 1 kHz, a small-signal power gain of 19 dB was measured, accompanied by a negative resistance input impedance. This performance is consistent with a quasistatic theory of the amplifier.
We have investigated the mutual phase interaction between a single detector junction and a phase-locked three-junction array of high-T/sub c/ Josephson junctions. The YBCO Josephson junctions were fabricated on a MgO bicrystal substrate. In the array junction structure, the dc bias current was fed to the junction array having parallel geometry with each pair of junctions shunted by a superconducting coplanar microstrip superconducting loop. The configuration of bias leads was a series connection of interlocking dc SQUIDs geometry which enables all junctions to oscillate at the same frequency. We measured the radiation power of a Josephson-junction array using an off-chip coupled to a radiometer and on-chip coupled to a single junction from centimeter- to millimeter-wave ranges. Clear self-induced Shapiro steps were observed in the current-voltage curves of the detector junction. The power transfered to the single detector junction was estimated from the modulation of Shapiro step height. The maximum detected power level was about 7 nW at frequency 150-250 GHz. The Josephson oscillation power excited the resonant mode coupled to the coplanar microstrip loops.
Mutual phase locking has been demonstrated in series arrays of two and four Josephson junctions at millimeter-wave frequencies. Experimental observations are in good agreement with theory reported earlier. This technique increases the output power available from a Josephson junction source. Available output power is expected to be proportional to the square of the number of junctions until the array impedance approaches the load impedance. The output frequency is voltage tunable over as much as an octave. Theory indicates that the technique can be extended to even larger arrays.
We have investigated experimentally a novel type of self-synchronizing two-dimensional Josephson junction array based on special symmetry breaking network architectures. The measurements confirm the theoretical prediction that in such "selector Josephson networks" the Josephson oscillators mutually synchronize into a coherent in-phase state and that such arrays are capable of emitting maximum coherent microwave power in the entire theoretically expected frequency range. The DC biased oscillator array operates coherently even without a reinjection of the generated microwave by an external load or by a resonator cavity. The selector network possesses a much higher tolerance against imperfections, perturbations and load parameter variations than conventional regular two-dimensional or one-dimensional arrays. The experimental sample has been fabricated in an industrial process using Nb-technology, in effect providing relatively large spreads in the array junction parameters. The measured frequency range goes from 85 GHz up to 380 GHz with a maximum microwave power of 0.16 /spl mu/W matched to a coupled load for an array with only 100 active Nb-AlO/sub x/-Nb junctions.
Mutual phase locking of Josephson oscillations in a two-junction cell and multijunction arrays is analysed. The locking is due to ac currents of Josephson frequency ω, which flow through some special coupling circuit. For the most important case of almost identical junctions, the locking frequency range is shown to be proportional to the imaginary part of the complex conductivity Y(ω) of the coupling circuit. The power and linewidth of coherent oscillations, as well as the coherence stability with respect to the junction intrinsic noise and parameter spread, have been calculated. The maximum locking range and, hence, the maximum parameter tolerances are shown to take place in the closed-loop-type (ring) structures with long-range junction interactions.