Trasporto acustoelettrico in dispositivi nin planari

La condivisione di informazioni riservate tramite canali di comunicazioni sicuri costituisce da sempre un tema di grande interesse che ha spinto negli anni ad ideare protocolli di crittografia sempre più sofisticati. Nessuno dei sistemi finori proposti garantisce tuttavia l'assoluta sicurezza delle informazioni scambiate. È oggi, pertanto, oggetto di intenso studio la possibilità di implementare algoritmi di comunicazione intrinsecamente sicuri basati sul trasferimento di informazione tramite quanti, piuttosto che tramite pacchetti classici. Il significato quantistico di misura garantisce che un eventuale intercettatore lascerebbe inevitabilmente una traccia della sua attività e la sua presenza potrebbe facilmente essere rivelata. Tra le possibili realizzazioni di un protocollo di questo tipo vi è quella basata sulla trasmissione di stati di polarizzazione di singolo fotone. Ad oggi, tuttavia, non esiste ancora una sorgente di singolo fotone sufficientemente efficiente e compatibile con protocolli di fabbricazione su larga scala. La mia tesi si sviluppa nell'ambito di un progetto di ricerca europeo (SECOQC) che mira alla realizzazione di una sorgente di singoli fotoni basata sul trasferimento controllato di singoli elettroni tramite onde acustiche di superficie (SAW) da una regione di tipo n ad una regione di tipo p di un'eterostruttura di GaAs/AlGaAs separate da una regione intrinseca. In quest'ultima verrà realizzata una punta di contatto in grado di controllare il trasporto di carica indotto dalla SAW tra la regione n e la regione p fino al limite del singolo elettrone. La maggior difficoltà ancora da risolvere per la realizzazione di una sorgente di singoli fotoni come quella sopra descritta consiste nella realizzazione di una giunzione n-i-p planare compatibile col trasporto di carica mediato da SAW: alle due interfacce delle giunzioni n-i e i-p si vengono a creare due barriere di potenziale che, per via della loro altezza (circa metà del gap del GaAs), rendono impossibile il trasporto indotto da SAW. Il lavoro da me svolto presso il laboratorio del NEST-SNS si inserisce in questo contesto: ho progettato e realizzato un opportuno schema di iniezione di carica attraverso una giunzione n-i all'interno di un'eterostruttura GaAs/AlGaAs che permetta alla SAW di trasportare elettroni fino ad una regione di raccolta attraverso una regione intrinseca di diverse decine di μm di lunghezza, compatibile quindi con la fabbricazione di una punta di contatto. Il funzionamento di questo schema si basa sul fatto, osservato e dimostrato per la prima volta durante il mio lavoro di tesi, che la SAW è in grado di estrarre elettroni da una corrente che attraversa una regione di tipo i e di trasportarli per una distanza macroscopica fino ad un contatto di raccolta. Questo fenomeno è di notevole interesse nell’ambito del progetto SECOQC in quanto permette l’iniezione di carica attraverso una giunzione n-i compatibile con trasporto mediato da SAW e potrà essere quindi implementato per la fabbricazione della sorgente di singoli fotoni descritta sopra

A gate- and flux-controlled supercurrent diode

Non-reciprocal charge transport in supercurrent diodes (SDs) polarized growing interest in the last few years for its potential applications in superconducting electronics (SCE). So far, SD effects have been reported in complex hybrid superconductor/semiconductor structures or metallic systems subject to moderate magnetic fields, thus showing a limited potentiality for practical applications in SCE. Here, we report the design and the realization of a monolithic SD by exploiting a Dayem bridge-based superconducting quantum interference device (SQUID). Our structure allows reaching rectification efficiencies ($\eta$) up to about 6%. Moreover, the absolute value and the polarity of $\eta$ can be selected on demand by the modulation of an external magnetic flux or by a gate voltage, thereby guaranteeing high versatility and improved switching speed. Furthermore, our SD operates in a wide range of temperatures up to about the 70% of the superconducting critical temperature of the titanium film composing the interferometer. Our SD can find extended applications in SCE by operating in synergy with widespread superconducting technologies, such as nanocryotrons, rapid single flux quanta (RSFQs) and memories.Comment: 5 pages, 4 figure

Acoustoelectric luminescence from a field-effect n-i-p lateral junction

A surface-acoustic-wave (SAW) driven light-emitting-diode structure that can implement a single-photon-source for quantum-cryptography applications is demonstrated. Our lateral n-i-p junction is realized starting from an undoped GaAs/AlGaAs quantum well by gating. It incorporates interdigitated transducers for SAW generation and lateral gates for current control. We demonstrate acoustoelectric transport and SAW-driven electroluminescence. The acoustoelectric current can be controlled down to complete pinch-off by means of the lateral gates

Surface-acoustic-wave driven planar light-emitting device

Electroluminescence emission controlled by means of surface acoustic waves (SAWs) in planar light-emitting diodes (pLEDs) is demonstrated. Interdigital transducers for SAW generation were integrated onto pLEDs fabricated following the scheme which we have recently developed. Current-voltage, light-voltage and photoluminescence characteristics are presented at cryogenic temperatures. We argue that this scheme represents a valuable building block for advanced optoelectronic architectures

Anti-bunched photons from a lateral light-emitting diode

We demonstrate anti-bunched emission from a lateral-light emitting diode. Sub-Poissonian emission statistic, with a g$^{(2)}$(0)=0.7, is achieved at cryogenic temperature in the pulsed low-current regime, by exploiting electron injection through shallow impurities located in the diode depletion region. Thanks to its simple fabrication scheme and to its modulation bandwidth in the GHz range, we believe our devices are an appealing substitute for highly-attenuated lasers in existing quantum-key-distribution systems. Our devices outperform strongly-attenuated lasers in terms of multi-photon emission events and can therefore lead to a significant security improvement in existing quantum key distribution systems

Acoustic charge transport in n-i-n three terminal device

We present an unconventional approach to realize acoustic charge transport devices that takes advantage from an original input region geometry in place of standard Ohmic input contacts. Our scheme is based on a n-i-n lateral junction as electron injector, an etched intrinsic channel, a standard Ohmic output contact and a pair of in-plane gates. We show that surface acoustic waves are able to pick up electrons from a current flowing through the n-i-n junction and steer them toward the output contact. Acoustic charge transport was studied as a function of the injector current and bias, the SAW power and at various temperatures. The possibility to modulate the acoustoelectric current by means of lateral in-plane gates is also discussed. The main advantage of our approach relies on the possibility to drive the n-i-n injector by means of both voltage or current sources, thus allowing to sample and process voltage and current signals as well.Comment: 9 pages, 3 figures. Submitted to Applied Physics Letter

Circuit-theoretic phenomenological model of an electrostatic gate-controlled bi-SQUID

A numerical model based on a lumped circuit element approximation for a bi-superconducting quantum interference device (bi-SQUID) operating in the presence of an external magnetic field is presented in this paper. Included in the model is the novel ability to capture the resultant behaviour of the device when a strong electric field is applied to its Josephson junctions by utilising gate electrodes. The model is used to simulate an all-metallic SNS (Al-Cu-Al) bi-SQUID, where good agreement is observed between the simulated results and the experimental data. The results discussed in this work suggest that the primary consequences of the superconducting field effect induced by the gating of the Josephson junctions are accounted for in our minimal model; namely, the suppression of the junctions super-current. Although based on a simplified model, our results can potentially help with the task of clarifying the microscopic origin of this effect. Also, the possible applications of this effect regarding the operation of SQUIDs as ultra-high precision sensors, where the performance of such devices can be improved via careful tuning of the applied gate voltages, are discussed at the end of the paper.Comment: 9 pages, 4 figure

Electrostatic field-driven supercurrent suppression in ionic-gated metallic Josephson nanotransistors

Recent experiments have shown the possibility to tune the electron transport properties of metallic nanosized superconductors through a gate voltage. These results renewed the longstanding debate on the interaction between intense electrostatic fields and superconductivity. Indeed, different works suggested competing mechanisms as the cause of the effect: unconventional electric field-effect or quasiparticle injection. By realizing ionic-gated Josephson field-effect nanotransistors (IJoFETs), we provide the conclusive evidence of electrostatic field-driven control of the supercurrent in metallic nanosized superconductors. Our Nb IJoFETs show bipolar giant suppression of the superconducting critical current up to $\sim45\%$ with negligible variation of both the critical temperature and the normal-state resistance, in a setup where both overheating and charge injection are impossible. The microscopic explanation of these results calls upon a novel theory able to describe the non-trivial interaction of static electric fields with conventional superconductivity.Comment: 9 pages, 6 figure