Understanding the saturation power of Josephson Parametric Amplifiers made from SQUIDs arrays

We report on the implementation and detailed modelling of a Josephson Parametric Amplifier (JPA) made from an array of eighty Superconducting QUantum Interference Devices (SQUIDs), forming a non-linear quarter-wave resonator. This device was fabricated using a very simple single step fabrication process. It shows a large bandwidth (45 MHz), an operating frequency tunable between 5.9 GHz and 6.8 GHz and a large input saturation power (-117 dBm) when biased to obtain 20 dB of gain. Despite the length of the SQUID array being comparable to the wavelength, we present a model based on an effective non-linear LC series resonator that quantitatively describes these figures of merit without fitting parameters. Our work illustrates the advantage of using array-based JPA since a single-SQUID device showing the same bandwidth and resonant frequency would display a saturation power 15 dB lower.Comment: 12 pages, 9 figures, Appendices include

A photonic crystal Josephson traveling wave parametric amplifier

An amplifier combining noise performances as close as possible to the quantum limit with large bandwidth and high saturation power is highly desirable for many solid state quantum technologies such as high fidelity qubit readout or high sensitivity electron spin resonance for example. Here we introduce a new Traveling Wave Parametric Amplifier based on Superconducting QUantum Interference Devices. It displays a 3 GHz bandwidth, a -102 dBm 1-dB compression point and added noise near the quantum limit. Compared to previous state-of-the-art, it is an order of magnitude more compact, its characteristic impedance is in-situ tunable and its fabrication process requires only two lithography steps. The key is the engineering of a gap in the dispersion relation of the transmission line. This is obtained using a periodic modulation of the SQUID size, similarly to what is done with photonic crystals. Moreover, we provide a new theoretical treatment to describe the non-trivial interplay between non-linearity and such periodicity. Our approach provides a path to co-integration with other quantum devices such as qubits given the low footprint and easy fabrication of our amplifier.Comment: 6 pages, 4 figures, Appendixe

Low-temperature quantum transport in CVD-grown single crystal graphene

Chemical vapor deposition (CVD) has been proposed for large-scale graphene synthesis for practical applications. However, the inferior electronic properties of CVD graphene are one of the key problems to be solved. In this study, we present a detailed study on the electronic properties of high-quality single crystal monolayer graphene. The graphene is grown by CVD on copper using a cold-wall reactor and then transferred to Si/SiO2. Our low-temperature magneto-transport data demonstrate that the characteristics of the measured single-crystal CVD graphene samples are superior to those of polycrystalline graphene and have a quality which is comparable to that of exfoliated graphene on Si/SiO2. The Dirac point in our best samples is located at back-gate voltages of less than 10V, and their mobility can reach 11000 cm2/Vs. More than 12 flat and discernible half-integer quantum Hall plateaus have been observed in high magnetic field on both the electron and hole side of the Dirac point. At low magnetic field, the magnetoresistance shows a clear weak localization peak. Using the theory of McCann et al., we find that the inelastic scattering length is larger than 1 {\mu}m in these samples even at the charge neutrality point

Low-temperature quantum transport in CVD-grown single crystal graphene

Chemical vapor deposition (CVD) is typically used for large-scale graphene synthesis for practical applications. However, the inferior electronic properties of CVD graphene are one of the key problems to be solved. Therefore, we present a detailed study on the electronic properties of high-quality single-crystal monolayer graphene. The graphene is grown via CVD on copper, by using a cold-wall reactor, and then transferred to Si/SiO2. Our low-temperature magneto-transport data demonstrate that the characteristics of the single-crystal CVD graphene samples are superior to those of polycrystalline graphene and have a quality which is comparable to that of exfoliated graphene on Si/SiO2. The Dirac point in our best samples occurs at back-gate voltages lower than 10 V, and a maximum mobility of 11,000 cm2/(V·s) is attained. More than 12 flat and discernible half-integer quantum Hall plateaus occur under a high magnetic field on both the electron and hole sides of the Dirac point. At a low magnetic field, the magnetoresistance exhibits a weak localization peak. Using the theory of McCann et al., we obtain inelastic scattering lengths of >1 µm, even at the charge neutrality point of the samples

Observation of two-mode squeezing in a traveling wave parametric amplifier

Traveling wave parametric amplifiers (TWPAs) have recently emerged as essential tools for broadband near quantum-limited amplification. However, their use to generate microwave quantum states still misses an experimental demonstration. In this letter, we report operation of a TWPA as a source of two-mode squeezed microwave radiation. We demonstrate broadband entanglement generation between two modes separated by up to 400 MHz by measuring logarithmic negativity between 0.27 and 0.51 and collective quadrature squeezing below the vacuum limit between 1.5 and 2.1 dB. This work opens interesting perspectives for the exploration of novel microwave photonics experiments with possible applications in quantum sensing and continuous variable quantum computing

A tunable Josephson platform to explore many-body quantum optics in circuit-QED

Coupling an isolated emitter to a single mode of the electromagnetic field is now routinely achieved and well understood. Current efforts aim to explore the coherent dynamics of emitters coupled to several electromagnetic modes (EM). freedom. Recently, ultrastrong coupling to a transmission line has been achieved where the emitter resonance broadens to a significant fraction of its frequency. In this work we gain significantly improved control over this regime. We do so by combining the simplicity of a transmon qubit and a bespoke EM environment with a high density of discrete modes, hosted inside a superconducting metamaterial. This produces a unique device in which the hybridisation between the qubit and up to 10 environmental modes can be monitored directly. Moreover the frequency and broadening of the qubit resonance can be tuned independently of each other in situ. We experimentally demonstrate that our device combines this tunability with ultrastrong coupling and a qubit nonlinearity comparable to the other relevant energy scales in the system. We also develop a quantitative theoretical description that does not contain any phenomenological parameters and that accurately takes into account vacuum fluctuations of our large scale quantum circuit in the regime of ultrastrong coupling and intermediate non-linearity. The demonstration of this new platform combined with a quantitative modelling brings closer the prospect of experimentally studying many-body effects in quantum optics. A limitation of the current device is the intermediate nonlinearity of the qubit. Pushing it further will induce fully developed many-body effects, such as a giant Lamb shift or nonclassical states of multimode optical fields. Observing such effects would establish interesting links between quantum optics and the physics of quantum impurities.Comment: Main paper and Supplementary Information combined in one file. List of the modifications in the final version: new abstract and introduction, comparison to RWA treatment, more precise capacitance mode

State preparation of a fluxonium qubit with feedback from a custom FPGA-based platform

We developed a versatile integrated control and readout instrument for experiments with superconducting quantum bits (qubits), based on a field-programmable gate array (FPGA) platform. Using this platform, we perform measurement-based, closed-loop feedback operations with $428 \, \mathrm{ns}$ platform latency. The feedback capability is instrumental in realizing active reset initialization of the qubit into the ground state in a time much shorter than its energy relaxation time $T_1$. We show experimental results demonstrating reset of a fluxonium qubit with $99.4\,\%$ fidelity, using a readout-and-drive pulse sequence approximately $1.5 \, \mathrm{\mu s}$ long. Compared to passive ground state initialization through thermalization, with the time constant given by $T_1 = ~ 80 \, \mathrm{\mu s}$, the use of the FPGA-based platform allows us to improve both the fidelity and the time of the qubit initialization by an order of magnitude.Comment: 3 pages, 2 figures. The following article has been submitted to the AIP Conference Proceedings of the Fifth International Conference on Quantum Technologies (ICQT-2019

Broadband parametric amplification for multiplexed SiMOS quantum dot signals

Spins in semiconductor quantum dots hold great promise as building blocks of quantum processors. Trapping them in SiMOS transistor-like devices eases future industrial scale fabrication. Among the potentially scalable readout solutions, gate-based dispersive radiofrequency reflectometry only requires the already existing transistor gates to readout a quantum dot state, relieving the need for additional elements. In this effort towards scalability, traveling-wave superconducting parametric amplifiers significantly enhance the readout signal-to-noise ratio (SNR) by reducing the noise below typical cryogenic low-noise amplifiers, while offering a broad amplification band, essential to multiplex the readout of multiple resonators. In this work, we demonstrate a 3GHz gate-based reflectometry readout of electron charge states trapped in quantum dots formed in SiMOS multi-gate devices, with SNR enhanced thanks to a Josephson traveling-wave parametric amplifier (JTWPA). The broad, tunable 2GHz amplification bandwidth combined with more than 10dB ON/OFF SNR improvement of the JTWPA enables frequency and time division multiplexed readout of interdot transitions, and noise performance near the quantum limit. In addition, owing to a design without superconducting loops and with a metallic ground plane, the JTWPA is flux insensitive and shows stable performances up to a magnetic field of 1.2T at the quantum dot device, compatible with standard SiMOS spin qubit experiments

Non-degenerate parametric amplifiers based on dispersion engineered Josephson junction arrays

Determining the state of a qubit on a timescale much shorter than its relaxation time is an essential requirement for quantum information processing. With the aid of a new type of non-degenerate parametric amplifier, we demonstrate the continuous detection of quantum jumps of a transmon qubit with 90% fidelity in state discrimination. Entirely fabricated with standard two-step optical lithography techniques, this type of parametric amplifier consists of a dispersion engineered Josephson junction (JJ) array. By using long arrays, containing $10^3$ JJs, we can obtain amplification at multiple eigenmodes with frequencies below $10~\mathrm{GHz}$, which is the typical range for qubit readout. Moreover, by introducing a moderate flux tunability of each mode, employing superconducting quantum interference device (SQUID) junctions, a single amplifier device could potentially cover the entire frequency band between 1 and $10~\mathrm{GHz}$.Comment: P.W. and I.T. contributed equally. 9 pages, 5 figures and appendice

Amplification paramétrique en résonance et à ondes progressives proche de la limite quantique

A requirement for quantum information experiments using superconducting quantum circuits is the readout of extremely weak microwave signals. Quantum limited amplifiers are essential for such task and this is precisely why they occupy an always-growing importance in such experiments, particularly for superconducting quantum bits readout.In this thesis, we present two quantum limited amplifiers architectures: the first one is based on a resonator while the second one leverages a traveling-wave construction. These amplifiers both take advantage of tunable Josephson junctions (squid). It is the ideal component for quantum limited parametric amplification: it is both nonlinear and dissipationless (given its superconducting nature). This thesis demonstrates the operation of these Josephson quantum limited amplifiers and how they go beyond the state-of-the-art thanks to original design choices.An important limitation of resonant Josephson parametric amplifiers is their poor dynamic range. We identified the Kerr nonlinearity as the main cause, and we successfully managed to tailor it using Josephson junction arrays. We also developed a model in good agreement with the experimental data, therefore validating our initial assumption that the Kerr nonlinearity and saturation are linked. However, most of the resonant Josephson parametric amplifiers suffer from a fundamental issue, arising from their architecture. It is the conservation of the gain bandwidth product, therefore limiting their bandwidth. Traveling-wave architectures are well suited to overcome this limitation. However, two new issues must be dealt with: impedance matching and phase matching. To tackle the former, we developed an original and simple fabrication process in order to obtain a 50ohm matched 2000 squid array. As for the phase, we matched it by periodically modulating the impedance of the squid transmission line in order to open a photonic band gap. Therefore, we locally distort the dispersion relation. This step does not add any complexity to the initial fabrication process. Moreover, we developed a model in good agreement with experimental data while considering the nonlinear behavior of the photonic gap in a Josephson metamaterial. In addition to demonstrating high performance amplifiers poised for numerous quantum technological applications, this thesis opens the door to more fundamental quantum optics experiments taking advantage of these highly nonlinear transmission lines.With the resonant amplifiers, we measured a 1dB compression point around -117dBm and 45MHz bandwidth (at 20 db gain). With the traveling-wave amplifiers, we measured 18dB gain on a 2.25GHz bandwidth, a 1dB compression point reaching -103dBm and near quantum-limited noise performances.Un des défis des expériences d’information quantique avec des circuits quantiques supraconducteurs est la lecture de signaux microondes de très basses énergies. Les amplificateurs opérant à la limite quantique du bruit sont indispensables à cette tâche. C’est donc pourquoi ils occupent une place toujours plus prépondérante dans de telles expériences, en particulier pour la lecture de bits quantiques supraconducteurs.Dans cette thèse, nous présentons deux architectures d’amplificateurs limités quantiquement : la première est une architecture employant un résonateur alors que la seconde est basée sur des ondes progressives. Ces deux types d’amplificateurs ont en commun d’avoir comme brique élémentaire une jonction Josephson accordable en flux (squid). Cette dernière est le support idéal pour une amplification paramétrique limitée quantiquement : elle est à la fois non-linéaire et non dissipative (de par son caractère supraconducteur). Cette thèse s’emploie à démontrer le fonctionnement des amplificateurs paramétriques Josephson limités quantiquement que nous avons conçus et qui, grâce à des architectures originales, présentent des performances qui vont au-delà de l’état de l’art.Un inconvénient majeur des amplificateurs Josephson en résonance est leur étendue dynamique très limitée. Nous avons identifié l’origine de cette limitation, la non-linéarité Kerr, et avons réussi à l’adapter via l’implémentation de chaines de jonctions Josephson. Nous avons développé un modèle en bon accord avec nos données expérimentales, validant l’effet de la non-linéarité Kerr sur la saturation. Malgré tout, la plupart des amplificateurs résonants souffrent d’une limitation intrinsèque à leur architecture : la conservation du produit gain bande passante, limitant cette dernière. L’architecture à ondes progressives est un candidat idéal pour passer outre cette loi de conservation. Cependant, elle fait face à deux problèmes : l’adaptation en impédance et en phase. Concernant l’adaptation en impédance, nous avons mis au point un procédé de fabrication, simple et inédit, permettant d’avoir une impédance de 50 ohms avec une chaîne de plus de 2000 squids. Concernant l’adaptation en phase, nous avons faiblement modulé périodiquement l’impédance de la ligne de squid pour ouvrir un gap photonique et déformer localement la relation de dispersion. Cette technique n’ajoute en rien à la complexité du processus de fabrication, qui se démarquait déjà par sa simplicité. De plus, un modèle en bon accord avec nos données expérimentales a été développé, prenant notamment en compte le comportement non-linéaire du gap photonique dans un métamatériaux Josephson. Au-delà des amplificateurs limités quantiquement démontrés et leurs nombreuses applications, cette thèse ouvre des perspectives d’expériences d’optique quantique fondamentales à faire sur ces lignes de transmission hautement non-linéaires.Avec les amplificateurs en résonance, nous avons atteint des points de compression à 1dB (à 20dB de gain) de -117dBm, pour 45MHz de bande passante. Avec les amplificateurs à ondes progressives nous avons atteint un gain maximal de 18dB pour une bande-passante de 2.25GHz et un points de compression à 1dB atteignant -103dBm, tout en restant proche de la limite quantique de bruit