Superconducting quantum circuits provide a versatile and powerful platform for quantum computing as well as for the study of fundamental physics and quantum technologies in general, such as radiation sensors. The development of novel detectors and measurement techniques is important for advancing quantum technologies by enabling detection schemes that are faster, more accurate, or less complex than the current state of the art. In this thesis, we improve upon the existing measurement techniques in superconducting quantum circuits by employing novel methods on a radiation sensor based on superconductor–normal-metal–superconductor (SNS) junctions, in addition to superconducting quantum bits (qubits). The presented methods are experimentally demonstrated in the five publications of this thesis. First, we demonstrate that an SNS sensor, essentially a power or a photon detector, has the capability to directly measure the variance of itinerant microwave photons. Experimentally, we show the measurement of the zero-delay second-order correlation function of interfering thermal and coherent photons displaying bunching with increasing proportion of thermal photons. We combine the signal field with a homodyne field and carry out Wigner function tomography by utilizing techniques from quantum optics. Second, we show a new readout technique for superconducting qubits by multichannel driving which utilizes effectively longitudinal coupling of the qubit and a readout cavity. We find that applying this technique reduces the readout error by 50%, while being roughly 20% faster than the conventional dispersive readout. We finish the thesis by displaying methods for making quantum circuits more compact. We demonstrate a rigorous process for the design of lumped-element resonators without utilizing any computer simulations. The fabricated resonators have a small physical footprint compared to that of coplanar waveguide resonators ubiquitous in superconducting circuits. We achieve an accuracy of 99% in the designed resonance frequency with a standard deviation of 1%, demonstrating the applicability of the model. The final publication demonstrates multiplexed readout of the SNS sensors, reducing the number of required control and measurement lines.
Superconducting quantum circuits provide a versatile and powerful platform for quantum computing as well as for the study of fundamental physics and quantum technologies in general, such as radiation sensors. The development of novel detectors and measurement techniques is important for advancing quantum technologies by enabling detection schemes that are faster, more accurate, or less complex than the current state of the art. In this thesis, we improve upon the existing measurement techniques in superconducting quantum circuits by employing novel methods on a radiation sensor based on superconductor–normal-metal–superconductor (SNS) junctions, in addition to superconducting quantum bits (qubits). The presented methods are experimentally demonstrated in the five publications of this thesis. First, we demonstrate that an SNS sensor, essentially a power or a photon detector, has the capability to directly measure the variance of itinerant microwave photons. Experimentally, we show the measurement of the zero-delay second-order correlation function of interfering thermal and coherent photons displaying bunching with increasing proportion of thermal photons. We combine the signal field with a homodyne field and carry out Wigner function tomography by utilizing techniques from quantum optics. Second, we show a new readout technique for superconducting qubits by multichannel driving which utilizes effectively longitudinal coupling of the qubit and a readout cavity. We find that applying this technique reduces the readout error by 50%, while being roughly 20% faster than the conventional dispersive readout. We finish the thesis by displaying methods for making quantum circuits more compact. We demonstrate a rigorous process for the design of lumped-element resonators without utilizing any computer simulations. The fabricated resonators have a small physical footprint compared to that of coplanar waveguide resonators ubiquitous in superconducting circuits. We achieve an accuracy of 99% in the designed resonance frequency with a standard deviation of 1%, demonstrating the applicability of the model. The final publication demonstrates multiplexed readout of the SNS sensors, reducing the number of required control and measurement lines.
This dissertation focuses on the development of a new type of thermal microwave photodetector based on superconductor–normal-metal–superconductor (SNS) junctions. We motivate the development of the detector mainly by microwave quantum optics applications in the context of superconducting qubits coupled to microwave transmission lines and resonators, i.e., in the context of circuit quantum electrodynamics (cQED). In cQED, single-photon microwave pulses naturally arise as a results of a qubit exchanging its excitation with a transmission line. While immense progress has been achieved in cQED in general, and in linear microwave amplification in particular, the challenge of developing an efficient and practical detector for single itinerant photons remains open, mostly due to the exceedingly small energy of individual microwave photons. This prevents microwave implementations of a certain class of quantum optical protocols, including the parity measurement protocol proposed in this dissertation. The core of this dissertation is dedicated to introducing our detector design, discussing the thermal properties of the detector in detail, and demonstrating the operation of the detector in a time-gated threshold detection mode. In particular, we demonstrate threshold detection of coherent 8.4 GHz microwave pulses containing roughly 200 photons, or 1.1 zJ of energy. Compared to other thermal detectors, this is an order of magnitude improvement in the energy of the detected pulses. In addition, we characterize the linear response of the SNS junctions as separate components. That is, we embed the junctions in a microwave circuit that is specifically designed to allow determining the electrical admittance of the SNS junctions from the response of the circuit as a whole.
This dissertation focuses on the development of a new type of thermal microwave photodetector based on superconductor–normal-metal–superconductor (SNS) junctions.
We motivate the development of the detector mainly by microwave quantum optics applications in the context of superconducting qubits coupled to microwave transmission lines and resonators, i.e., in the context of circuit quantum electrodynamics (cQED). In cQED, single-photon microwave pulses naturally arise as a results of a qubit exchanging its excitation with a transmission line. While immense progress has been achieved in cQED in general, and in linear microwave amplification in particular, the challenge of developing an efficient and practical detector for single itinerant photons remains open, mostly due to the exceedingly small energy of individual microwave photons. This prevents microwave implementations of a certain class of quantum optical protocols, including the parity measurement protocol proposed in this dissertation.
The core of this dissertation is dedicated to introducing our detector design, discussing the thermal properties of the detector in detail, and demonstrating the operation of the detector in a time-gated threshold detection mode. In particular, we demonstrate threshold detection of coherent 8.4 GHz microwave pulses containing roughly 200 photons, or 1.1 zJ of energy. Compared to other thermal detectors, this is an order of magnitude improvement in the energy of the detected pulses.
In addition, we characterize the linear response of the SNS junctions as separate components. That is, we embed the junctions in a microwave circuit that is specifically designed to allow determining the electrical admittance of the SNS junctions from the response of the circuit as a whole.; Tama vaitoskirja keskittyy uudentyyppisen mikroaaltofotoni-ilmaisimen kehitykseen. Ilmaisin perustuu suprajohde–normaalimetalli–suprajohde-liitoksiin (SNS).
Ilmaisin on suunnattu lahinna mikroaaltotaajuuksilla tapahtuviin kvanttioptiikan sovellutuksiin, joissa suprajohtavat kvanttibitit (eli kubitit) kytketaan mikroaaltosiirtolinjoihin. Tallaisissa niin sanotuissa piirikvanttisahkodynamiikan kokeissa yhden fotonin mikroaaltopulsseja syntyy luonnollisesti virittyneen kubitin luovuttaessa energiansa mikroaaltosiirtolinjaan. Yhden fotonin liikkuvia mikroaaltopulsseja ei kuitenkaan kyeta havaitsemaan tehokkaasti nykyaan kaytossa olevalla tekniikalla, lahinna yksittaisten mikroaaltofotonien haviavan pienen energian vuoksi. Tama siitakin huolimatta, etta lineaariset vahvistimet, joita useimmiten kaytetaan mikroaaltomittauksissa, ovat kehittyneet viime vuosina vahvasti. Ilman yksifotoni-ilmaisia eraita kvanttioptiikan menetelmia ei voida kuitenkaan hyodyntaa, mukaan lukien pariteettimittausmenetelma, jota ehdotetaan tassa vaitoskirjassa.
Vaitoskirjan keskeisimmassa osassa esitetaan ilmaisimen toimintaperiaate ja tarkeimmat termodynaamiset ominaisuudet seka tutkitaan kuinka heikkoja mikroaaltopulsseja ilmaisin kykenee havaitsemaan. Jos pulssin…
Superconducting circuits constitue a promising platform for physically realizing a quantum computer. One of the most fundamental operations of a quantum computer is to measure the state of its quantum bits (qubits). The measurement process should be as fast as possible, in order to minimize errors in the computation caused by noise. Conventionally, the state of a qubit in a superconducting circuit is determined by coupling the qubit to a readout resonator and measuring how the state of the resonator changes when a readout pulse is applied to it. Recently, a method was proposed that makes it possible to read out the state of a qubit faster by applying pulses to both the resonator and the qubit simultaneously. In this thesis, we study whether this so-called multichannel technique could be used to unconditionally drive the readout resonator back to its initial state after the readout has been finished. Using numerical simulations, we find that compared to just waiting passively, the time it takes to initialize the resonator can be reduced by up to 86%. We also implement a part of the initialization protocol experimentally and find the results encouraging for the future implementation of the full protocol.
Superconducting circuits provide a controllable and versatile platform for studying quantum phenomena and advancing quantum technologies. Josephson junctions and resonators are the central circuit elements, as they enable the realization of macroscopic quantum systems—such as qubits—that can be conveniently controlled, probed, and integrated with non-superconducting components. The implementation of a thermal reservoir through the integration of a dissipative element, such as a normal-metal resistor, offers an experimental platform for investigating quantum thermodynamics, heat management in circuits, on-chip thermal detectors, and the realization of quantum thermal machines. In this thesis, we integrate a mesoscopic normal metal and its associated thermometers as an on-chip bolometer within circuit quantum electrodynamics devices for thermal detection and quantum thermodynamics experiments. The on-chip bolometer is utilized as a sensitive and broadband thermal spectrometer to characterize the properties of a superconducting resonator and to investigate its dominant loss channels. We demonstrate the capability of our thermal detector to resolve the resonator modes over a wide frequency range extending above 20 GHz. When two thermal baths are connected through a qubit–resonator system, a quantum heat valve can be realized. This device has been demonstrated using both transmon and flux qubits. Photonic heat transport between the two baths is controlled by tuning the qubit energy via an applied static magnetic field. To model these experiments, we propose an approach based on electromagnetic simulation and microwave circuit theory, restricted to the linear-response regime. Despite this limitation, the method successfully captures the experimental observations and provides a practical tool for analysing and designing circuits for future experiments. A periodically driven qubit coupled to two unequal resonators, each terminated by a heat reservoir, enables the realization of a quantum Otto refrigerator. The use of a transmon qubit for this purpose is limited by its weak anharmonicity. We therefore explore alternative platforms for the experimental implementation of a quantum refrigerator based on charge and flux qubits, which exhibit strong anharmonicity. As an important step, we perform experiments on a driven flux qubit galvanically coupled to a resonator that is, in turn, weakly capacitively coupled to a heat reservoir. We observe interference patterns in the heat current arising from driving-induced coherence. In particular, resonance peaks in the heat transferred to the bath appear at driving frequencies that are integer fractions of the resonator frequency. These quantum features are governed by the relative phase accumulated by the qubit states under driving.
After years of academic development, the circuit quantum electrodynamics is entering the age of applications. This thesis was realized in this context of creating tools to bridge the gap between an astounding academic quantum system, superconducting circuits, and a grand goal, the universal quantum computer. A likely blueprint for quantum processors consists in the assembly of a large number of elementary modules arranged in a network.In this experimental thesis, a possible node for such a network, the quantum node, was developed and fabricated using state-of-the-art techniques for 2D superconducting microwave circuits. This node was first used to implement a novel sequential readout method for a superconducting qubit. This experiment, first proposed in 2013 by Sete et al., potentially allows for faster, more accurate read out of superconducting qubits. The read out of qubits is one of the several bottlenecks limiting the development of fault-tolerant superconducting quantum computers, which made this project both useful as a demonstration of the quantum node and for applications. This novel readout method achieves readout performances close to state-of-the-art of superconducting qubit readout even though the chip was not optimized for that purpose.During this work, we also contributed to two other experiments engineering quantum measurement and dissipation with superconducting circuits. First, a dedicated circuit was developed to demonstrate a new form of quantum measurement: the multiplexed photon number measurement. In that experiment led by A. Essig and Q. Ficheux, a superconducting transmon quantum bit is read out at multiple frequencies simultaneously to extract more than one bit of information about the number of photons contained in a microwave resonator coupled to that quantum bit.Second, we contributed to the experimental demonstration of the exponential suppression of bit-flips in a qubit encoded in a Schrödinger Cat state of a microwave mode. This experiment led by R. Lescanne and Z. Leghtas, demonstrates the improvement by a factor 300 of the lifetime of a qubit thanks to the autonomous error correction realized through the engineering of the dissipation of a microwave mode.
Quantum computers have huge potential applications, but do not currently exist. It has already been proven that a quantum computer would outperform the best classical supercomputers in certain problems, some of which have vital connections with our daily lives. For example, quantum computers efficiently solve the prime number factoring problem, which in turn is the foundation of the RSA algorithm behind most online transactions. There is a great deal of current effort to implement quantum computers, and we have seen good progress in platforms including superconducting circuits, ion traps, and photons in cavity QED systems and spins in semiconductors. These machines include up to roughly 50 quantum bits at present, but they are not very useful as quantum errors quickly decohere the computer's state and prevent computation. These errors can be mitigated via quantum error correction at the cost of additional size and complexity. Progress in the field towards error corrected, large-scale quantum machines requires us to require new tools for controlling, coupling, and reading out qubits. In this thesis, I will focus on such explorations in superconducting circuits. In this thesis, we seek to expand the already flexible toolkit of quantum circuits by exploring the uses of parametric couplings based on third-order nonlinearities. This type of nonlinearities has only been used in quantum-limited amplifiers before, here we try to further explore their applications by creating new methods for controlling and measuring qubits that based on it. In the first experiment, we address the problem of implementing a highly efficient quantum non-demolition qubit readout. With the use of two-mode squeezed (TMS) light and combined with phase-preserving parametric amplifiers into an interferometer for dispersive qubit readout, we demonstrate a measurement scheme with a 44% improvement in power signal-to-noise ratio. We also investigate the back-action of the measurement scheme. In the second experiment, we create an effective chemical potential for photons with parametric system-bath coupling. In particular, we use a lossy Superconducting Nonlinear Asymmetric Inductive eLement (SNAIL) as both the bath and coupler. The bath engineering is realized by combining the multiple parametric drives and the dissipation together.