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 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.
We theoretically propose and experimentally implement a method of measuring a qubit by driving it close to the frequency of a dispersively coupled bosonic mode. The separation of the bosonic states corresponding to different qubit states begins essentially immediately at maximum rate, leading to a speedup in the measurement protocol. Also the bosonic mode can be simultaneously driven to optimize measurement speed and fidelity. We experimentally test this measurement protocol using a superconducting qubit coupled to a resonator mode. For a certain measurement time, we observe that the conventional dispersive readout yields close to 100% higher average measurement error than our protocol. Finally, we use an additional resonator drive to leave the resonator state to vacuum if the qubit is in the ground state during the measurement protocol. This suggests that the proposed measurement technique may become useful in unconditionally resetting the resonator to a vacuum state after the measurement pulse.
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 circuits are one of the most promising and rapidly developing platforms for implementing quantum bits and their control operations for the purpose of large-scale quantum computing. A quantum algorithm is run by executing a series of precise operations on the quantum bits, or qubits. The most important of these operations are the quantum gates along with the measurement and initialization of the states of the qubits and auxiliary components.
In this dissertation, we develop techniques to enhance each of these three operations. The work employs systems of coupled qubits and resonators in the framework of circuit quantum electrodynamics.
Firstly, we derive a theoretical lower bound for the error of a single-qubit gate implemented with a linear oscillator mode and show how to reach this bound. Secondly, we experimentally demonstrate a technique for measuring the state of a qubit by simultaneously driving the qubit and the accompanying readout resonator. Compared to our method, the standard technique involving only the resonator drive yields up to 100% larger error. Moreover, we experimentally and theoretically develop quantum devices and protocols that may be used to initialize or reset auxiliary quantum circuits such as resonators. By coupling a quantum circuit to a strongly dissipative environment through a coupler, the decay rate of the circuit may be tuned on demand by several orders of magnitude. Finally, we propose a general method to analytically solve the dynamics of a complex, open bosonic system that may be used to describe, for example, the experimental systems of this dissertation.
The experimental techniques developed in this dissertation may potentially be utilized in future implementations of a quantum computer, and the proposed theoretical protocols pave the way for future experiments aiming to improve qubit control and heat management in large-scale quantum processors.; Suprajohtavat kvanttipiirit ovat yksi lupaavimmista ja nopeimmin kehittyvista alustoista, joilla voi mahdollisesti toteuttaa suuren kokoluokan kvanttilaskentaa. Kvanttilaskennassa keskeisessa asemassa ovat kvanttibitit eli kubitit seka niilla tehtavat tarkat operaatiot, joista kvanttialgoritmit muodostuvat. Operaatioista tarkeimmat ovat loogiset kvanttiportit seka kubittien ja muiden tarvittavien komponenttien tilojen mittaus ja alustus.Tassa vaitoskirjassa kehitetaan tapoja naiden kolmen operaation parantamiseksi. Tyossa kasitellaan kvanttielektrodynamiikkaan perustuvia, kubittien ja resonaattorien muodostamia kytkettyja systeemeja.
Ensimmaiseksi johdamme teoreettisen alarajan lineaarisella varahtelymoodilla toteutetulle yhden kubitin kvanttiportin virheelle seka naytamme, miten alaraja on mahdollista saavuttaa. Toiseksi toteutamme kokeellisesti menetelman, jolla voi mitata kubitin tilan ajamalla kubittia ja siihen kytkettya lukuresonaattoria samanaikaisesti. Yleisesti kaytetty mittaustapa, jossa ajetaan ainoastaan resonaattoria, tuottaa jopa 100% suuremman virheen kuin toteuttamamme menetelma.…
Fast, precise, and quantum-non-demolition (QND) readout of superconducting qubits is a fundamental component of high-fidelity quantum sensing and computation. Conventional approaches typically operate in the dispersive regime, where the qubit-resonator coupling g is weak compared to the detuning Δ. While exhibiting good QND properties, the readout rate is limited to ∼g2N/Δ≪g, where N is the number of photons in the resonator. QND readout in the nondispersive regime, where the readout rate reaches its full potential ∼g, relies on parameter sweeps that may encounter resonances, leading to measurement-induced state transitions (MIST). In this work, we study a nondispersive readout protocol that replaces these sweeps by sudden quenches of the coupling constant, using a resonator that is preloaded with photons. We call this protocol bang-bang readout, and show that it realizes single-shot projective measurements. The fidelity and QNDness of the qubit post-measurement are remarkably high, with an error that decreases like 1/N. We show that the protocol can also be implemented without preloading the resonator by instead strongly driving the qubit, e.g., with a classical flux drive.
A key challenge in quantum computing is speeding up measurement and initialization. Here, we experimentally demonstrate a dispersive measurement method for superconducting qubits that simultaneously measures the qubit and returns the readout resonator to its initial state. The approach is based on universal analytical pulses and requires knowledge of the qubit and resonator parameters, but needs no direct optimization of the pulse shape, even when accounting for the nonlinearity of the system. Moreover, the method generalizes to measuring an arbitrary number of modes and states. For the qubit readout, we can drive the resonator to ∼102 photons and back to ∼10−3 photons in less than 3κ−1, while still achieving a T1-limited assignment error below 1\%. We also present universal pulse shapes and experimental results for qutrit readout.
A key challenge in quantum computing is speeding up measurement and initialization. Here, we experimentally demonstrate a dispersive measurement method for superconducting qubits that simultaneously measures the qubit and returns the readout resonator to its initial state. The approach is based on universal analytical pulses and requires knowledge of the qubit and resonator parameters, but needs no direct optimization of the pulse shape, even when accounting for the nonlinearity of the system. Moreover, the method generalizes to measuring an arbitrary number of modes and states. For the qubit readout, we can drive the resonator to ∼102 photons and back to ∼10−3 photons in less than 3κ−1, while still achieving a T1-limited assignment error below 1\%. We also present universal pulse shapes and experimental results for qutrit readout.