For the field of quantum computing and technology to continue its growth and begin to solve truly useful problems, it must overcome the limitations imposed by errors affecting quantum devices. Leading modern devices can be classed as “noisy intermediate-scale quantum” or NISQ devices. These are devices that have not realised fault-tolerance nor utilise error correction, but nonetheless could solve genuinely useful, classically intractable problems. However, the scalability and reliability of current NISQ devices is limited by the impact of errors. This thesis uses the principles of quantum control to work towards improving quantum technology reliability. By characterising the errors affecting a quantum device and tailoring robust, dynamic control solutions, I am able to achieve superior performance in a trapped-ion quantum device compared to its baseline or primitive operation. I begin by characterising our experimental system, a trapped 171Yb+ ion. Following on, I demonstrate three quantum control techniques. First, I work to improve the measurement fidelity of a trapped-ion hyperfine qubit using electron shelving to a metastable level. Second, this work develops and demonstrates an error characterisation tool to diagnose the correlation properties of errors affecting a quantum device. Building from this, I demonstrate the use of dynamically corrected gates to both improve single-qubit gate fidelities and reduce error correlations temporally between gates and spatially between qubits. Finally, I discuss and implement a more robust and flexible two-qubit entangling Mølmer-Sørensen gate using phase modulation of the interaction laser. All of this work adds to the “toolkit” of quantum control and can be used to improve both the reliability of modern device performance -- in particular by reducing susceptibility to noise -- and assist with building up to larger numbers of qubits and gates by tailoring more robust, scalable entangling gates between qubits.
For the field of quantum computing and technology to continue its growth and begin to solve truly useful problems, it must overcome the limitations imposed by errors affecting quantum devices. Leading modern devices can be classed as “noisy intermediate-scale quantum” or NISQ devices. These are devices that have not realised fault-tolerance nor utilise error correction, but nonetheless could solve genuinely useful, classically intractable problems. However, the scalability and reliability of current NISQ devices is limited by the impact of errors. This thesis uses the principles of quantum control to work towards improving quantum technology reliability. By characterising the errors affecting a quantum device and tailoring robust, dynamic control solutions, I am able to achieve superior performance in a trapped-ion quantum device compared to its baseline or primitive operation. I begin by characterising our experimental system, a trapped 171Yb+ ion. Following on, I demonstrate three quantum control techniques. First, I work to improve the measurement fidelity of a trapped-ion hyperfine qubit using electron shelving to a metastable level. Second, this work develops and demonstrates an error characterisation tool to diagnose the correlation properties of errors affecting a quantum device. Building from this, I demonstrate the use of dynamically corrected gates to both improve single-qubit gate fidelities and reduce error correlations temporally between gates and spatially between qubits. Finally, I discuss and implement a more robust and flexible two-qubit entangling Mølmer-Sørensen gate using phase modulation of the interaction laser. All of this work adds to the “toolkit” of quantum control and can be used to improve both the reliability of modern device performance -- in particular by reducing susceptibility to noise -- and assist with building up to larger numbers of qubits and gates by tailoring more robust, scalable entangling gates between qubits.
Achieving precise control over quantum systems has groundbreaking ap-plications, such as simulating complex quantum systems, or implementing quantum algorithms with an improved efficiency over classical algorithms. Quantum computing has been under development for the last decades, re-sulting in the so-called Noisy Intermediate-Scale Quantum (NISQ) devices with about 50 qubits. While these devices are built with the purpose of achieving practical advantages over classical processors, they are vulnerable to noise sources. In order to account for error sources, one needs to im-plement Quantum Error Correction (QEC) techniques. These techniques protect quantum information by encoding a logical qubit into several phys-ical ones. For the implementation of NISQ devices and QEC techniques, it is vital to keep improving the quantum operations and states involved. Here we develop calibration protocols based on the application of Bayesian inference. We show their advantages compared to the more traditional frequentist approaches, such as their ability to maximise the information gained with each measurement, or their straightforward automation. In our first line of research, we develop a Bayesian protocol for the correction of unwanted phases appearing in the experimental preparation of Steane code states. This protocol requires 13 times less measurements than the frequentist approach. In our second line of research, we develop a proto-col for locking a laser to the qubit transition frequency in a trapped-ion architecture. This calibration is vital, since the correct implementation of single-qubit gates depends on it. In our final line of research, we study the Mølmer-Sørensen entangling gate, used in trapped-ion systems. First we introduce the calibration parameters, and derive a semianalytical model of the effects of one of them, for which there was no previous analytical understanding. Finally, we develop a Bayesian protocol for the calibration of these parameters, which was successfully implemented and tested in an experimental trapped-ion quantum processor.
High-fidelity two-qubit entangling gates play an important role in many quantum information processing tasks and are a necessary building block for constructing a universal quantum computer. Such high-fidelity gates have been demonstrated on trapped-ion qubits; however, control errors and noise in gate parameters may still lead to reduced fidelity. Here we propose and demonstrate a general family of two-qubit entangling gates which are robust to different sources of noise and control errors. These gates generalize the renowned Mølmer-Sørensen gate by using multitone drives. We experimentally implemented several of the proposed gates on ^{88}Sr^{+} ions trapped in a linear Paul trap and verified their resilience.
Qubits encoded within internal energy levels of atoms and ions have been used to demonstrate high-performance primitives of quantum computing for small numbers of qubits. Yet, the path to achieving sufficient size and fidelity for useful fault-tolerant quantum computation remains daunting. In this dissertation, I present several techniques I developed as steps along this path. These results include high-fidelity, low-loss detection for alkali atoms in optical tweezers, crosstalk-mitigated parallel one-qubit gates, robust entangling gates on trapped ions, and leakage-to-erasure conversion for non-destructive detection of atom loss errors. Through this collection of techniques, I have sought to make full use of the underlying atomic physics to alleviate technical burden in scaling up to large systems while maintaining or improving on qubit control fidelity. I emphasize that all these results are achieved with low technical overhead, allowing for direct implementation in systems at scale.
Quantum computers promise to solve models of important physical processes, optimize complex cost functions, and challenge cryptography in ways that are intractable using current computers. In order to achieve these promises, quantum computers must both increase in size and decrease error rates. To increase the system size, we report on the design, construction, and operation of an integrated trapped ion quantum computer consisting of a chain of 15 171Yb+ ions with all-to-all connectivity and high-fidelity gate operations. In the process, we identify a physical mechanism that adversely affects gate fidelity in long ion chains. Residual heating of the ions from noisy electric fields creates decoherence due to the weak confinement of the ions transverse to a focused addressing laser. We demonstrate this effect in chains of up to 25 ions and present a model that accurately describes the observed decoherence. To mitigate this noise source, we first propose a new sympathetic cooling scheme to periodically re-cool the ions throughout a quantum circuit, and then demonstrate its capability in a proof-of-concept experiment. One path to suppress error rates in quantum computers is through quantum error correction schemes that combine multiple physical qubits into logical qubits that robustly store information within an entangled state. These extra degrees of freedom enable the detection and correction of errors. Fault-tolerant circuits contain the spread of errors while operating the logical qubit and are essential for realizing error suppression in practice. We demonstrate fault-tolerant preparation, measurement, rotation, and stabilizer measurement of a distance-3 Bacon-Shor logical qubit in our quantum computer. The result is an encoded logical qubit with error rates lower than the error of the entangling operations required to operate it.
Physical qubits in experimental quantum information processors are inevitably exposed to different sources of noise and imperfections, which lead to errors that typically accumulate hindering our ability to perform long computations reliably. Progress towards scalable and robust quantum computation relies on exploiting quantum error correction (QEC) to actively battle these undesired effects. In this work, we present a comprehensive study of crosstalk errors in a quantum-computing architecture based on a single string of ions confined by a radio-frequency trap, and manipulated by individually-addressed laser beams. This type of errors affects spectator qubits that, ideally, should remain unaltered during the application of single- and two-qubit quantum gates addressed at a different set of active qubits. We microscopically model crosstalk errors from first principles and present a detailed study showing the importance of using a coherent vs incoherent error modelling and, moreover, discuss strategies to actively suppress this crosstalk at the gate level. Finally, we study the impact of residual crosstalk errors on the performance of fault-tolerant QEC numerically, identifying the experimental target values that need to be achieved in near-term trapped-ion experiments to reach the break-even point for beneficial QEC with low-distance topological codes.
Quantum computing promises to address problems in simulation, optimization, and chemistry, but current hardware operates in the noisy intermediate-scale (NISQ) regime where coherence limits, gate imperfections, and readout errors restrict performance. Improving calibration and mitigating errors therefore remain essential. This thesis analyzes error reduction on superconducting transmon qubits through calibration, gate analysis, and circuit-level mitigation. It compares single-qubit amplitude calibration methods and analyzes two-qubit gates based on the cross-resonance interaction, identifying differences in precision, convergence, and performance. In addition, it develops Inverted-Circuit Zero-Noise-Extrapolation (IC-ZNE) which measures effective error strength using inverted circuits. Compared to standard ZNE, IC-ZNE achieves results closer to ideal values and reduces observable errors. Overall, the presented methods improve the reliability of near-term quantum processors.