![]() ![]() This prescription, however, belies issues that only arise as qubit coherence and operational fidelity improve. In essence, by improving qubit coherence, error rates may be reduced below fault-tolerant thresholds, and, in principle, QEC may be employed to suppress errors at arbitrary system scales. 14– 16 Achieving targets such as these-once thought all but impossible-has led many to recognise that scalable, error-corrected quantum computation may become a reality. As a result, many qubit technologies 7 have witnessed remarkable performance improvements, with dephasing times approaching milliseconds or even seconds (depending on the qubit modality), 6, 8– 13 and operational fidelities reaching ~99.999%. In turn, experimentalists have primarily focused their efforts on reducing both the level of environmental noise and the qubit sensitivity to that which remains (c.f., refs 4– 6). Dephasing in qubit systems is commonly attributed to environmental fluctuations of a qubit bias or control parameter, e.g., an external magnetic field that modulates the qubit-state energy splitting and hence its dynamic phase evolution. A familiar aspect of this challenge is decoherence, a process by which even idle qubits undergoing free evolution (i.e., the identity operator) will gradually lose their stored quantum information, rendering them useless in subsequent computation.Ī prevalent component of decoherence is dephasing, a randomisation of the relative phase between the basis states that form a coherent superposition state. 1, 2 The need for higher fidelity performance motivates research at all architectural levels, 3 from theoretical studies of fault tolerance and analyses of quantum error correction (QEC) implementations down to experimental improvements in the operational fidelity of elemental devices. We discuss the relevance of these bounds for quantum error correction in contemporary experiments and future large-scale quantum information systems, and consider potential means to improve master clock stability.Ī fundamental challenge to the broad application of quantum information science is the management of error in fragile quantum hardware. We then use representative lab-grade and performance-grade oscillator specifications to calculate operational fidelity bounds on trapped-ion and superconducting qubits with relatively slow and fast operation times. We first connect standard oscillator phase-noise metrics to their corresponding qubit dephasing spectral densities. In this manuscript, we articulate the impact of instabilities in the master clock on qubit phase coherence and provide tools to calculate the contributions to qubit error arising from these processes. In the case of the phase degree of freedom in a quantum superposition, however, the coherence that must be preserved is not solely internal to the qubit, but rather necessarily includes that of the qubit relative to the ‘master clock’ (e.g., a local oscillator) that governs its control system. Experimentalists seeking to improve the coherent lifetimes of quantum bits have generally focused on mitigating decoherence mechanisms through, for example, improvements to qubit designs and materials, and system isolation from environmental perturbations. ![]()
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