SummaryWe report an 8.7-second qubit coherence time in a ytterbium ion (171Yb+) using a microwave synthesis system derived from an ultra-low phase noise cryogenic sapphire oscillator (CSO). Our measurement provides evidence that the phase noise of a local oscillator that serves as the master clock plays a significant role in the operation fidelities of a trapped-ion quantum information processor.
SummaryWe report an 8.7-second qubit coherence time in a ytterbium ion (171Yb+) using a microwave synthesis system derived from an ultra-low phase noise cryogenic sapphire oscillator (CSO). Our measurement provides evidence that the phase noise of a local oscillator that serves as the master clock plays a significant role in the operation fidelities of a trapped-ion quantum information processor.
Abstract 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. 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. 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. We first connect standard oscillator phase-noise metrics to their corresponding qubit dephasing spectral densities. 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 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.
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. 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. 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. We first connect standard oscillator phase-noise metrics to their corresponding qubit dephasing spectral densities. 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 discuss the relevance of these bounds for quantum error correction in contemporary experiments and future large-scale quantum information systems, and discuss potential means to improve master clock stability.
Qubit implementations like trapped ions can be controlled by microwave signals in the gigahertz range, which require a local oscillator (LO) for upconversion. The LO superimposes a static frequency offset and phase noise, both leading to errors in qubit gate operation. Both error terms can be reduced by implementing the LO as an integer-N frequency synthesizer, where a phase-locked loop controls and stabilizes the frequency. An exemplary integer-N frequency synthesizer is realized and its performance for quantum computers is characterized in this work. The qubit fidelity calculated based on phase noise measurement results is above 99.943% for primitive π-gates.
Quantum systems promise to revolutionize information processing science and technology [1-3]. The preservation of quantum coherence, the defining property of qubits, fundamentally constrains the performance of quantum information processing with quantum memories [4]. While trapped atomic ions theoretically support million-year coherence based on spontaneous emission [5-7], experimental demonstrations have reached far less, only about an hour [8-13]. Here we combine clock-state qubits with decoherence-free subspace (DFS) encoding to achieve coherence exceeding ten hours. Using correlation-based phase tracking in 171Yb+ ion pairs sympathetically cooled by 138Ba+ ion, we demonstrate this without magnetic shielding or enhanced microwave phase stabilization that previously limited coherence times. DFS encoding references the qubit phase to the inter-ion energy difference to reject microwave phase noise and common-mode magnetic fluctuations, while clock states provide environmental insensitivity. Throughout measurements extended to 1600 seconds, we observe minimal coherence decay, with exponential fits yielding a coherence time of (3.77 +/- 1.09) x 10^4 seconds. Our results establish DFS encoding as a form of passive error correction that eliminates technical noise constraints, unlocking the million-year coherence potential of atomic ions for scalable quantum information processing.
Quantum computation is a promising technology owing to the unique physical nature of quantum systems. Quantum computers have the potential to be as much as exponentially more efficient than today’s most powerful classical computer at certain tasks. One of the major challenges the scientific community must overcome, arises due to the intrinsic tendency of quantum information to be highly susceptible to environmental noise. This thesis presents work in developing and characterizing methods for precision quantum control in the presence of decoherence inducing noise, using trapped 171Yb+ ions as a model experimental platform. A flexible , robust microwave system is used to access the 12.6 GHz hyperfine qubit manifold in the outer S-shell orbital. The low phase noise characteristics of a microwave oscillator allow free-evolution coherence times in excess of three seconds, and operational fidelities F > 99.99%, characterized by randomized benchmarking. Starting from this relatively clean baseline, control techniques may be implemented in order to simulate a variety of realistic, time-dependent noise models, via arbitrary IQ modulation of the microwave source. A formalism for calculating the spectral noise filter functions of piece-wise constant control sequences is developed. The efficacy of this formalism is demonstrated by experimentally characterizing a variety of control protocols in the presence of engineered noise.
This work describes the design and implementation of a new apparatus to achieve quantum information processing in trapped ions using microwave methods. The apparatus involves many design improvements over a previous quantum processing experiment which achieved the highest single-qubit gate fidelity, longest coherence time, and highest microwave based two-qubit fidelity. The goal of this new experiment is a two-qubit gate speed and a fidelity improvement of a factor of 10, making microwave based gates much more feasible as quantum processors. To this end, a novel clock qubit within 43Ca+ is chosen, and the ion height is nearly halved. An anticipated heating rate increase is counteracted by the use of cryogenics. A novel ion chip design is implemented via wafer-scale fabrication and subsequently attached using a novel eutectic bonding technique. The implementation required many design and fabrication problems to be solved; these are described. The vacuum system reaches < 10−11 mbar even at room temperature, lowering experimental difficulty and allowing for performance comparisons at a wide temperature range. A new experiment control system, ARTIQ, and corresponding Sinara hardware is used for control. Experimental results achieved so far include a study of contributing factors to the ion loading rate, and a comparison between the designed and measured microwave fields. All results achieved to date are compatible with the main speed and fidelity goal, which should be achieved in the near future.