Wideband laser locking to an atomic reference with modulation transfer spectroscopy

We demonstrate that conventional modulated spectroscopy apparatus, used for laser frequency stabilization in many atomic physics laboratories, can be enhanced to provide a wideband lock delivering deep suppression of frequency noise across the acoustic range. Using an acousto-optic modulator driven with an agile oscillator, we show that wideband frequency modulation of the pump laser in modulation transfer spectroscopy produces the unique single lock-point spectrum previously demonstrated with electro-optic phase modulation. We achieve a laser lock with 100 kHz feedback bandwidth, limited by our laser control electronics. This bandwidth is sufficient to reduce frequency noise by 30 dB across the acoustic range and narrows the imputed linewidth by a factor of five.Comment: 11 pages, 3 figures, 1 table; v2: additional laser frequency noise data demonstrating greater linewidth reduction, many textual improvements, accepted for publication in Optics Expes

Sustained state-independent quantum contextual correlations from a single ion

We use a single trapped-ion qutrit to demonstrate the quantum-state-independent violation of noncontextuality inequalities using a sequence of randomly chosen quantum nondemolition projective measurements. We concatenate 53 × 106 sequential measurements of 13 observables, and unambiguously violate an optimal noncontextual bound. We use the same data set to characterize imperfections including signaling and repeatability of the measurements. The experimental sequence was generated in real time with a quantum random number generator integrated into our control system to select the subsequent observable with a latency below 50 μs, which can be used to constrain contextual hiddenvariable models that might describe our results. The state-recycling experimental procedure is resilient to noise and independent of the qutrit state, substantiating the fact that the contextual nature of quantum physics is connected to measurements and not necessarily to designated states. The use of extended sequences of quantum nondemolition measurements finds applications in the fields of sensing and quantum information

All-solid-state continuous-wave laser systems for ionization, cooling and quantum state manipulation of beryllium ions

We describe laser systems for photoionization, Doppler cooling, and quantum state manipulation of beryllium ions. For photoionization of neutral beryllium, we have developed a continuous-wave 235nm source obtained by two stages of frequency doubling from a diode laser at 940nm. The system delivers up to 400 mW at 470nm and 28 mW at 235nm. For control of the beryllium ion, three laser wavelengths at 313nm are produced by sum-frequency generation and second-harmonic generation from four infrared fiber lasers. Up to 7.2 W at 626nm and 1.9 W at 313nm are obtained using two pump beams at 1051 and 1551nm. Intensity drifts of around 0.5% per hour have been measured over 8h at a 313nm power of 1W. These systems have been used to load beryllium ions into a segmented ion trap

MaRCoS, an open-source electronic control system for low-field MRI

Every magnetic resonance imaging (MRI) device requires an electronic control system that handles pulse sequences and signal detection and processing. Here we provide details on the architecture and performance of MaRCoS, a MAgnetic Resonance COntrol System developed by an open international community of low-field MRI researchers. MaRCoS is inexpensive and can handle cycle-accurate sequences without hard length limitations, rapid bursts of events, and arbitrary waveforms. It can also be easily adapted to meet further specifications required by the various academic and private institutions participating in its development. We describe the MaRCoS hardware, firmware and software that enable all of the above, including a Python-based graphical user interface for pulse sequence implementation, data processing and image reconstruction.Comment: 10 pages, 4 figure

A compact ion-trap quantum computing demonstrator

Quantum information processing is steadily progressing from a purely academic discipline towards applications throughout science and industry. Transitioning from lab-based, proof-of-concept experiments to robust, integrated realizations of quantum information processing hardware is an important step in this process. However, the nature of traditional laboratory setups does not offer itself readily to scaling up system sizes or allow for applications outside of laboratory-grade environments. This transition requires overcoming challenges in engineering and integration without sacrificing the state-of-the-art performance of laboratory implementations. Here, we present a 19-inch rack quantum computing demonstrator based on $^{40}\textrm{Ca}^+$ optical qubits in a linear Paul trap to address many of these challenges. We outline the mechanical, optical, and electrical subsystems. Further, we describe the automation and remote access components of the quantum computing stack. We conclude by describing characterization measurements relevant to digital quantum computing including entangling operations mediated by the Molmer-Sorenson interaction. Using this setup we produce maximally-entangled Greenberger-Horne-Zeilinger states with up to 24 ions without the use of post-selection or error mitigation techniques; on par with well-established conventional laboratory setups

Feedback-stabilised quantum states in a mixed-species ion system

Trapped ions are among the leading platforms for realising quantum information processing (QIP). One major challenge in constructing a large-scale QIP device will be to incorporate feedback techniques for performing quantum error correction. This thesis describes the development of a novel classical control system for ion trap quantum computing incorporating powerful real-time processing, and its use in performing a number of experiments involving such processing which form crucial building blocks for stabilizing large-scale ion trap systems. A second major component is the demonstration of multi-qubit quantum control in mixed-species ion chains, which allowed low-crosstalk error-check operations to be performed over tens of cycles in a multi-qubit system for the first time. Combined with feedback this allowed the stabilisation of entanglement over extended sequences of operations. The technical advances in the thesis are a set of control hardware and related firmware and software that is specifically designed to meet the needs of quantum error correction. It enables advanced sequences of measurement, real-time decision making and parameter adjustment needed for scalable experiments, with feedback a core element in its design. Together the feedback-capable system and mixed-species setup were used to test new protocols including a single-qubit adaptive phase estimation scheme relying on rapid real-time classical computation and low-latency parameter updates to optimally extract information, outperforming standard non-adaptive fitting in speed and flexibility. Single- and mixed-species gates between calcium and beryllium and associated experimental techniques were investigated using registers of two and three ions, leading to the first gates between qubits encoded in optical and hyperfine transitions, which reached two-qubit fidelities above 96% and three-qubit fidelities of 93.8(5)%. In preparatory steps for further work, a single-species dissipative protocol was used to prepare an entangled steady-state using a new approach devised in our group, while ion transport and separation experiments with up to four single-species and two mixed-species ions into wells 800μm apart at excitations below ten quanta was implemented and optimised. The main scientific result of the thesis is the demonstration of the repeated extraction of quantum correlations from a pair of beryllium ions using a calcium ancilla qubit. This type of correlation measurement is critical for performing fault-tolerant algorithms. The measurement was then combined with real-time feedback in order to stabilize beryllium qubits in both subspaces and in entangled states, for sequences including up to fifty rounds of feedback, an order of magnitude more than previous work. Information on the major infidelities in the protocols was extracted from the measurement outcome correlations. This thesis concludes with an outlook for extending the role of both classical and quantum feedback in trapped-ion QIP experiments. This is the second edition of the thesis, released on the 27th of September 2018, incorporating minor corrections. The first edition was released on the 13th of July 2018