arXiv subjects

Jonathan P. Home

Publications and source records attributed to Jonathan P. Home.

At least 19 recordsLinked to original sources

arXiv2026

Error Correction of Beamsplitter-Generated Entangled GKP States

To be useful, quantum computers will be required to successfully correct errors occurring at the hardware level. Bosonic codes provide a hardware-efficient option for error correction, but fault-tolerance further requires that the available gate interactions be compatible with the code. A promising bosonic code is the Gottesman-Kitaev-Preskill (GKP) code, for which a linear beamsplitter-like coupling between two bosonic modes is fault-tolerant, making this a key primitive for building larger systems. Here, using two motional modes of a trapped ion, we demonstrate the generation of entangled states of GKP qubits by interfering two qunaught states, which have a grid structure but carry no logical information, on a beamsplitter. We generate all four Bell states with an average fidelity of 69%, and subsequently demonstrate an extension of the entangled state lifetime through the use of quantum error correction. These results complete the set of Gaussian operations required for quantum computing with GKP codes and enable explorations of multi-mode bosonic encodings as well as fundamental tests of information channels.

quant-ph↗

arXiv2024

Quantum control of a single $\mathrm{H}_2^+$ molecular ion

Science is founded on the benchmarking of theoretical models against experimental measurements, with the challenge that for all but the simplest systems, the calculations required for high precision become extremely challenging. $\mathrm{H}_2^+$ is the simplest stable molecule, and its structure is calculable to high precision. However, studying $\mathrm{H}_2^+$ experimentally presents significant challenges: Standard control methods such as laser cooling are not applicable due to the long lifetimes of its rotational and vibrational states. Here we solve this issue by combining buffer gas cooling to quench the $\mathrm{H}_2^+$ rovibrational excitation with quantum logic operations between $\mathrm{H}_2^+$ and a co-trapped 'helper' ion to control the molecule's hyperfine structure. This enables us to perform pure quantum state preparation, coherent control, and non-destructive readout, which we use to demonstrate high-resolution microwave spectroscopy in the hyperfine structure of $\mathrm{H}_2^+$ with a precision of 2 Hz. Our results pave the way for high precision spectroscopy of $\mathrm{H}_2^+$ in both the microwave and optical domains. Due to the wide applicability of buffer gas cooling, our method provides a general tool for molecular ion species that are hard to control with quantum logic tools alone.

physics.atom-ph↗

arXiv2024

Physical coherent cancellation of optical addressing crosstalk in a trapped-ion experiment

We present an experimental investigation of coherent crosstalk cancellation methods for light delivered to a linear ion chain cryogenic quantum register. The ions are individually addressed using focused laser beams oriented perpendicular to the crystal axis, which are created by imaging each output of a multi-core photonic-crystal fibre waveguide array onto a single ion. The measured nearest-neighbor native crosstalk intensity of this device for ions spaced by 5 $\mu$m is found to be $\sim 10^{-2}$. We show that we can suppress this intensity crosstalk from waveguide channel coupling and optical diffraction effects by a factor $>10^3$ using cancellation light supplied to neighboring channels which destructively interferes with the crosstalk. We measure a rotation error per gate on the order of $\epsilon_{x} \sim 10^{-5}$ on spectator qubits, demonstrating a suppression of crosstalk error by a factor of $> 10^2$. We compare the performance to composite pulse methods for crosstalk cancellation, and describe the appropriate calibration methods and procedures to mitigate phase drifts between these different optical paths, including accounting for problems arising due to pulsing of optical modulators.

quant-ph↗

arXiv2024

Time-adaptive phase estimation

Phase estimation is known to be a robust method for single-qubit gate calibration in quantum computers, while Bayesian estimation is widely used in devising optimal methods for learning in quantum systems. We present Bayesian phase estimation methods that adaptively choose a control phase and the time of coherent evolution based on prior phase knowledge. In the presence of noise, we find near-optimal performance with respect to known theoretical bounds, and demonstrate some robustness of the estimates to noise that is not accounted for in the model of the estimator, making the methods suitable for calibrating operations in quantum computers. We determine the utility of control parameter values using functions of the prior probability of the phase that quantify expected knowledge gain either in terms of expected narrowing of the posterior or expected information gain. In particular, we find that by maximising the rate of expected gain we obtain phase estimates having standard deviation a factor of 1.43 larger than the Heisenberg limit using a classical sequential strategy. The methods provide optimal solutions accounting for available prior knowledge and experimental imperfections with minimal effort from the user. The effect of many types of noise can be specified in the model of the measurement probabilities, and the rate of knowledge gain can easily be adjusted to account for times included in the measurement sequence other than the coherent evolution leading to the unknown phase, such as times required for state preparation or readout.

quant-ph↗

arXiv2024

Multi-zone trapped-ion qubit control in an integrated photonics QCCD device

Multiplexed operations and extended coherent control over multiple trapping sites are fundamental requirements for a trapped-ion processor in a large scale architecture. Here we demonstrate these building blocks using a surface-electrode trap with integrated photonic components which are scalable to larger numbers of zones. We implement a Ramsey sequence using the integrated light in two zones, separated by 375 $\mu$m, performing transport of the ion from one zone to the other in 200 $\mu$s between pulses. In order to achieve low motional excitation during transport, we developed techniques to measure and mitigate the effect of the exposed dielectric surfaces used to deliver the integrated light to the ion. We also demonstrate simultaneous control of two ions in separate zones with low optical crosstalk, and use this to perform simultaneous spectroscopy to correlate field noise between the two sites. Our work demonstrates the first transport and coherent multi-zone operations in integrated photonic ion trap systems, forming the basis for further scaling in the trapped-ion QCCD architecture.

quant-ph↗

arXiv2023

Grating design methodology for tailored free-space beam-forming

We present a design methodology for free-space beam-forming with general profiles from grating couplers which avoids the need for numerical optimization, motivated by applications in ion trap physics. We demonstrate its capabilities through a variety of gratings using different wavelengths and waveguide materials, designed for new ion traps with all optics fully integrated, including UV and visible wavelengths. We demonstrate designs for diffraction-limited focusing without restriction on waveguide taper geometry, emission angle, or focus height, as well as focused higher order Hermite-Gaussian and Laguerre-Gaussian beams. Additional investigations examine the influence of grating length and taper angle on beam-forming, indicating the importance of focal shift in apertured beams. The design methodology presented allows for efficient design of beamforming gratings with the accuracy as well as the flexibility of beam profile and operating wavelength demanded by application in atomic systems.

physics.optics↗

arXiv2023

Integration of a high finesse cryogenic build-up cavity with an ion trap

We report on the realization of a hemispherical optical cavity with a finesse of F = 13000 sustaining inter-cavity powers of 10 kW, which we operate in a closed-cycle cryostat vacuum system close to 4 Kelvin. This was designed and built with an integrated radio-frequency Paul trap, in order to combine optical and radio-frequency trapping. The cavity provides a power build-up factor of 2250. We describe a number of aspects of the system design and operation, including low-vibration mounting and locking including thermal effects at high powers. Thermal self-locking in the high intracavity power regime was observed to enhance the passive stability below 1 kHz. Observations made over repeated cool-downs over a course of a year show a repeatable shift between the ion trap center and the cavity mode.

physics.atom-ph↗

arXiv2022

Control of an atomic quadrupole transition in a phase-stable standing wave

Using a single calcium ion confined in a surface-electrode trap, we study the interaction of electric quadrupole transitions with a passively phase-stable optical standing wave field sourced by photonics integrated within the trap. We characterize the optical fields through spatial mapping of the Rabi frequencies of both carrier and motional sideband transitions as well as AC Stark shifts. Our measurements demonstrate the ability to engineer favorable combinations of sideband and carrier Rabi frequency as well as AC Stark shifts for specific tasks in quantum state control and metrology.

physics.atom-ph↗

arXiv2022

Pure circularly polarized light emission from waveguide microring resonators

Circularly polarized light plays a key role in many applications including spectroscopy, microscopy, and control of atomic systems. Particularly in the latter, high polarization purity is often required. Integrated technologies for atomic control are progressing rapidly, but while integrated photonics can generate fields with pure linear polarization, integrated generation of highly pure circular polarization states has not been addressed. Here, we show that waveguide microring resonators, perturbed with azimuthal gratings and thereby emitting beams carrying optical orbital angular momentum, can generate radiated fields of high circular polarization purity. We achieve this in a passive device by taking advantage of symmetries of the structure and radiated modes, and directly utilizing both transverse and longitudinal field components of the guided modes. On the axis of emission and at maximum intensity, we measure an average polarization impurity of $1.0 \times 10^{-3}$ in relative intensity across the resonance FWHM, and observe impurities below $10^{-4}$ in this range. This constitutes a significant improvement over the ${\sim}10^{-2}$ impurity demonstrated in previous work on integrated devices. Photonic structures allowing high circular polarization purity may assist in realizing high-fidelity control and measurement in atomic quantum systems.

physics.optics↗

arXiv2021

Design, fabrication and characterisation of a micro-fabricated double-junction segmented ion trap

We describe the implementation of a three-dimensional Paul ion trap fabricated from a stack of precision-machined silica glass wafers, which incorporates a pair of junctions for 2-dimensional ion transport. The trap has 142 dedicated electrodes which can be used to define multiple potential wells in which strings of ions can be held. By supplying time-varying potentials, this also allows for transport and re-configuration of ion strings. We describe the design, simulation, fabrication and packaging of the trap, including explorations of different parameter regimes and possible optimizations and design choices. We give results of initial testing of the trap, including measurements of heating rates and junction transport.

quant-ph↗

arXiv2020

Cooperative Breakdown of the Oscillator Blockade in the Dicke Model

The Dicke model, which describes the coupling of an ensemble of spins to a harmonic oscillator, is known for its superradiant phase transition, which can both be observed in the ground state in a purely Hamiltonian setting, as well as in the steady state of an open-system Dicke model with dissipation. We demonstrate that, in addition, the dissipative Dicke model can undergo a second phase transition to a nonstationary phase, characterized by unlimited heating of the harmonic oscillator. Identifying the mechanism of the phase transition and deriving the scaling of the critical coupling with the system size we conclude that the novel phase transition can be understood as a cooperative breakdown of the oscillator blockade which otherwise prevents higher excitation of the system. We discuss an implementation with trapped ions and investigate the role of cooling, by which the breakdown can be suppressed.

quant-ph↗

arXiv2020

Engineering generalized Gibbs ensembles with trapped ions

The concept of generalized Gibbs ensembles (GGEs) has been introduced to describe steady states of integrable models. Recent advances show that GGEs can also be stabilized in nearly integrable quantum systems when driven by external fields and open. Here, we present a weakly dissipative dynamics that drives towards a steady-state GGE and is realistic to implement in systems of trapped ions. We outline the engineering of the desired dissipation by a combination of couplings which can be realized with ion-trap setups and discuss the experimental observables needed to detect a deviation from a thermal state. We present a novel mixed-species motional mode engineering technique in an array of micro-traps and demonstrate the possibility to use sympathetic cooling to construct many-body dissipators. Our work provides a blueprint for experimental observation of GGEs in open systems and opens a new avenue for quantum simulation of driven-dissipative quantum many-body problems.

quant-ph↗

arXiv2020

Integrated optical multi-ion quantum logic

Practical and useful quantum information processing (QIP) requires significant improvements with respect to current systems, both in error rates of basic operations and in scale. Individual trapped-ion qubits' fundamental qualities are promising for long-term systems, but the optics involved in their precise control are a barrier to scaling. Planar-fabricated optics integrated within ion trap devices can make such systems simultaneously more robust and parallelizable, as suggested by previous work with single ions. Here we use scalable optics co-fabricated with a surface-electrode ion trap to achieve high-fidelity multi-ion quantum logic gates, often the limiting elements in building up the precise, large-scale entanglement essential to quantum computation. Light is efficiently delivered to a trap chip in a cryogenic environment via direct fibre coupling on multiple channels, eliminating the need for beam alignment into vacuum systems and cryostats and lending robustness to vibrations and beam pointing drifts. This allows us to perform ground-state laser cooling of ion motion, and to implement gates generating two-ion entangled states with fidelities $>99.3(2)\%$. This work demonstrates hardware that reduces noise and drifts in sensitive quantum logic, and simultaneously offers a route to practical parallelization for high-fidelity quantum processors. Similar devices may also find applications in neutral atom and ion-based quantum-sensing and timekeeping.

quant-ph↗

arXiv2020

Scalable arrays of micro-Penning traps for quantum computing and simulation

We propose the use of 2-dimensional Penning trap arrays as a scalable platform for quantum simulation and quantum computing with trapped atomic ions. This approach involves placing arrays of micro-structured electrodes defining static electric quadrupole sites in a magnetic field, with single ions trapped at each site and coupled to neighbors via the Coulomb interaction. We solve for the normal modes of ion motion in such arrays, and derive a generalized multi-ion invariance theorem for stable motion even in the presence of trap imperfections. We use these techniques to investigate the feasibility of quantum simulation and quantum computation in fixed ion lattices. In homogeneous arrays, we show that sufficiently dense arrays are achievable, with axial, magnetron and cyclotron motions exhibiting inter-ion dipolar coupling with rates significantly higher than expected decoherence. With the addition of laser fields these can realize tunable-range interacting spin Hamiltonians. We also show how local control of potentials allows isolation of small numbers of ions in a fixed array and can be used to implement high fidelity gates. The use of static trapping fields means that our approach is not limited by power requirements as system size increases, removing a major challenge for scaling which is present in standard radio-frequency traps. Thus the architecture and methods provided here appear to open a path for trapped-ion quantum computing to reach fault-tolerant scale devices.

quant-ph↗

arXiv2020

Robust dynamical exchange cooling with trapped ions

We investigate theoretically the possibility for robust and fast cooling of a trapped atomic ion by transient interaction with a pre-cooled ion. The transient coupling is achieved through dynamical control of the ions' equilibrium positions. To achieve short cooling times we make use of shortcuts to adiabaticity by applying invariant-based engineering. We design these to take account of imperfections such as stray fields, and trap frequency offsets. For settings appropriate to a currently operational trap in our laboratory, we find that robust performance could be achieved down to $6.3$ motional cycles, comprising $14.2\ \mathrm{μs}$ for ions with a $0.44\ \mathrm{MHz}$ trap frequency. This is considerably faster than can be achieved using laser cooling in the weak coupling regime, which makes this an attractive scheme in the context of quantum computing.

quant-ph↗

arXiv2019

Direct characteristic-function tomography of quantum states of the trapped-ion motional oscillator

We implement direct readout of the symmetric characteristic function of quantum states of the motional oscillation of a trapped calcium ion. Using suitably chosen internal electronic state-dependent displacements based on bi-chromatic laser fields we map the expectation value of the real or imaginary part of the displacement operator to the internal states, which are subsequently read out using fluorescence detection. Combining the two readout results provides full information about the symmetric characteristic function. We characterize the performance of the technique by applying it to a range of archetypal quantum oscillator states, including displaced and squeezed Gaussian states as well as two and three component superpositions of displaced squeezed states which have applications in continuous variable quantum computing. For each, we discuss relevant features of the characteristic function and Wigner phase-space quasi-probability distribution. The direct reconstruction of these highly non-classical oscillator states using a reduced number of measurements is an essential tool for understanding and optimizing the control of these systems for quantum sensing and quantum information applications.

quant-ph↗

arXiv2019

Segmented ion-trap fabrication using high precision stacked wafers

We describe the use of laser-enhanced etching of fused silica in order to build multi-layer ion traps. This technique offers high precision of both machining and alignment of adjacent wafers. As examples of designs taking advantage of this possibility, we describe traps for realizing two key elements of scaling trapped ion systems. The first is a trap for a cavity-QED interface between single ions and photons, in which the fabrication allows shapes that provide good electro-static shielding of the ion from charge build-up on the mirror surfaces. The second incorporates two X-junctions allowing two-dimensional shuttling of ions. Here we are able to investigate designs which explore a trade-off between pseudo-potential barriers and confinement at the junction center. In both cases we illustrate the design constraints arising from the fabrication.

physics.app-ph↗

arXiv2018

Repeated multi-qubit readout and feedback with a mixed-species trapped-ion register

Quantum error correction will be essential for realizing the full potential of large-scale quantum information processing devices. Fundamental to its experimental realization is the repetitive detection of errors via projective measurements of quantum correlations among qubits, and correction using conditional feedback. Performing these tasks repeatedly requires a system in which measurement and feedback decision times are short compared to qubit coherence times, where the measurement reproduces faithfully the desired projection, and for which the measurement process has no detrimental effect on the ability to perform further operations. Here we demonstrate up to 50 sequential measurements of correlations between two beryllium-ion qubits using a calcium ion ancilla, and implement feedback which allows us to stabilize two-qubit subspaces as well as Bell states. Multi-qubit mixed-species gates are used to transfer information from qubits to the ancilla, enabling quantum state detection with negligible crosstalk to the stored qubits. Heating of the ion motion during detection is mitigated using sympathetic recooling. A key element of the experimental system is a powerful classical control system, which features flexible in-sequence processing to implement feedback control. The methods employed here provide a number of essential ingredients for scaling trapped-ion quantum computing, and provide new opportunities for quantum state control and entanglement-enhanced quantum metrology.

quant-ph↗