arXiv subjects

D. Leibfried

Publications and source records attributed to D. Leibfried.

At least 19 recordsLinked to original sources

arXiv2025

Compatibility of trapped ions and dielectrics at cryogenic temperatures

We study the impact of an unshielded dielectric $\unicode{x2013}$ here, a bare optical fiber $\unicode{x2013}$ on a $^{40}$Ca${^+}$ ion held several hundred $\mu$m away in a cryogenic surface electrode trap. We observe distance-dependent stray electric fields of up to a few kV/m due to the dielectric, which drift on average less than 10% per month and can be fully compensated with reasonable voltages on the trap electrodes. We observe ion motional heating rates attributable to the dielectric of $\approx$30 quanta per second at an ion-fiber distance of 215(4) $\mu$m and $\approx$1.5 MHz motional frequency. These results demonstrate the viability of using unshielded, trap-integrated dielectric objects such as miniature optical cavities or other optical elements in cryogenic surface electrode ion traps.

physics.atom-ph↗

arXiv2024

Electromagnetically-Induced-Transparency Cooling with a Tripod Structure in a Hyperfine Trapped Ion with Mixed-Species Crystals

Cooling of atomic motion is a crucial tool for many branches of atomic physics, ranging from fundamental physics explorations to quantum information and sensing. For trapped ions, electromagnetically-induced-transparency (EIT) cooling has received attention for the relative speed, low laser power requirements, and broad cooling bandwidth of the technique. However, in applications where the ion used for cooling has hyperfine structure to enable long coherence times, it is difficult to find a closed three-level system in which to perform standard EIT cooling. Here, we demonstrate successful EIT cooling on 25Mg+ by the addition of an extra laser frequency; this method can be applied to any ion with non-zero nuclear spin. Furthermore, we demonstrate simultaneous EIT cooling of all axial modes in mixed-species crystals 9Be+ - 25Mg+ and 9Be+ - 25Mg+ - 9Be+ through the 25Mg+ ion.

physics.atom-ph↗

arXiv2023

Experimental speedup of quantum dynamics through squeezing

We show experimentally that a broad class of interactions involving quantum harmonic oscillators can be made stronger (amplified) using a unitary squeezing protocol. While our demonstration uses the motional and spin states of a single trapped $^{25}$Mg$^{+}$ ion, the scheme applies generally to Hamiltonians involving just a single harmonic oscillator as well as Hamiltonians coupling the oscillator to another quantum degree of freedom such as a qubit, covering a large range of systems of interest in quantum information and metrology applications. Importantly, the protocol does not require knowledge of the parameters of the Hamiltonian to be amplified, nor does it require a well-defined phase relationship between the squeezing interaction and the rest of the system dynamics, making it potentially useful in instances where certain aspects of a signal or interaction may be unknown or uncontrolled.

quant-ph↗

arXiv2021

Motional squeezing for trapped ion transport and separation

Transport, separation, and merging of trapped ion crystals are essential operations for most large-scale quantum computing architectures. In this work, we develop a theoretical framework that describes the dynamics of ions in time-varying potentials with a motional squeeze operator, followed by a motional displacement operator. Using this framework, we develop a new, general protocol for trapped ion transport, separation, and merging. We show that motional squeezing can prepare an ion wave packet to enable transfer from the ground state of one trapping potential to another. The framework and protocol are applicable if the potential is harmonic over the extent of the ion wave packets at all times. As illustrations, we discuss two specific operations: changing the strength of the confining potential for a single ion, and separating same-species ions with their mutual Coulomb force. Both of these operations are, ideally, free of residual motional excitation.

quant-ph↗

arXiv2021

High-fidelity laser-free universal control of two trapped ion qubits

Universal control of multiple qubits -- the ability to entangle qubits and to perform arbitrary individual qubit operations -- is a fundamental resource for quantum computation, simulation, and networking. Here, we implement a new laser-free scheme for universal control of trapped ion qubits based on microwave magnetic fields and radiofrequency magnetic field gradients. We demonstrate high-fidelity entanglement and individual control by creating symmetric and antisymmetric two-qubit maximally entangled states with fidelities in the intervals [0.9983, 1] and [0.9964, 0.9988], respectively, at 68% confidence, corrected for state initialization error. This technique is robust against multiple sources of decoherence, usable with essentially any trapped ion species, and has the potential to perform simultaneous entangling operations on many pairs of ions without increasing control signal power or complexity.

quant-ph↗

arXiv2020

Quantum amplification of boson-mediated interactions

Strong and precisely-controlled interactions between quantum objects are essential for quantum information processing, simulation, and sensing, and for the formation of exotic quantum matter. A well-established paradigm for coupling otherwise weakly-interacting quantum objects is to use auxiliary bosonic quantum excitations to mediate the interactions. Important examples include photon-mediated interactions between atoms, superconducting qubits, and color centers in diamond, and phonon-mediated interactions between trapped ions and between optical and microwave photons. Boson-mediated interactions can in principle be amplified through parametric driving of the boson channel; the drive need not couple directly to the interacting quantum objects. This technique has been proposed for a variety of quantum platforms, but has not to date been realized in the laboratory. Here we experimentally demonstrate the amplification of a boson-mediated interaction between two trapped-ion qubits by parametric modulation of the trapping potential. The amplification provides up to a 3.25-fold increase in the interaction strength, validated by measuring the speedup of two-qubit entangling gates. This amplification technique can be used in any quantum platform where parametric modulation of the boson channel is possible, enabling exploration of new parameter regimes and enhanced quantum information processing.

quant-ph↗

arXiv2020

State Readout of a Trapped Ion Qubit Using a Trap-Integrated Superconducting Photon Detector

We report high-fidelity state readout of a trapped ion qubit using a trap-integrated photon detector. We determine the hyperfine qubit state of a single $^9$Be$^+$ ion held in a surface-electrode rf ion trap by counting state-dependent ion fluorescence photons with a superconducting nanowire single-photon detector (SNSPD) fabricated into the trap structure. The average readout fidelity is 0.9991(1), with a mean readout duration of 46 $μ$s, and is limited by the polarization impurity of the readout laser beam and by off-resonant optical pumping. Because there are no intervening optical elements between the ion and the detector, we can use the ion fluorescence as a self-calibrated photon source to determine the detector quantum efficiency and its dependence on photon incidence angle and polarization.

quant-ph↗

arXiv2020

Laser-free trapped-ion entangling gates with simultaneous insensitivity to qubit and motional decoherence

The dominant error sources for state-of-the-art laser-free trapped-ion entangling gates are decoherence of the qubit state and the ion motion. The effect of these decoherence mechanisms can be suppressed with additional control fields, or with techniques that have the disadvantage of reducing gate speed. Here, we propose using a near-motional-frequency magnetic field gradient to implement a laser-free gate that is simultaneously resilient to both types of decoherence, does not require additional control fields, and has a relatively small cost in gate speed.

quant-ph↗

arXiv2020

VECSEL systems for quantum information processing with trapped beryllium ions

Two vertical-external-cavity surface-emitting laser (VECSEL) systems producing ultraviolet (UV) radiation at 235 nm and 313 nm are demonstrated. The systems are suitable for quantum information processing applications with trapped beryllium ions. Each system consists of a compact, single-frequency, continuous-wave VECSEL producing high-power near-infrared light, tunable over tens of nanometers. One system generates 2.4 W at 940 nm, using a gain mirror based on GaInAs/GaAs quantum wells, which is converted to 54 mW of 235 nm light for photoionization of neutral beryllium atoms. The other system uses a novel gain mirror based on GaInNAs/GaAs quantum-wells, enabling wavelength extension with manageable strain in the GaAs lattice. This system generates 1.6 W at 1252 nm, which is converted to 41 mW of 313 nm light that is used to laser cool trapped $^{9}$Be$^{+}$ ions and to implement quantum state preparation and detection. The 313 nm system is also suitable for implementing high-fidelity quantum gates, and more broadly, our results extend the capabilities of VECSEL systems for applications in atomic, molecular, and optical physics.

physics.atom-ph↗

arXiv2020

Ion transport and reordering in a two-dimensional trap array

Scaling quantum information processors is a challenging task, requiring manipulation of a large number of qubits with high fidelity and a high degree of connectivity. For trapped ions, this could be realized in a two-dimensional array of interconnected traps in which ions are separated, transported and recombined to carry out quantum operations on small subsets of ions. Here, we use a junction connecting orthogonal linear segments in a two-dimensional (2D) trap array to reorder a two-ion crystal. The secular motion of the ions experiences low energy gain and the internal qubit levels maintain coherence during the reordering process, therefore demonstrating a promising method for providing all-to-all connectivity in a large-scale, two- or three-dimensional trapped-ion quantum information processor.

quant-ph↗

arXiv2020

Quantum logic spectroscopy with ions in thermal motion

A mixed-species geometric phase gate has been proposed for implementing quantum logic spectroscopy on trapped ions that combines probe and information transfer from the spectroscopy to the logic ion in a single pulse. We experimentally realize this method, show how it can be applied as a technique for identifying transitions in currently intractable atoms or molecules, demonstrate its reduced temperature sensitivity, and observe quantum-enhanced frequency sensitivity when it is applied to multi-ion chains. Potential applications include improved readout of trapped-ion clocks and simplified error syndrome measurements for quantum error correction.

physics.atom-ph↗

arXiv2019

Far-from-equilibrium noise heating and laser cooling dynamics in radio-frequency Paul traps

We study the stochastic dynamics of a particle in a periodically driven potential. For atomic ions trapped in radio-frequency Paul traps, noise heating and laser cooling typically act slowly in comparison with the unperturbed motion. These stochastic processes can be accounted for in terms of a probability distribution defined over the action variables, which would otherwise be conserved within the regular regions of the Hamiltonian phase space. We present a semiclassical theory of low-saturation laser cooling applicable from the limit of low-amplitude motion to large-amplitude motion, accounting fully for the time-dependent and anharmonic trap. We employ our approach to a detailed study of the stochastic dynamics of a single ion, drawing general conclusions regarding the nonequilibrium dynamics of laser-cooled trapped ions. We predict a regime of anharmonic motion in which laser cooling becomes diffusive (i.e., it is equally likely to cool the ion as it is to heat it), and can also turn into effective heating. This implies that a high-energy ion could be easily lost from the trap despite being laser cooled; however, we find that this loss can be counteracted using a laser detuning much larger than Doppler detuning.

physics.atom-ph↗

arXiv2019

Can a periodically driven particle resist laser cooling and noise?

Studying a single atomic ion confined in a time-dependent periodic anharmonic potential, we find large amplitude trajectories stable for millions of oscillation periods in the presence of stochastic laser cooling. The competition between energy gain from the time-dependent drive and damping leads to the stabilization of such stochastic limit cycles. Instead of converging to the global minimum of the averaged potential, the steady-state phase-space distribution develops multiple peaks in the regions of phase space where the frequency of the motion is close to a multiple of the periodic drive. Such distinct nonequilibrium behaviour can be observed in realistic radio-frequency traps with laser-cooled ions, suggesting that Paul traps offer a well-controlled test-bed for studying transport and dynamics of microscopically driven systems.

physics.atom-ph↗

arXiv2019

Trapped-ion spin-motion coupling with microwaves and a near-motional oscillating magnetic field gradient

We present a new method of spin-motion coupling for trapped ions using microwaves and a magnetic field gradient oscillating close to the ions' motional frequency. We demonstrate and characterize this coupling experimentally using a single ion in a surface-electrode trap that incorporates current-carrying electrodes to generate the microwave field and the oscillating magnetic field gradient. Using this method, we perform resolved-sideband cooling of a single motional mode to its ground state.

quant-ph↗

arXiv2019

Versatile laser-free trapped-ion entangling gates

We present a general theory for laser-free entangling gates with trapped-ion hyperfine qubits, using either static or oscillating magnetic-field gradients combined with a pair of uniform microwave fields symmetrically detuned about the qubit frequency. By transforming into a `bichromatic' interaction picture, we show that either ${\hatσ_ϕ\otimes\hatσ_ϕ}$ or ${\hatσ_{z}\otimes\hatσ_{z}}$ geometric phase gates can be performed. The gate basis is determined by selecting the microwave detuning. The driving parameters can be tuned to provide intrinsic dynamical decoupling from qubit frequency fluctuations. The ${\hatσ_{z}\otimes\hatσ_{z}}$ gates can be implemented in a novel manner which eases experimental constraints. We present numerical simulations of gate fidelities assuming realistic parameters.

quant-ph↗

arXiv2018

Quantum amplification of mechanical oscillator motion

Detection of the weakest forces in nature and the search for new physics are aided by increasingly sensitive measurements of the motion of mechanical oscillators. However, the attainable knowledge of an oscillator's motion is limited by quantum fluctuations that exist even if the oscillator is in its lowest possible energy state. Here we demonstrate a widely applicable technique for amplifying coherent displacements of a mechanical oscillator with initial magnitudes well below these zero-point fluctuations. When applying two orthogonal "squeezing" interactions before and after a small displacement, the displacement is amplified, ideally with no added quantum noise. We implement this protocol with a trapped-ion mechanical oscillator and measure an increase of up to 17.5(3) decibels in sensitivity to small displacements.

quant-ph↗

arXiv2018

Phase-space study of surface-electrode Paul traps: Integrable, chaotic, and mixed motions

We present a comprehensive phase-space treatment of the motion of charged particles in electrodynamic traps. Focusing on five-wire surface-electrode Paul traps, we study the details of integrable and chaotic motion of a single ion. We introduce appropriate phase-space measures and give a universal characterization of the trap effectiveness as a function of the parameters. We rigorously derive the commonly used (time-independent) pseudopotential approximation, quantify its regime of validity and analyze the mechanism of its breakdown within the time-dependent potential. The phase space approach that we develop gives a general framework for describing ion dynamics in a broad variety of surface Paul traps. To probe this framework experimentally, we propose and analyze, using numerical simulations, an experiment that can be realized with an existing four-wire trap. We predict a robust experimental signature of the existence of trapping pockets within a mixed regular and chaotic phase-space structure. Intricately rich escape dynamics suggest that surface traps give access to exploring microscopic Hamiltonian transport phenomena in phase space.

quant-ph↗

arXiv2017

Efficient eigenvalue determination for arbitrary Pauli products based on generalized spin-spin interactions

Effective spin-spin interactions between N+1 qubits enable the determination of the eigenvalue of an arbitrary Pauli product of dimension N with a constant, small number of multi-qubit gates that is independent of N and encodes the eigenvalue in the measurement basis states of an extra ancilla qubit. Such interactions are available whenever qubits can be coupled to a shared harmonic oscillator, a situation that can be realized in several physical qubit implementations. For example, suitable interactions have already been realized for up to 14 qubits in ion traps. It should be possible to implement stabilizer codes for quantum error correction with a constant number of multi-qubit gates, in contrast to typical constructions using a number of two-qubit gates that increases as a function of N. The special case of finding the parity of N qubits only requires a small number of operations that is independent of N. This compares favorably to algorithms for computing the parity on conventional machines, which implies a genuine quantum advantage.

quant-ph↗