Rajibul Islam

Quantum simulation of open quantum systems offers a pathway towards better understanding various non-equilibrium physics that would otherwise be challenging to study. Most open quantum systems studied are modeled as obeying the Markov approximation, where the bath into which the system dissipates information is assumed to be unaffected by the system-bath interaction. However, real baths are in general influenced by this interaction to some degree, and some systems which exist in structured non-Markovian environments can display novel behavior as a result. Here we utilize a trapped ion quantum simulator to simulate a single spin-$1/2$ driven-dissipative system with a non-Markovian dissipation channel, and experimentally compare steady-states to those from an analogous Markovian bath. We observe that a non-Markovian dissipative channel can dramatically change the steady-state even for a single qubit, to a regime inaccessible for Markovian dissipation. This demonstrates the added richness available to quantum systems in structured environments. The techniques used here are compatible with many-body extensions of the model, which can not be simulated efficiently on a classical computer in general. Our work also opens up new possibilities in quantum reservoir engineering beyond the Markovian regime.

Holographic beam shaping is a powerful approach for generating individually addressable optical spots for controlling atomic qubits, such as those in trapped-ion quantum processors. However, its application in qubit control is limited by residual intensity crosstalk at neighboring sites and by a nonzero background floor in the far wings of the addressing beam, leading to accumulated errors from many exposed qubits. Here, we present an all-optical scheme that mitigates both effects using a single digital micromirror device (DMD) operated in a double-pass configuration, in which light interacts with two separate regions of the same device. In the first pass, one region of the DMD is placed in a Fourier plane and implements a binary-amplitude hologram for individual addressing, while in the second pass a different region serves as a programmable intermediate image-plane aperture for spatial filtering. By multiplexing the Fourier-plane hologram to include secondary holograms, we generate weak auxiliary fields that interfere destructively with unwanted light at selected sites, while image-plane filtering suppresses the residual tail at larger distances. Together, these techniques maintain relative intensity crosstalk at or below $10^{-5}$ ($-50\,\mathrm{dB}$) across the full field of view relevant for qubit addressing, and further reduce the far-wing background to approximately $10^{-6}$ at large distances from the addressed qubit, approaching the detection limit. These results provide a compact, DMD-based solution for low-crosstalk optical holographic qubit addressing that is directly applicable to trapped ions and other spatially ordered quantum systems.

We present a room-temperature Extreme High Vacuum (XHV) system engineered to support the long-duration operation of a trapped-ion quantum processor. Background-gas collisions impose limitations on trapped-ion performance and scalability by interrupting algorithmic execution and, in some cases, ejecting ions from the trap. Using molecular-flow simulations, we optimize the chamber geometry, conductance pathways, and pumping configuration to maximize the effective pumping speed at the ion location. We perform high-temperature heat treatment of stainless steel vacuum components to achieve the desired outgassing rate, guided by quantitative relations of bulk diffusive processes, allowing us to reduce the \(\mathrm{H_2}\) outgassing load to the \(10^{-15}\,\mathrm{mbar\,l\,s^{-1}\,cm^{-2}}\) level. The final pressure in our chamber, measured by a hot cathode gauge, is \(1.5\times10^{-12}\,\mathrm{mbar}\), corresponding to the gauge's measurement limit. We measure the local pressure at the ion location by observing collision-induced reordering events in a long ion chain of mixed-isotope Yb\(^+\). From the observed reordering frequency, we extract the average interval between collisions to be \((1.9 \pm 0.1)\,\mathrm{hrs/ion}\). This corresponds to a local pressure of \((3.9 \pm 0.3)\times10^{-12}\,\mathrm{mbar}\) at the ion location, assuming that all collisions arise from background H\(_2\) molecules at room temperature. Our demonstration extends the continuous operation time of a quantum processor while maintaining the simplicity of a room-temperature system that does not require cryogenic apparatus.

Optical systems capable of generating fields with sub-wavelength spatial features have become standard in science and engineering research and industry. Pertinent examples include atom- and ion-based quantum computers and optical lithography setups. So far, no tools exist to characterize such fields - both intensity and polarization - at sub-wavelength length scales. We use a single trapped atomic ion, confined to approximately 40 nm X 40 nm X 180 nm to sense a laser light field at a wavelength of 370 nm. With its spatial extent smaller than the absorption cross-section of a resonant detector, the ion-sensor operates at the fundamental limit of spatial resolution. Our technique relies on developing an analytical model of the ion-light interaction and using the model to extract the intensity and polarization. An important insight provided in this work is also that the inverse of this model can be learned, in a restricted sense, on a deep neural network, speeding up the intensity and polarization readout by five orders of magnitude. This speed-up makes the technique field-deployable to characterize optical instruments by probing light at the sub-wavelength scale.

Trapped ions for quantum information processing has been an area of intense study due to the extraordinarily high fidelity operations that have been reported experimentally. Specifically, barium trapped ions have been shown to have exceptional state-preparation and measurement (SPAM) fidelities. The $^{133}\mathrm{Ba}^+$ ($I = 1/2$) isotope in particular is a promising candidate for large-scale quantum computing experiments. However, a major pitfall with this isotope is that it is radioactive and is thus generally used in microgram quantities to satisfy safety regulations. We describe a new method for creating microgram barium chloride ($\mathrm{BaCl}_2$) ablation targets for use in trapped ion experiments and compare our procedure to previous methods. We outline two recipes for fabrication of ablation targets that increase the production of neutral atoms for isotope-selective loading of barium ions. We show that heat-treatment of the ablation targets greatly increases the consistency at which neutral atoms can be produced and we characterize the uniformity of these targets using trap-independent techniques such as energy dispersive x-ray spectroscopy (EDS) and neutral fluorescence collection. Our comparison between fabrication techniques and demonstration of consistent neutral fluorescence paves a path towards reliable loading of $^{133}\mathrm{Ba}^+$ in surface traps and opens opportunities for scalable quantum computing with this isotope.

We numerically investigate a hybrid trapping architecture for 2D ion crystals using static electrode voltages and optical cavity fields for in-plane and out-of-plane confinements, respectively. By studying the stability of 2D crystals against 2D-3D structural phase transitions, we identify the necessary trapping parameters for ytterbium ions. Multiple equilibrium configurations for 2D crystals are possible, and we analyze their stability by estimating potential barriers between them. We find that scattering to anti-trapping states limits the trapping lifetime, which is consistent with recent experiments employing other optical trapping architectures. These 2D ion crystals offer an excellent platform for quantum simulation of frustrated spin systems, benefiting from their 2D triangular lattice structure and phonon-mediated spin-spin interactions. Quantum information processing with tens of ions is feasible in this scheme with current technologies.

We propose and experimentally demonstrate an analog scheme for generating XY-type ($J_{ij}^x σ_x^i σ_x^j \;$ + $J_{ij}^y σ_y^i σ_y^j \;$) Hamiltonians on trapped ion spins with independent control over the $J_{ij}^x$ and $J_{ij}^y$ terms. The Ising-type interactions $σ_x^i σ_x^j \;$ and $σ_y^i σ_y^j \;$ are simultaneously generated by employing two spin-dependent forces operating in parallel on the same set of normal modes. We analytically calculate the region of validity of this scheme, and provide numerical and experimental validation with $^{171}\rm{Yb}^+\;$ ions. This scheme inherits the programmability and scalability of the Ising-type interactions with trapped ions that have been explored in numerous quantum simulation experiments. Our approach extends the capabilities of existing trapped ion quantum simulators to access a large class of spin Hamiltonians relevant for exploring exotic quantum phases such as superfluidity and spin liquids.

Trapped ions are one of the leading platforms for quantum information processing, exhibiting the highest gate and measurement fidelities of all contending hardware. In order to realize a universal quantum computer with trapped ions, independent and parallel control over the state of each qubit is necessary. The manipulation of individual qubit states in an ion chain via stimulated Raman transitions generally requires light focused on individual ions. In this manuscript, we present a novel, guided-light individual addressing system for hyperfine Ba$^+$ qubits. The system takes advantage of laser-written waveguide technology, enabled by the atomic structure of Ba$^+$, allowing the use of visible light to drive Raman transitions. Such waveguides define the spatial mode of light, suppressing aberrations that would have otherwise accumulated in a free-space optics set up. As a result, we demonstrate a nearest neighbour relative intensity crosstalk on the order of 10$^{-4}$, without any active aberration compensation. This is comparable to or better than other previous demonstrations of individual addressing. At the same time, our modular approach provides independent and agile control over the amplitude, frequency, and phase of each channel; combining the strengths of previous implementations.

Despite the progress in building sophisticated microfabricated ion traps, Paul traps employing needle electrodes retain their significance due to the simplicity of fabrication while producing high-quality systems suitable for quantum information processing, atomic clocks etc. For low noise operations such as minimizing `excess micromotion', needles should be geometrically straight and aligned precisely with respect to each other. Self-terminated electrochemical etching, previously employed for fabricating ion trap needle electrodes employs a sensitive and time-consuming technique resulting in a low success rate of usable electrodes. Here we demonstrate an etching technique for quick fabrication of straight and symmetric needles with a high success rate and a simple apparatus with reduced sensitivity to alignment imperfections. The novelty of our technique comes from using a two-step approach employing turbulent etching for fast shaping and slow etching/polishing for subsequent surface finish and tip cleaning. Using this technique, needle electrodes for an ion-trap can be fabricated within a day, significantly reducing the setup time for a new apparatus. The needles fabricated via this technique have been used in our ion-trap to achieve trapping lifetimes of several months.

Linear ion trap chains are a promising platform for quantum computation and simulation. The XY model with long-range interactions can be implemented with a single side-band Molmer-Sorensen scheme, giving interactions that decay as $1/r^α$, where $α$ parameterises the interaction range. Lower $α$ leads to longer range interactions, allowing faster long-range gate operations for quantum computing. However, decreasing $α$ causes an increased generation of coherent phonons and appears to dephase the effective XY interaction model. We characterise and show how to correct for this effect completely, allowing lower $α$ interactions to be coherently implemented. Ion trap chains are thus shown to be a viable platform for spatial quantum search in optimal $O(\sqrt{N})$ time, for $N$ ions. Finally, we introduce a $O(\sqrt{N})$ quantum state transfer protocol, with a qubit encoding that maintains a high fidelity.

The rise of programmable quantum devices has motivated the exploration of circuit models which could realize novel physics. A promising candidate is a class of hybrid circuits, where entangling unitary dynamics compete with disentangling measurements. Novel phase transitions between different entanglement regimes have been identified in their dynamical states, with universal properties hinting at unexplored critical phenomena. Trapped ion hardware is a leading contender for the experimental realization of such physics, which requires not only traditional two-qubit entangling gates, but a constant rate of local measurements accurately addressed throughout the circuit. Recent progress in engineering high-precision optical addressing of individual ions makes preparing a constant rate of measurements throughout a unitary circuit feasible. Using tensor network simulations, we show that the resulting class of hybrid circuits, prepared with native gates, exhibits a volume-law to area-law transition in the entanglement entropy. This displays universal hallmarks of a measurement-induced phase transition. Our simulations are able to characterize the critical exponents using circuit sizes with tens of qubits and thousands of gates. We argue that this transition should be robust against additional sources of experimental noise expected in modern trapped ion hardware, and will rather be limited by statistical requirements on post selection. Our work highlights the powerful role that tensor network simulations can play in advancing the theoretical and experimental frontiers of critical phenomena.

We propose an experimental architecture where an array of optical tweezers affords site-dependent control over the confining potential of a conventional radio-frequency ion trap. The site-dependent control enables programmable manipulation of phonon modes of ions, with many potential applications in quantum information processing (QIP) and thermodynamics. We describe protocols for programming the array of optical tweezers to attain a set of target phonon modes with high accuracy. We propose applications of such controls in simulating quantum thermodynamics of a particle of programmable effective mass via Jarzynski's equality and improving the efficiency of sympathetic cooling and quantum logic gates in a multi-species ion system of disparate masses. We discuss the required optical parameters in a realistic ion trap system and potential adverse effects of optical tweezers in QIP. Our scheme extends the utility of trapped-ions as a platform for quantum computation and simulation.

Trapped ions have emerged as one of the highest quality platforms for the quantum simulation of interacting spin models of interest to various fields of physics. In such simulators, two effective spins can be made to interact with arbitrary strengths by coupling to the collective vibrational or phonon states of ions, controlled by precisely tuned laser beams. However, the task of determining laser control parameters required for a given spin-spin interaction graph is a type of inverse problem, which can be highly mathematically complex. In this paper, we adapt a modern machine learning technique developed for similar inverse problems to the task of finding the laser control parameters for a number of interaction graphs. We demonstrate that typical graphs, forming regular lattices of interest to physicists, can easily be produced for up to 50 ions using a single GPU workstation. The scaling of the machine learning method suggests that this can be expanded to hundreds of ions with moderate additional computational effort.

Controlling the interaction graph between spins or qubits in a quantum simulator allows user-controlled tailoring of native interactions to achieve a target Hamiltonian. The flexibility of engineering long-ranged phonon-mediated spin-spin interactions in a trapped ion quantum simulator offers such a possibility. Trapped ions, a leading candidate for simulating computationally hard quantum many-body dynamics, are most readily trapped in a linear 1D chain, limiting their utility for readily simulating higher dimensional spin models. In this work, we introduce a hybrid method of analog-digital simulation for simulating 2D spin models and dynamically changing interactions to achieve a new graph using a linear 1D chain. The method relies on time domain Hamiltonian engineering through a successive application of Stark shift gradient pulses, and wherein the pulse sequence can simply be obtained from a Fourier series decomposition of the target Hamiltonian over the space of lattice couplings. We focus on engineering 2D rectangular nearest-neighbor spin lattices, demonstrating that the required control parameters scale linearly with ion number. This hybrid approach offers compelling possibilities for the use of 1D chains in the study of Hamiltonian quenches, dynamical phase transitions, and quantum transport in 2D and 3D. We discuss a possible experimental implementation of this approach using real experimental parameters.

Quantization of energy is a quintessential characteristic of quantum systems. Here we analyze its effects on the operation of Otto cycle heat machines and show that energy quantization alone may alter and increase machine performance in terms of output power, efficiency, and even operation mode. Our results demonstrate that quantum thermodynamics enable the realization of classically inconceivable Otto machines, such as those with an incompressible working fluid. We propose to measure these effects experimentally using a laser-cooled trapped ion as a microscopic heat machine.

A trapped ion transported along a periodic potential is studied as a paradigmatic nanocontact frictional interface. The combination of the periodic corrugation potential and a harmonic trapping potential creates a one-dimensional energy landscape with multiple local minima, corresponding to multistable stick-slip friction. We measure the probabilities of slipping to the various minima for various corrugations and transport velocities. The observed probabilities show that the multislip regime can be reached dynamically at smaller corrugations than would be possible statically, and can be described by an equilibrium Boltzmann model. While a clear microscopic signature of multislip behavior is observed for the ion motion, the frictional force and dissipation are only weakly affected by the transition to multistable potentials.

High-resolution addressing of individual ultracold atoms, trapped ions or solid state emitters allows for exquisite control in quantum optics experiments. This becomes possible through large aperture magnifying optics that project microscopic light patterns with diffraction limited performance. We use programmable amplitude holograms generated on a digital micromirror device to create arbitrary microscopic beam shapes with full phase and amplitude control. The system self-corrects for aberrations of up to several $λ$ and reduces them to $λ/50$, leading to light patterns with a precision on the $10^{-4}$ level. We demonstrate aberration-compensated beam shaping in an optical lattice experiment and perform single-site addressing in a quantum gas microscope for $^{87}$Rb.

A collection of trapped atomic ions represents one of the most attractive platforms for the quantum simulation of interacting spin networks and quantum magnetism. Spin-dependent optical dipole forces applied to an ion crystal create long-range effective spin-spin interactions and allow the simulation of spin Hamiltonians that possess nontrivial phases and dynamics. Here we show how appropriate design of laser fields can provide for arbitrary multidimensional spin-spin interaction graphs even for the case of a linear spatial array of ions. This scheme uses currently existing trap technology and is scalable to levels where classical methods of simulation are intractable.