Papers by Robert de Keijzer
Academia Quantum, Oct 23, 2024
Parametrized gate circuits are used in plentiful applications in the current NISQ era ofquantum c... more Parametrized gate circuits are used in plentiful applications in the current NISQ era ofquantum computing. These parametrized gates are chiefly implemented using analyticallyfound pulse protocols, often yielding suboptimal gate times, and consequently, fidelities.Alternatively, gate optimization algorithms are designed to construct high fidelity pulsesfor individual, fixed points in continuous parameter space. Gates for intermediate parameterscan subsequently be found by some form of interpolation between previouslyconstructed pulses. Nevertheless, it is not guaranteed (as with analytic protocols) thatthe pulses found by the optimization algorithms belong to the same family of solutionsand thus show resemblance. Interpolation between two pulses of differing solution familiesoften leads to high infidelities, as the pulse strays away from the minimum in theparameter/fidelity landscape. In this work, we introduce a spectral clustering method tosort high-fidelity, optimized pulses in families, and interpolating solely between pulses ofthe same family. Accordingly, interpolations will always approach maximal fidelity. Furthermore,as more than one pulse family is constructed, the parameter space can be partitionedaccording to which family prevails fidelity-wise. This work provides a meticulousdemonstration of our constitutive continuous gate family construction by applying it to auniversal gate set for Rydberg and Cat qubits under noise.
arXiv (Cornell University), Sep 18, 2023
The state |ψ(t)⟩ of a closed quantum system evolves under the Schrödinger equation, where the rev... more The state |ψ(t)⟩ of a closed quantum system evolves under the Schrödinger equation, where the reversible evolution of the state is described by the action of a unitary operator U (t) on the initial state |ψ0⟩, i.e. |ψ(t)⟩ = U (t)|ψ0⟩. However, realistic quantum systems interact with their environment, resulting in non-reversible evolutions, described by Lindblad equations. The solution of these equations give rise to quantum channels Φt that describe the evolution of density matrices according to ρ(t) = Φt(ρ0), which often results in decoherence and dephasing of the state. For many quantum experiments, the time until which measurements can be done might be limited, e.g. by experimental instability or technological constraints. However, further evolution of the state may be of interest. For instance, to determine the source of the decoherence and dephasing, or to identify the steady state of the evolution. In this work, we introduce a method to approximate a given target quantum channel by means of variationally approximating equivalent unitaries on an extended system, invoking the Stinespring dilation theorem. We report on an experimentally feasible method to extrapolate the quantum channel on discrete time steps using only data on the first time steps. Our approach heavily relies on the ability to spatially transport entangled qubits, which is unique to the neutral atom quantum computing architecture. Furthermore, the method shows promising predictive power for various non-trivial quantum channels. Lastly, a quantitative analysis is performed between gate-based and pulse-based variational quantum algorithms.
Physical Review A
In a neutral-atom quantum computer, the qubits are individual neutral atoms trapped in optical tw... more In a neutral-atom quantum computer, the qubits are individual neutral atoms trapped in optical tweezers. Excitations to Rydberg states form the basis for the entanglement procedure that is at the basis of multiqubit quantum gates. However, these Rydberg atoms are often antitrapped, leading to decoherence and atom loss. In this work, we give a quantum-mechanical description of the antitrapping loss rates and determine the recapture probability after Rydberg excitation, distinguishing between having the laser traps turned on and off. We find that ample time (≈30 µs, in a 88 Sr system) is needed for the wave functions to expand out of the trap. Therefore, even with traps on, ≈100% recapture probabilities can be expected for times in which significant entanglement operations between atoms can be performed. We find that for two-dimensional radial traps with bosonic 88 Sr atoms, the time in which perfect recapture can be achieved is of the same order of magnitude for traps turned on and off.
Neutral atoms in tweezer arrays for hybrid Rydberg quantum computing
arXiv (Cornell University), Dec 20, 2022
We investigate the robustness of two-qubit gates to deviations of experimental controls, on a neu... more We investigate the robustness of two-qubit gates to deviations of experimental controls, on a neutral atom platform utilizing Rydberg states. We construct robust CZ gates -employing techniques from quantum optimal control -that retain high Bell state fidelity F > 0.999 in the presence of significant deviations of the coupling strength to the Rydberg state. Such deviations can arise from laser intensity noise and atomic motion in an inhomogeneous coupling field. We also discuss methods to mitigate errors due to deviations of the laser detuning. The designed pulses operate on timescales that are short compared to the fundamental decay timescale set by spontaneous emission and blackbody radiation. We account for the finite lifetime of the Rydberg state in both the optimisation and fidelity calculations -this makes the gates conducive to noisy intermediate-scale quantum experiments, meaning that our protocols can reduce infidelity on near-term quantum computing devices. We calculate physical properties associated with infidelity for strontium-88 atoms -including lifetimes, polarisabilities and blockade strengths -and use these calculations to identify optimal Rydberg states for our protocols, which allows for further minimisation of infidelity.
Optimization of variational quantum eigensolver
Pulse based Variational Quantum Optimal Control for hybrid quantum computing
This work studies pulse based variational quantum algorithms (VQAs), which are designed to determ... more This work studies pulse based variational quantum algorithms (VQAs), which are designed to determine the ground state of a quantum mechanical system by combining classical and quantum hardware. In contrast to more standard gate based methods, pulse based methods aim to directly optimize the laser pulses interacting with the qubits, instead of using some parametrized gate based circuit. Using the mathematical formalism of optimal control, these laser pulses are optimized. This method has been used in quantum computing to optimize pulses for quantum gate implementations, but has only recently been proposed for full optimization in VQAs. Pulse based methods have several advantages over gate based methods such as faster state preparation, simpler implementation and more freedom in moving through the state space. Based on these ideas, we present the development of a novel adjoint based variational method. This method can be tailored towards and applied in neutral atom quantum computers. This method of pulse based variational quantum optimal control is able to approximate molecular ground states of simple molecules up to chemical accuracy and is able to compete with the gate based variational quantum eigensolver in terms of total number of quantum evaluations. The total evolution time $T$ and the form of the control Hamiltonian $H_c$ are important factors in the convergence behavior to the ground state energy, both having influence on the quantum speed limit and the controllability of the system.
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Papers by Robert de Keijzer