arXiv subjects

C. H. Valahu

Publications and source records attributed to C. H. Valahu.

6 recordsLinked to original sources

arXiv2024

Universal Quantum Gate Set for Gottesman-Kitaev-Preskill Logical Qubits

The realisation of a universal quantum computer at scale promises to deliver a paradigm shift in information processing, providing the capability to solve problems that are intractable with conventional computers. A key limiting factor of realising fault-tolerant quantum information processing (QIP) is the large ratio of physical-to-logical qubits that outstrip device sizes available in the near future. An alternative approach proposed by Gottesman, Kitaev, and Preskill (GKP) encodes a single logical qubit into a single harmonic oscillator, alleviating this hardware overhead in exchange for a more complex encoding. Owing to this complexity, current experiments with GKP codes have been limited to single-qubit encodings and operations. Here, we report on the experimental demonstration of a universal gate set for the GKP code, which includes single-qubit gates and -- for the first time -- a two-qubit entangling gate between logical code words. Our scheme deterministically implements energy-preserving quantum gates on finite-energy GKP states encoded in the mechanical motion of a trapped ion. This is achieved by a novel optimal control strategy that dynamically modulates an interaction between the ion's spin and motion. We demonstrate single-qubit gates with a logical process fidelity as high as 0.960 and a two-qubit entangling gate with a logical process fidelity of 0.680. We also directly create a GKP Bell state from the oscillators' ground states in a single step with a logical state fidelity of 0.842. The overall scheme is compatible with existing hardware architectures, highlighting the opportunity to leverage optimal control strategies as a key accelerant towards fault tolerance.

quant-ph↗

arXiv2024

Experimental Quantum Simulation of Chemical Dynamics

Accurate simulation of dynamical processes in molecules and reactions is among the most challenging problems in quantum chemistry. Quantum computers promise efficient chemical simulation, but the existing quantum algorithms require many logical qubits and gates, placing practical applications beyond existing technology. Here, we carry out the first quantum simulations of chemical dynamics by employing a more hardware-efficient encoding scheme that uses both qubits and bosonic degrees of freedom. Our trapped-ion device accurately simulates the dynamics of non-adiabatic chemical processes, which are among the most difficult problems in computational chemistry because they involve strong coupling between electronic and nuclear motions. We demonstrate the programmability and versatility of our approach by simulating the dynamics of three different molecules as well as open-system dynamics in the condensed phase, all with the same quantum resources. Our approach requires orders of magnitude fewer resources than equivalent qubit-only quantum simulations, demonstrating the potential of using hybrid encoding schemes to accelerate quantum simulations of complex chemical processes, which could have applications in fields ranging from energy conversion and storage to biology and drug design.

quant-ph↗

arXiv2024

Robust and Deterministic Preparation of Bosonic Logical States in a Trapped Ion

Encoding logical qubits in bosonic modes provides a potentially hardware-efficient implementation of fault-tolerant quantum information processing. Here, we demonstrate high-fidelity and deterministic preparation of highly non-classical bosonic states in the mechanical motion of a trapped ion. Our approach implements error-suppressing pulses through optimized dynamical modulation of laser-driven spin-motion interactions to generate the target state in a single step. We demonstrate logical fidelities for the Gottesman-Kitaev-Preskill (GKP) state as high as $\bar{\mathcal{F}}=0.940(8)$, a distance-3 binomial state with an average fidelity of $\mathcal{F}=0.807(7)$, and a 12.91(5) dB squeezed vacuum state.

quant-ph↗

arXiv2024

Simulating open-system molecular dynamics on analog quantum computers

Interactions of molecules with their environment influence the course and outcome of almost all chemical reactions. However, classical computers struggle to accurately simulate complicated molecule-environment interactions because of the steep growth of computational resources with both molecule size and environment complexity. Therefore, many quantum-chemical simulations are restricted to isolated molecules, whose dynamics can dramatically differ from what happens in an environment. Here, we show that analog quantum simulators can simulate open molecular systems by using the native dissipation of the simulator and injecting additional controllable dissipation. By exploiting the native dissipation to simulate the molecular dissipation -- rather than seeing it as a limitation -- our approach enables longer simulations of open systems than are possible for closed systems. In particular, we show that trapped-ion simulators using a mixed qudit-boson (MQB) encoding could simulate molecules in a wide range of condensed phases by implementing widely used dissipative processes within the Lindblad formalism, including pure dephasing and both electronic and vibrational relaxation. The MQB open-system simulations require significantly fewer additional quantum resources compared to both classical and digital quantum approaches.

quant-ph↗

arXiv2024

Ultrasensitive single-ion electrometry in a magnetic field gradient

Hyperfine energy levels in trapped ions offer long-lived spin states. In addition, the motion of these charged particles couples strongly to external electric field perturbations. These characteristics make trapped ions attractive platforms for the quantum sensing of electric fields. However, the spin states do not exhibit a strong intrinsic coupling to electric fields. This limits the achievable sensitivities. Here, we amplify the coupling between electric field perturbations and the spin states by using a static magnetic field gradient. Displacements of the trapped ion resulting from the forces experienced by an applied external electric field perturbation are thereby mapped to an instantaneous change in the energy level splitting of the internal spin states. This gradient mediated coupling of the electric field to the spin enables the use of a range of well-established magnetometry protocols for electrometry. Using our quantum sensor, we demonstrate AC sensitivities of $\mathrm{S^{AC}_{min}=960(10)\times 10^{-6}~V m^{-1}Hz^{-\frac{1}{2}}}$ at a signal frequency of $ω_ε/2π=5.82~\mathrm{Hz}$, and DC sensitivities of $\mathrm{S^{DC}_{min}=1.97(3)\times 10^{-3} ~V m^{-1}Hz^{-\frac{1}{2}}}$ with a Hahn-echo type sensing sequence. We also employ a rotating frame relaxometry technique, with which our quantum sensor can be utilised as an electric field noise spectrum analyser. We measure electric field signals down to a noise floor of $\mathrm{S_{E}(ω)=6.2(5)\times 10^{-12}~V^2 m^{-2}Hz^{-1}}$ at a frequency of $\mathrm{30.0(3)~kHz}$. We therefore demonstrate unprecedented electric field sensitivities for the measurement of both DC signals and AC signals across a frequency range of sub-Hz to $\sim\mathrm{500~kHz}$. Finally, we describe a set of hardware modifications that are capable of achieving a further improvement in sensitivity by up to six orders of magnitude.

quant-ph↗

arXiv2021

Robust entanglement by continuous dynamical decoupling of the J-coupling interaction

We propose a $σ_z\otimes σ_z$ laser-free entangling gate which uses the intrinsic J-coupling of ions in a static magnetic gradient. Dephasing of the interaction is suppressed by means of continuous dynamical decoupling using pairs of microwave fields. The gate is virtually insensitive to common amplitude noise of the microwave fields and enables high fidelities despite qubit frequency fluctuations, while the J-coupling interaction's inherent robustness to motional decoherence is retained. Errors far below the fault-tolerant threshold can be achieved at high initial temperatures, negating the requirement of sideband cooling below the Doppler temperature. By adjusting the powers of the continuous microwave fields, the J-coupling interaction can be tuned and can be used to implement parallel entangling gates within an ion chain.

quant-ph↗