arXiv subjects

Fei Zhuang

Publications and source records attributed to Fei Zhuang.

6 recordsLinked to original sources

arXiv2022

Designing globally optimal entangling gates using geometric space curves

High-fidelity entangling gates are essential for quantum computation. Currently, most approaches to designing such gates are based either on simple, analytical pulse waveforms or on ones obtained from numerical optimization techniques. In both cases, it is typically not possible to obtain a global understanding of the space of waveforms that generate a target gate operation, making it challenging to design globally optimal gates. Here, we show that in the case of weakly coupled qubits, it is possible to find all pulses that implement a target entangling gate. We do this by mapping quantum evolution onto geometric space curves. We derive the minimal conditions these curves must satisfy in order to guarantee a gate with a desired entangling power is implemented. Pulse waveforms are extracted from the curvatures of these curves. We illustrate our method by designing fast, CNOT-equivalent entangling gates for silicon quantum dot spin qubits with fidelities exceeding 99%. We show that fidelities can be further improved while maintaining low bandwidth requirements by using geometrically derived pulses as initial guesses in numerical optimization routines.

quant-ph↗

arXiv2022

Noise-resistant Landau-Zener sweeps from geometrical curves

Landau-Zener physics is often exploited to generate quantum logic gates and to perform state initialization and readout. The quality of these operations can be degraded by noise fluctuations in the energy gap at the avoided crossing. We leverage a recently discovered correspondence between qubit evolution and space curves in three dimensions to design noise-robust Landau-Zener sweeps through an avoided crossing. In the case where the avoided crossing is purely noise-induced, we prove that operations based on monotonic sweeps cannot be robust to noise. Hence, we design families of phase gates based on non-monotonic drives that are error-robust up to second order. In the general case where there is an avoided crossing even in the absence of noise, we present a general technique for designing robust driving protocols that takes advantage of a relationship between the Landau-Zener problem and space curves of constant torsion.

quant-ph↗

arXiv2021

Doubly geometric quantum control

In holonomic quantum computation, single-qubit gates are performed using driving protocols that trace out closed loops on the Bloch sphere, making them robust to certain pulse errors. However, dephasing noise that is transverse to the drive, which is significant in many qubit platforms, lies outside the family of correctable errors. Here, we present a general procedure that combines two types of geometry -- holonomy loops on the Bloch sphere and geometric space curves in three dimensions -- to design gates that simultaneously suppress pulse errors and transverse noise errors. We demonstrate this doubly geometric control technique by designing explicit examples of such dynamically corrected holonomic gates.

quant-ph↗

arXiv2021

Dynamically corrected gates from geometric space curves

Quantum information technologies demand highly accurate control over quantum systems. Achieving this requires control techniques that perform well despite the presence of decohering noise and other adverse effects. Here, we review a general technique for designing control fields that dynamically correct errors while performing operations using a close relationship between quantum evolution and geometric space curves. This approach provides access to the global solution space of control fields that accomplish a given task, facilitating the design of experimentally feasible gate operations for a wide variety of applications.

quant-ph↗

arXiv2004

Propagation of atomic matter waves inside an atom wave guide

The phenomenological description of propagation of atomic matter waves inside a curved atom wave guide is presented based on the effective action principle. The evolutions in both temporal and spatial domains for the atomic matter wave in the presence of guiding potential fields (classical and quantized) are considered. The coherent control of three-level atomic matter waves in a wave guide by an external controlling light is briefly discussed. The concepts of {\it atomic matter-wave bandgap structure} in a spatially periodic guiding field ({\it e.g.}, the interior potential of carbon peapod which can trap both atoms and light) and {\it optical lattice bandgap medium} are suggested.

quant-ph↗

arXiv2003

Frequency-independent effective rest mass of photons in the 2TDLM model

A physically interesting {\it effective rest mass} of photons in electromagnetic media, which is independent of wave frequency $ω$, is defined in the present paper. It is verified that this frequency-independent effective rest mass of photons can be easily read off from the optical refractive index squared $n^{2}(ω) $ of commonly-seen electromagnetic media. As an illustrative example, we extract the frequency-independent effective rest mass of photons from $n^{2}(ω) $ in the {\it two time derivative Lorentz material} (2TDLM) model. The connection between effective rest mass and electromagnetic parameters of electric permittivity ($ε$) and magnetic permeability ($μ$) in left-handed media is also briefly discussed.

cond-mat.mtrl-sci↗