Session M39: Semiconductor Qubits VI

Quantum state transfer in quantum-dot spin chains

Electron spins in semiconductor quantum dots are a leading platform for quantum computing because they have extremely long coherence times and are compatible with advanced semiconductor manufacturing techniques. In recent years, large-scale arrays of electron spins in gate-defined quantum dots have emerged as key elements of spin-based quantum information processors. Electron spin qubits naturally interact with each other via nearest-neighbor exchange coupling. However, the ability to transmit quantum states over long distances is beneficial for fault-tolerant quantum computing. In this talk, we discuss the experimental realization of various ways to transfer spin states in quantum dots and how such methods of state transfer might be useful for quantum computing.   

Quantum simulation of Thouless pump on quantum hardware

We demonstrate the simulation of Thouless pump in a XY spin chain in the presence of staggered field on IonQ quantum hardware. The important ingredient of the Thouless pump is that the system should be evolving adiabatically. Therefore, the conventional quantum simulations based on Trotter decomposition would result in a circuit with large depth, which complicates achieving good results even for a short chain formed from several qubits. Here we map the original spin model to a free fermion model, which is identical to the Rice-Mele model, so that we can utilize a powerful compiling method to construct fixed depth circuits and simulate the Thouless pump with arbitrary small Trotter step. We show that with this technique, the clear features of Thouless pump can be identified on quantum hardware.   

Collective coherent scattering from quantum dots in a photonic crystal waveguide

The light-matter interface of a coherent field scattering from a quantum dot coupled to a nanophotonic waveguide enables efficient nonlinear photon-photon interactions and the realization of exotic states of light, such as two-photon bound states. The exploration of collective quantum phenomena with multiple dots within a waveguide is also of great interest, but the spectral inhomogeneity of quantum dots has been a persistent challenge. We overcome this inhomogeneity by strain-tuning InAs quantum dots into resonance, and experimentally demonstrate collective coherent scattering in a photonic crystal waveguide. We show that collective scattering results in an enhanced optical nonlinearity, demonstrated with both the intensity and photon statistics of transmitted coherent light. In addition to providing a means of manipulating quantum optical nonlinearities, collective coherent scattering in this platform may also allow the creation of high-fidelity two-qubit gates, the study of non-Markovian effects, and the creation of subradiant states.   

Antisite defect qubits in monolayer transition metal dichalcogenides

Quantum bit as the heart of quantum information technology brings unprecedented capability of computation that is expected to transform science and society in unimaginable ways. Being atomically thin and amendable to external controls, two-dimensional (2D) materials offer a new paradigm for the realization of patterned qubit fabrication and operation at room temperature. Using high-throughput atomistic simulations and a symmetry-based hypothesis, we identify six neutral anion-antisite defects in transition metal dichalcogenide (TMD) monolayers that host a paramagnetic triplet ground state. Our in-depth analysis reveals the nature of optical transitions and triplet-singlet intersystem crossings in the qubit, which provides a complete cycle for initialization, manipulation and redout of the qubit. As an illustrative example, the operational principles of the antisite qubit in WS₂ are discussed in details. We also demonstrate that the antisite defect qubit system is stable against interlayer interactions in a multilayer structure for qubit isolation and protection in future defect-based devices. The key characters of host materials that give rise to defects with a triplet ground state are discussed, which suggests a feasible strategy for continued discovery of promising defect qubits in diverse classes of 2D materials. Our study opens a new pathway for creating scalable and controllable spin qubits in 2D TMDs.    

All-optical Raman-based noise spectroscopy of a solid-state spin

The development of spin qubits with long coherence times for quantum information processing, communication and sensing requires sources of spin noise to be identified and mitigated. Although microwave-based spin control is typically used for noise spectroscopy of solid-state spin qubits, this approach becomes infeasible when high frequency noise components are stronger than the available microwave powers. Here, we introduce an all-optical approach for noise spectroscopy of spin qubits, which enables the characterization of spin systems for which microwave control is challenging. Our approach involves Raman-based spin rotations to optically implement the Carr-Purcell-Meiboom-Gill pulse sequences inspired by nuclear magnetic resonance spectroscopy. Analyzing the temporal dynamics of a spin of interest under the application of these sequences allows us to extract the spectral densities of the noise sources that interact with the spin and lead to its decoherence. To demonstrate the capabilities of our all-optical approach for noise spectroscopy, we use it to measure the noise spectral density of a single spin confined in an InAs/GaAs quantum dot. By leveraging the high bandwidths (over 100 MHz) provided by the all-optical approach, we extract the high frequency spectral density of the noise of a single spin interacting with a large ensemble of nuclear spins broadened by strain. Our measurements confirm previous theoretical modeling of such hyperfine interactions, and shed light on the ability to extend quantum dot coherence times utilizing dynamical decoupling sequences. As such, our Raman-based approach for noise spectroscopy provides insights for the development of optically-active semiconductor spin qubits with long coherence times for quantum information processing, communication and sensing.   

Spin response in self-assembled quantum dots beyond the g-tensor model

Self-assembled quantum dots, with both electrical tunability and optical addressability, have shown promise as a semiconductor qubit architecture. The use of hole spins as qubit basis increases lifetime for weak nuclear interactions. Furthermore, hole spins in self-assembled dot have strong spin-orbit interactions, which we seek to utilize as a control scheme. In this talk, we discuss the interplay of spin-orbit effects and higher order magnetic fields terms. The resulting effects, such as atomic-level spin texture, cannot be fully captured by existing g-tensor models. As such, we propose second order diamagnetic corrections to the linear g-tensor model. We compare the corrected model to a fully atomistic tight-binding model to show experimentally realizable regimes of said corrections. Finally, we have a brief discussion of double dot systems and alloyed systems, which exhibit a much stronger diamagnetic shift.   

Quantum Simulation of Quantized Plasmons in 1D Systems: The critical role of on-site repulsion and Coulomb exchange

Linear atomic chains, such as atom chains on surfaces, linear arrays of dopant atoms in semiconductors, or linear molecules, provide ideal testbeds for studying single-particle, collective (plasmonic) and strongly correlated excitations in the quantum limit for interacting matter systems. We use exact diagonalization to find the many-body excitations of finite (4-26) atom chains, including hopping, long-range electron-electron repulsion and the corresponding electron-core attraction in an extended-range Hubbard model. For spinless electrons, we showed previously that quantized plasmonic excitations can be identified in chains as short as 8 atoms and can be launched one-by-one by quantum emitters attached to the chain. When spin is included, we find that quantized plasmons still can exist in short chains for a wide range of on-site repulsion of opposite spins U. However, for U large enough that the same spins cluster locally, the excitations cease to be plasmonic. Short range Coulomb exchange also plays a key role. Quantized plasmonic excitations disappear in the absence of this exchange. Systems with and without spin will be compared to highlight the essential physics that determines the quantization of 1D excitations in each regime.   

Lieb-Robinson correlation functions in 1D, 2D, and 3D qubit networks

The Lieb-Robinson correlation function provides a state-independent measure of quantum entanglement between two qubits. An important and well-known result is that this quantum correlation between qubits is local and its spatial spread in a network of interacting qubits is limited by a finite velocity, the Lieb-Robinson velocity. We consider qubits inspired by quantum-dot cellular automata whose parameters can be electronically tuned. The resulting Hamiltonian is of the transverse-field Ising model form, which has been realized in multiple systems including superconducting qubit arrays. Our focus is on the early-time behavior which grows with a power-law dependence on time. The power-law exponent increases linearly with distance--here characterized by the number of interacting qubit links connecting the two correlated qubits. We deduce an analytic form for the early-time correlation function for a network of qubits with arbitrary connectivity in one, two, and three dimensions.  For regular square arrays we calculate the dependence of the Lieb-Robinson velocity on the direction of propagation.   

Spin qubits in carbon nanotubes

Spin qubits hosted in carbon nanotubes is a promising scalable platform for high fidelity qubits. Carbon nanotubes can be interfaced with on-chip microwave circuits [1] and the electronic spin of a double quantum dots coupled to microwave photon [2,3,4,5]. I will present the decoherence mechanisms associated to the hybrid charge-spin spectrum [6] and give their estimation in the case of suspended carbon nanotubes.

[1] T. Cubaynes et al., Appl. Phys. Lett.  (2020)

[2] J.J Viennot et al., Science (2015)

[3] T. Cubaynes et al, NJP Quantum Inf (2019)

[4]  F. Borjans et al, Nature (2020)

[5] P.H. Collard et al,  arXiv:2108.01206 (2021)

[6] M. Beniot et al, Phys. Rev B (2019)   

Polaron effects on optical properties of semiconductor based spin-photon interfaces

Understanding the effects of vibrational modes on solid-state quantum bits proves a major challenge in developing robust spin-photon interfaces for semiconductor quantum computing architectures. Donor spins in silicon are known to exhibit remarkably long-coherence times making them attractive candidates for qubits, however the semiconductor environment introduces strong electron-phonon couplings which adversely effect the fidelity of the spin-photon interface, and therefore our ability to entangle qubits and perform quantum gate computations. In order to better understand the role of electron-phonon couplings in these systems, we study a microscopic model that captures the physical mechanisms inherent to these interactions in indirect bandgap semiconductors like silicon. In particular, we focus on the role played by non-local electron-phonon couplings which we find to have a substantial effect on the calculated lifetimes of the optical transitions pertaining to the donor atoms. We report on calculated fluorescence emission spectra that closely resemble phonon sideband formation and zero-phonon line characteristics in experimentally observed spectra, as well as identifing the physical mechanisms by which non-local electron-phonon couplings lead to zero-phonon line broadening.    

Quantum Information Scrambling in Extended Hubbard Models

This study investigates entanglement and quantum information scrambling (QIS) by the example

of a many-body Extended Hubbard Model (EHM) that described a special type of quantum dot array

(interacting V-shapes). The concept of QIS is used in the framework of quantum information processing

by quantum circuits and quantum channels. In general, QIS is seen as the de-localization of quantum

information over the entire quantum system. Recently, connections of QIS to quantum information

processing and machine learning have been made.

We firstly make an introduction on the concept of quantum information scrambling and its connection

with the 4-point out-of-time-order (OTO) correlators. In order to have a quantitative measure of QIS,

we use the tripartite mutual information, that measures the mutual information between 3

different spacetime partitions of the system; this is used to quantify the dynamical spreading

of quantum entanglement in the system. Then, we investigate scrambling in the quantum many-body Extended Hubbard Model with external

magnetic field Bz for both uniform and thermal quantum channel inputs and show that it

scrambles for specific external tuning parameters (e.g. tunneling amplitudes, on-site potentials, magnetic

field). In addition, we compare different Hilbert space sizes (different number of

qubits) and show the qualitative and quantitative differences in quantum scrambling, as we increase the

number of quantum dots in the system. Moreover, we find a "scrambling phase transition" for a threshold

temperature in the thermal case, that is, the temperature of the model that the channel starts to

scramble quantum information. Finally, we make comparisons with the Transverse Field Ising model

(TFI) and investigate connections to Quantum Machine Learning (QML) and draw potential parallels

between quantum learning and information scrambling.   

Tailoring Quantum Oscillations of Excitonic Schrodinger’s Cats as Qubits

We report [https://arxiv.org/abs/2107.13518] experimental detection and control of Schrodinger’s Cat like macroscopically large, quantum coherent state of a two-component Bose-Einstein condensate of spatially indirect electron-hole pairs or excitons. Phase coherent periodic oscillations in photo generated capacitance as a function of applied voltage bias and light intensity over a macroscopically large area are measured. Coherent resonant tunneling in this well-dot heterostructure restricts the available momentum space of the charge carriers within this quantum well. Consequently, the average electric polarization vector of the associated indirect excitons collectively orients along the direction of applied bias and these excitons undergo Bose-Einstein condensation below ~100 K. Finally, we observe collective Rabi oscillations of these macroscopically large, ‘multipartite’, two-level, coupled and uncoupled quantum states of excitonic condensate as qubits. Moreover, some of these excitons undergoing BEC can be addressed independently of others using applied bias over a localized region to satisfy, in principle, the DiVincenzo’s criteria for scalable quantum computation.