Session K35: Semiconducting Qubits: Interface Characterization

A silicon metal-oxide-semiconductor quantum dot patterned with nano-imprint lithography

We have transferred the metal gate pattern of a nanoscale depletion mode quantum dot onto a Si/SiO₂ substrate with nano-imprint lithography (NIL), eliminating the use of e-beam lithography (EBL) and, consequently, reducing the effects of charge impurities. Critical features with 50 nm scale and separation can be dependably reproduced without making substantial changes to the device design. By studying charge transport through a quantum point contact and quantum dot, the prevalence of impurities was diminished in NIL devices when compared to similar EBL counterparts. Furthermore, 1/f charge noise was measured with an average of 1.4 µeV/√Hz at 1 Hz, equivalent to previous measurements made on EBL devices. This work offers a path toward reliable quantum dot operation in MOS by improving fabrication techniques to reduce charge impurities.   

Vibrational modes at the Si/SiO2 interface detected by pulse electron spin resonance

Electron spin relaxation mechanisms for dangling bonds (P_b centers) at the Si/SiO₂ interface in silicon nanowires produced by metal assisted chemical etching have been investigated by pulse electron spin resonance. The increased interface area in the nanowires provides the signal to noise ratio required to detect non-exponential decay of spin magnetization The spin-lattice relaxation rates are reported in the temperature range 4-300 K and the experimentally observed temperature dependence is explained in terms of the excess vibrational modes which manifest as tunneling two level systems (TTLSs) giving rise to the so called “boson peak” of the amorphous interface. The experiment together with a theory of the spin relaxation mechanism which involves TTLSs provide a novel method to address the boson peak and other issues related to the role of the TTLSs in determining noise and decoherence in qubits and other sensitive devices.   

Fidelity of strongly driven electric dipole spin resonance

Coherent control of a single spin confined in a quantum dot (QD) by the microwave electric field (electric dipole spin resonance; EDSR) is an active field of recent research. One of the required mechanisms to couple the orbital motion to the spin degree of freedom is to utilize the slanting magnetic field [1]. J. Yoneda et al.[2] had reported EDSR with a large Rabi frequency up to 100 MHz by optimizing the design of the micro-magnet as well as inducing larger power of microwave. In the largest achievable microwave amplitude, the Rabi frequency is saturated from the expected linear dependence with the microwave amplitude and the fidelity of the Rabi oscillation becomes degraded. Possible origin of this saturation is the non-parabolicity of the confinement potential [3]. In this report, we study the effect of the noise by strongly-driven orbital motion on the fidelity of the spin. In particular, the first (second)-order spin-orbital coupling induces enhanced spin-relaxations (decoherence) with increasing microwave amplitude.
[1] Y. Tokura, et al., Phys. Rev. Lett. 96, 047202 (2006). [2] J. Yoneda, et al., Phys. Rev. Lett. 113, 267601 (2014). [3] Y. Tokura, et al., JPS Conf. Proc. 1, 012022 (2014).   

Low power electric dipole spin resonance in silicon: theory

Control of individual electron spins is one of the cornerstones of spin-based quantum technology. The application of ac magnetic fields allows to drive coherent spin rotations in electrons placed in gate-defined quantum dots, but there is a strong incentive to avoid magnetic driving since it is technically demanding and limits the Rabi frequency due to sample heating issues. Electric dipole spin resonance techniques, which harness some type of spin-orbit coupling to electrically control the electron spin state, provide a more robust possibility [1,2]. Since the intrinsic spin- orbit coupling for electrons in silicon is very weak, the development of novel efficient tools for spin control applicable to silicon based quantum devices is desirable [2-5]. Here, we present a theoretical investigation of an efficient novel mechanism in silicon quantum dots to induce single electron coherent spin rotations relying on an external magnetic field gradient.

[1] K. C. Nowack et al., Science 318, 1430 (2007).
[2] M. Pioro-Ladriere et al., Nat. Phys. 4, 776 (2008).
[3] A. Corna et al., npj Quantum Information 6, 4 (2018).
[4] E. Kawakami et al., Nat. Nanotech. 9, 666 (2014).
[5] Yoneda et al., Nat. Nanotech. 13, 102 (2018).   

Low power electric dipole spin resonance in silicon: experiment

Spin-based quantum information processing requires exquisite single spin control. While electron spin resonance is a natural means of controlling spin qubits in gate-defined quantum dots, generating localized ac magnetic fields large enough to drive coherent oscillations is challenging. As an alternative, electric dipole spin resonance (EDSR) can be employed [1]. Spin-orbit coupling often mediates EDSR: however, intrinsic spin-orbit coupling in silicon is small. To overcome this, on-chip micromagnets can be used to generate synthetic spin-orbit interactions [2], with EDSR having now been demonstrated in silicon [3, 4]. Here we present experimental results using a new technique for low-power EDSR in silicon with micromagnets, in a step towards scalable power budgets for large-scale spin-based quantum processors.

[1] E. I. Rashba, Sov. Phys. Solid State 2, 1109 (1960)
[2] M. Pioro-Ladriere et al., Nat. Phys. 4, 776 (2008)
[3] E. Kawakami et al., Nat. Nanotech. 9, 666 (2014)
[4] J. Yoneda et al., Nat. Nanotech. 13, 102 (2018)   

Charge offset drift in single electron devices containing aluminum oxide

Single electron devices (SEDs) suffer from a long-time instability, referred to as charge offset drift (ΔQ₀), that hampers integration of SEDs in applications such as quantum metrology and computing. Previous measurements of ΔQ₀ show SEDs containing aluminum oxide (AlOx) have been less stable than comparable SEDs containing only silicon dioxide (SiO2). Here, we have fabricated two different types of SEDs: all-metal Al/AlOx/Al tunnel junction-based devices and tunable barrier silicon metal-oxide-semiconductor (MOS) devices. The all-metal SEDs contain plasma-oxidized AlOx as the tunnel barrier. The charge offset stability measured on these devices is better than any other reported metallic SEDs, displaying a very small linear ΔQ₀ of 0.1e over 7.5 days and two times lower standard deviation. The MOS SEDs are made on a silicon-on-insulator (SOI) substrate with a thermal SiO2 gate oxide, Al gates, and thermal AlOx as an isolation oxide. Four of the five MOS SEDs measured show a linear charge offset drift of less than 0.07 e over 7 days and standard deviations less than 0.02 e. These results suggest the stability of devices made with AlOx may be significantly better than previously thought and that other factors, such as geometry, are playing as large a role as materials.   

Probing decoherence at an atom-defect quantum interface

The atomic-scale quantum properties of interfaces play a central role in the ubiquitous surface-mediated decoherence that currently limits a wide variety of quantum technologies. We have constructed a novel instrument for quantitatively studying decoherence at active quantum surfaces. This hybrid quantum system combines neutral atoms adsorbed onto a diamond surface in ultrahigh vacuum conditions and subsurface nitrogen vacancy (NV) centers. We present measurements of the decoherence and relaxation rates of shallow NV centers interacting with atomic adsorbates deposited at thicknesses varying from angstroms to nanometers. As adsorbate atoms are added, we observe a significant reduction in spin relaxation time. We present progress towards a detailed understanding of coherence at this model quantum interface, with important consequences for a broad range of quantum interfaces.   

Ultra-thin body buried oxide 28nm FD-SOI platform for silicon quantum dots

Silicon spin qubits, with their long coherence time and their compatibility with CMOS industrial technology shows great promise for large scale quantum computing and co-integration. Here we present a UTBB FD-SOI platform that is designed to operate as a transistor and host quantum dots. This platform is composed of NMOS structures with wire gate and split enhancement gate geometries, and is entirely fabricated inside STMicroelectronics’ standard process line. We explore various regimes in gate voltage space and demonstrate reproducible operation of multiples devices at 1.5 K and 10m K. We identify various regime for formation of quantum dots including electrostatic single and double quantum dots. We also present a cryogenic characterizsation of classical FD-SOI transistors that involves variation of mobility, kink effect and split C-V analysis down to 10 mK. These results set a pathway towards improved FD-SOI devices and quantum dots fully fabricated in standard process line.
  

Magnetotransport of metal-oxide-semiconductor devices fabricated on highly enriched 28Si

Isotopically enriched ²⁸Si is regarded as an ideal environment for quantum computation (QC) as elimination of unpaired nuclear spins can result in low error rates for QC. At NIST we have developed a method to grow isotopically enriched ²⁸Si, which provides the unique advantage of targeting a desired enrichment anywhere between natural abundance and the highest possible enrichment > 99.99998 % ²⁸Si isotopic fractions. To explore the electrical properties of ²⁸Si, we fabricate gated Hall bar devices and study the magnetotransport at magnetic fields (B) 12 T and temperatures (T) ranging from 1.2 K to 10 K. The magnetoresistance at |B| ≤ 0.25 T shows maximum mobilities of ≈ 1700 cm²/(V×s) and ≈ 6000 cm²/(V×s) at an electron density of ≈ 2.5×10¹² cm^-2 for devices fabricated on ²⁸Si and ^nat.Si, respectively. We use the T dependence of weak-localization and Shubnikov-de Haas oscillations to deduce the dominant scattering mechanisms in these devices. We believe that the lower mobility observed for the devices fabricated on ²⁸Si is due to the dilute adventitious C, N and O detected in ²⁸Si. We will also discuss the preliminary results of fabrication and measurement of gate defined quantum dot devices in ²⁸Si epilayers.   

New Linear-Optical Approach to Quantum Information Processing and Quantum Simulation

Linear optical networks formed from beam splitters and phase shifters have been shown capable of carrying out all quantum information processing tasks, but at the cost of rapid growth in resources as the complexity of the task increases. Here, we show that the use of recently introduced directionally unbiased optical multiports (N≥3), where input ports can double as output ports, can achieve results with much more compact setups and with substantial savings in resources. The behavior of such multiports is highly flexible, with a variety of possible behaviors being controlled by a set of parameters that can be changed or tuned in real time. Here we give an overview of the properties and potential applications of these multiports. Applications include their use as high-dimensional coins and lattice sites for quantum walks, new type of logical gates for two-photon entangled states, and elements of optical simulators for solid state systems. We illustrate that the band structures and electronic behavior of a variety of solid state systems, including those with nontrivial topological behavior, can be simulated using relatively compact linear-optical systems based on such directionally unbiased multiports.   

Manipulation of entanglement sudden death in an all-optical experimental setup

Entanglement sudden death (ESD) is the phenomenon wherein a multipartite entangled state disentangles in finite time even when individual qubits decohere only asymptotically in time due to noise. Prolonging the entanglement is essential for the practical realization of entanglement-based quantum information and computation protocols. For this purpose, local NOT operation in the computational basis on one or both qubits has been proposed to combat the amplitude damping noise. Here, we aim to discuss an all-optical-experimental implementation of the NOT operations that can hasten, delay, or avert ESD, all depending on when it is applied during the process of decoherence for the polarization entangled photonic qubit system[1]. Further, the preparation and characterization of a polarization entangled photon source instrumental in obtaining the exciting experimental results on the manipulation of ESD along with the attendant theory will be presented.
Reference [1]: Ashutosh Singh, Siva Pradyumna, A. R. P. Rau, and Urbasi Sinha, "Manipulation of entanglement sudden death in an all-optical setup," J. Opt. Soc. Am. B 34, 681-690 (2017).   

Implementation and Simulation of Electrostatically Controlled Quantum Dots in CMOS Technology

A new architecture of a charge qbit suitable for implementation in large scale CMOS circuits is presented. We demonstrate techniques for time-independent / time-dependent simulations of quantum states and transport in such quantum dots. The time-independent technique makes use of a semi-analytical approach in the Schrodinger formalism combining the calculations of the electric field in CMOS structures with a piece-wise potential approximation of eight or more potential wells. The time-dependent technique leverages the semi-analytical approach to obtain the evolution of eigen states in explicit form. The techniques are upgraded to calculations based on the density matrix. This approach allows estimations of transition frequencies, leakage of the wavefunction between quantum states, and de-coherence due to finite potentials and asymmetries of CMOS structures. Based on the height of the barrier between the wells, the behavior can be interpreted as semi-classical single electron transport, CNOT quantum gate operation, or quantum annealing. The presented approach allows the design of quantum gates compatible with conventional CMOS technologies and operating at 4K. The structure was standard foundry solution. We describe the implementation including integrated control structure