Session S07: Quantum Computing with Defects

Towards fault-tolerant quantum error correction with spins in diamond

Quantum error correction (QEC) is essential for reliable large-scale quantum information processing. Pioneering experiments have demonstrated QEC codes that could only correct specific types of errors using various physical platforms [1,2]. However, an experimental demonstration of a fault-tolerant QEC code that can correct any type of single-qubit error remains an open challenge. Here, I will present our progress towards the implementation of a fault-tolerant QEC code using a solid-state spin register in diamond. Recently, we have demonstrated that such a register can hold up to 10 qubits with high-fidelity universal control, coherence times up to one minute, and genuine multipartite entanglement [3]. Building upon these promising results, I will show how we can use non-destructive repeated parity measurements to encode a logical state in multiple C13 nuclear-spin qubits in diamond. These parity measurements might be further used to detect and correct arbitrary single-qubit errors on the logically encoded state, and are therefore an important step towards fault-tolerant quantum information processing.

[1] Kelly et al., Nature, 519, 7541, 2015
[2] Cramer et al., Nat. Commun., 7, 11526, 2016
[3] Bradley et al., Phys. Rev. X, 9, 031045, 2019   

Hardware-Efficient Quantum Error Correction with NV Center

The near-term intermediate-scale quantum (NISQ) era dawns with the demonstration of `quantum supremacy’. In the NISQ era, it is under debate if quantum error correction (QEC) is required. Although QEC is essential towards scalable universal quantum computation, it imposes a prohibitively high overhead for NISQ devices. To reduce the overhead, a hardware-efficient QEC approach has recently been employed and enjoyed experimental success [1]. Applying the same philosophy to our system—a quantum register consisting of one NV electronic spin and neighboring nuclear spins, we have carefully characterized the system and recently identified its dominant decoherence source [2]. Moving forward, we developed a hardware-efficient QEC code for such noise, which requires exponentially less overhead [3], and we are progressing in experiments towards one logical qubit consisting of two physical qubits.

[1] N. Ofek et al., Nature 536, 441 (2016)
[2] M. Chen et al., New J. Phys. 20, 063011 (2018)
[3] D. Layden et al., arXiv: 1903.01046

  

Detection and control of large systems of nuclear-spin qubits in diamond

Nuclear spins in diamond are promising for their use as qubits in quantum computers and quantum networks, and for simulating many-body physics phenomena. Recently, we demonstrated the 3D imaging of a system of 27-nuclear-spin qubits using a nitrogen vacancy (NV) center in diamond [1], and a universally connected 10-qubit register formed of 9 nuclear spins combined with the NV center electron spin [2].

Building on these recent results, I will present new methods that allow us to extend control over more nuclear spin qubits. By combining precise knowledge of the nuclear spin environment with dynamic nuclear polarization techniques and selective readout protocols, we can prepare and measure individual nuclear spin qubits within a large interacting cluster. As well as extending the number of qubits available for quantum information applications, these techniques open the door to the quantum simulation of complex many-body physics phenomena using nuclear spins in diamond.

[1] M. H. Abobeih et al. Nature (in press), preprint: arXiv:1905.02095 (2019)
[2] C. E. Bradley et al. Phys. Rev. X 9, 031045 (2019)   

Hidden Silicon-Vacancy Centers in Diamond

Color centers in diamond—in particular, negatively charged silicon-vacancy (SiV^-) centers—have generated excitement recently as potential hardware elements in quantum networks and devices. The attention is due in part to the protective influence of diamond’s wide bandgap and weak magnetic susceptibility, and in part to the technology available for manipulating and detecting light at these photon energies. In spite of this, open questions remain concerning the optical properties of SiV^- centers and related defects, and there exist significant opportunities for elucidating these properties using nonlinear optical spectroscopy. Here we report measurements on a high-density sample of negatively charged SiV^- centers in diamond through the use of collinear optical multidimensional coherent spectroscopy (MDCS). Using the technique, we have uncovered a hidden population of centers that not typically observed in photoluminescence, and which exhibit a high degree of spectral inhomogeneity and longer-than-expected single-particle electronic T₂ dephasing times. The phenomenon is likely caused by strain, indicating opportunities for controllably mediating electronic coherence in color-center-based quantum devices.   

Second-order Nonlinear Frequency Conversion and Integrated Color Centers in Silicon Carbide Nanophotonics

4H-Silicon carbide photonics offer the unique prospect of monolithic generation and frequency conversion of quantum light on-chip, as the material hosts color centers with favorable spin coherence properties and has a strong second-order optical nonlinearity. We integrate single color centers into thin-film nanophotonic devices and demonstrate efficient second-harmonic generation using a doubly resonant microring resonator scheme, which may be modified for quantum frequency conversion to the telecommunications band. We introduce color centers into thin films via electron irradiation and study the optical stability of single defects.   

Controlling the Silicon Vacancy in Silicon Carbide via Electric and Magnetic Fields

The Silicon Vacancy in Silicon Carbide is an optically-active, spin-3/2 defect with a long spin coherence and potential for integration into large-scale nanophotonic circuits due to its narrow, stable optical transitions and small inhomogenous broadening. We demonstrate the optical transitions of the Silicon Vacancy are widely tunable via electric fields, which may enable multi-emitter scalability. We perform magnetic-field spectroscopy on single defects, and discuss the cavity-assisted spin-initialization protocols enabled by its fine structure.   

Computational identification of defect qubits in transition metal dichalcogenide WSe2

Nitrogen-vacancy (NV) center in diamond serves as a leading solid-state qubit system due to its fidelity of manipulation at room temperature, while the search for novel solid-state systems with NV-like defects is highly desirable for the future development of solid-state quantum technologies. Two-dimensional solid-state systems are superior platforms to implement controlled manipulation of qubits. Our computational studies focus on point defects in WSe₂ as one of the well-known compounds in the family of transition metal dichalcogenides (TMDs). First-principle calculations based on density functional theory are adopted to study the formation energetics and electronic properties of point defects including intrinsic defects such as V_W and Se antisite (W_Se), and extrinsic defects such as V_W-N_Se, V_W-P_Se, and V_W-As_Se. Our calculations show that both intrinsic and extrinsic defects can exhibit high magnetic moments and triplet ground states, which offer a set of defect candidates as qubits in this 2D TMD system. In addition, optical transition paths between ground and excited states will be discussed, which provide potential optical signatures for experimental verification.   

Probing the Coherent Spin Dynamics of Divacancies in Silicon Carbide with Spin Correlated Low-Field Magnetoresistance

Silicon carbide has attracted attention in the quantum information community due to remarkably long room temperature spin coherence times [1] and the potential for integration with the photonics and communications sectors due to divacancy energies in the near-infrared regime [2]. Isolated neutral divacancies are realizable and addressable via optically detected magnetic resonance [3]. The long coherence times of these individual deep centers suggest that they are ideal candidates for single spin sensing and quantum memory applications. We describe an approach we predict will allow exploration of the coherent spin dynamics of these divacancies through low-field magnetoresistance by addressing an individual divacancy with a spin-polarized scanning tunneling microscope (SP-STM) [4]. Measurement of the spin coherence time should be feasible and signatures of the local hyperfine interactions and single-spin exchange interactions should be resolvable.
[1] A. L. Falk, et al., Nat. Commun. 4:1819 (2013)
[2] N. T. Son, et al. Phys. Rev. Lett., 96, 055501 (2006)
[3] D. J. Christle, et al., Nature Materials 14, 160 (2015)
[4] S. R. McMillan, et al., arXiv:1907.05509 [cond-mat.mes-hall]   

Site-controlled generation of tin-vacancy centers in diamond via shallow ion implantation and subsequent diamond growth

Color centers in diamond have garnered much interest in recent years as potential solid-state spin qubits. Paramount to implementing these color centers in scalable photonic systems is the development of techniques to generate high-quality, site-controlled emitters. This challenge is amplified for color centers with larger group-IV impurity atoms, which have emerged as otherwise promising emitters due to predictions of long spin coherence times without a dilution refrigerator. In the case of the tin-vacancy (SnV-) center, conventional site-controlled color center generation methods either damage the diamond surface or yield bulk spectra with unexplained features. In this talk we present a novel method to generate site-controlled SnV- centers with clean, consistent bulk spectra. We shallowly implant Sn ions and subsequently grow a layer of diamond via chemical vapor deposition. This method is compatible with nanophotonic device fabrication and can be extended to other color centers.   

Integrated Photonic Circuit for Generation and Isolation of Single Photons from Quantum Dot Ensembles

Practical quantum photonic technology must be scalable, highly isolated from noise, low loss, and have high stability of optical elements. These needs have motivated the development of photonic integrated circuits (PICs) for quantum optical devices. While significant progress has been made in silicon PICs, generating single photons depends on heralded, probabilistic processes such as spontaneous down-conversion. III-V quantum dots (QDs) have demonstrated deterministic, high-purity, single-photon and entangled-pair emission, including electrically-pumped QDs, providing a path towards source integration. We have previously demonstrated tungsten silicide waveguide-integrated superconducting single-photon detectors with low dark counts (~1 count per 1000 s) on a III-V PIC with waveguide-coupled LEDs. These detectors may allow in-situ measurement of on-chip single-photon sources. We will demonstrate on-chip generation of single photons from a QD ensemble, using ring resonators to isolate the emission of single dots. These circuits may be tuned thermally, and cascaded ring resonators may have sufficiently high Q-factors to isolate single transitions. On-chip detectors may be used to quantify single-photon purity using an integrated Hanbury-Brown and Twiss interferometer.   

Ab-inito and crystal-field calculations of defect properties of Er3+ in yttria

Wide band-gap oxides with rare-earth impurities exhibit narrow transitions with long coherence times and high quantum efficiency due to partially filled f orbitals. This allows the fabrication and usage of these materials in highly efficient optical amplifiers, high power lasers, data, and quantum information processing.
We study electronic, structural, and spin properties of trivalent erbium impurities in yttria. First, we perform ab-initio calculations of the structural properties using the generalized gradient approximation and the pseudopotential method. We also calculate the electronic band structure and the density of states of the bulk and doped yttria. Then we proceed to compute the formation energies of erbium impurities. We also compute level splittings and g-tensors using a crystal field Hamiltonian. We show that the g-tensor is highly anisotropic, with a large component in one direction. The presence of an abundant isotope with a non-zero nuclear spin and anisotropic g-tensor suggest that an experimental control of the spin dynamics of this system is possible through the Zeeman and hyperfine interaction.   

Simultaneous manipulation of multiple diamond color centers for multiplexed repeaters

Quantum networks will require scalable quantum information processing architectures for fast data rates and reliable communication. Recently, the first memory-enhanced quantum repeater that beats direct photon transmission between two communication nodes was realized[1]. However, achieving the range and speed demanded by proposed quantum applications on the quantum internet requires quantum repeater nodes with many multiplexed memory qubits[2]. In particular, it is necessary to optically excite and measure multiple quantum memories fast and efficiently. In this talk, we will discuss recent experimental progress on multiplexed repeaters based on color centers in diamond, with a particular focus on the efficient fluorescence collection towards high-speed entanglement distribution.

1. M. Bhaskar, R. Riedinger, B. Machielse, D. Levonian, C. Nguyen, E. Knall, H. Park, D. Englund, M. Loncar, D. Sukachev, and M. Lukin, arXiv:1909.01323 (2019)
2. M. Pant, H. Krovi, D. Towsley, L. Tassiulas, L. Jiang, P. Basu, D. Englund, and S. Guha, Npj Quantum Information 5, 25 (2019)   

Atomic-scale control of tunneling in few-donor quantum dots

Donor-based quantum devices in silicon are a promising candidate for spin-based solid-state quantum computing and analog quantum simulation. Carefully designing the tunneling strength in tunnel-coupled quantum dots is critical to high fidelity performance of spin initialization, readout, spin-exchange operations. This presentation covers our results in atomic-scale control and characterization of tunneling in STM-patterned devices in the few-donor quantum dots regime. We present resonant tunneling spectroscopy analysis of the tunnel junctions in few-donor single-electron transistors and double-dot devices where the tunnel gaps are defined at the atomic-scale. We characterize the tunneling rates between few-donor quantum dots and atomically aligned single electron charge sensors and report their impact on spin-selective tunneling for spin initialization and readout in few-donor quantum dots.
  

Seeking superconductivity with new, two-dimensional dopants super-saturated in silicon

Realizing superconducting Josephson junctions within group-IV semiconductors would herald a new era of solid-state quantum computing. For some acceptors, super-saturated doping has been demonstrated to superconduct, e.g. boron-doped silicon having a T_c of 0.6 K [1]. Here we explore the possibility of superconductivity in similar material systems using both n-type (antimony) and p-type (aluminum) delta-doped layers. Low-temperature molecular beam epitaxy (MBE) has been used to embed atomically thin (2D) layers of antimony and aluminum atoms with dopant densities on the order of (10¹³ - 10¹⁴) cm^-2. Mesa-etched Hall bar devices are fabricated on these delta layers and magnetotransport measurements at 4 K will be presented to study the material properties, such as carrier concentration and mobility. Temperature dependent measurements below 500 mK will be carried out in a dry dilution refrigerator to test for superconductivity. Comparisons between different dopant systems will also be presented.

[1] Marcenat, C. et al. Phys. Rev. B 81, 020501–020504(R) (2010).
  

EPR spectroscopy of Er:CaWO4 at millikelvin temperatures

Rare-earth-ions are interesting physical systems because they have long lived states and record coherence times, due the screening of the 4f valence shell by the 5s and 5p filled shells. Rare-earths with an odd number of electrons are also paramagnetic, with an electron-spin transition at GHz frequencies in magnetic fields less than 1 Tesla. For such transitions, coherence times around 50-100 us have been measured at temperatures as low as 1.4K [1]. Here we present results from our recent Electron-Paramagnetic-Resonance (EPR) studies of 0.005% Er:CaWO₄. These measurements were recorded in a novel regime for this material: sub-Kelvin temperature down to 10mK, using a superconducting micro-resonator fabricated directly on the material surface and using a superconducting parametric amplifier for the microwave signals [2], [3]. We observe the longest recorded Hahn-echo decay for an electronic spin transition in an Erbium doped material, up to 1 ms.
[1] S. Bertaina et al., Nature nanotechnology, vol. 2, no 1, p. 39, (2007)
[2] A. Bienfait et al., Nature nanotechnology, vol. 11, no 3, p. 253, (2016)
[3] C. Macklin et al., science, vol. 350, issue 6258, p307, (2015)