Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session L29: Semiconductor Qubits - Quantum Computing with Donor Spins IIFocus Live
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Sponsoring Units: DQI Chair: Xiqiao Wang |
Wednesday, March 17, 2021 8:00AM - 8:36AM Live |
L29.00001: A two-qubit gate between phosphorus donor electrons in silicon Invited Speaker: Samuel Gorman Electron spin qubits formed by atoms in silicon have large orbital energies and weak spin-orbit coupling giving rise to isolated electron spin ground states with seconds long coherence times [1]. The exchange interaction promises fast two-qubit gate operations between single-spin qubits [2]. However, creating a tuneable exchange interaction between two electrons bound to phosphorus atom qubits has not been possible. This reflects the challenges in knowing how far apart to place the atoms to turn on and off the exchange interaction, whilst aligning atomic circuitry for high-fidelity independent readout of the spins. Here, we report a ~800 ps sqrt(SWAP) gate between phosphorus donor electron spin qubits in silicon with independent ~94 % fidelity single-shot spin readout [3]. By engineering qubit placement on the atomic scale, we provide a route to the realisation and efficient characterisation of multi-qubit quantum circuits based on donor qubits in silicon. Towards this end, we present recent results on the role of quantised nuclear spins on the exchange dynamics of atomic-scale devices. |
Wednesday, March 17, 2021 8:36AM - 8:48AM Live |
L29.00002: Hardware-efficient error-correcting codes for large nuclear spins Jonathan Gross, Clément Godfrin, Alexandre Blais, Eva Dupont-Ferrier Improving the performance of near-term quantum devices involves correcting dominant sources of error. Donor nuclear spins in silicon are attractive qubits as they are compact, robust, and show record coherence time for solid-state systems. Amazingly, these coherence times are still “brief” with respect to the near-infinite relaxation times of the donors’ spins. This observation motivates a hardware-efficient approach to error correction that corrects the dominant dephasing errors. Here we present a protocol consisting of experimentally feasible operations that leverages the extended Hilbert space of a large nuclear spin to correct dephasing errors. Simulations, using state-of-the-art manipulation fidelities, predict significant improvement in reachable logical fidelity over existing spin quantum-error-correction protocols. These results provide a realizable blueprint for a corrected spin-based qubit using built-in error correction schemes. |
Wednesday, March 17, 2021 8:48AM - 9:00AM Live |
L29.00003: Rydberg Entangling Gates in Silicon Eleanor Crane, Alexander Schuckert, Nguyen Le, Andrew James Fisher We propose a new Rydberg entangling gate scheme which we demonstrate theoretically to have an order of magnitude improvement in fidelities and speed over existing protocols. We find that applying this gate to donors in silicon would help overcome the strenuous requirements on atomic precision donor placement and substantial gate tuning, which so far has hampered scaling. We calculate multivalley Rydberg interactions for several donor species using the Finite Element Method, and show that induced electric dipole and Van der Waals interactions, calculated here for the first time, are important even for low-lying excited states. We show that Rydberg gate operation is possible within the lifetime of donor excited states with 99.9% fidelity for the creation of a Bell state in the presence of decoherence. |
Wednesday, March 17, 2021 9:00AM - 9:12AM Live |
L29.00004: The adsorption of AlCl3 and incorporation of Al on Si(100) for Atomic Precision Fabrication Azadeh Farzaneh, Sungha Baek, Matthew Radue, Kevin J Dwyer, Yifei Mo, Robert E Butera Superconducting semiconductors like Si, have the potential to combine aspects of spin-based qubits with superconducting devices and circuits within one material. Al-doped Si is predicted to have higher Tc than B-doped Si prompting an exploration of atomic precision (AP) compatible Al precursors. We investigated the adsorption of AlCl3 on Si(100) with scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, and density functional theory calculations to reveal reaction pathways of Al atom incorporation in Si. Annealing AlCl3-exposed surface at temperatures<450°C produced three-atom-wide chlorinated aluminum chains (CACs) elongated along Si(100) dimer row. These CACs break up upon annealing at temperatures>450°C and are incorporated in Si. Subsequently, Al δ-layers were overgrown with epitaxial Si and secondary ion mass spectrometry profiling revealed an Al atom concentration > 1020 cm-3 within a ~2nm delta layer region. These results promote AlCl3 as a viable precursor for Al doping processes of AP advanced manufacturing in Si devices. |
Wednesday, March 17, 2021 9:12AM - 9:24AM Live |
L29.00005: Characterizing electron temperature and a cryogenic printed circuit board for spin-based quantum computing in Silicon Ranjit Kashid, Pradeep Namboodiri, Alessandro Restelli, Xiqiao Wang, Jonathan Wyrick, Fan Fei, Neil Zimmerman, Richard Silver Phosphorous donors in silicon and gate-defined quantum dots are a primary candidate for spin-based quantum computing. For high fidelity quantum state readout, electron temperature should be significantly less than the Zeeman energy difference; high-frequency wiring is needed to allow for high-speed gate operation including qubit manipulation. To satisfy these operational frequency and low electron temperature requirements, we have developed a 6-layer cryogenic high frequency printed circuit board (PCB) with onboard filtering. The high-frequency PCB consists of 14 DC filtered lines, 16 medium frequency lines (up to a few GHz) for readout using DC charge sensing or RF reflectometry, 2 tank circuits, and 2 high-frequency microwave lines for transmission of signals up to 40 GHz. We will discuss, transmission characteristics, crosstalk, tank circuit characterization, and electron temperature measurements of single-electron transistors at cryogenic temperatures. Our current work is focused on using this cryogenic PCB with improved inline filtering to demonstrate robust charge sensing and single-shot spin readout using the RF-Reflectometry technique and DC charge sensing. |
Wednesday, March 17, 2021 9:24AM - 9:36AM Live |
L29.00006: Decoherence of Dipole Coupled Flip-Flop Qubits John Truong, Xuedong Hu A recent proposal for a scalable donor-based qubit scheme promises excellent coherence properties, fast qubit couplings and insensitivity to donor placement. The suggested system consists of two different types of qubits per donor: a flip-flop qubit consisting of the electron and nuclear spin states, and a charge qubit of the donor electron tunneling between the donor and an interface quantum dot. In this scheme, the qubits can be coupled to each other via the electric dipole interaction between their respective charge qubits. Here we study this effective coupling, especially the effect of charge noise on two-qubit gates utilizing this coupling. We find that due to the proximity of the charge excited states to the flip-flop logical states, the presence of charge noise could greatly reduce the fidelity of two-qubit operations under otherwise ideal conditions. We calculate the qubit-noise interaction strengths and identify leakage from the qubit Hilbert space as the main culprit of the reduced gate fidelity. To mitigate this decoherence channel, we identify bias conditions when charge qubit leakage can be suppressed while spin qubit coupling remains reasonably strong. |
Wednesday, March 17, 2021 9:36AM - 9:48AM Live |
L29.00007: Magnetotransport Characterization of Atomic-scale B-doped δ-layer devices in Si Sungha Baek, Azadeh Farzaneh, Kevin J Dwyer, James Williams, Robert E Butera Atomic precision advanced manufacturing (APAM) processes in silicon have advanced significantly over the past decade as researchers strive towards the ultimate realization of a single dopant qubit based Kane quantum computer architecture. Here, we demonstrate the fabrication of atomic-scale, B-doped δ-layer devices in Si through area selective deposition of BCl3 on Si(100) and measure the magnetotransport properties of resulting Hall bar devices. Magneto and Hall measurements conducted at 3 K revealed a sheet resistance of 1.9 kΩ without performing incorporation or activation anneal. Relatively high hole concentrations, 1.9 x 1014 cm2, and mobilities, ∼39 cm2 V −1 s −1, within a sub-nanometer transport layer were routinely obtained on multiple samples. We further demonstrate STM-based lithography techniques to create B-doped wires and tunnel junctions and report on their electrical characterization. These findings provide a pathway towards the realization of atomic-scale acceptor-based devices and the exploration of superconductivity in silicon. |
Wednesday, March 17, 2021 9:48AM - 10:00AM Live |
L29.00008: Area-selective deposition of BCl3 on Si(100) for B-doped δ-layer device fabrication Kevin Dwyer, Azadeh Farzaneh, Sungha Baek, Michael Dreyer, Robert E Butera Atomically-precise, δ-doped structures forming electronic devices in Si can be fabricated using depassivation lithography in a scanning tunneling microscope (STM). Incorporation of dopant atoms from gaseous precursors into lithographic patterns enables metallic wires, and precisely placed single atom qubits for quantum information. We present results on the adsorption and incorporation of B from area-selective deposition of BCl3 onto the Si(100) surface. We show that BCl3 readily adsorbs onto Si(100) and is selective to H- and Cl-based resists, which can both be patterned using STM. We explore the effects of annealing temperature on B incorporation and the resulting electrical activation of B δ-layers with peak concentrations >1020 B/cm3. Finally, we perform low-temperature electrical characterizations of B-delta layers and demonstrate fabrication of atomic-scale, B-doped devices. |
Wednesday, March 17, 2021 10:00AM - 10:12AM Live |
L29.00009: Stochastic atomistic disorder in atomic-precision doping Justin Koepke, Jeffrey Ivie, Quinn Campbell, Mitchell Brickson, Peter Schultz, Richard Muller, Andrew D Baczewski, Andrew M Mounce, Ezra Bussmann, Shashank Misra Atomic-precision advanced manufacturing (APAM) enables the near-deterministic placement of single donors with near-atomic precision, which has been used to produce surprising device demonstrations, e.g. the single-atom transistor. However, APAM incorporates dopants into silicon using surface chemistry, where competing chemical processes lead to stochastic outcomes. Through statistical studies on arrays of nominally identical structures ranging down to single-atom features, we have measured the probability to achieve desired target outcomes for few and single-atom structures. Significant variability is measured in single- and few-donor structures. For example, we find a success rate of 68% for single-donor sites. We will explain how the results pose a significant challenge for engineering future quantum simulators and circuits. |
Wednesday, March 17, 2021 10:12AM - 10:24AM Live |
L29.00010: Dynamical decoupling of P1 centers in diamond Ethan Williams, Chandrasekhar Ramanathan The coherence times (T2) of nitrogenic color centers in diamond are often limited by the T2 of substitutional nitrogen defects (P1 centers) present in the lattice. In ensemble quantum sensing experiments, the strength of the signal increases with nitrogen concentration, however T2 begins to decrease for nitrogen concentrations above about 1 ppm. Increasing the coherence times of dense P1 centers could lead to improved sensitivity for quantum sensing of ac signals. We explore the use of pulsed dynamical decoupling sequences to control the coherence times of P1 centers in diamond as a function of the P1 concentration. The control of P1 centers is also key to improving the fidelity of Hamiltonian engineering schemes for quantum simulation in diamond. |
Wednesday, March 17, 2021 10:24AM - 10:36AM Live |
L29.00011: Three Dimensional Control of Monolothic Atomic-Scale Devices in Silicon Matthew Donnelly, Joris Keizer, Brandur Thorgrimsson, Michelle Simmons Building on work that has demonstrated the viability of using vertically separated gates in 3D monolithic phosphorus-doped-silicon (Si:P) devices to couple to single electron transistors [1] and nanowires [2], we will show how reducing the footprint of these epitaxial gates coupled with a novel alignment procedure can be used to finely tune the electrostatic potential in donor atom devices. In the fashion of electrostatically defined quantum dots, we can tune parameters such as qubit-reservoir tunnel rates and inter-qubit tunnel coupling, which were hitherto fixed by the exact dopant atom positions. This work opens up new avenues of investigation in atom based qubits including two-qubit exchange gates in a symmetric noise-suppression regime and tuneable couplings for analog quantum simulators. |
Wednesday, March 17, 2021 10:36AM - 10:48AM Not Participating |
L29.00012: Comparing effective mass models of the phoshorus donor in silicon Luke Pendo, Xuedong Hu Evaluating effective mass models of an isolated phosphorus donor in silicon requires disentangling various sources of error. In an effort to suppress the error resulting from state approximation, we construct states using envelope functions expanded in freely extensible basis sets equipped with tunable parameters. This robust basis set allows us, in principle, to compute arbitrarily precise approximate eigenstates of the effective mass Hamiltonian as confirmed by near zero values of the energy variance. This means we can, in principle, closely approximate the exact electronic structure of the donor for a broad class of model potentials, which in turn allows us to evaluate and compare these potential models. |
Wednesday, March 17, 2021 10:48AM - 11:00AM Live |
L29.00013: Low charge noise in atom qubits in silicon Ludwik Kranz, Samuel Gorman, Brandur Thorgrimsson, Yu He, Daniel Keith, Joris Keizer, Michelle Simmons Despite electron spins in silicon offering a competitive, scalable quantum computing platform two-qubit gate fidelities to date have fallen short of the 99% threshold required for error-corrected processors. In the past few years there has been a growing realization that the critical obstacle in meeting this threshold is charge noise, which results in J fluctuations during the two-qubit gate operation, limiting the gate fidelity. Here, we show a significantly reduced magnitude of charge noise in all-epitaxial, precision placed phosphorus atom-based platform, in which the qubits are naturally separated from surfaces and interface states. We perform charge noise measurements using both the charge sensor and the qubits themselves. We find a consistent charge noise spectrum over 4 frequency decades in the sub-Hz frequency regime, with the noise level of S0 = 0.0088 ± 0.0004 µeV2/Hz, one order of magnitude lower than that reported in other systems. We believe that studying the origins of charge noise and its spectrum in our system will enable the design of noise resilient multi-qubit devices and protocols. |
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