Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F52: Semiconducting QC: Charge Noise and Electrical Control |
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Sponsoring Units: GQI Chair: Michael Stewart, NIST Room: 399 |
Tuesday, March 14, 2017 11:15AM - 11:27AM |
F52.00001: Mitigating the effects of charge noise and improving the coherence of a quantum dot hybrid qubit Brandur Thorgrimsson, Dohun Kim, Yuan-Chi Yang, C.B. Simmons, Daniel R. Ward, Ryan H. Foote, D. E. Savage, M. G. Lagally, Mark Friesen, S. N. Coppersmith, M. A. Eriksson The quantum dot hybrid qubit, which can be viewed as a hybrid between a charge and spin qubit, is formed with three electrons in a double dot. The qubit is operated without any magnetic fields and exhibits both spin-qubit-like stability and charge-qubit-like speeds. Here we show that charge noise is the main source of decoherence for the hybrid qubit, and demonstrate that its effect can be mitigated in two ways: by modifying the qubit’s internal parameters or by changing its operating regime. By combining these methods, we have increased a hybrid qubit’s free induction decay time from 11 ns to 127 ns, and its Rabi decay time from 33 ns to over 1 $\mu$s. Additionally, we show that the longest Rabi decay times are not limited by fluctuations of the qubit energy but by fluctuations of the Rabi frequency (both of which arise from charge noise). This work was supported in part by ARO (W911NF-12-0607) and by NSF (DMR-1206915 and PHY-1104660). Development and maintenance of the growth facilities used for fabricating samples was supported by DOE (DE-FG02-03ER46028). This research utilized NSF-supported shared facilities at the University of Wisconsin-Madison. [Preview Abstract] |
Tuesday, March 14, 2017 11:27AM - 11:39AM |
F52.00002: Charge-noise-insensitive gate operations for always-on, exchange-only qubits Yun-Pil Shim, Charles Tahan Exchange-only qubit in a triple quantum dot allows for exchange-only implementation of all gate operations. In addition to the long gate pulses to implement qubit gate operations, exchange operation makes qubit gates susceptible to charge noise since the charge and spin degrees of freedom are coupled during exchange operations. There have been efforts to find sweet spots for qubit operations that are insensitive to charge noise, such as resonant exchange (RX) qubit which operates on a partial sweet spot and symmetric operation point (SOP) that offers a dynamical sweet-spot for pair-wise exchange interactions. We present an always-on, exchange-only (AEON) qubit that offers a true sweet spot to charge noise on the quantum dot energy levels [1]. Further, our qubit system allows for all single- and two-qubit gate operations to be done at sweet spots using only DC-pulses to tune the couplings between the dots, while only taking one pulse for an encoded two-qubit entangling operation. We show that AEON qubit can be considered as a generalization of SOP to three spin system, and present numerical simulations for SOP operations. [1] Yun-Pil Shim and Charles Tahan, Phys. Rev. B 93, 121410(R) (2016). [Preview Abstract] |
Tuesday, March 14, 2017 11:39AM - 11:51AM |
F52.00003: Three-spin qubits under the influence of tunneling noise Maximilian Russ, Guido Burkard We investigate the behavior of qubits consisting of three electron spins in double and triple quantum dots subject to external electric fields \footnote{M. Russ, F. Ginzel, and G. Burkard, arXiv:1607.02351 (2016) (accepted for Phys. Rev. B)}. Our model includes two independent bias parameters, $\varepsilon$ and $\varepsilon_{M}$, and two independent tunnel couplings, $t_l$ and $t_r$, which all couple to external electromagnetic fields and can be controlled in experiments by gate voltages applied to the quantum dot structures. By varying the detuning parameters one can switch the qubit type by shifting the energies in the single quantum dots thus changing the electron occupancy in each dot resulting in different qubit encodings. We focus on random electromagnetic field fluctuations, i.e., ``charge noise'', at each gate resulting in dephasing of the qubit. We pay special attention to charge noise with respect to the tunnel couplings due to recent interest in symmetric gate operations where the tunnel barrier is controlled. We search for sweet spots and double sweet spots, working points which are least susceptible to noise and compare the results to detuning noise. As a result, we show the absence of non-trivial double sweet spots in the case for tunneling noise. [Preview Abstract] |
Tuesday, March 14, 2017 11:51AM - 12:03PM |
F52.00004: Effects of charge noise on a pulse-gated singlet-triplet $S-T_-$ qubit Zhenyi Qi, Mark Friesen, Susan Coppersmith, Maxim Vavilov We study the dynamics of a pulse-gated semiconductor double quantum dot qubit. In recent experiments on $S-T_-$ qubits, the coherence times are relatively long, but the visibility of the quantum oscillations is low. We argue that these observations are consistent with a theory that incorporates decoherence arising from charge noise that gives rise to detuning fluctuations of the double dot. Because effects from charge noise are largest near the singlet-triplet avoided level crossing, the visibility of the oscillations are low when the singlet-triplet avoided level crossing occurs in the vicinity of the charge degeneracy point crossed during the manipulation, but there is only modest dephasing at the large detuning value at which the quantum phase accumulates. This theory agrees well with experimental data and predicts that the visibility can be increased greatly by appropriate tuning of the interdot tunneling rate. [Preview Abstract] |
Tuesday, March 14, 2017 12:03PM - 12:15PM |
F52.00005: Comparison of charge offset drift in Si/SiO$_{2}$ based single electron devices of differing geometry Binhui Hu, Neil M. Zimmerman, M. D. Stewart, Jr. Practical applications of single electron devices (SEDs) require that each SED is stable during operation. However, a low-frequency time instability known as charge offset drift is present in real SEDs. Experimentally, it is well established that the charge offset drift is large in Al/AlO$_{x}$ based SEDs ($\Delta $Q$_{0}$\textgreater 1e) and minimal in mesa-etched Si/SiO$_{2}$ based silicon on insulator (SOI) devices ($\Delta $Q$_{0}$\textless 0.01e) [1]. This result has been interpreted to be a consequence of intrinsic material properties. Specifically, the level of interaction between TLS defects present in the amorphous insulators, AlO$_{x}$ and SiO$_{2}$, is distinctly different [1]. We will present recent measurements on Si/SiO$_{2}$-based single-layer SEDs fabricated on bulk wafers which show appreciable charge offset drift, in discrepancy with the above interpretation. We will discuss these results in the context of the origin of charge offset drift in the Si/SiO$_{2}$ material system and the role being played by device structure. [1] M. D. Stewart, Jr. and Neil M. Zimmerman, Appl. Sci. 2016, 6(7), 187, and references therein. [Preview Abstract] |
Tuesday, March 14, 2017 12:15PM - 12:27PM |
F52.00006: Lifting of Spin Blockade by Charged Impurities in Si-MOS Double Quantum Dot Devices Cameron King, Joshua Schoenfield, M. J. Calder\'{o}n, Belita Koiller, Andr\'{e} Saraiva, Xuedong Hu, Hong-Wen Jiang, Mark Friesen, S. N. Coppersmith Fabricating quantum dots in silicon metal-oxide-semiconductor (MOS) for quantum information processing applications is attractive because of the long spin coherence times in silicon and the potential for leveraging the massive investments that have been made for scaling of the technology for classical electronics. One obstacle that has impeded the development of electrically gated MOS singlet-triplet qubits is the lack of observed spin blockade, where the tunneling of a second electron into a dot is fast when the two-electron state is a singlet and slow when the two-electron state is a triplet, even in samples with large singlet-triplet energy splittings. We show that this is a commonly exhibited problem in MOS double quantum dots, and present evidence that the cause is stray positive charges in the oxide layer inducing accidental dots near the device’s active region that allow spin blockade lifting. [Preview Abstract] |
Tuesday, March 14, 2017 12:27PM - 12:39PM |
F52.00007: Cobalt micro-magnet integration on silicon MOS quantum dots Julien Camirand Lemyre, Sophie Rochette, John Anderson, Ronald P. Manginell, Tammy Pluym, Dan Ward, Malcom S. Carroll, Michel Pioro-Ladri\`ere Integration of cobalt micro-magnets on silicon metal-oxide-semiconductor (MOS) quantum dot devices has been investigated. The micro-magnets are fabricated in a lift-off process with e-beam lithography and deposited directly on top of an etched poly-silicon gate stack. Among the five resist stacks tested, one is found to be compatible with our MOS specific materials (Si and SiO$_{\mathrm{2}})$. Moreover, devices with and without additional Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ insulating layer show no additional gate leakage after processing. Preliminary transport data indicates electrostatic stability of our devices with integrated magnets. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the U. S. Department of Energy under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 14, 2017 12:39PM - 12:51PM |
F52.00008: Coherent Oscillations in Silicon Double Quantum Dots due to Meissner-screened Magnetic Field Gradients Devin Underwood We report on observation of coherent singlet-triplet oscillations of isotopically enhanced Si/SiGe quantum-dot qubits due to the screening of an applied magnetic field by superconducting aluminum gates. The qubits employ overlapping accumulation-mode gates [1] for which magnetostatic modeling indicates an expected dot-to-dot field gradient of order 1 mT per tesla of applied field. The observed oscillations are consistent with these estimates, correspondingly showing frequencies of a few MHz for applied magnetic fields less than about 50 mT. In the 10-100 mT field range, oscillation frequencies change dramatically as the magnetic field is changed, consistent with crossing critical fields for the various geometries of superconducting gates. At much higher magnetic fields, weaker paramagnetic gradients due to other sources are observed as previously reported [2]. Static Meissner gradients are superimposed over 1/f magnetic noise, which is likely due to residual 29-Si nuclear spins [2]. These superconducting gradients may be used to help evaluate sources of magnetic noise in spin qubits. [1] M.G. Borselli et al., Nanotechnology \textbf{26}, 375202 (2015) [2] K. Eng et al., Sci. Adv. \textbf{1}, e1500214 (2015) [Preview Abstract] |
Tuesday, March 14, 2017 12:51PM - 1:03PM |
F52.00009: Vibration-induced electrical noise in a cryogen-free dilution refrigerator: Characterization, mitigation, and impact on qubit coherence Arne Laucht, Rachpon Kalra, Juan P. Dehollain, Daniel Bar, Solomon Freer, Stephanie Simmons, Juha T. Muhonen, Andrea Morello Cryogen-free low-temperature setups are becoming more prominent in experimental science due to their convenience and reliability, and concern about the increasing scarcity of helium as a natural resource. Despite not having any moving parts at the cold end, pulse tube cryocoolers introduce vibrations that can be detrimental to the experiments. We characterize the coupling of these vibrations to the electrical signal observed on cables installed in a cryogen-free dilution refrigerator. The dominant electrical noise is in the 5–10 kHz range and its magnitude is found to be strongly temperature dependent. We test the performance of different cables designed to diagnose and tackle the noise, and find triboelectrics to be the dominant mechanism coupling the vibrations to the electrical signal. Flattening a semi-rigid cable or jacketing a flexible cable in order to restrict movement within the cable, successfully reduces the noise level by over an order of magnitude. Furthermore, we characterize the effect of the pulse tube vibrations on an electron spin qubit device in this setup. Coherence measurements are used to map out the spectrum of the noise experienced by the qubit, revealing spectral components matching the spectral signature of the pulse tube. [Preview Abstract] |
Tuesday, March 14, 2017 1:03PM - 1:15PM |
F52.00010: Silicon based cryogenic platform for the integration of qubit and classical control chips T. Leonhardt, A. Hollmann, D. Jirovec, R. Neumann, B. Klemt, S. Kindel, M. Kucharski, G. Fischer, D. Bougeard, H. Bluhm, L. R. Schreiber Electrostatically confined electron-spin-qubits proved viable for quantum information processing [1-3]. Yet their up-scaling not only demands improvement of physical qubits, but also the development and cryogenic integration of classical control hardware. Therefore, we created a platform to integrate quantum chips and classical electronics. These multilayer interposer chips incorporate passive circuit elements, high bandwidth coplanar wave guides and interconnects for electron spin resonant qubit control as well as low impedance DC microstrips reducing EM-crosstalk from AC to DC lines. We used the interposer for measurements of a Si/SiGe quantum dot at 30 mK. We also characterized a commercial voltage controlled oszillator (VCO) based on hetero-bipolar transistors [4]. Tunable about 30 GHz it is ideal for electron spin resonant qubit control. Cooled from 300 to 4 K it exhibits a slightly increased output power and frequency, while the phase noise level is constant. The device remains functional up to magnetic fields of 6 T. [1] M.Veldhorst et al. Nature 526(2015) [2] E.Kawakami et al. PNAS 113(2016) [3] K.Takeda et al. Sci.Adv. 2(2016) [4] M.Kucharski et al. Proc. EuMIC (2016) [Preview Abstract] |
Tuesday, March 14, 2017 1:15PM - 1:27PM |
F52.00011: Testing of a bottom-up approach for electrically contacting nanoscale quantum electronic devices Aruna Ramanayaka, Hyun-Soo Kim, Ke Tang, M. D. Stewart, Jr., J. M. Pomeroy We present a complementary metal--oxide--semiconductor (CMOS) technology compatible bottom-up approach for realizing electrical connections to nanoscale quantum electronic devices. State of the art fabrication methods used for making external electrical contacts to nanoscale devices utilizes complex, time consuming, and expensive fabrication techniques. In order to simplify the fabrication of these electrical contacts, we create pre-pattered electrical contact wires that are degenerately doped regions in the substrate and extend down to micrometer scale using photolithography and low energy ion implantation. With this approach we are able not only to bring external electrical contacts close to a single field of view of a scanning tunneling microscope (STM), approximately 10 $\mu$ m x 10 $\mu$ m area, allowing the STM to draw direct electrical connections between these external electrical lines and the nanoscale device, but also to cut down the fabrication time considerably by fabricating pre-patterned electrical contacts at wafer scale. However, proximity of these implant lines has to be restricted due to the diffusion of implanted ions during high temperature processing of Si substrates, e.g., substrate preparation, implant activation, and oxidation. We will discuss the limitations for different high temperature processing methods, and electrical measurements of a nanoscale device using pre-defined external contacts. [Preview Abstract] |
Tuesday, March 14, 2017 1:27PM - 1:39PM |
F52.00012: Single Electron Charge Pumping in CMOS Devices Roy Murray, Justin K. Perron, MD Stewart, Jr., Neil M. Zimmerman Achieving a large current simultaneously with low uncertainty remains the central challenge for electrical current metrology. Silicon based single electron devices offer a unique opportunity to increase current by parallelizing devices due to their superb temporal stability. As a first step on this path, we present results on individual devices fabricated at NIST in the silicon on insulator architecture operated as single electron pumps. We will present results from devices operated in both turnstile and ratchet pumping mode, where the former has a bias applied across the device, and the latter can be operated with no bias. We will discuss error rates both as a function of device operation mode and temperature of the device. [Preview Abstract] |
Tuesday, March 14, 2017 1:39PM - 1:51PM |
F52.00013: Charge noise in quantum dot qubits: beyond the Markovian approximation. Yuan-Chi Yang, Mark Friesen, S. N. Coppersmith Charge noise is a limiting factor in the performance of semiconductor quantum dot qubits, including both spin and charge qubits. In this work, we develop an analytical formalism for treating semiclassical noise beyond the Markovian approximation, which allows us to investigate noise models relevant for quantum dots, such as $1/f$ noise. We apply our methods to both charge qubits and quantum dot hybrid qubits, and study the effects of charge noise on single-qubit rotations in these systems. The formalism is also directly applicable to the case of strong microwave driving, for which the rotating wave approximation breaks down. [Preview Abstract] |
Tuesday, March 14, 2017 1:51PM - 2:03PM |
F52.00014: Low frequency charge noise comparison in Si/SiO$_2$ and Si/SiGe quantum dots to assess suitability for quantum computing Blake Freeman, Joshua Schoenfield, HongWen Jiang We directly compare the low frequency charge noise in Si/SiO$_2$ and Si/SiGe gate defined quantum dots by using devices with identically patterned gates and similar fabrication procedures. Charge noise figures are obtained by measuring the low frequency $1/f$ current noise through the biased quantum dots in the coulomb blockade regime. The current noise is normalized and used to extract a measurement of the potential energy noise in the system. The temperature dependence of this noise and other recent measurements will be discussed. Ultimately we find the measured charge noise in Si/SiO$_2$ compares favorably with that of the SiGe device as well as previous measurements made on other substrates suggesting Si/SiO$_2$ is a viable candidate for spin based quantum computing. [Preview Abstract] |
Tuesday, March 14, 2017 2:03PM - 2:15PM |
F52.00015: Spin qubit transport in a double quantum dot Xinyu Zhao, Xuedong Hu Long distance spin communication is a crucial ingredient to scalable quantum computer architectures based on electron spin qubits.~ One way to transfer spin information over a long distance on chip is via electron transport.~ Here we study the transport of an electron spin qubit in a double quantum dot by tuning the interdot detuning voltage.~ We identify a parameter regime where spin relaxation hot-spots can be avoided and high-fidelity spin transport is possible. Within this parameter space, the spin transfer fidelity is determined by the operation speed and the applied magnetic field.~ In particular, near zero detuning, a proper choice of operation speed is essential to high fidelity. In addition, we also investigate the modification of the effective g-factor by the interdot detuning, which could lead to a phase error between spin up and down states. The results presented in this work could be a useful guidance for experimentally achieving high-fidelity spin qubit transport. [Preview Abstract] |
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