Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Q27: Focus Session: Semiconductor Qubits- Quantum Control |
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Sponsoring Units: GQI Chair: Sergey Frolov, Delft University of Technology, Netherlands Room: C155 |
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q27.00001: Control and Manipulation of Two-Electron Spin Qubits in GaAs Quantum Dots Invited Speaker: Amir Yacoby We have developed means to both couple and decouple a two electron spin qubit from its environment. Using dynamic nuclear polarization we are able to suppress fluctuations in the nuclear environment and prolong T2* by nearly an order of magnitude reaching 150 nano seconds. Our polarization scheme employs a quantum feedback mechanism that directly conditions the rate at which the qubit polarizes its nuclear environment on a quantum limited measurement of the hyperfine field seen by the same qubit. In addition, the stabilized state of the nuclear environment allows us to perform controlled X rotations and thereby demonstrate full control over the entire Bloch sphere as well as full quantum state tomography. Using dynamic decoupling of the two electron spin qubit from its environment we are able to prolong T2 by nearly three orders of magnitude reaching nearly 300 micro seconds. Our results indicate that gate fidelities of up to 99.99{\%} are within reach despite the fluctuating nuclear environment. Moreover, the demonstrated ultra long coherence time allows for more than 10$^5$ coherent gate operations which exceed the estimated threshold for quantum error corrections by a substantial margin. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q27.00002: Quantum gates for the singlet-triplet $\mathrm{T}_+$ qubit Hugo Ribeiro, J.R. Petta, Guido Burkard We theoretically show that hyperfine interactions can be harnessed for quantum gate operations in GaAs semiconductor quantum dots [1]. In the presence of an external magnetic field $B$, which splits the triplet states, the hyperfine interaction results in an avoided crossing between the spin singlet $\textrm{S}$ and spin triplet $\textrm{T}_{+}$, which form the basis of a new type of spin qubit. Coherent quantum control for this qubit is achieved through Landau-Zener-St\"uckelberg transitions at the S-T$_{+}$ avoided crossing [2]. A set of suitable transitions allows to build any single qubit gates on timescales shorter than the decoherence time $T_2^* \sim 16\mathrm{ns}$ [1]. We also show how to build a conditional two-qubit gate by capacitively coupling two S-T$_{+}$ qubits. \\[4pt] [1] H. Ribeiro, J. R. Petta, and G. Burkard, Phys. Rev. B 82, 115445 (2010). \newline [2] H. Ribeiro and G. Burkard, Phys. Rev. Lett. 102, 216802 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q27.00003: Coherent spin manipulation with a triple quantum dot Ghislain Granger, Louis Gaudreau, Alicia Kam, Sergei Studenikin, Piotr Zawadzki, Geof Aers, Michel Pioro-Ladri\`ere, Zbigniew Wasilewski, Andrew Sachrajda Recently, Landau-Zener-Stuckelberg (LZS) oscillations have been demonstrated in a double quantum dot device [1]. In this talk we demonstrate LZS oscillations in a triple quantum dot environment. Our triple quantum dot design allows us to tune to either the charge or spin qubit regimes. Using a pulsing technique in the spin qubit regime, we create a superposition of triple quantum dot states, allow for phase accumulation, and interfere. We demonstrate coherent LZS oscillations with three spins across the triple quantum dot structure. We investigate their dependence on pulse rise time, separation time, energy detuning, and magnetic field. \\[4pt] [1] J. R. Petta et al., Science 327, 669 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q27.00004: Novel Coherent Spin Oscillations in a Triple Quantum Dot Circuit Andrew Sachrajda, Ghislain Granger, Louis Gaudreau, Alicia Kam, Sergei Studenikin, Piotr Zawadzki, Geof Aers, Michel Pioro-ladriere, Zbig Wasilewski We have demonstrated Landau-Zener-Stuckelberg oscillations in a triple quantum dot circuit related to pairs of triple quantum dot states. Different initialization schemes and pulse shapes involving all three dots will be discussed. However, the complexity of a triple quantum dot system suggests that in general coherent behaviour can be expected from interplays between various combinations of states. Here we demonstrate both experimentally and theoretically in a triple quantum circuit containing three spins, a coherent interplay between two coexisting qubits as a function of pulse amplitude and rise time. To further clarify the behaviour within the system we also observe and study coherent oscillations after a fourth spin has been added to the system in one of the relevant dots. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q27.00005: A Single Electron Charge Qubit in the Strong Driving Limit J. Stehlik, Y. Dovzhenko, J. R. Petta, H. Lu, A. C. Gossard The dynamics of strongly driven two-level systems in the presence of dissipation have been thoroughly studied using theoretical models.$[1]$ We use a model system, a GaAs double quantum dot (DQD) containing a single electron, to experimentally explore the strong-driving regime. We measured the transport through the DQD as a function of detuning and applied microwave power and compare with the Tien-Gordon model. In contrast with previous experiments, we directly access the occupation of the DQD using a quantum point contact charge sensor. In the high frequency regime ($\hbar \omega_{driving} \gg \Delta$, where $\Delta$ is the tunnel coupling) we observe up to 9-photon transitions and clear Bessel function behavior of the DQD occupation with applied microwave power. We also studied the intermediate frequency regime, observing 18-photon transitions. The data are modeled using the time-dependent Schrodinger equation.$[2]$ By comparing the data with the simulations, we estimate $T_1\sim 15$ ns and $T_2\sim3$ ns. \\ \noindent $[1]$ A. J. Leggett \textit{et al.}, Rev. Mod. Phys. \textbf {59}, 1 (1987).\\ \noindent $[2]$ S. N. Shevchenko, S. Ashhab, F. Nori, Phys. Rep. {\bf492}, 1 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q27.00006: Non-adiabatic Quantum Control of a Semiconductor Charge Qubit Yuliya Dovzhenko, Jiri Stehlik, Karl Petersson, Jason Petta, Hong Lu, Arthur Gossard A GaAs double quantum dot is configured in the single-electron regime and operated as a charge qubit. The two basis states correspond to the electron being in either the left or the right dot. Non-adiabatic voltage pulses are applied to the depletion gates to drive coherent rotations, and the double dot occupation is read out using a nearby quantum point contact charge sensor. In contrast with previous work, where a single non-adiabatic pulse was applied for quantum control, we apply multiple pulses working towards a charge echo.[1,2] Data for $\frac{\pi}{2}$ - $\tau$ - $\frac{\pi}{2}$ and the $\frac{\pi}{2}$ - $\tau_1$ - $\pi$ - $\tau_2$ - $\frac{\pi}{2}$ ``charge echo" pulse sequences are obtained and compared with numerical simulations of the charge qubit evolution. \noindent References:\\ \noindent [1] K. D. Petersson \textit{et al.}, Phys. Rev. Lett. (in press).\\ \noindent [2] Y. Nakamura \textit{et al.}, Phys. Rev. Lett. {\bf88}, 047901 (2002).\\ [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q27.00007: Extended coherence of exchange operations in double quantum dot spin qubits using Hahn echo Michael Shulman, Hendrik Bluhm, Oliver Dial, Vladimir Umansky, Amir Yacoby Semiconductor spin qubits are promising candidates for quantum computation because of their long coherence times and potential for scalability. The exchange interaction is a powerful resource in these qubits, as it can drive single qubit rotations and inter-qubit entanglement. However, spin qubits driven by exchange become sensitive to charge noise, which in free induction decay experiments has lead to dephasing after a few coherent exchange oscillations. We perform a Hahn echo measurement in two-electron spin qubits in GaAs quantum dots. The $\pi$-pulse is applied by means of a stabilized nuclear gradient in the quantum dots. We find an exponential dephasing with a time constant of up to 10$\mu$s, which is more than an order of magnitude larger than $T_{2}^{*}$, and corresponds to 500 coherent exchange operations within $T_{2}$. This increase in $T_{2}$ is expected to allow for several cPHASE operations between two charge coupled two-electron qubits within $T_{2}$. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q27.00008: Two-qubit operations of two-electron spin qubits in GaAs quantum dots Hendrik Bluhm, Michael Shulman, Oliver E. Dial, Vladimir Umansky, Amir Yacoby The realization of two-qubit entangling gates is one of the most important milestones for the development of quantum-dot based electron spin qubits. Our measurements and simulations of the coupling strength and the relevant coherence time indicate very favorable prospects for the realization of such gates using the Coulomb interaction between adjacent spin qubits. This operation can be protected against dephasing due to low frequency electric noise by simultaneously applying a $\pi$-pulse to both qubits, which is essential to achieve the required coherence time. We report the experimental realization of this echo operation in a two-qubit device, conditional evolution of one qubit depending on the charge state of the neighboring double dot, and further progress toward two-qubit entanglement. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q27.00009: Spin Manipulation in InAs Nanowire Double Quantum Dots M.D. Schroer, M. Jung, K.D. Petersson, C.M. Quintana, J.R. Petta Recently, much effort has been devoted to the development of physical qubits for integration into quantum computers. Qubits allowing control with electric fields are attractive, as ac magnetic fields are more difficult to generate and localize on the nanoscale. The material properties of InAs allow efficient driving of electron spin resonance via the spin--orbit interaction. Our work has focused on developing quantum dots in InAs nanowires as fully characterized and controllable qubits. We have optimized our nanowire growth to eliminate the presence of planar defects, which impede the predictable formation of quantum dots. Using a bottom--gated architecture [1], we demonstrate tunable InAs nanowire double quantum dots, with the occupation controllable to the last electron. Pauli blockade is observed in the two-electron regime, demonstrating spin-dependent transport. We are able to drive single spin rotations by applying microwaves to one of the local metallic gates; from the electron spin resonance condition we extract a g--factor of $\sim$9. Finally, we demonstrate full electrical control of the two-electron system and characterize gate fidelities.\\[4pt] [1] S. Nadj-Perge {\it et al.}, arXiv:1011.0064v1 [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q27.00010: On-demand single-electron transfer between distant quantum dots with nanosecond pulses of surface acoustic waves R.P.G. McNeil, M. Kataoka, C.J.B. Ford, C.H.W. Barnes, J.P. Griffiths, G.A.C. Jones, I. Farrer, D.A. Ritchie Quantum dots (QDs) provide a useful system for manipulating and storing quantum information. Methods for moving quantum information (spin) between processor and storage, or to a region of holes for conversion to photon qubits, will be required. Tunnelling of electrons over long distances between QDs is not viable. We show controlled long-range transfer of single electrons between QDs through a depleted 1D channel using pulses of surface acoustic waves (SAWs). In our device, two QDs are connected by a 4$\mu$m channel with QD occupancy monitored by 1D charge detectors. Electrons may be trapped and raised above the Fermi energy by stepping gate voltages. Having set the first QD to be `full' and the other QD `empty', a short SAW pulse is sent to transfer the electron to the opposite QD. This bi-directional process may be repeated over 100 times with the same electron. SAW power and pulse-width dependences suggest that transfer is achieved during the first few SAW cycles allowing sub-20ns pulses to be used. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q27.00011: A proposed all-electrical spin qubit CNOT gate robust against charge noise Sankar Das Sarma, Jason Kestner We shall propose an alternative to the Loss-DiVincenzo implementation of the CNOT gate in a quantum dot spin qubit system. Our all-electrical proposal has the advantage of being robust against uncertainties and fluctuations in the tunnel coupling, barrier gate voltage pulse area, and interwell detuning which typically arise due to charge noise. The core idea is to introduce an auxiliary dot and use an analog to the stimulated Raman adiabatic passage (STIRAP) pulse sequence in three-level atomic systems, often referred to in the context of electron transport in quantum dot systems as CTAP (Coherent Tunneling by Adiabatic Passage). Spin-dependent tunneling opens the possibility of performing entangling two-qubit gates by this method. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q27.00012: Sensitivity to electronics error in coupled double quantum dot qubits Erik Nielsen, Richard Muller, Malcolm Carroll Reducing the effects of electronics control error in double quantum dot (DQD) quantum bits (qubit) is a central challenge to the creation of a solid-state quantum computing architecture. We investigate a system of capacitively coupled DQDs which implement a variant of the controlled phase gate when using each DQD as a singlet-triplet qubit. We identify regimes in which the gate action is more robust to sources of noise such as error around the applied bias point due to electronics or charge noise. Energy spectra are found using a configuration interaction (CI) method that accurately captures the (2,0) configuration of the DQD system, which is important for operating in these potentially low-noise regimes. This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin company, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q27.00013: Gate Capacitance Reproducibility and Modeling in Silicon Double Quantum Dots Ted Thorbeck, Akira Fujiwara, Neil Zimmerman For many applications the ability to design quantum dots with a specific set of gate capacitances and then rely on the reproducibility of those capacitances is crucial. For quantum computing, the ability to design our gate capacitances would help in reaching the few electron regime and in coupling multiple devices. For other applications the ability to design our gate capacitances would enable higher temperature operation. Our double quantum dots are formed by electrostatic gates on a silicon nanowire. We have measured 20 similar devices with 8 different sets of lithographic parameters. We will report on the reproducibility of the gate capacitances. For example, the range of capacitances is typically within 10{\%} of the average. We will also compare our measured capacitances to simulations based on lithographic parameter. This simulation could then be used to design new devices. [Preview Abstract] |
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