Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session J17: Focus Session: Superconducting Phase Qubits |
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Sponsoring Units: GQI Chair: Rob McDermott, University of Wisconsin Room: 318 |
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J17.00001: High-fidelity gates in Josephson phase qubits Invited Speaker: Erik Lucero Complex algorithms for a quantum computer require error correction and robust calibration protocols for extended pulse sequences. We present significant progress towards both of these goals with our detailed measurements of gate fidelity and coupled qubit experiments with multi-pulse sequences. We measure single qubit gate fidelities of 0.98, limited by energy relaxation; and by carefully separating out gate and measurement error we construct a complete error budget. Using the new metrological technique of ``Ramsey filtering'' we show how one important error process can be measured and reduced to a level of 10$^{-4}$, a magnitude believed to be near the fault tolerant threshold. This measurement demonstrates that our quantum system remains in the two-state qubit manifold during our single qubit operations. This precision and accuracy is made possible by custom control electronics that can create arbitrarily shaped microwave pulses. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J17.00002: Universal Quantum Gates in Josephson Junction Phase Qubits Radoslaw Bialczak, M. Ansmann, M. Hofheinz, E. Lucero, M. Neeley, A. O'Connell, D. Sank, M. Steffen, H. Wang, J. Wenner, A. Cleland, J. Martinis Josephson junction phase qubits are at a point where they can be used to create more complex operations such as quantum gates. Here we present work where we have tuned capacitively coupled Josephson junction phase qubits on and off resonance to generate and characterize a SQiSW gate using quantum process tomography (QPT). The SQiSW is the most fundamental universal gate for our system because it arises directly from the Hamiltonian for the physical circuit of our coupled qubits. In order to create more complex gates such as the CNOT, the SQiSW gate must be used to generate the entanglement. We perform QPT and obtain the Chi matrix, from which quantitative measures such as the gate fidelities can be calculated. We also show how to correct for measurement crosstalk and reduced visibilities present in our system and we perform measurements that quantitatively characterize the on/off ratio of our coupling scheme. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J17.00003: Engineering Tripartite Entangled States of Two Phase Qubits Coupled via a Cavity Jae Park, Fabio Altomare, Ray Simmonds We present an experimentally inexpensive scheme for preparing certain tripartite entangled states. We suggest ways to test the degree to which such target states have been successfully prepared. We present a convenient geometrical interpretation of the resonant unitary dynamics which gives a natural interpretation for the characteristic frequencies and provides intuition for the pulse sequence necessary to achieve a desired target state. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J17.00004: Two Qubits and a Cavity: Three's Company Fabio Altomare, Michael Allman, Katarina Cicak, Jae A. Park, Mika A. Sillanpaa, Adam Sirois, Joshua Strong, Jed Whittaker, Raymond W. Simmonds Quantum information theory suggests that there are two inequivalent classes of tripartite entanglement under stochastic local operations and classical communications (PRA, 62, 062314). Representative of these classes are the GHZ state and the W states, respectively. In this talk I will describe our experimental results on two superconducting phase qubits coupled through a cavity: one of the few cases where three is company and not a crowd. This system, effectively three coupled qubits if we restrict the cavity excitation to the single photon manifold, has allowed us to observe the spectroscopic signature and dynamics of Tripartite Entanglement. The rich dynamics of this system has allowed us to also observe a) Bell state between two qubits (with the third one disentangled), and b) W state between the three qubits. Future possibilities include the observation of GHZ state, particularly interesting for its practical applications, and for testing the non-locality of quantum mechanics. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J17.00005: Relaxation Dynamics of Fock States in a High Q Microwave Resonator Coupled to a Superconducting Phase Qubit Haohua Wang, Max Hofheinz, Markus Ansmann, Radoslaw Bialczak, Erik Lucero, Matthew Neeley, Aaron O'Connell, Daniel Sank, Jim Wenner, Andrew Cleland, John Martinis We have improved the lifetime of our high $Q$ microwave resonator that is coupled to a superconducting phase qubit. Using high speed electronics, we have successfully generated Fock states with up to 15 photons. We analyze the resonator number state using the qubit to verify the high purity of the Fock states. Finally we monitor the subsequent decay of the Fock states in time, and show that the decay matches that expected from theory, with the $n$-photon lifetime scaling as $T_{1}$/$n$, where $T_{1}$ is the one-photon lifetime. Measurements on the decay of the coherent states, generated in the resonator using classical pulses, are also in agreement with theory. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J17.00006: Controllable Coherent Population Transfers in Superconducting Qubits for Quantum Computing L.F. Wei, J.R. Johansson, L.X. Cen, S. Ashhab, F. Nori We propose an approach to coherently transfer populations between selected quantum states in one- and two-qubit systems by using controllable Stark-chirped rapid adiabatic passages. These evolution-time insensitive transfers, assisted by easily implementable single-qubit phase-shift operations, could serve as elementary logic gates for quantum computing. Specifically, this proposal could be conveniently demonstrated with existing Josephson phase qubits. Our proposal can find an immediate application in the readout of these qubits. Indeed, the broken parity symmetries of the bound states in these artificial atoms provide an efficient approach to design the required adiabatic pulses. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J17.00007: Three and Four Coupled Josephson Junction Phase Qubits Zechariah Thrailkill, Joseph Lambert, Sam Kennerly, Roberto Ramos The Josephson junction phase qubit has been shown to be a viable candidate for quantum computation. The two coupled phase qubit system has been extensively studied theoretically and experimentally. We have analyzed the quantum behavior of systems with more, three and four capacitively-coupled phase qubits, with different possible configurations. We have used anharmonic oscillators to model the systems. We will discuss some of the properties of these simple networks. The focus is on natural state evolution using a time independent, or adiabatically changing Hamiltonian. Analyzing how to transfer quantum information from one qubit to another and performing operations to change the overall state of these systems will give a better understanding of how to utilize the different qubit configurations. We will report on the progress of spectroscopic measurements for the three phase qubit systems. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J17.00008: Tunable Cavity QED with Josephson Phase Qubits Joshua Strong We have designed a tunable Josephson resonator and have coupled it to two phase qubits. The resonator can act as a cavity for QED-type experiments. We discuss results. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J17.00009: Emulation of spin dynamics using a superconducting phase qudit Matthew Neeley, M. Ansmann, R. Bialczak, M. Hofheinz, E. Lucero, A. O'Connell, D. Sank, H. Wang, J. Wenner, John Martinis, Andrew Cleland In superconducting quantum circuits, the nonlinearity of the Josephson junction allows energy-level transitions to be addressed individually by their unique frequencies. Typically this is used to operate the system as an effective two-level system, a qubit. In a recent experiment, we extended our coherent control of a phase qubit to the first five energy levels, allowing us to operate the device as a qu{\it d}it with $d = 3$, $4$, or $5$. We use this system to emulate the dynamics of single spins with spin quantum number $s = 1/2$, $1$ and $3/2$. We show that the phase acquired by a spin under rotation around a closed path follows the theoretical prediction. In particular, we confirm the even (odd) parity of integer (half-integer) spins under $2\pi$ rotation. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J17.00010: Multiplexed Phase qubit readout using SQUID-resonators Jed Whittaker, Michael Allman, Fabio Altomare, Katarina Cicak, Dale Li, Jae Park, Adam Sirois, Joshua Strong, Raymond Simmonds Flux biased phase qubits have traditionally been read out using a critical current switching technique of a coupled DC SQUID. This method has three limitations: it is extremely slow (orders of magnitude longer than typical energy relaxation times), difficult to multiplex, and by exceeding the critical current, it is dissipative and feeds broadband radiation back into the qubit, decohering its state. We are developing a SQUID-resonator readout method that addresses all three of these limitations. By operating the SQUID as a resonator, we can measure the state of the qubit quickly (on the order of its coherence time), we can multiplex resonant readout lines, and we can operate on the SQUID's supercurrent branch eliminating dissipation and decohering radiation. This faster, quieter readout should allow us to use measured results for real-time quantum feedback. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J17.00011: Berry's Phase of a Current-Biased Josephson Junction Anthony Tyler, Roberto Ramos, Zechariah Thrailkill, Sam Kennerly A quantum system, prepared in an eigenstate, can accumulate a geometric phase known as Berry's phase in addition to the expected dynamic phase. This occurs when there are adiabatic changes to the Hamiltonian which trace a closed loop in parameter space. A common example of this phase is an electron in a slowly varying magnetic field which traces a closed path. From this adiabatic variation, the electron's spin state has acquired a Berry's phase in addition to the dynamic phase. Due to the similarities between spin-1/2 particles, such as the electron, and solid state quantum bits (qubits), there should be an analogous process by which these system can gain a Berry's phase. Such processes have been tested in the charge qubit and has been derived for the flux qubit. Here, we will derive the Berry's phase for a phase qubit which can be found experimentally using quantum state tomography. We then utilize this to explore the possibility of creating topological gates with phase qubits. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J17.00012: Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ intrinsic SQUIDs as candidates of high-T$_c$ phase qubits X.Y. Jin, J. Lisenfeld, Y. Koval, A. Lukashenko, A.V. Ustinov, P. M\"uller An intrinsic SQUID is a superconducting ring made of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystal, intercepted by two intrinsic Josephson junction stacks. The inductance parameter $\beta_L$ can be tuned in a wide range by changing the height and the cross-section area of the stacks. When biased with dc current, the device showed typical properties of hysteretic dc-SQUIDs. When a device was coupled with a coil and a Nb readout dc-SQUID, typical rf-SQUID behavior was observed. By choosing a proper reset field, quantum escape from a single minimum has been measured on a sample of $\beta_L\sim10$. The escape rate can be fine-tuned by applying short pulses down to 1 ns, which allows a fast readout technique. With these prerequisites achieved, our experiments have opened the path to directly using these intrinsic SQUIDs as high-T$_c$ phase qubits. The first attempts to measure Rabi oscillations on these devices will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J17.00013: GHZ protocols for superconducting qubits Andrei Galiautdinov, John Martinis Superconducting circuits with Josephson junctions gained considerable attention as promising candidates for scalable solid state quantum computing architectures. While macroscopic quantum behavior of such circuits has already been demonstrated (e.g., Rabi oscillations, high fidelity state preparation and measurement, various logic gate operations, etc.), further progress in developing a workable quantum computer will depend crucially on architecture's ability to implement various multiqubit entangled states. Here we show how Greenberger-Horne-Zeilinger states can be generated in tripartite systems with capacitive and inductive couplings. Generalization to architectures containing arbitrary numbers of qubits is also discussed. [Preview Abstract] |
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