M. Lenander

Implementing the Quantum von Neumann Architecture with Superconducting Circuits

A quantum version of a central processing unit was created with superconducting circuits and elements. The von Neumann architecture for a classical computer comprises a central processing unit and a memory holding instructions and data. We demonstrate a quantum central processing unit that exchanges data with a quantum random-access memory integrated on a chip, with instructions stored on a classical computer. We test our quantum machine by executing codes that involve seven quantum elements: Two superconducting qubits coupled through a quantum bus, two quantum memories, and two zeroing registers. Two vital algorithms for quantum computing are demonstrated, the quantum Fourier transform, with 66% process fidelity, and the three-qubit Toffoli-class OR phase gate, with 98% phase fidelity. Our results, in combination especially with longer qubit coherence, illustrate a potentially viable approach to factoring numbers and implementing simple quantum error correction codes.

Minimizing quasiparticle generation from stray infrared light in superconducting quantum circuits

We find that quasiparticle generation from stray infrared light creates a significant loss mechanism in superconducting resonators and qubits. We show that resonator quality factors and qubit energy relaxation times are limited by a quasiparticle density of approximately 200 μm−3, induced by 4 K blackbody radiation from the environment. We demonstrate how this influence can be fully removed by isolating the devices from the radiative environment using multistage shielding.R. Barends, J. Wenner, M. Lenander, Y. Chen, R. C. Bialczak, J. Kelly, E. Lucero, P. O’Malley, M. Mariantoni, D. Sank, H. Wang, T. C. White, Y. Yin, J. Zhao, A. N. Cleland, John M. Martinis, and J. J. A. Baselmans Department of Physics, University of California, Santa Barbara, CA 93106, USA SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands (Dated: January 25, 2013)

Improving the coherence time of superconducting coplanar resonators

The quality factor and energy decay time of superconducting resonators have been measured as a function of material, geometry, and magnetic field. Once the dissipation of trapped magnetic vortices is minimized, we identify surface two-level states (TLS) as an important decay mechanism. A wide gap between the center conductor and the ground plane, as well as use of the superconductor Re instead of Al, are shown to decrease loss. We also demonstrate that classical measurements of resonator quality factor at low excitation power are consistent with single-photon decay time measured using qubit-resonator swap experiments.

Deterministic Entanglement of Photons in Two Superconducting Microwave Resonators

Quantum entanglement, one of the defining features of quantum mechanics, has been demonstrated in a variety of nonlinear spinlike systems. Quantum entanglement in linear systems has proven significantly more challenging, as the intrinsic energy level degeneracy associated with linearity makes quantum control more difficult. Here we demonstrate the quantum entanglement of photon states in two independent linear microwave resonators, creating N-photon NOON states (entangled states |N0> + |0N>) as a benchmark demonstration. We use a superconducting quantum circuit that includes Josephson qubits to control and measure the two resonators, and we completely characterize the entangled states with bipartite Wigner tomography. These results demonstrate a significant advance in the quantum control of linear resonators in superconducting circuits.

Photon shell game in three-resonator circuit quantum electrodynamics

The ability to coherently switch a state between two systems is a key requirement for quantum information processing. Such control is now demonstrated by shifting the quantum state of a microwave photon between any one of three superconducting-circuit resonators: in analogy to the classic three cups and a ball game.

Quantum process tomography of two-qubit controlled-Z and controlled-NOT gates using superconducting phase qubits

We experimentally demonstrate quantum process tomography of controlled-Z and controlled-NOT gates using capacitively-coupled superconducting phase qubits. These gates are realized by using the j2i state of the phase qubit. We obtain a process fidelity of 0.70 for the controlled-phase and 0.56 for the controlled-NOT gate, with the loss of fidelity mostly due to single-qubit decoherence. The controlled-Z gate is also used to demonstrate a two-qubit Deutsch-Jozsa algorithm with a single function query. Quantum computation and quantum communication rely on excellent control of the underlying quantum system [1]. Reasonable control has been achieved with a variety of quantum systems, with superconducting qubits emerging as one of the most promising candidates [2]. Recent experiments using superconducting architectures include demonstrations of quantum algorithms using two qubits [3] and the entanglement of three qubits [4, 5]. A key element in these experiments is a two-qubit entangling gate, such as the p iSWAP [4] and the controlledZ (CZ) gates [3, 5]. Because the CZ gate is simple to implement, has high fidelity, and can readily generate controlled-NOT (CNOT) logic [6], it likely will be an important component in more complex algorithms such as quantum error correction. At present, however, the CZ gate functionality has only been directly tested for a subset of the possible input states. In this Letter, we demonstrate the operation of a CZ gate in a superconducting phase qubit, and fully characterize this gate as well as a CNOT gate using quantum process tomography (QPT). We additionally use the CZ gate to perform the Deutsch-Jozsa algorithm [3], here with a single-shot evaluation of the function. The use of QPT provides a more complete gate evaluation than, for example, measuring the truth table for the corresponding CNOT gate [7, 8], as it verifies that the gate will properly transform any possible input state. QPT for two- or three-qubit gates has been reported in NMR [9], optics [10–12], and in ion traps [13, 14]. In solid state systems, QPT has been implemented for the p iSWAP gate with the phase qubit [15].

Fast tunable coupler for superconducting qubits

A major challenge in the field of quantum computing is the construction of scalable qubit coupling architectures. Here, we demonstrate a novel tuneable coupling circuit that allows superconducting qubits to be coupled over long distances. We show that the inter-qubit coupling strength can be arbitrarily tuned over nanosecond timescales within a sequence that mimics actual use in an algorithm. The coupler has a measured on/off ratio of 1000. The design is self-contained and physically separate from the qubits, allowing the coupler to be used as a module to connect a variety of elements such as qubits, resonators, amplifiers, and readout circuitry over long distances. Such design flexibility is likely to be essential for a scalable quantum computer.

Wirebond crosstalk and cavity modes in large chip mounts for superconducting qubits

We analyze the performance of a microwave chip mount that uses wirebonds to connect the chip and mount grounds. A simple impedance ladder model predicts that transmission crosstalk between two feedlines falls off exponentially with distance at low frequencies, but rises to near unity above a resonance frequency set by the chip to ground capacitance. Using SPICE simulations and experimental measurements of a scale model, the basic predictions of the ladder model were verified. In particular, by decreasing the capacitance between the chip and box grounds, the resonance frequency increased and transmission decreased. This model then influenced the design of a new mount that improved the isolation to − 65 dB at 6 GHz, even though the chip dimensions were increased to 1 cm × 1 cm, three times as large as our previous devices. We measured a coplanar resonator in this mount as preparation for larger qubit chips, and were able to identify cavity, slotline, and resonator modes.

Measurement of energy decay in superconducting qubits from nonequilibrium quasiparticles

M. Lenander, H. Wang, Radoslaw C. Bialczak, Erik Lucero, Matteo Mariantoni, M. Neeley, A. D. O’Connell, D. Sank, M. Weides, J. Wenner, T. Yamamoto, Y. Yin, J. Zhao, A. N. Cleland, and John M. Martinis Department of Physics, University of California, Santa Barbara, CA 93106, USA Department of Physics, Zhejiang University, Hangzhou 310027, China and Green Innovation Research Laboratories, NEC Corporation, Tsukuba, Ibaraki 305-8501, Japan

Phase qubits fabricated with trilayer junctions

We have developed a novel Josephson junction geometry with minimal volume of lossy isolation dielectric, suitable for higher quality trilayer junctions implemented in qubits. The junctions are based on in situ deposited trilayers with thermal tunnel oxide, have micron-sized areas and a low subgap current. In qubit spectroscopy only a few avoided level crossings are observed, and the measured relaxation time of T1≈400 ns is in good agreement with the usual phase qubit decay time, indicating low loss due to the additional isolation dielectric.