Session D27: Focus Session: Superconducting Qubits - Gates and Algorithms

Scaling Superconducting Qubits with the ResQu Architecture

This abstract not available.   

Quantum Logic Gates for Coupled Superconducting Resonators

Superconducting resonators are a promising element for many applications in quantum information processing, such as memory, state transfer, and qubit-qubit coupling. Here I introduce a new application---multi-level quantum logic using superpositions of Fock states. A circuit-QED implementation of single and coupled-resonator gates will be presented and theoretically analyzed. This scheme, using experimentally demonstrated interactions, will be compared with traditional qubit operations.   

Two-qubit gates and coupling with low-impedance flux qubits

We experimentally demonstrate the coupling of two low-impedance flux qubits mediated via a transmission line resonator. We explore the viability of experimental coupling protocols which involve selective microwave driving on the qubits independently as well as fast frequency tuning through on-chip flux-bias. Pulse-shaping techniques for single-qubit and two-qubit gates are employed for reducing unwanted leakage and phase errors. A joint readout through the transmission line resonator is used for characterizing single-qubit and two-qubit states.   

Entangling ISWAP gate using frequency shifted anharmonic qubits

In this talk, we examine the coupling between frequency separated qubits, typical of superconducting implementations. We show how to correct for errors coming from finite turn-on time (corresponding to bringing the qubits into resonance) as well as leakage error (corresponding to exciting population out of the qubit manifold), namely by bringing the qubits in and out of resonance repeatedly to cancel out the unwanted parts of the Hamiltonian. The gates presented are smooth and robust and represent a whole class of analytic and numeric solutions for the evolution of the composite system.   

Analytic control methods for high fidelity unitary operations in a weakly nonlinear oscillator

In qubits made from a weakly anharmonic oscillator the leading source of error at short gate times is leakage of population out of the two dimensional Hilbert space that forms the qubit. In this talk we explore a general technique based on an adiabatic expansion to find pulse shapes that correct this type of error. This leads to a family of solutions that can be further refined based on what is feasible for a particular application. This set of pulses contains and improves upon the previously developed DRAG solution [F. Motzoi, et. al., Phys. Rev. Lett. 103, 110501 (2009)] and can be further generalized to more complicated systems with additional leakage channels.   

CNOT gate for superconducting qubits biased at their symmetry points

A number of different techniques have been proposed and demonstrated in the past few years for implementing two-qubit gates in a system of two coupled superconducting qubits biased at their symmetry points. Most of these techniques implement the iSWAP gate. I will discuss a new technique that implements the CNOT gate. The two qubits are driven at the frequency of the target qubit, and the amplitudes applied to the two qubits are chosen such that the target qubit undergoes Rabi oscillations for only one of the two possible states of the control qubit. As a result a CNOT gate can be implemented.   

Controlled-NOT logic gate based on conditional spectroscopy

A controlled-NOT logic gate based on conditional rotation of a target qubit by applying a microwave pulse of appropriate frequency has been demonstrated experimentally for a pair of superconducting~flux qubits [Plantenberg et. al., Nature 447, 836 (2007)] . Here we discuss a related construction appropriate for coupled phase qubits~or a phase~qubit coupled to a resonator.~Our results show that an intrinsic fidelity of more than 99{\%} is~achievable~in about 45ns.   

Quantum Logic Gates for the Rezqu Architecture

A promising quantum computing architecture has been recently proposed by the UCSB superconducting quantum computation group. In this architecture, n phase qubits are capacitively coupled to individual memory resonators as well as a common bus. In this talk we discuss the design of quantum logic gates for this architecture and discuss the intrinsic fidelities.   

Idling error and SWAP/MOVE operation in RezQu architecture for phase qubits

We analyze several basic operations in the RezQu architecture for superconducting phase qubits recently proposed by John Martinis, concentrating on the idling error, generation of single-excitation states, and the single-excitation transfer (which we call MOVE) between a phase qubit and its memory. We show that the idling error is negligible, being proportional to the sixth power of the coupling strength. We also show that in the rotating wave approximation the MOVE operation, which is simpler than the usual SWAP, can be realized perfectly using a tune/detune pulse with four adjustable parameters. The pulse consists of the front ramp (with proper shaping), a constant near-resonant overshoot, and an arbitrary rear ramp.   

Experimental demonstration of quantum algorithms on a 4-qubit/5-resonator quantum microprocessor utilizing superconducting qubits in the RezQu architecture

We present our newly designed and fabricated 4-qubit/5-resonator quantum microprocessor composed of ``off-the-shelf" qubit and resonator components in the RezQu (``rez-(,)kyoo") architecture. The RezQu architecture uses resonators with qubits in the zero state to turn off stray coupling. Each qubit is coupled to a $\lambda /4$ memory resonator and coupling between the qubits is mediated by a common $\lambda /2$ resonator bus. Eight microwave lines drive the individual qubits, memory resonators, and coupling resonator. We demonstrate control over the quantum microprocessor via small scale quantum algorithms that require executing high- fidelity single qubit gates, quantum Fourier transform, Toffoli, CNOT, and other entangling gates.   

Efficient Toffoli Gate in Circuit Quantum Electrodynamics

The fidelity of quantum gates in circuit quantum electrodynamics is typically limited by qubit decoherence.  As such, significant improvements can be realized by shortening gate duration [1, 2]. The three-qubit Toffoli gate, also called the controlled-controlled NOT, is an important operation in basic quantum error correction.  We report a scheme for a Toffoli gate that exploits interactions with non-computational excited states of transmon qubits which can be executed faster than an equivalent construction using one- and two-qubit gates.  The application of this gate to efficient measurement-free quantum error correction will be discussed. \\[4pt] [1] DiCarlo, et al. Nature 467, 574 (2010). \\[0pt] [2] Chow, et al. Phys. Rev. Lett. 102, 090502 (2009).   

Progress towards a microwave-based high-fidelity Toffoli gate with superconducting qubits

We describe recent progress at IBM towards a microwave-based implementation of the Toffoli gate using three capacitively shunted flux qubits dispersively coupled to a resonator. We discuss the device architecture and the microwave protocol, along with expected limits to gate fidelity and scaling.