Berlin 2005 – scientific programme

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TT: Tiefe Temperaturen

TT 14: Symposium Superconducting Quantum Systems

TT 14.2: Invited Talk

Saturday, March 5, 2005, 14:15–14:45, TU H104

Single-Shot State Measurement of Coupled Phase Qubits — •John Martinis — Physics Department, University of California, Santa Barbara, CA 93106, USA

The Josephson junction is an ideal solid-state system for building electrical No-dqatomsNo-dq that can function as quantum bits for a quantum computer. I will discuss experimental work based on large-area No-dqphaseNo-dq qubits, where experiments have demonstrated qubit state preparation and measurement with good fidelity, as well as manipulation of the qubit state via the observation of Rabi oscillations.

A more advanced measurement technique, based on a fast pulse in the current bias, has reduced the measurement time for our phase qubits from 100 ns to less than 2 ns. With this improvement, we have recently been able to perform experiments on capacitively-coupled qubits, demonstrating coherent oscillations corresponding to the i-swap operation 01<->i10. This is the first experiment with Josephson qubits to achieve simultaneous single-shot state measurements of both qubit states. During the oscillations, the states 01 and 10 are found to be anti-correlated, as expected for an entangled quantum state.

Detailed measurements show that the two measurement pulses have to be timed within about 1 ns of each other in order for large crosstalk in the state probabilities not to appear in the measurement data. A semi-classical theory explaining this phenomenon will be presented.

With these developments, we will shortly be able to demonstrate CNOT gates with full state analysis using process tomography. I will present the simple theory describing these future experiments.