Quantum computation requires quantum logic gates that use the interaction within pairs of quantum bits (qubits) to perform conditional operations. Superconducting qubits may offer an attractive route towards scalable quantum computing. In previous experiments on coupled superconducting qubits, conditional gate behaviour and entanglement were demonstrated. Here we demonstrate selective execution of the complete set of four different controlled-NOT (CNOT) quantum logic gates, by applying microwave pulses of appropriate frequency to a single pair of coupled flux qubits. All two-qubit computational basis states and their superpositions are used as input, while two independent single-shot SQUID detectors measure the output state, including qubit-qubit correlations. We determined the gate's truth table by directly measuring the state transfer amplitudes and by acquiring the relevant quantum phase shift using a Ramsey-like interference experiment. The four conditional gates result from the symmetry of the qubits in the pair: either qubit can assume the role of control or target, and the gate action can be conditioned on either the 0-state or the 1-state. These gates are now sufficiently characterized to be used in quantum algorithms, and together form an efficient set of versatile building blocks.

The authors have studied low-frequency resistance fluctuations in shadow-evaporated Al/ AlO x /Al tunnel junctions. Between 300 and 5 K the spectral density follows a 1 / f law. Below 5 K, individual defects distort the 1 / f shape of the spectrum. The spectral density decreases linearly with temperature between 150 and 1 K and saturates below 0.8 K. At 4.2 K, it is about two orders of magnitude lower than expected from a recent survey ͓D. J. Van 4 Decoherence due to external sources such as the measurement devices has been studied extensively and is by now well understood, 5 permitting qubit dephasing times of up to several microseconds. 6 Future progress in this field of research depends crucially on understanding and controlling decoherence due to defects in the devices. 7-9 Superconducting qubits contain Josephson junctions, whose Josephson energy, E J = ⌽ 0 I C / ͑2 ͒, determines the potential landscape of the qubit ͑I C is the critical current and ⌽ 0 = h /2e is the superconducting flux quantum͒. Due to imperfections of the tunnel barrier, E J fluctuates in time, leading to fluctuations in the qubit potential. Therefore, the qubit energy splitting is not constant during an experiment, which leads to decoherence.

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