Short dephasing times pose one of the main challenges in realizing a quantum computer. Different approaches have been devised to cure this problem for superconducting qubits, a prime example being the operation of such devices at optimal working points, so-called "sweet spots." This latter approach led to significant improvement of T2 times in Cooper pair box qubits [D. Vion et al., Science 296, 886 (2002)]. Here, we introduce a new type of superconducting qubit called the "transmon." Unlike the charge qubit, the transmon is designed to operate in a regime of significantly increased ratio of Josephson energy and charging energy EJ /EC. The transmon benefits from the fact that its charge dispersion decreases exponentially with EJ /EC , while its loss in anharmonicity is described by a weak power law. As a result, we predict a drastic reduction in sensitivity to charge noise relative to the Cooper pair box and an increase in the qubit-photon coupling, while maintaining sufficient anharmonicity for selective qubit control. Our detailed analysis of the full system shows that this gain is not compromised by increased noise in other known channels.

Neutrinos with energies above 10(8) GeV are expected from cosmic ray interactions with the microwave background and are predicted in many speculative models. Such energetic neutrinos are difficult to detect, as they are shadowed by Earth, but rarely interact in the atmosphere. Here we propose a novel detection strategy: Earth-skimming neutrinos convert to charged leptons that escape Earth, and these leptons are detected in ground level fluorescence detectors. With the existing HiRes detector, neutrinos from some proposed sources are marginally detectable, and improvements of 2 orders of magnitude are possible at the proposed Telescope Array.

The role of mixed state entanglement in liquid-state nuclear magnetic resonance (NMR) quantum computation is not yet well-understood. In particular, despite the success of quantum information processing with NMR, recent work has shown that quantum states used in most of those experiments were not entangled. This is because these states, derived by unitary transforms from the thermal equilibrium state, were too close to the maximally mixed state. We are thus motivated to determine whether a given NMR state is entanglable -that is, does there exist a unitary transform that entangles the state? The boundary between entanglable and nonentanglable thermal states is a function of the spin system size N and its temperature T . We provide new bounds on the location of this boundary using analytical and numerical methods; our tightest bound scales as N ∼ T , giving a lower bound requiring at least N ∼ 22, 000 proton spins to realize an entanglable thermal state at typical laboratory NMR magnetic fields. These bounds are tighter than known bounds on the entanglability of effective pure states.

We demonstrate the ability to control spontaneous emission from a superconducting qubit coupled to a cavity. The time domain profile of the emitted photon is shaped into a symmetric truncated exponential. The experiment is enabled by a qubit coupled to a cavity, with a coupling strength that can be tuned in tens of nanoseconds while maintaining a constant dressed state emission frequency. Symmetrization of the photonic wave packet will enable use of photons as flying qubits for transferring the quantum state between atoms in distant cavities.Transferring information on a classical computer is enabled by the data bus. The analog for shuttling quantum information is a more challenging problem. One approach uses itinerant photons to couple a single quantum emitter (a qubit) with a distant absorber. This can be accomplished by coupling an emitter to a cavity, allowing the generation of exotic states of light such as those containing exactly one photon resulting from a qubit relaxation event [1]. Photons are well suited for carrying information over long distances due to their long coherence lengths. However, encoding information from the qubit to the photon requires accurate control over the interaction between the two systems.When a qubit in some arbitrary superposition of ground and excited states relaxes radiatively through the cavity, its state is mapped onto the basis of zero-and one-photon states. If the qubit's dipole coupling strength is held constant, the cavity emission will have a wave packet shaped with a steep leading-edge front followed by an exponentially decaying tail. The shape is characteristic of the random Poissonian relaxation process. However, this wave-packet shape is unsuitable for quantum state transfer because it will be reflected at the destination cavity's input port due to what is effectively an impedance mismatch. This occurs even if the source and destination cavities are identical.To overcome this reflection and loss in transfer fidelity, one can engineer the wave packet emitted by the source to be symmetric in time. Then to absorb this wave packet, the destination qubit-cavity system properties are time reversed so absorption at the destination and emission at the source are mirror images of one another. Theoretical estimates for this technique predict near unit transfer fidelity [2,3].One can achieve time-symmetric wave packets by loading the cavity with a single photon and then vary the cavity leak rate with a superconducting quantum interference device-based coupler circuit to match the desired emission profile [4]. This technique has shown considerable promise and has been used to absorb classical states of light as well [5]. At optical frequencies, careful control of the pump laser has been used to produce single photons with a desired shape [6]. In another experiment, an electro-optic modulator has been used to modulate a single photon from a biphoton pair generated by spontaneous parametric down-conversion, with the modulation time reference set by the second photon of the pair...