We propose to use a single mesoscopic ensemble of trapped polar molecules for quantum computing. A "holographic quantum register" with hundreds of qubits is encoded in collective excitations with definite spatial phase variations. Each phase pattern is uniquely addressed by optical Raman processes with classical optical fields, while one-and two-qubit gates and qubit read-out are accomplished by transferring the qubit states to a stripline microwave cavity field and a Cooper pair box where controllable two-level unitary dynamics and detection is governed by classical microwave fields.PACS numbers: 03.67. Lx, 33.90.+h, 85.25.Cp, 42.70.Ln In classical computer science holographic data storage is poised to provide the next generation in digital media [1,2]. The defining characteristic of this method is that information is stored globally rather than on specific sites in a storage medium. Current investigations of quantum memory components include similar ideas for storage of optical information in ensembles of atoms [3,4,5,6] and molecules [7].In the quantum version of holographic storage one can envisage N atoms or molecules in a lattice initially all populating the same internal quantum state |g [see Fig. 1(b)]. The quantum information in an incident weak field Ω 1 e i k1· x is, by the assistance of a control field Ω 2 (t)e

We propose to encode quantum information in rotational excitations in a molecular ensemble. Using a stripline cavity field for quantum state transfer between the molecular ensemble and a Cooper pair box two-level system, our proposal offers a linear scaling of the number of qubits in our register with the number of rotationally excited states available in the molecules.PACS numbers: 03.67. Lx, 85.25.Cp, 33.90.+h One obstacle which transcends all implementations of a quantum computer [1,2,3,4] concerns the extension of current proof-of-principle operations beyond a handful of qubits. At the heart of this obstacle lies the exponential scaling of the Hilbert space dimension with the number of qubits. If one chooses to work with qubits encoded in separate particles, the available state space is exponentially large in the particle number, but selective access to individual particles and precise control of the interactions among individual quantum particles presents a formidable challenge. This has spurred interest in quantum systems that intrinsically support a vast Hilbert space. Obvious candidates are molecular quantum systems which easily provide 100 accessible internal rotational and vibrational levels [5,6]. The quantum information capacity of such systems corresponds, however, to a mere log 2 (100) ≈ 6 qubits, and most molecular implementations to date have not exploited the rich internal structure, but have focussed on other advantages provided by molecular systems such as the large intermolecular dipole-dipole coupling [7], switchable interactions [8], and long coherence times [6]. These advantages also make molecules very attractive for hybrid quantum computing schemes involving solid state, optical and molecular quantum degrees of freedom simultaneously. Notably, in [9], it has been proposed to trap a mesoscopic molecular ensemble at an antinode of the quantized field of a stripline cavity with a Cooper pair box (CPB) placed at the adjacent antinode. This setup is illustrated in Fig. 1a. The energy scale for the stripline cavity mode matches typical energies for rotational excitations of polar molecules, providing a natural interface between the cavity and molecular degrees of freedom. The large electric dipole moment of polar molecules makes the strong coupling regime relatively easy to achieve while strong coupling of the field to the CPB has been demonstrated experimentally in [10,11]. Furthermore, by using an ensemble of N molecules one achieves a √ N enhancement of the coupling to the weak quantum field compared to the single molecule vacuum Rabi frequency g. In [9], the essential idea is to counteract the rapid decoherence in a Cooper-pair box by transferring the quantum state to the molecular ensemble for storage of the qubit in a collective

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We present a method for performing quantum state reconstruction on qubits and qubit registers in the presence of decoherence and inhomogeneous broadening. The method assumes only rudimentary single qubit rotations as well as knowledge of decoherence and loss mechanisms. We show that full state reconstruction is possible even in the case where single qubit rotations may only be performed imperfectly. Furthermore we show that for ensemble quantum computing proposals, quantum state reconstruction is possible even if the ensemble experiences inhomogeneous broadening and if only imperfect qubit manipulations are available during state preparation and reconstruction.

We assess the feasibility of a collective district heating and cooling network based on a foundation pile heat exchanger in a new urban area in Vejle, Denmark. A thermogeological model for the area is developed based on geophysical investigations and borehole information. In tandem with a building energy demand model, the subsurface thermal properties serve as the input for a newly developed computational temperature model for collective heating and cooling with energy piles. The purpose of the model is to estimate the long-term performance and maximum liveable area that the energy piles are able to support. We consider two case studies where residential and office buildings dominate the building mass. We find that three to four floors can be supplied with heating and cooling from the energy piles, depending on the use and design of the buildings.