Nanocrystalline (anatase), mesoporous TiO2 thin films were functionalized with [Ru(bpy)2(deebq)](PF6)2, [Ru(bq)2(deeb)](PF6)2, [Ru(deebq)2(bpy)](PF6)2, [Ru(bpy)(deebq)(NCS)2], or [Os(bpy)2(deebq)](PF6)2, where bpy is 2,2'-bipyridine, bq is 2,2'-biquinoline, and deeb and deebq are 4,4'-diethylester derivatives. These compounds bind to the nanocrystalline TiO2 films in their carboxylate forms with limiting surface coverages of 8 (+/- 2) x 10(-8) mol/cm2. Electrochemical measurements show that the first reduction of these compounds (-0.70 V vs SCE) occurs prior to TiO2 reduction. Steady state illumination in the presence of the sacrificial electron donor triethylamine leads to the appearance of the reduced sensitizer. The thermally equilibrated metal-to-ligand charge-transfer excited state and the reduced form of these compounds do not inject electrons into TiO2. Nanosecond transient absorption measurements demonstrate the formation of an extremely long-lived charge separated state based on equal concentrations of the reduced and oxidized compounds. The results are consistent with a mechanism of ultrafast excited-state injection into TiO2 followed by interfacial electron transfer to a ground-state compound. The quantum yield for this process was found to increase with excitation energy, a behavior attributed to stronger overlap between the excited sensitizer and the semiconductor acceptor states. For example, the quantum yields for [Os(bpy)2(dcbq)]/TiO2 were phi(417 nm) = 0.18 +/- 0.02, phi(532.5 nm) = 0.08 +/- 0.02, and phi(683 nm) = 0.05 +/- 0.01. Electron transfer to yield ground-state products occurs by lateral intermolecular charge transfer. The driving force for charge recombination was in excess of that stored in the photoluminescent excited state. Chronoabsorption measurements indicate that ligand-based intermolecular electron transfer was an order of magnitude faster than metal-centered intermolecular hole transfer. Charge recombination was quantified with the Kohlrausch-Williams-Watts model.

Generating quantum entanglement in large systems on time scales much shorter than the coherence time is key to powerful quantum simulation and computation. Trapped ions are among the most accurately controlled and best isolated quantum systems [1] with low-error entanglement gates operated via the vibrational motion of a few-ion crystal within tens of microseconds [2]. To exceed the level of complexity tractable by classical computers the main challenge is to realise fast entanglement operations in large ion crystals [3,4]. The strong dipole-dipole interactions in polar molecule [5] and Rydberg atom [6,7] systems allow much faster entangling gates, yet stable state-independent confinement comparable with trapped ions needs to be demonstrated in these systems [8]. Here, we combine the benefits of these approaches: we report a 700 ns two-ion entangling gate which utilises the strong dipolar interaction between trapped Rydberg ions and produce a Bell state with 78% fidelity. The sources of gate error are identified and a total error below 0.2% is predicted for experimentally-achievable parameters. Furthermore, we predict that residual coupling to motional modes contributes ∼ 10 −4 gate error in a large ion crystal of 100 ions. This provides a new avenue to significantly speed up and scale up trapped ion quantum computers and simulators. Trapped atomic ions are one of the most promising architectures for realizing a universal quantum computer [1]. The fundamental single-and two-qubit quantum gates have been demonstrated with errors less than 0.1% [2], sufficiently low for fault-tolerant quantum errorcorrection schemes [10]. Nevertheless, a scalable quantum computer requires a large number of qubits and a large number of gate operations to be conducted within the coherence time.Most established gate schemes using a common motional mode are slow (typical gate times are between 40 and 100 µs) and difficult to scale up since the motional spectrum becomes more dense with increasing ion number. Many new schemes have been proposed [11][12][13][14], with the fastest experimentally-achieved gate being 1.6 µs (99.8% fidelity) and 480 ns (60% fidelity) [15], realised by driving multiple motional modes simultaneously. Although the gate speed is not limited by the trap frequencies, the gate protocol requires the phase-space trajectories of all modes to close simultaneously at the end of the pulse sequence [15]. In long ion strings with a large number of vibrational modes, it becomes increasingly challenging to find and implement laser pulse parameters that execute this gate with a low error. Thus, a slow-down of gate speed appears inevitable.Two-qubit entangling gates in Rydberg atom systems are substantially faster, owing to strong dipole-dipole interactions. The gate fidelities in recent experiments using neutral atoms are fairly high [16,17]. However, the atom traps need to be turned off during Rydberg excitation. This can cause unwanted coupling between qubits and atom motion as well as atom loss [8,18]. Employing blue-detune...

Trapped Rydberg ions are a promising novel approach to quantum computing and simulations [1][2][3]. They are envisaged to combine the exquisite control of trapped ion qubits [4] with the fast two-qubit Rydberg gates already demonstrated in neutral atom experiments [5][6][7]. Coherent Rydberg excitation is a key requirement for these gates. Here, we carry out the first coherent Rydberg excitation of an ion and perform a single-qubit Rydberg gate, thus demonstrating basic elements of a trapped Rydberg ion quantum computer.Systems of trapped ion qubits have set numerous benchmarks for single-qubit preparation, manipulation, and readout [8]. They can perform low error entanglement operations [9,10] with up to 14 ion qubits [11]. Still, a major limitation towards realizing a large-scale trapped ion quantum computer or simulator is the scalability of entangling quantum logic gates [12].Arrays of neutral atoms in dipole traps offer another promising approach to quantum computation and simulation. Here, qubits are stored in electronically low-lying states and multi-qubit gates may be realized by exciting atoms to Rydberg states [6,7,13,14]. Rydberg states are exotic states of matter in which the valence electron is excited to high principal quantum numbers. They can have extremely high dipole moments and may interact strongly with each other, which has allowed entanglement generation [15,16] and fast two-qubit Rydberg gates [5] in neutral atom systems.A system of trapped Rydberg ions may combine the advantages of both technologies. Electronically lowlying states may be used as qubit states and fast multiqubit gates are envisaged by coherently exciting ions to Rydberg states and employing dipolar interactions between them [1,17]. Multi-qubit gates commonly used in trapped ion systems suffer scalability restrictions due to spectral crowding of motional modes [12]. This issue does not affect multi-qubit Rydberg gates thus a trapped Rydberg ion quantum computer offers an alternate approach to a scalable system. An unanswered question was whether trapped ions can be excited to Rydberg states in a coherent fashion as is required for multi-qubit Rydberg gates. In our experiment we study a single 88 Sr + ion confined in a linear Paul trap. Three atomic levels in a ladder configuration are coupled using two UV lasers (Fig. 1). The qubit state |0 is coupled to the excited state |e by the pump laser at 243 nm with Rabi frequency Ω P . |e is coupled in turn to the Rydberg state |r (42S 1/2 , m J = −1/2) using the Stokes laser at 307 nm with Rabi frequency Ω S . The experimental setup is described in detail in the Methods section and in [3].We can use the two-photon coupling for coherent control of the Rydberg excitation. At two-photon resonance (|0 to |r ) the coupling Hamiltonian has a "dark" eigenstate |Φ dark ∼ Ω S e iφ |0 − Ω P |r (Methods), which is named so because it does not contain any component of |0-|rRydberg excitation by STIRAP shown by comparing application of the single and the double STIRAP pulse sequences. The sin...

Trapped Rydberg ions are a promising new system for quantum information processing. They have the potential to join the precise quantum operations of trapped ions and the strong, longrange interactions between Rydberg atoms. Technically, the ion trap will need to stay active while exciting the ions into the Rydberg state, else the strong Coulomb repulsion will quickly push the ions apart. Thus, a thorough understanding of the trap effects on Rydberg ions is essential for future applications. Here we report the observation of two fundamental trap effects. First, we investigate the interaction of the Rydberg electron with the quadrupolar electric trapping field. This effect leads to Floquet sidebands in the spectroscopy of Rydberg D-states whereas Rydberg S-states are unaffected due to their symmetry. Second, we report on the modified trapping potential in the Rydberg state compared to the ground state which results from the strong polarizability of the Rydberg ion. We observe the resultant energy shifts as a line broadening which can be suppressed by cooling the ion to the motional ground state in the directions orthogonal to the excitation laser.PACS numbers: 32.80. Ee, 37.10.Ty, 32.60.+i Trapped ions are one of the most mature implementations of a quantum computer. The trapped ion approach has set several benchmarks with qubit lifetimes up to minutes [1], entanglement operations with error probabilities smaller than 10 −3 [2, 3], and with up to 14 entangled qubits [4]. Trapped ions also assume a leading role in the implementation of quantum algorithms [5][6][7][8], quantum error correction [9][10][11], and quantum simulations [12][13][14][15].The standard method to realize quantum information processing with trapped ions employs the common motion for entanglement operations between the ion qubits [16]. A current limitation of trapped ion quantum computation is the limited storage capacity as it becomes more difficult to perform entanglement operations in large ion crystals due to the increasingly complex motional mode structure. Possible schemes to reach larger quantum systems include segmented ion traps [17], ionphoton networks [18], and, trapped Rydberg ions [19,20].Trapped Rydberg ions are a novel quantum system. Here, the outermost electron of an ion is excited into Rydberg states far away from the atomic core. Due to large generated dipole moments, Rydberg ions are envisioned to sense each other by means of a dipolar interaction. The advantage of the Rydberg interaction is that it does not depend on the motional mode structure, thus it may be used in larger ion crystals for entanglement operations [19,21,22]. A similar entanglement method has been demonstrated with neutral atoms [23][24][25]. In this sense trapped Rydberg ions promise to join the advantages of both technologies: they combine the strong dipolar interaction between Rydberg atoms with the precise quantum control and long storage times of trapped ions.Recently, trapped Rydberg ions have been realized for the first time using a single-photon ...