Integrated optical waveplates for arbitrary operations on polarization-encoded single-qubits

Integrated photonic technologies applied to quantum optics have recently enabled a wealth of breakthrough experiments in several quantum information areas. Path encoding was initially used to demonstrate operations on single or multiple qubits. However, a polarization encoding approach is often simpler and more effective. Two-qubits integrated logic gates as well as complex interferometric structures have been successfully demonstrated exploiting polarization encoding in femtosecond-laser-written photonic circuits. Still, integrated devices performing single-qubit rotations are missing. Here we demonstrate waveguide-based waveplates, fabricated by femtosecond laser pulses, capable to effectively produce arbitrary single-qubit operations in the polarization encoding. By exploiting these novel components we fabricate and test a compact device for the quantum state tomography of two polarization-entangled photons. The integrated optical waveplates complete the toolbox required for a full manipulation of polarization-encoded qubits on-chip, disclosing new scenarios for integrated quantum computation, sensing and simulation, and possibly finding application also in standard photonic devices

Szilard Engine Reversibility as Quantum Gate Function

A quantum gate is a logically and thermodynamically reversible situation that effects a unitary transformation of qubits of superimposed information, and essentially constitutes a situation for a reversible quantum decision. A quantum decision is a symmetry break, and the effect of the function of a Szilard engine is a symmetry break. A quantum gate is a situation in which a reversible quantum decision can be made, and so if a logically and thermodynamically reversible Szilard engine can be theoretically constructed then it would function as a quantum gate. While the traditionally theorized Szilard engine is not thermodynamically reversible, if one of the bounding walls of a Szilard engine were to be constructed out of the physical information by which it functions in such a manner as to make that information available to both sides of the wall simultaneously, then such a Szilard engine would be both logically and thermodynamically reversible, and thus capable of function as a quantum gate. A theoretical model of the special case of a reversible Szilard engine functioning as a quantum gate is presented and discussed, and since a quantum decision is made when the shutter of a Szilard engine closes, the coherence of linked reversible Szilard engines should be considered as a state during which all of the shutters of linked Szilard engines are open simultaneously

Creation, storage, and on-demand release of optical quantum states with a negative Wigner function

Highly nonclassical quantum states of light, characterized by Wigner functions with negative values, have been created so far only in a heralded fashion. In this case, the desired output emerges rarely and randomly from a quantum-state generator. An important example is the heralded production of high-purity single-photon states, typically based on some nonlinear optical interaction. In contrast, on-demand single-photon sources were also reported, exploiting the quantized level structure of matter systems. These sources, however, lead to highly impure output states, composed mostly of vacuum. While such impure states may still exhibit certain single-photon-like features such as anti-bunching, they are not enough nonclassical for advanced quantum information processing. On the other hand, the intrinsic randomness of pure, heralded states can be circumvented by first storing and then releasing them on demand. Here we propose such a controlled release, and we experimentally demonstrate it for heralded single photons. We employ two optical cavities, where the photons are both created and stored inside one cavity, and finally released through a dynamical tuning of the other cavity. We demonstrate storage times of up to 300 ns, while keeping the single-photon purity around 50% after storage. This is the first demonstration of a negative Wigner function at the output of an on-demand photon source or a quantum memory. In principle, our storage system is compatible with all kinds of nonclassical states, including those known to be essential for many advanced quantum information protocols.Comment: 14 pages, 5 figure

Research on nonlinear and quantum optics at the photonics and quantum information group of the University of Valladolid

We outline the main research lines in Nonlinear and Quantum Optics of the Group of Photonics and Quantum Information at the University of Valladolid. These works focus on Optical Solitons, Quantum Information using Photonic Technologies and the development of new materials for Nonlinar Optics. The investigations on optical solitons cover both temporal solitons in dispersion managed fiber links and nonparaxial spatial solitons as described by the Nonlinear Helmholtz Equation. Within the Quantum Information research lines of the group, the studies address new photonic schemes for quantum computation and the multiplexing of quantum data. The investigations of the group are, to a large extent, based on intensive and parallel computations. Some associated numerical techniques for the development of the activities described are briefly sketched

High Speed Control of Atom Transfer Sequence from Magneto-Optical to Dipole Trap for Quantum Computing

Two circuits were designed, built, and tested for the purpose of aiding in the transfer of 87Rb atoms from a MOT to dipole traps and for characterizing the final dipole traps. The first circuit was a current switch designed to quickly turn the magnetic fields of the MOT off. The magnetic coil switch was able to reduce the magnetic field intensity to 5 % of its initial value after 81 μs. The second circuit was an analog signal switch designed to turn the modulation signal of an AOM off. The analog switch was able to reduce the modulation signal intensity to 8.5 % of its initial magnitude in ~25μs. The performances of the two circuits discussed in this paper are sufficient for future atom transfer research based on predicted experimental requirements

Real-time capable individual-ion qubit measurement

Scaling up the number of qubits for quantum simulation and quantum computation and reaching a low error rate of the involved quantum logic operations are the major challenges in the development of a fault-tolerant universal quantum computer. Trapped ions in surface-electrode traps are a promising candidate that could satisfy both criteria. These traps can be extended into a two-dimensional array, a so-called quantum charge-coupled device, in which the ions can be moved around to different zones that fulfill a specific task. In this way an architecture can be developed that could perform multiple quantum-logic operations with many ions simultaneously. These operations were first implemented with laser beams and have evolved in the past years to a point where they are almost reaching the threshold for fault tolerance. However, this approach is fundamentally limited by spontaneous emission and is hard to scale up to a large number of qubits. An alternative approach using microwave radiation can overcome these problems. The microwave conductors can be integrated into the surface-electrode traps and therefore feature the same scalability as the trap itself. A small distance between the ions and the microwave conductors in the trap-surface is desirable to reach a strong field to drive the quantum logic operations. A downside of a reduced distance is an increased motional heating rate of the ions. To counteract this effect, the trap can be cooled down to cryogenic temperatures. Cooling the trap and its surrounding also helps to achieve excellent vacuum conditions that are required to reduce the collision rate of the ions with background gas molecules. These collisions would be fatal during a sequence of quantum logic operations. Another important factor for the operation of a quantum simulator or quantum computer is the ability to prepare the ions in a known state and to detect the state of the ions. In this thesis, we explain how an EMCCD camera can be used as a spatially resolving detector to readout the state of each ion simultaneously and we discuss the benefits and limitations of this technique. We could show that the combined error-rate of state preparation and camera-based detection is on the order of 0.4% which is comparable with a photomultiplier-based detection for a single ion. We also demonstrated that the camera-based detection outperforms the photomultiplier when the state of two ions should be detected. Here we determined the amount of crosstalk between two ions to be so low that the error-rate is basically independent of the number of simultaneously detected qubits. We also discuss options for future improvements of the state preparation and detection system to further reduce the error rate