Solid state optical interconnect between distant superconducting quantum chips

We propose a design for a quantum interface exploiting the electron spins in crystals to swap the quantum states between the optical and microwave. Using sideband driving of a superconducting flux qubit and a combined cavity/solid-state spin ensemble Raman transition, we demonstrate how a stimulated Raman adiabatic passage (STIRAP)-type operation can swap the quantum state between a superconducting flux qubit and an optical cavity mode with a fidelity higher than $90\%$. We further consider two distant superconducting qubits with their respective interfaces joined by an optical fiber and show a quantum transfer fidelity exceeding $90\%$ between the two distant qubits.Comment: 5 figures, 5 page

Detection of a weak magnetic field via cavity enhanced Faraday rotation

We study the sensitive detection of a weak static magnetic field via Faraday rotation induced by an ensemble of spins in a bimodal degenerate microwave cavity. We determine the limit of the resolution for the sensitivity of the magnetometry achieved using either single-photon or multiphoton inputs. For the case of a microwave cavity containing an ensemble of Nitrogen-vacancy defects in diamond, we obtain a magnetometry sensitivity exceeding 0.5~\text{\nano\tesla}/\sqrt{\text{\hertz}}, utilizing a single photon probe field, while for a multiphoton input we achieve a sensitivity about 1 \text{\femto\tesla}/\sqrt{\text{\hertz}}, using a coherent probe microwave field with power of P_\text{in}=1~\text{\nano\watt}.Comment: 8 pages, 7 figure

Optimised control of Stark-shift-chirped rapid-adiabatic-passage in a lambda-type three-level system

Inhomogeneous broadening of energy levels is one of the principal limiting factors for achieving "slow" or "stationary" light in solid state media by means of electromagnetically induced transparency (EIT), a quantum version of stimulated Raman adiabatic passage (STIRAP). Stark-shift-chirped rapid-adiabatic-passage (SCRAP) has been shown to be far less sensitive to inhomogeneous broadening than STIRAP, a population transfer technique to which it is closely related. We further optimise the pulses used in SCRAP to be even less sensitive to inhomogeneous broadening in a lambda-type three-level system. The optimised pulses perform at a higher fidelity than the standard gaussian pulses for a wide range of detunings (i.e. large inhomogeneous broadening).Comment: 6 pages, 6 figures, 1 tabl

Quantum switching networks for perfect qubit routing

We develop the work of Christandl et al. [M. Christandl, N. Datta, T. C. Dorlas, A. Ekert, A. Kay, and A. J. Landahl, Phys. Rev. A 71, 032312 (2005)], to show how a d-hypercube homogenous network can be dressed by additional links to perfectly route quantum information between any given input and output nodes in a duration which is independent of the routing chosen and, surprisingly, size of the network

Cavity-free nondestructive detection of a single optical photon

Detecting a single photon without absorbing it is a long standing challenge in quantum optics. All experiments demonstrating the nondestructive detection of a photon make use of a high quality cavity. We present a cavity free scheme for nondestructive single-photon detection. By pumping a nonlinear medium we implement an inter-field Rabi-oscillation which leads to a ?pi phase shift on weak probe coherent laser field in the presence of a single signal photon without destroying the signal photon. Our cavity-free scheme operates with a fast intrinsic time scale in comparison with similar cavity-based schemes. We implement a full real-space multimode numerical analysis of the interacting photonic modes and confirm the validity of our nondestructive scheme in the multimode case.Comment: 4 figures, 5 page

Unambiguous discrimination among oracle operators

We address the problem of unambiguous discrimination among oracle operators. The general theory of unambiguous discrimination among unitary operators is extended with this application in mind. We prove that entanglement with an ancilla cannot assist any discrimination strategy for commuting unitary operators. We also obtain a simple, practical test for the unambiguous distinguishability of an arbitrary set of unitary operators on a given system. Using this result, we prove that the unambiguous distinguishability criterion is the same for both standard and minimal oracle operators. We then show that, except in certain trivial cases, unambiguous discrimination among all standard oracle operators corresponding to integer functions with fixed domain and range is impossible. However, we find that it is possible to unambiguously discriminate among the Grover oracle operators corresponding to an arbitrarily large unsorted database. The unambiguous distinguishability of standard oracle operators corresponding to totally indistinguishable functions, which possess a strong form of classical indistinguishability, is analysed. We prove that these operators are not unambiguously distinguishable for any finite set of totally indistinguishable functions on a Boolean domain and with arbitrary fixed range. Sets of such functions on a larger domain can have unambiguously distinguishable standard oracle operators and we provide a complete analysis of the simplest case, that of four functions. We also examine the possibility of unambiguous oracle operator discrimination with multiple parallel calls and investigate an intriguing unitary superoperator transformation between standard and entanglement-assisted minimal oracle operators.Comment: 35 pages. Final version. To appear in J. Phys. A: Math. & Theo