Improved readout of qubit-coupled Gottesman-Kitaev-Preskill states

The Gottesman-Kitaev-Preskill encoding of a qubit in a harmonic oscillator is a promising building block towards fault-tolerant quantum computation. Recently, this encoding was experimentally demonstrated for the first time in trapped-ion and superconducting circuit systems. However, these systems lack some of the Gaussian operations which are critical to efficiently manipulate the encoded qubits. In particular, homodyne detection, which is the preferred method for readout of the encoded qubit, is not readily available, heavily limiting the readout fidelity. Here, we present an alternative read-out strategy designed for qubit-coupled systems. Our method can improve the readout fidelity with several orders of magnitude for such systems and, surprisingly, even surpass the fidelity of homodyne detection in the low squeezing regime.Comment: 5 pages, 2 figure

Discrimination of binary coherent states using a homodyne detector and a photon number resolving detector

We investigate quantum measurement strategies capable of discriminating two coherent states probabilistically with significantly smaller error probabilities than can be obtained using non- probabilistic state discrimination. We apply a postselection strategy to the measurement data of a homodyne detector as well as a photon number resolving detector in order to lower the error probability. We compare the two different receivers with an optimal intermediate measurement scheme where the error rate is minimized for a fixed rate of inconclusive results. The photon number resolving (PNR) receiver is experimentally demonstrated and compared to an experimental realization of a homodyne receiver with postselection. In the comparison it becomes clear, that the perfromance of the new PNR receiver surpasses the performance of the homodyne receiver, which we proof to be optimal within any Gaussian operations and conditional dynamics.Comment: 10 pages, 6 figure

Heralded generation of a micro-macro entangled state

Using different optical setups based on squeezed state and photon subtraction we show how optical entanglement between a macroscopic and a microscopic state - the so-called Schrodinger cat state or micro-macro state - can be generated. The entangled state is heralded and is thus produced a priori in contrast to previous proposals. We define the macroscopicity of the macroscopic part of the state as their mean distance in phase space and the success rate in discriminating them with homodyne detection, and subsequently, based on these measures we investigate the macroscopicity of different states. Furthermore, we show that the state can be used to map a microscopic qubit onto a macroscopic one thereby linking a qubit processor with a qumode processor

Correction: Jacobsen, C.S., et al. Continuous Variable Quantum Key Distribution with a Noisy Laser. Entropy 2015, 17, 4654–4663

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Controlled Coupling of a Single Nitrogen-Vacancy Center to a Silver Nanowire

We report on the controlled coupling of a single nitrogen-vacancy (NV) center to a surface plasmon mode propagating along a chemically grown silver nanowire (NW). We locate and optically characterize a single NV center in a uniform dielectric environment before we controllably position this emitter in the close proximity of the NW. We are thus able to control the coupling of this particular emitter to the NW and directly compare the photon emission properties before and after the coupling. The excitation of single plasmonic modes is witnessed and a total rate enhancement by a factor of up to 4.6 is demonstrated