Entanglement is considered to be one of the most profound features of quantum mechanics 1,2 . An entangled state of a system consisting of two subsystems cannot be described as a product of the quantum states of the two subsystems 9,10,16,17 . In this sense the entangled system is considered inseparable and nonlocal. It is generally believed that entanglement manifests itself mostly in systems consisting of a small number of microscopic particles. Here we demonstrate experimentally the entanglement of two objects, each consisting of about 10 12 atoms. Entanglement is generated via interaction of the two objects -more precisely, two gas samples of cesium atoms -with a pulse of light, which performs a non-local Bell measurement on collective spins of the samples 14 . The entangled spin state can be maintained for 0.5 millisecond. Besides being of fundamental interest, the robust, long-lived entanglement of material objects demonstrated here is expected to be useful in quantum information processing, including teleportation 3-5 of quantum states of matter and quantum memory. In this Letter we describe an experiment on the generation of entanglement between two separate samples of atoms containing 10 12 atoms each, along the lines of a recent proposal 14 . Besides the fact that we demonstrate a quantum entanglement at the level of macroscopic objects, our experiment proves feasible a new approach to the quantum interface between light and atoms suggested in 14,15 and paves the road towards the other protocols proposed there, such as the teleportation of atomic states and quantum memory. The entanglement is generated through a non-local Bell measurement on the two samples' spins performed by transmitting a pulse of light through the samples.The ideal EPR entangled state of two sub-systems described by continuous non- . Recently in 16,17 , the necessary and sufficient condition for the entanglement or inseparability for such Gaussian quantum variables has been cast in a form of an inequality involving only the variances of variables:

The information carrier of today's communications, a weak pulse of light, is an intrinsically quantum object. As a consequence, complete information about the pulse cannot, even in principle, be perfectly recorded in a classical memory. In the field of quantum information this has led to a long standing challenge: how to achieve a high-fidelity transfer of an independently prepared quantum state of light onto the atomic quantum state 1-4 ? Here we propose and experimentally demonstrate a protocol for such quantum memory based on atomic ensembles. We demonstrate for the first time a recording of an externally provided quantum state of light onto the atomic quantum memory with a fidelity up to 70%, significantly

Quantum teleportation is an important ingredient in distributed quantum networks, and can also serve as an elementary operation in quantum computers. Teleportation was first demonstrated as a transfer of a quantum state of light onto another light beam; later developments used optical relays and demonstrated entanglement swapping for continuous variables. The teleportation of a quantum state between two single material particles (trapped ions) has now also been achieved. Here we demonstrate teleportation between objects of a different nature--light and matter, which respectively represent 'flying' and 'stationary' media. A quantum state encoded in a light pulse is teleported onto a macroscopic object (an atomic ensemble containing 10 caesium atoms). Deterministic teleportation is achieved for sets of coherent states with mean photon number (n) up to a few hundred. The fidelities are 0.58 +/- 0.02 for n = 20 and 0.60 +/- 0.02 for n = 5--higher than any classical state transfer can possibly achieve. Besides being of fundamental interest, teleportation using a macroscopic atomic ensemble is relevant for the practical implementation of a quantum repeater. An important factor for the implementation of quantum networks is the teleportation distance between transmitter and receiver; this is 0.5 metres in the present experiment. As our experiment uses propagating light to achieve the entanglement of light and atoms required for teleportation, the present approach should be scalable to longer distances.

We present time-resolved spontaneous emission measurements of single quantum dots embedded in photonic crystal waveguides. Quantum dots that couple to a photonic crystal waveguide are found to decay up to 27 times faster than uncoupled quantum dots. From these measurements beta-factors of up to 0.89 are derived, and an unprecedented large bandwidth of 20 nm is demonstrated. This shows the promising potential of photonic crystal waveguides for efficient single-photon sources. The scaled frequency range over which the enhancement is observed is in excellent agreement with recent theoretical proposals taking into account that the light-matter coupling is strongly enhanced due to the significant slow-down of light in the photonic crystal waveguides.

1 arXiv:1507.06831v1 [quant-ph] Jul 2015The detection and characterization of paramagnetic species by electron-spin resonance (ESR) spectroscopy is widely used throughout chemistry, biology, and materials science [1], from in-vivo imaging [2] to distance measurements in spinlabeled proteins [3]. ESR typically relies on the inductive detection of microwave signals emitted by the spins into a coupled microwave resonator during their Larmor precession -however, such signals can be very small, prohibiting the application of ESR at the nanoscale, for example, at the single-cell level or on individual nanoparticles. In this work, using a Josephson parametric microwave amplifier combined with high-quality factor superconducting micro-resonators cooled at millikelvin temperatures, we improve the state-of-the-art sensitivity of inductive ESR detection by nearly 4 orders of magnitude. We demonstrate the detection of 1700 bismuth donor spins in silicon within a single Hahn [4] echo with unit signal-to-noise (SNR) ratio, reduced to just 150 spins by averaging a single Carr-Purcell-Meiboom-Gill sequence [5]. This unprecedented sensitivity reaches the limit set by quantum fluctuations of the electromagnetic field instead of thermal or technical noise, which constitutes a novel regime for magnetic resonance. The detection volume of our resonator is ∼0.02 nl, and our approach can be readily scaled down further to improve sensitivity, providing a new and versatile toolbox for ESR at the nanoscale.A wide variety of techniques are being actively explore to push the limits of sensitivity of ESR to the nanoscale, including approaches based on optical [6,7] or electrical [8,9] detection, as well as scanning probe methods [10,11]. Our focus in this work is to maximise the sensitivity of inductively detected pulsed ESR, in order to maintain the broad applicability to different spin species as well as fast high-bandwidth detection. Pulsed ESR spectroscopy proceeds by probing a sample coupled to a microwave resonator of frequency ω 0 and quality factor Q with sequences of microwave pulses that perform successive spin rotations, triggering the emission of a microwave signal called a spin-echo whose amplitude and shape contain the desired information about the number and properties of paramagnetic species. The spectrometer sensitivity is conveniently quantified by the minimal number of spins N min that can be detected within a single echo [4]. Conventional ESR spectrometers use 3D resonators with moderate quality factors in which the spins are only weakly coupled to the microwave photons and thus obtain a sensitivity of N min ∼ 10 13 spins at T = 300 K and X-band fre-2 quencies (ω 0 /2π ∼ 9 − 10 GHz). To increase the sensitivity, micro-fabricated metallic planar resonators with smaller mode volumes have been used, resulting in larger spin-microwave coupling [12,13]. Combined with operation at T = 4 K and the use of low-noise cryogenic amplifiers and superconducting high-Q thin-film resonators, sensitivities up to N min ∼ 10 7 spins have ...

A density matrix ρ(t) yields probabilistic information about the outcome of measurements on a quantum system. We introduce here the past quantum state, which, at time T, accounts for the state of a quantum system at earlier times t

We propose a multi-mode quantum memory protocol able to store the quantum state of the field in a microwave resonator into an ensemble of electronic spins. The stored information is protected against inhomogeneous broadening of the spin ensemble by spin-echo techniques resulting in memory times orders of magnitude longer than previously achieved. By calculating the evolution of the first and second moments of the spin-cavity system variables for realistic experimental parameters, we show that a memory based on NV center spins in diamond can store a qubit encoded on the |0 and |1 Fock states of the field with 80% fidelity.PACS numbers: 03.67. Lx, 42.50.Ct, 42.50.Pq Ensembles of electronic spins have been proposed as quantum memories in hybrid architectures for quantum computing including superconducting qubits [1][2][3][4]. Progress in this direction was reported in a number of experiments, demonstrating first strong coupling of an ensemble of spins in a crystal to a superconducting resonator [5][6][7][8][9][10][11], and more recently reversible storage of a single microwave photon in the spin ensemble [12,13]. From these results it clearly appears that inhomogeneous broadening of the spin ensemble is a major obstacle, which needs to be overcome for hybrid quantum circuits to fully benefit from the long spin-coherence times. Due to inhomogeneous broadening, quantum information leaks from the "bright" collective degree of freedom coupled to the cavity into dark modes of the spin ensemble [14][15][16]. An appealing possibility is to actively and coherently restore it using refocusing techniques, inspired from magnetic-resonance methods [17] and based on the application of π pulses to the spins acting as time reversal. However, these ideas face a number of challenges: (i) The spatial inhomogeneity of the microwave resonator field may make it difficult to apply a π pulse efficiently to each spin, (ii) after the π-pulse inversion, the spin ensemble should remain stable despite its coupling to the cavity, and (iii) the whole statistics of the collective spin must be restored at the single quantum level. The present work proposes a protocol, which addresses all these issues, and we exemplify its feasibility for the specific case of NV centers in diamond [18], using currently available experimental techniques and realistic parameters. The proposed memory extends the storage times by several orders of magnitude compared to [12,13]. It is intrinsically multi-mode and thus allows to store reversibly a number of quantum states, paving the way to the realization of a genuine quantum Turing machine [2,19].In our proposal the π pulses are performed by rapid adiabatic passage [20] through the electron spin resonance, a method known to tolerate an inhomogeneous microwave field. Stability of the ensemble after inversion is ensured provided the cavity quality factor is sufficiently low [21]. Since this is incompatible with a faithful transfer of quantum information from the cavity into the spins, we propose to use a cavity with ...

International audienceA quantum memory at microwave frequencies, able to store the state of multiple superconducting qubits for long times, is a key element for quantum information processing. Electronic and nuclear spins are natural candidates for the storage medium as their coherence time can be well above 1 s. Benefiting from these long coherence times requires one to apply the refocusing techniques used in magnetic resonance, a major challenge in the context of hybrid quantum circuits. Here, we report the first implementation of such a scheme, using ensembles of nitrogen-vacancy centers in diamond coupled to a superconducting resonator, in a setup compatible with superconducting qubit technology. We implement the active reset of the nitrogen-vacancy spins into their ground state by optical pumping and their refocusing by Hahn-echo sequences. This enables the storage of multiple microwave pulses at the picowatt level and their retrieval after up to 35 μs, a 3 orders of magnitude improvement compared to previous experiments