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(A) Schematic of experimental setup. PBS1-6, polarizing beam splitters, λ/2, half waveplate, polarization state transformers, R s (θ s , φ s ) and R i (θ i , φ i ), (D1,D2,D3), single photon detectors, DM, dichroic mirror. The inset illustrates the timing of the write and read pulses. (B) The relevant atomic level structure.
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Matsukevich and Kuzmich (1) describe an interesting experiment directed toward the realization of scalable quantum networks through the protocol of (2). In particular, they demonstrate coherence between two atomic ensembles comprising two cylindrical volumes of cold rubidium atoms within a single magnetooptical trap. The authors claim that their me...
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Citations
... However, caution should be exercised in claiming entanglement for the state of the entire atomic system. The existence of higher-order terms, even with a much smaller probability, may hinder the creation of entanglement between the two atomic ensembles [3,19]. As a matter of fact, without altering the probability distribution of various numbers of atomic excitations in the current scheme, e.g. ...
... As a matter of fact, without altering the probability distribution of various numbers of atomic excitations in the current scheme, e.g. reducing the probability of multiple atomic excitations, it is impossible to achieve entanglement between the two atomic ensembles [3,19]. One effective way of altering the probability distribution is to make a projective measurement onto the single-excitation subspace, in a similar way as the DLCZ scheme. ...
In spontaneous Raman scattering in an atomic ensemble, a collective atomic spin wave is created in correlation with the Stokes field. When the Stokes photons from two or more such atomic ensembles are made indistinguishable, a ‘which-way’ collective atomic spin excitation is generated among the independent atomic ensembles. We demonstrate this phenomenon experimentally by reading out the atomic spin excitations and observing interference between the read-out beams. When a single-photon projective measurement is made on the indistinguishable Stokes photons, this simple scheme can be used to entangle independent atomic ensembles. Compared to other currently used methods, this scheme can be easily scaled up and has greater efficiency.
... One approach to realizing a polarization-qubit memory is to first transform the polarization encoding into path encoding and then store it into two distant ensembles. This approach has been implemented in both atomic ensembles [4] [5] [7] and atomic vapors [23]. The abovementioned realizations suffer from several practical drawbacks due to the extra step of spatially splitting the input state. ...
Faithfully storing an unknown quantum light state is essential to advanced
quantum communication and distributed quantum computation applications. The
required quantum memory must have high fidelity to improve the performance of a
quantum network. Here we report the reversible transfer of photonic
polarization states into collective atomic excitation in a compact solid-state
device. The quantum memory is based on an atomic frequency comb (AFC) in
rare-earth ion doped crystals. We obtain up to 0.998 process fidelity for the
storage and retrieval process of single-photon-level coherent pulse. This
reliable quantum memory is a crucial step toward quantum networks based on
solid-state devices.
... A subsequent pair of read pulses was followed by a polarization measurement of the signal photon, which exhibited Bell-type correlations with the idler. This work was criticized by van Enk and Kimble[84,85]for the postselected character of the measurement. An experiment in a similar setting proving the presence of entanglement without resorting to postselection was later reported by the same group[86]. ...
Quantum memory is important to quantum information processing in many ways: a synchronization device to match various processes within a quantum computer, an identity quantum gate that leaves any state unchanged, and a tool to convert heralded photons to photons-on-demand. In addition to quantum computing, quantum memory would be instrumental for the implementation of long-distance quantum communication using quantum repeaters. The importance of this basic quantum gate is exemplified by the multitude of optical quantum memory mechanisms being studied: optical delay lines, cavities, electromagnetically-induced transparency, photon-echo, and off-resonant Faraday interaction. Here we report on the state-of-the-art in the field of optical quantum memory, including criteria for successful quantum memory and current performance levels. Comment: This is the original version of the review paper published in Nature Photonics. The review reflects the state of the art in the field as of September 2009. The sections on DLCZ and off-resonant Faraday interaction, which were shortened in the Nature Photonics version due to space limitations, are presented here in their original form. v3: minor error fixed.
... Even if the uncertainty measure from the preceding Section would identify the presence of four-mode entanglement in the state ρ 1 , this does not guarantee that ρ W itself carries any entanglement. The standard counterexample [18] is a four-mode state of the (unnormalized) form ...
We construct a method for verifying mode entanglement of N-mode W states. The ideal W state contains exactly one excitation symmetrically shared between N modes, but our method takes the existence of higher numbers of excitations into account, as well as the vacuum state and other deviations from the ideal state. Moreover, our method distinguishes between full N-party entanglement and states with M-party entanglement with M<N, including mixtures of the latter. We specialize to the case N=4 for illustrative purposes. In the optical case, where excitations are photons, our method can be implemented using linear optics. Comment: 11 pages, 12 figures
... Two ensembles can also be correlated by joint projective measurements 3,6,14 . This has been used to generate coherence between two macro-atoms within a single atomic cloud 14 , although entanglement between the ensembles was not verified 21,22 . For two remote atomic ensembles, a similar projective measurement has been used to generate probabilistic, but heralded 23 , entanglement 6 . ...
Generation of non-classical correlations (or entanglement) between atoms, photons or combinations thereof is at the heart of quantum information science. Of particular interest are material systems serving as quantum memories that can be interconnected optically. An ensemble of atoms can store a quantum state in the form of a magnon-which is a quantized collective spin excitation-that can be mapped onto a photon with high efficiency. Here, we report the phase-coherent transfer of a single magnon from one atomic ensemble to another via an optical resonator serving as a quantum bus that in the ideal case is only virtually populated. Partial transfer deterministically creates an entangled state with one excitation jointly stored in the two ensembles. The entanglement is verified by mapping the magnons onto photons, whose correlations can be directly measured. These results should enable deterministic multipartite entanglement between atomic ensembles.
... If we assume equal transfer efficiencies at Site A, we find cos [11], quantum network protocols eliminate the vacuum component of Eq.(35) and only the entanglement characteristics of |ψ are relevant [109, 110]. In our experiment, atomic qubits were stored for a time 500 ns at Site A and 200 ns at Site B. It should be possible to extend the qubit storage times to longer than 10 µs, as the single-quanta storage results suggest [48]. ...
... Polarization correlations between the idler fields produced at the remote sites are recorded and analyzed for the presence of entanglement. The contributions of the vacuum and single photon idler excitations are excluded in the observed photoelectric coincidences between the remote sites [109, 110]. Since quantum state transfer is a local process, it cannot generate entanglement. ...
Quantum communication networks enable secure transmission of information between remote sites. However, at present, photon losses in the optical fiber limit communication distances to less than 150 kilometers. The quantum repeater idea allows extension of these distances. In practice, it involves the ability to store quantum information for a long time in atomic systems and coherently transfer quantum states between matter and light. Previously known schemes involved atomic Raman transitions in the UV or near-infrared and suffered from severe loss in optical fiber that precluded long-distance quantum communication. Ph.D. Committee Chair: Kuzmich, Alex; Committee Member: Chapman, Michael; Committee Member: Kennedy, Brian; Committee Member: Raman, Chandra; Committee Member: Voss, Paul
... Of course, the states in Eqs. 1-2 are idealizations that must be generalized to describe our actual experiment [6, 27, 29]. Specifically, the presence of various sources of noise necessarily transforms these pure states into mixed states. ...
A critical requirement for diverse applications in quantum information science is the capability to disseminate quantum resources over complex quantum networks. For example, the coherent distribution of entangled quantum states together with quantum memory (for storing the states) can enable scalable architectures for quantum computation, communication and metrology. Here we report observations of entanglement between two atomic ensembles located in distinct, spatially separated set-ups. Quantum interference in the detection of a photon emitted by one of the samples projects the otherwise independent ensembles into an entangled state with one joint excitation stored remotely in 10(5) atoms at each site. After a programmable delay, we confirm entanglement by mapping the state of the atoms to optical fields and measuring mutual coherences and photon statistics for these fields. We thereby determine a quantitative lower bound for the entanglement of the joint state of the ensembles. Our observations represent significant progress in the ability to distribute and store entangled quantum states.
In 1935 Einstein, Podolsky and Rosen used the assumption of local realism to conclude in a Gedankenexperiment with two entangled particles that quantum mechanics is not complete. For this reason EPR motivated an extension of quantum mechanics by so-called local hidden variables. Based on
this idea in 1964 Bell constructed a mathematical inequality whereby experimental tests could distinguish between quantum mechanics and local-realistic theories. Many experiments have since been done that are consistent with quantum mechanics, disproving the concept of local realism. But all these tests suffered from loopholes allowing a local-realistic explanation of the experimental observations by exploiting either the low detector efficiency or the fact that the detected particles were not observed space-like separated. In this context, of special interest is entanglement between different quantum objects like atoms and photons, because it allows one to entangle distant atoms by the interference of photons. The resulting space-like separation together with the almost perfect detection efficiency of the atoms allows a first loophole-free test of Bell's inequality.
The primary goal of the present thesis is the experimental realization of entanglement between a single localized atom and a single spontaneously emitted photon at a wavelength suitable for the transport over long distances. In the experiment a single optically trapped Rb87 atom is excited to a state which has two selected decay channels. In the following spontaneous decay a photon is emitted coherently with equal probability into both decay channels. This accounts for perfect correlations between the polarization state of the emitted photon and the Zeeman state of the atom
after spontaneous decay. Because these decay channels are spectrally and in all other degrees of freedom indistinguishable, the spin state of the atom is entangled with the polarization state of the photon. To verify entanglement, appropriate correlation measurements in complementary bases of the photon polarization and the internal quantum state of the atom are performed. It is shown, that the generated atom-photon state yields an entanglement fidelity of 0.82.
The experimental results of this work mark an important step towards the generation of entanglement between space-like separated atoms for a first loophole-free test of Bell's inequality. Furthermore entanglement between a single atom and a single photon is an important tool for new quantum
communication and information applications, e.g. the remote state preparation of a single atom over large distances.
We analyze the efficiency and scalability of the Duan-Lukin-Cirac-Zoller (DLCZ) protocol for quantum repeaters focusing on the behavior of the experimentally accessible measures of entanglement for the system, taking into account crucial imperfections of the stored entangled states. We calculate then the degradation of the final state of the quantum-repeater linear chain for increasing sizes of the chain, and characterize it by a lower bound on its concurrence and the ability to violate the Clausner-Horne-Shimony-Holt inequality. The states are calculated up to an arbitrary number of stored excitations, as this number is not fundamentally bound for experiments involving large atomic ensembles. The measurement by avalanche photodetectors is modeled by “ON/OFF” positive operator-valued measure operators. As a result, we are able to consistently test the approximation of the real fields by fields with a finite number of excitations, determining the minimum number of excitations required to achieve a desired precision in the prediction of the various measured quantities. This analysis finally determines the minimum purity of the initial state that is required to succeed in the protocol as the size of the chain increases. We also provide a more accurate estimate for the average time required to succeed in each step of the protocol. The minimum purity analysis and the new time estimates are then combined to trace the perspectives for implementation of the DLCZ protocol in present-day laboratory setups.
van Enk and Kimble criticize several aspects of our study but do not challenge our main result, the achievement of quantum state transfer between matter and light. Instead, their critique focuses on the quantitative amount of entanglement present in our experiment and how, the vacuum should be accounted for in these measures, both in our experiment and in others. Although a careful discussion of this topic has some value for the field, it does not alter the conclusions of our paper.
