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arXiv:1707.02837 · Quantum Science and Technology

Frequency-bin entanglement of ultra-narrow band non-degenerate photon pairs

Daniel Rieländer, Andreas Lenhard, Osvaldo Jime`nez Farìas, Alejandro Máttar, Daniel Cavalcanti, Margherita Mazzera, Antonio Acín, Hugues de Riedmatten

Published Nov 2, 2017• 25 citations
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Abstract

Abstract We demonstrate frequency-bin entanglement between ultra-narrowband photons generated by cavity enhanced spontaneous parametric down conversion. Our source generates photon pairs in widely non-degenerate discrete frequency modes, with one photon resonant with a quantum memory material based on praseodymium doped crystals and the other photon at telecom wavelengths. Correlations between the frequency modes are analyzed using phase modulators and narrowband filters before detection. We show high-visibility two photon interference between the frequency modes, allowing us to infer a coherent superposition of the modes. We develop a model describing the state that we create and use it to estimate optimal measurements to achieve a violation of the Clauser–Horne (CH) Bell inequality under realistic assumptions. With these settings we perform a Bell test and show a significant violation of the CH inequality, thus proving the entanglement of the photons. Finally we demonstrate the compatibility with a quantum memory material by using a spectral hole in the praseodymium (Pr) doped crystal as spectral filter for measuring high-visibility two-photon interference. This demonstrates the feasibility of combining frequency-bin entangled photon pairs with Pr-based solid state quantum memories.

DOI
10.1088/2058-9565/aa97b6

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Abstract We demonstrate frequency-bin entanglement between ultra-narrowband photons generated by cavity enhanced spontaneous parametric down conversion. Our source generates photon pairs in widely non-degenerate discrete frequency modes, with one photon resonant with a quantum memory material based on praseodymium doped crystals and the other photon at telecom wavelengths. Correlations between the frequency modes are analyzed using phase modulators and narrowband filters before detection. We show high-visibility two photon interference between the frequency modes, allowing us to infer a coherent superposition of the modes. We develop a model describing the state that we create and use it to estimate optimal measurements to achieve a violation of the Clauser–Horne (CH) Bell inequality under realistic assumptions. With these settings we perform a Bell test and show a significant violation of the CH inequality, thus proving the entanglement of the photons. Finally we demonstrate the compatibility with a quantum memory material by using a spectral hole in the praseodymium (Pr) doped crystal as spectral filter for measuring high-visibility two-photon interference. This demonstrates the feasibility of combining frequency-bin entangled photon pairs with Pr-based solid state quantum memories.
We present a source of ultra-narrow band, widely non degenerate photon pairs source compatible with quantum repeaters architectures. One photon is compatible with a solid state Praseodymium (Pr3+) doped crystal spin wave quantum memory, and the other photon is at telecommunication wavelengths, such that it can be transmitted over large distances in optical fibers. Pr3+doped solids absorbing photons at 606 nm have shown very promising properties as QM for light. Despite this fact, so far no experiment demonstrated storage of quantum light in Pr3+doped crystals. The main challenge to realize such a source is that the bandwidth of the memory is limited to a few MHz by the excited state spacing of the ions. To achieve the required narrow linewidth we use widely non-degenerate cavity enhanced spontaneous down conversion (SPDC). We measure a correlation time of 104 ns, the longest reported to date for SPDC emitted photons, corresponding to around 2 MHz bandwidth. In addition, the number of longitudinal modes emitted by the source is strongly reduced, thanks to a clustering effect, which facilitates the filtering to reach single mode operation.
A monochromatic laser pumping a parametric down-conversion crystal generates frequency-entangled photon pairs. We study this experimentally by addressing such frequency-entangled photons at telecommunication wavelengths (around 155015501550 nm) with fiber-optics components such as electro-optic phase modulators and narrow-band frequency filters. The theory underlying our approach uses the notion of frequency-bin entanglement. Our results show that the phase modulators address coherently up to eleven frequency bins, leading to an interference pattern which can violate by more than five standard deviations a Bell inequality adapted to our setup.
Multiplexing is essential for improving entanglement distribution rates in quantum communication. Frequency multiplexing provides a promising and scalable path toward large-capacity quantum networks. Further progress requires increasing the number of frequency modes and developing broadband photon-pair sources and quantum memories that are spectrally compatible. Here, we report the integration of a cavity-enhanced spontaneous parametric down-conversion source in the telecom C-band with a frequency-multiplexed atomic frequency comb memory. The bow-tie cavity source was simultaneously resonant at 606 nm and 1550 nm, generating non-degenerate photon pairs exhibiting a clustered frequency-comb spectrum. The atomic frequency comb memory, implemented in Praseodymium-doped Yttrium Orthosilicate crystals, provided up to 83 frequency modes with 123 MHz spacing and enabled broadband storage of 606 nm signal photons. By filtering the main cluster, we obtained 32.7±4.832.7 \pm 4.832.7±4.8 effective modes, as confirmed from coincidence measurements. Importantly, we observed strong nonclassical correlations after storage, with cross-correlation values of gs,i(2)=8.1±0.7g_{s,i}^{(2)} = 8.1\pm0.7gs,i(2)​=8.1±0.7. Our experimental results demonstrate the feasibility of integrating cavity-enhanced photon-pair sources with rare-earth-ion-doped solid-state memories. The integration reveals a high frequency multiplicity that is essential for scalable quantum networks.
We report on quantum frequency conversion of memory-compatible narrow-bandwidth photons at 606 nm to the telecom C-band at 1552 \,nm. The 200 \,ns long photons, compatible with Praseodymium-based solid-state quantum memories are frequency converted using a single-step difference frequency generation process in a periodically poled Lithium Niobate waveguide. We characterize the noise processes involved in the conversion and by applying strong spectral filtering of the noise, we demonstrate high signal-to-noise ratio conversion at the single photon level (SNR$\,>\,100forameaninputphotonnumberperpulseof1).Wefinallyobservethatamemorycompatibleheraldedsinglephotonwithabandwidthof1.8100 for a mean input photon number per pulse of 1). We finally observe that a memory compatible heralded single photon with a bandwidth of 1.8100forameaninputphotonnumberperpulseof1).Wefinallyobservethatamemorycompatibleheraldedsinglephotonwithabandwidthof1.8\,$MHz, obtained from a spontaneous parametric down conversion pair source, still shows a strong non-classical behavior after conversion. We first demonstrate that correlations between heralding and converted heralded photons stay in the non-classical regime. Moreover, we measured the heralded autocorrelation function of the heralded photon using the converter device as a frequency-domain beam splitter, yielding a value of 0.19±0.070.19\pm0.070.19±0.07. The presented work represents a step towards the connection of several quantum memory systems emitting narrow-band visible photons to the telecommunication wavelengths.
Summary form only given. The ability to transfer entanglement over large distance is a main requirement for quantum networks. Another fundamental ingredient is the storage of such a quantum state in a quantum memory. To this end a useful resource is a photon pair for which one photon is resonant with the memory while its partner is at a low-loss telecommunication wavelength. We recently presented a photon pair source, compatible with a Pr:YSO solid state memory [1]. The photons have a narrow bandwidth for compatibility with the memory transition. Generating and transmitting qubits based on such photons is not straight forward, as usual implementations as e.g. polarisation qubits suffer distortion in fibres and time-bin qubits on the other hand require long interferometers for qubit analysis due to the long coherence time. Here we report on the implementation of frequency-bin entanglement [2, 3] which has neither of these drawbacks.
Entanglement between photons at telecommunication wavelengths and long-lived quantum memories is one of the fundamental requirements of long-distance quantum communication. Quantum memories featuring on-demand readout and multimode operation are additional precious assets that will benefit the communication rate. In this Letter, we report the first demonstration of entanglement between a telecom photon and a collective spin excitation in a multimode solid-state quantum memory. Photon pairs are generated through widely nondegenerate parametric down-conversion, featuring energy-time entanglement between the telecom-wavelength idler and a visible signal photon. The latter is stored in a Pr^{3+}:Y_{2}SiO_{5} crystal as a spin wave using the full atomic frequency comb scheme. We then recall the stored signal photon and analyze the entanglement using the Franson scheme. We measure conditional fidelities of 92(2)% for excited-state storage, enough to violate a Clauser-Horne-Shimony-Holt inequality, and 77(2)% for spin-wave storage. Taking advantage of the on-demand readout from the spin state, we extend the entanglement storage in the quantum memory for up to 47.7 μs, which could allow for the distribution of entanglement between quantum nodes separated by distances of up to 10 km.