Light-matter entanglement between telecom photons and solid-state quantum memories

We demonstrate the storage of light-matter entanglement between a telecom photon and an integrated solid-state quantum memory. The quantum memory is a fibre-coupled waveguide written in a rare-earth doped crystal.

We demonstrate the storage of light-matter entanglement between a telecom photon and an integrated solid-state quantum memory. The quantum memory is a fibre-coupled waveguide written in a rare-earth doped crystal.

Quantum memory is a fundamental building block for large-scale quantum networks. On-demand optical storage with a large bandwidth, a high multimode capacity and an integrated structure simultaneously is crucial for practical application. However, this has not been demonstrated yet. Here, we fabricate an on-chip waveguide in a $\mathrm {^{151}Eu^{3+}:Y_2SiO_5}$ crystal with insertion losses of 0.2 dB, and propose a novel pumping scheme to enable spin-wave atomic frequency comb (AFC) storage with a bandwidth of 11 MHz inside the waveguide. Based on this, we demonstrate the storage of 200 temporal modes using the AFC scheme and conditional on-demand storage of 100 temporal modes using the spin-wave AFC scheme. The interference visibility between the readout light field and the reference light field is $99.0\% \pm 0.6\%$ and $97\% \pm 3\%$ for AFC and spin-wave AFC storage, respectively, indicating the coherent nature of this low-loss, multimode and integrated storage device.

An efficient multimode quantum memory is a crucial resource for long-distance quantum communication based on quantum repeaters. We propose a quantum memory based on spectral shaping of an inhomogeneously broadened optical transition into an atomic frequency comb (AFC). The spectral width of the AFC allows efficient storage of multiple temporal modes without the need to increase the absorption depth of the storage material, in contrast to previously known quantum memories. Efficient readout is possible thanks to rephasing of the atomic dipoles due to the AFC structure. Long-time storage and on-demand readout is achieved by use of spin states in a lambda-type configuration. We show that an AFC quantum memory realized in solids doped with rare-earth-metal ions could store hundreds of modes or more with close to unit efficiency, for material parameters achievable today.

In this paper, we propose a slow light-enhanced quantum memory with high efficiency based on atomic frequency comb (AFC) in ion-doped photonic crystal waveguide (PCW). The performance of the quantum memory is investigated theoretically, considering the impact of the signal bandwidth. Both the forward and backward retrieval schemes are analyzed. In the forward retrieval scheme, the analysis shows that a moderate slow light effect can improve the retrieval efficiency to above 50% with very high fidelity, even when the intrinsic optical depth is very low and the signal bandwidth is comparable with the AFC bandwidth. In the backward retrieval scheme, retrieval efficiency larger than 90% can be obtained and fidelity can remain above 90% for signal with bandwidth much narrower than AFC bandwidth, when moderate slow light is introduced into waveguide with low intrinsic optical depth. Although the phase mismatching effect limits the slow light enhancement on retrieval efficiency and decreases the fidelity for signal with bandwidth approaching AFC bandwidth, we design a modified atomic frequency comb structure (MAFC) based on which a moderate slow light can make the retrieval efficiency larger than 85% and keep the fidelity above 80%. Our calculations show that the proposed scheme provides a promising way to realize high efficiency on-chip quantum memory.

We created high-resolution atomic frequency combs (AFCs) and optimized the pulse sequence used in AFC memory to realize highly efficient AFC quantum memory using the 167Er3+:Y2SiO5 hyperfine level system. As a result, the formation and relaxation times of AFCs were revealed, and AFCs with a resolution of 800 kHz were created by using an appropriate pump light intensity. An efficiency of 0.16% with a memory time of 0.5 µs was achieved even under a zero magnetic field and a 10 ppm Er concentration sample. This result paves the way for realizing highly efficient communication-wavelength AFC quantum memory.

Quantum memories for light are essential building blocks for quantum repeaters and quantum networks. Integrated operations of quantum memories could enable scalable application with low-power consumption. However, the photonic quantum storage lifetime in integrated devices has so far been limited to tens of microseconds, falling short of the requirements for practical applications. Here, we demonstrate quantum storage of photonic qubits for 1.021 milliseconds based on a laser-written optical waveguide fabricated in a 151Eu3+:Y2SiO5 crystal. Spin dephasing of 151Eu3+ is mitigated through dynamical decoupling applied via on-chip electric waveguides, and we obtain a storage efficiency of 12.0±0.5% at 1.021 milliseconds, which is a demonstration of integrated quantum memories that outperforms the efficiency of a simple fiber delay line. Such long-lived waveguide-based quantum memory could support applications in quantum repeaters, and further combination with critical magnetic fields could enable potential application as transportable quantum memories.

Quantum storage and distribution of entanglement are the key ingredients for realizing a global quantum internet. Compatible with existing fiber networks, telecom-wavelength entangled photons and corresponding quantum memories are of central interest. Recently, 167Er3+ ions have been identified as a promising candidate for an efficient telecom quantum memory. However, to date, no storage of entangled photons, the crucial step of quantum memory using these promising ions, 167Er3+, has been reported. Here, we demonstrate the storage and retrieval of the entangled state of two telecom photons generated from an integrated photonic chip. Combining the natural narrow linewidth of the entangled photons and long storage time of 167Er3+ ions, we achieve storage time of 1.936 μs, more than 387 times longer than in previous works. Successful storage of entanglement in the crystal is certified using entanglement witness measurements. These results pave the way for realizing quantum networks based on solid-state devices.

Feasible distribution of quantum entanglement over long distances remains a fundamental step towards quantum secure communication and quantum network implementations. Quantum repeater nodes based on quantum memories promise to overcome exponential signal decay inherent to optical implementations of quantum communication. While performance of current quantum memories hinders their practical application, multimode solutions with multiplexing can offer tremendous increase in entanglement distribution rates. We propose to use a wavevector-multiplexed atomic quantum memory (WV-MUX-QM) as a fundamental block of a multiplexed quantum repeater architecture. We show the WV-MUX-QM platform to provide quasi-deterministic entanglement generation over extended distances, mitigating the fundamental issue of optical loss even with currently available quantum memory devices, and exceeding performance of repeaterless solutions as well as other repeater-based protocols such as temporal multiplexing. We establish the entangled-bit (ebit) rate per number of employed nodes as a practical figure of merit reflecting the cost-efficiency of larger inter-node distances.

Spin echo techniques are essential for achieving long coherence times in\nsolid-state quantum memories for light because of inhomogeneous broadening of\nthe spin transitions. It has been suggested that unrealistic levels of\nprecision for the radio frequency control pulses would be necessary for\nsuccessful decoherence control at the quantum level. Here we study the effects\nof pulse imperfections in detail, using both a semi-classical and a fully\nquantum-mechanical approach. Our results show that high efficiencies and low\nnoise-to-signal ratios can be achieved for the quantum memories in the\nsingle-photon regime for realistic levels of control pulse precision. We also\nanalyze errors due to imperfect initial state preparation (optical pumping),\nshowing that they are likely to be more important than control pulse errors in\nmany practical circumstances. These results are crucial for future developments\nof solid-state quantum memories.\n

Entangling quantum systems with different characteristics through the\nexchange of photons is a prerequisite for building future quantum networks.\nProving the presence of entanglement between quantum memories for light working\nat different wavelengths furthers this goal. Here, we report on a series of\nexperiments with a thulium-doped crystal, serving as a quantum memory for 794\nnm photons, an erbium-doped fibre, serving as a quantum memory for\ntelecommunication-wavelength photons at 1535 nm, and a source of photon pairs\ncreated via spontaneous parametric down-conversion. Characterizing the photons\nafter re-emission from the two memories, we find non-classical correlations\nwith a cross-correlation coefficient of $g^{(2)}_{12} = 53\\pm8$; entanglement\npreserving storage with input-output fidelity of\n$\\mathcal{F}_{IO}\\approx93\\pm2\\%$; and non-locality featuring a violation of\nthe Clauser-Horne-Shimony-Holt Bell-inequality with $S= 2.6\\pm0.2$. Our\nproof-of-principle experiment shows that entanglement persists while\npropagating through different solid-state quantum memories operating at\ndifferent wavelengths.\n

In the domain of quantum communication, one of the main challenges is the distribution of entanglement over long distances. A scheme, the so-called quantum repeater, has been proposed in order to overcome the problem of channel losses. In this thesis we adresse different problems related to quantum repeaters, both theoretically and experimentally. We present an alternative scheme of quantum repeater based on atomic ensembles and linear optics. It is based on the use of a qubit amplifier and two photon interference. The main features of the protocol are that the distributed entangled state does not need post selection and the entanglement distribution rates approach the optimal rates one might expect from quantum repeaters using probabilistic swapping. An important concern of the whole class of quantum repeaters based on one photon interference is the phase stability in the communication channels. We performed an experiment evaluating the realistic phase noise in installed optical fibers in the Geneva urban area. We found fluctuations of order of 0.1 rad on the time scale of order of 100 $mu$s. The feasibility of the phase stabilization is discussed. One of the essential building blocks of quantum repeater is a quantum memory. We focus our attention on quantum memories based on solid state atomic ensembles and photon echo like techniques. In particular, we show, that the atomic coherence in eryso can be controlled to a very high degree using an external electric field. This allowed us to implement the CRIB protocol in this material and realize the first storage of light at the single photon level at telecommunication wavelength. A necessary step for the implementation of CRIB and AFC storage protocols, in order to achieve long storage times and unit efficiencies, is the use of control light pulses. We developped a numerical Maxwell-Bloch simulator allowing us to study a coherent control of atomic excitations. In particular, we investigate the use of adiabatic chirped pulses in the context of AFC protocol. We study the relationship between the pulse characteristics and the echo properties. The comparison with the conventional $pi$ pulses and the impacts on the multimode capacity of the AFC protocol are also discussed.

I present quantum nano-photonic devices based on nanophotonic resonators coupled to rare-earth-ions in crystals. The rare-earth ions exhibit long coherence times on optical transitions, which makes them suitable for optical quantum memories. We demonstrate a high-fidelity nanophotonic quantum memory based on a mesoscopic rare-earth ensemble coupled to a photonic crystal cavity. The nanocavity enables >95% spin polarization for efficient initialization of the atomic frequency comb memory, and time-bin-selective readout via enhanced optical Stark shift of the comb frequencies. Besides ensemble memories, single rare-earth-ions coupled to nano-resonators can be used as single optically addressable quantum bits where the quantum state is mapped on their Zeeman or hyperfine levels with long coherence time. Our solid-state nano-photonic quantum light-matter interfaces can be integrated with other chip-scale photon source and detector devices for multiplexed quantum and classical information processing at the nodes of quantum networks.

Optically active defects in solids---colour centres---are one of the most promising platforms for implementing quantum technologies. Their spin degrees of freedom serve as quantum memories that in some cases can operate at room temperature. Their control can be achieved with microwave spin control and resonant optical excitation but is hindered by the broadening of optical transitions from thermal phonons and spectral diffusion. Furthermore, spin-qubit optical transitions are often outside the telecommunications wavelength band required for long-distance fiber optic transmission. Harnessing the coupling between mechanical degrees of freedom and spins has emerged as an alternative route for controlling spin-qubits. However, connecting spin-mechanical interfaces to optical links to realize a spin-photon interface has remained a challenge. Here we demonstrate such an interface using a diamond optomechanical cavity that does not depend on optical transitions and can be applied to a wide range of spin qubits. Our device consists of a diamond microdisk resonator studied in [Optica 3, 963-970 (2016)]. The microdisk is fabricated from optical grade diamond that contains ensembles of NV centres. We use an optical mode at 1564 nm with the quality factor Qo = 150k. The mechanical mode which we use to couple to the NV spin state is a radial breathing mode with a frequency of around 2.1 GHz with the quality factor Qm = 4k. The device operates in the sideband resolved regime enabling optomechanical self-induced oscillations for sufficiently high optical input power of a blue detuned laser. These oscillations can produce the stress of a few MPa, large enough to drive the electronic spins of NV centers. We use a standard diamond NV confocal microscope to initialize and readout the NV state. The MW pulses transfer the population between |-1⟩|0⟩ and |+1⟩|0⟩ state. We wait for 0.7 us with the mechanical drive that drives the |-1⟩|+1⟩ transition. In our measurements, we observe a coinciding dip in the |+1⟩ population and a peak |-1⟩ population, that verifies that the spins are being optomechanically driven. On calibrating this signal with the MW Rabi contrast and the background signal, we estimate a driving rate of 2pix170kHz and ~45% transfer of spin population between |±1⟩ states. Feasible improvements in device geometry will increase the optomechanically-induced driving rate by a few orders of magnitude allowing for coherent control of NV spins using an optomechanical resonator.

In this work, we fabricate a multimode quantum memory out of a thuliumdoped crystal and demonstrate storage of laser pulses of up to 100 µsec. A significant step forward for creating quantum memories with long optical storage times.