Hybrid Quantum Networks Research Articles

Nonreciprocal quantum-state conversion between microwave and optical photons

Optomechanical quantum interfaces can be utilized to connect systems with distinctively different frequencies in hybrid quantum networks. Here we present a scheme of nonreciprocal quantum state conversion between microwave and optical photons via an optomechanical interface. By introducing an auxiliary cavity and manipulating the phase differences between the linearized optomechanical couplings, uni-directional state transmission can be achieved. The interface can function as an isolator, a circulator, and a two-way switch that routes the input states to a selected output channel. We show that under a generalized impedance matching condition, the state conversion can reach high fidelity and is robust against the thermal fluctuations in the mechanical mode.

Highly efficient frequency conversion with bandwidth compression of quantum light

Hybrid quantum networks rely on efficient interfacing of dissimilar quantum nodes, as elements based on parametric downconversion sources, quantum dots, colour centres or atoms are fundamentally different in their frequencies and bandwidths. Although pulse manipulation has been demonstrated in very different systems, to date no interface exists that provides both an efficient bandwidth compression and a substantial frequency translation at the same time. Here we demonstrate an engineered sum-frequency-conversion process in lithium niobate that achieves both goals. We convert pure photons at telecom wavelengths to the visible range while compressing the bandwidth by a factor of 7.47 under preservation of non-classical photon-number statistics. We achieve internal conversion efficiencies of 61.5%, significantly outperforming spectral filtering for bandwidth compression. Our system thus makes the connection between previously incompatible quantum systems as a step towards usable quantum networks.

Ion-photon entanglement and quantum frequency conversion with trapped Ba+ ions.

Trapped ions are excellent candidates for quantum nodes, as they possess many desirable features of a network node including long lifetimes, on-site processing capability, and production of photonic flying qubits. However, unlike classical networks in which data may be transmitted in optical fibers and where the range of communication is readily extended with amplifiers, quantum systems often emit photons that have a limited propagation range in optical fibers and, by virtue of the nature of a quantum state, cannot be noiselessly amplified. Here, we first describe a method to extract flying qubits from a Ba+ trapped ion via shelving to a long-lived, low-lying D-state with higher entanglement probabilities compared with current strong and weak excitation methods. We show a projected fidelity of ≈89% of the ion-photon entanglement. We compare several methods of ion-photon entanglement generation, and we show how the fidelity and entanglement probability varies as a function of the photon collection optic's numerical aperture. We then outline an approach for quantum frequency conversion of the photons emitted by the Ba+ ion to the telecommunication range for long-distance networking and to 780 nm for potential entanglement with rubidium-based quantum memories. Our approach is significant for extending the range of quantum networks and for the development of hybrid quantum networks compromised of different types of quantum memories.

Low-pump sum frequency generation of frequency-stabilized 453 nm blue laser for photonic quantum interface

Generation of a cavity-enhanced nondegenerate narrow-band photon pair source is a potential way to realize a perfect photonic quantum interface for a hybrid quantum network. However, to ensure the high quality of the photon source, the pump laser for the narrow-band photon source should be generated in a special way. Here, we experimentally generate the blue 453 nm laser with a sum frequency generation process in a periodically poled lithium niobate waveguide. A 13 mW laser at 453 nm can be achieved with a low-power 880 nm laser and 935 nm laser input, and the internal conversion efficiency is 21.6% after calculation. The frequency of a 453 nm laser is stabilized by locking two pump lasers on one ultrastable optical cavity. The single pass process without employing cavity enhancement can ensure a good robustness of the whole system.

Generation of single photons with highly tunable wave shape from a cold atomic ensemble.

The generation of ultra-narrowband, pure and storable single photons with widely tunable wave shape is an enabling step toward hybrid quantum networks requiring interconnection of remote disparate quantum systems. It allows interaction of quantum light with several material systems, including photonic quantum memories, single trapped ions and opto-mechanical systems. Previous approaches have offered a limited tuning range of the photon duration of at most one order of magnitude. Here we report on a heralded single photon source with controllable emission time based on a cold atomic ensemble, which can generate photons with temporal durations varying over three orders of magnitude up to 10 μs without a significant change of the readout efficiency. We prove the nonclassicality of the emitted photons, show that they are emitted in a pure state, and demonstrate that ultra-long photons with nonstandard wave shape can be generated, which are ideally suited for several quantum information tasks.

Programmable atom-photon quantum interface

We present the implementation of a programmable atom-photon quantum interface, employing a single trapped $^{40}$Ca$^+$ ion and single photons. Depending on its mode of operation, the interface serves as a bi-directional atom-photon quantum-state converter, as a source of entangled atom-photon states, or as a quantum frequency converter of single photons. The interface lends itself particularly to interfacing ions with single or entangled photons in hybrid quantum networks.

Wavelength-tunable sources of entangled photons interfaced with atomic vapours.

The prospect of using the quantum nature of light for secure communication keeps spurring the search and investigation of suitable sources of entangled photons. A single semiconductor quantum dot is one of the most attractive, as it can generate indistinguishable entangled photons deterministically and is compatible with current photonic-integration technologies. However, the lack of control over the energy of the entangled photons is hampering the exploitation of dissimilar quantum dots in protocols requiring the teleportation of quantum entanglement over remote locations. Here we introduce quantum dot-based sources of polarization-entangled photons whose energy can be tuned via three-directional strain engineering without degrading the degree of entanglement of the photon pairs. As a test-bench for quantum communication, we interface quantum dots with clouds of atomic vapours, and we demonstrate slow-entangled photons from a single quantum emitter. These results pave the way towards the implementation of hybrid quantum networks where entanglement is distributed among distant parties using optoelectronic devices.

Generation of Nondegenerate Narrow-Band Photon Pairs for a Hybrid Quantum Network

A hybrid quantum network is built of nodes based on different physical systems, each good for a particular task. The trick is, they must all play together nicely, even though their requirements for photonic linking generally do not match. The authors solve this problem by using a conjoined double cavity to create nondegenerate, narrow-band photon pairs, to accommodate the different frequencies and bandwidths that must be meshed. This setup is robust to environmental noise and can link an ion-trap computing node to a solid-state memory node, for example\hskip-0.22em.

Multi-level cascaded electromagnetically induced transparency in cold atoms using an optical nanofibre interface

Ultrathin optical fibres integrated into cold atom setups are proving to be ideal building blocks for atom-photon hybrid quantum networks. Such optical nanofibres (ONFs) can be used for the demonstration of nonlinear optics and quantum interference phenomena in atomic media. Here, we report on the observation of multilevel cascaded electromagnetically induced transparency (EIT) using an optical nanofibre to interface cold 87Rb atoms. Intense evanescent fields can be achieved at ultralow probe (780 nm) and coupling (776 nm) powers when the beams propagate through the nanofibre. The observed multipeak transparency spectra of the probe beam could offer a method for simultaneously slowing down multiple wavelengths in an optical nanofibre or for generating ONF-guided entangled beams, showing the potential of such an atom-nanofibre system for quantum information. We also demonstrate all-optical-switching in the all-fibred system using the obtained EIT effect.

Optomechanical interfaces for hybrid quantum networks

Abstract Recent advances on optical control of mechanical motion in an optomechanical resonator have stimulated strong interests in exploring quantum behaviors of otherwise classical, macroscopic mechanical systems and especially in exploiting mechanical degrees of freedom for applications in quantum information processing. In an optomechanical resonator, an optically- active mechanical mode can couple to any of the optical resonances supported by the resonator via radiation pressure. This unique property leads to a remarkable phenomenon: mechanically-mediated conversion of optical fields between vastly different wavelengths. The resulting optomechanical interfaces can play a special role in a hybrid quantum network, enabling quantum communication between disparate quantum systems. In this review, we introduce the basic concepts of optomechanical interactions and discuss recent theoretical and experimental progresses in this field. A particular emphasis is on taking advantage of mechanical degrees of freedom, while avoiding detrimental effects of thermal mechanical motion.

Role of thermal noise in tripartite quantum steering

The influence of thermal noise on bipartite and tripartite quantum steering induced by a short laser pulse in a hybrid three-mode optomechanical system is investigated. The calculation is carried out under the bad cavity limit, the adiabatic approximation of a slowly varying amplitude of the cavity mode, and with the assumption of driving the cavity mode with a blue detuned strong laser pulse. Under such conditions, explicit expressions of the bipartite and tripartite steering parameters are obtained, and the concept of collective tripartite quantum steering, recently introduced by He and Reid [Phys. Rev. Lett. 111, 250403 (2013)], is clearly explored. It is found that both bipartite and tripartite steering parameters are sensitive functions of the initial state of the modes and distinctly different steering behavior could be observed depending on whether the modes were initially in a thermal state or not. For the modes initially in a vacuum state, the bipartite and tripartite steering occur simultaneously over the entire interaction time. This indicates that collective tripartite steering cannot be achieved. The collective steering can be achieved for the modes initially prepared in a thermal state. We find that the initial thermal noise is more effective in destroying the bipartite rather than the tripartite steering which, on the other hand, can persist even for a large thermal noise. For the initial vacuum state of a steered mode, the tripartite steering exists over the entire interaction time even if the steering modes are in very noisy thermal states. When the steered mode is initially in a thermal state, it can be collectively steered by the other modes. There are thresholds for the average number of the thermal photons above which the existing tripartite steering appears as the collective steering. Finally, we point out that the collective steering may provide a resource in a hybrid quantum network for quantum secret sharing protocol.

Photon-photon interactions in a largely detuned optomechanical cavity

We propose to realize effective beam-splitter-like and two-mode-squeezing photon-photon interactions in a strong coupling optomechanical interface by exploiting detuned driving lasers. In this interface, the transitions between the optical system and the mechanical oscillator are suppressed by the large energy offsets, therefore protecting the photon-photon interactions from mechanical dissipations. Moreover, the destructive quantum interference between the eigenmodes of the interface is capable of further reducing the effects of initial mechanical thermal occupations. The interface can serve as a universal block for photon state engineering and hybrid quantum networks in high-temperature thermal bath and without the requirement of cooling the mechanical oscillator to the ground state.

Downconversion quantum interface for a single quantum dot spin and 1550-nm single-photon channel

Long-distance quantum communication networks require appropriate interfaces between matter qubit-based nodes and low-loss photonic quantum channels. We implement a downconversion quantum interface, where the single photons emitted from a semiconductor quantum dot at 910 nm are downconverted to 1560 nm using a fiber-coupled periodically poled lithium niobate waveguide and a 2.2-μm pulsed pump laser. The single-photon character of the quantum dot emission is preserved during the downconversion process: we measure a cross-correlation g(2)(τ = 0) = 0.17 using resonant excitation of the quantum dot. We show that the downconversion interface is fully compatible with coherent optical control of the quantum dot electron spin through the observation of Rabi oscillations in the downconverted photon counts. These results represent a critical step towards a long-distance hybrid quantum network in which subsystems operating at different wavelengths are connected through quantum frequency conversion devices and 1.5-μm quantum channels.

Entangling spins by measuring charge: A parity-gate toolbox

The parity gate emerged recently as a promising resource for performing universal quantum computation with fermions using only linear interactions. Here we analyze the parity gate ($P$ gate) from a theoretical point of view in the context of quantum networks. We present several schemes for entanglement generation with $P$ gates and show that native networks simplify considerably the resources required for producing multiqubit entanglement, such as $n$--Greenberg-Horne-Zellinger (GHZ) states. Other applications include a Bell-state analyzer and teleportation. We also show that cluster state fusion can be performed deterministically with parity measurements. We then extend this analysis to hybrid quantum networks containing spin and mode qubits. Starting from an easy-to-prepare resource (spin-mode entanglement of single electrons) we show how to produce a spin $n$-GHZ state with linear elements (beam splitters and local spin flips) and charge-parity detectors; this state can be used as a resource in a spin quantum computer or as a precursor for constructing cluster states. Finally, we construct an alternative spin-controlled-$Z$ gate by using the mode degrees of freedom as ancill\ae{}.