Modular entanglement of atomic qubits using photons and phonons

Abstract

PACS numbers:Quantum entanglement is the central resource behind applications in quantum information science, from quantum computers [1] and simulators of complex quantum systems [2] to metrology [3] and secure communication [1]. All of these applications require the quantum control of large networks of quantum bits (qubits) to realize gains and speedups over conventional devices. However, propagating quantum entanglement generally becomes difficult or impossible as the system grows in size, owing to the inevitable decoherence from the complexity of connections between the qubits and increased couplings to the environment. Here, we demonstrate the first step in a modular approach [4] to scaling entanglement by utilizing a hierarchy of quantum buses on a collection of three atomic ion qubits stored in two remote ion trap modules. Entanglement within a module is achieved with deterministic near-field interactions through phonons [5], and remote entanglement between modules is achieved through a probabilistic interaction through photons [6]. This minimal system allows us to address generic issues in synchronization and scalability of entanglement with multiple buses, while pointing the way toward a modular large-scale quantum computer architecture that promises less spectral crowding and less decoherence. We generate this modular entanglement faster than the observed qubit decoherence rate, thus the system can be scaled to much larger dimensions by adding more modules.Small modules of qubits have been entangled through native local interactions in many physical platforms, such as trapped atomic ions through their Coulomb interaction [5], Rydberg atoms through their electric dipoles [7,8], nitrogen-vacancy centers in diamond through their magnetic dipoles [9], and superconducting Josephson junctions through capacitive or inductive couplings [10,11]. However, each of these systems is confronted with practical limits to the number of qubits that can be reliably controlled, stemming from inhomogeneities, the complexity and density of the interactions between the qubits, or quantum decoherence. Scaling beyond these limits can be achieved by invoking a second type of interaction that can extend the entanglement to other similar qubit modules. Such an architecture should therefore exploit both the local interactions within the qubit modules, and also remote interactions between modules (an example architecture is shown in Fig 1). Optical interfaces provide ideal buses for this purpose [12,13], as optical photons can propagate over macroscopic distances with negligible loss. Several qubit systems have been entangled through remote optical buses, such as atomic ions [14], neutral atoms [15], and nitrogen-vacancy centers in diamond [16].In the experiment reported here, we juxtapose local and remote entanglement buses utilizing trapped atomic ion qubits, balancing the requirements of each interface within the same qubit system. The observed entanglement rate within and between modules is faster than the observed entangled qub...

Co-designing a scalable quantum computer with trapped atomic ions

“…Although efficient light collection and detector arrays will be necessary for the measurement of many trapped ion qubits through state-dependent fluorescence, it will be crucial for the single-photon linking of ELU modules as discussed above. With high numerical-aperture collection optics, 10% of the emitted photons can be collected, 51 and more could be extracted through an optical cavity 77,78 integrated with the ion trap. 79 Highly efficient photonic Bell-state detectors with near-ideal mode-matching can be realised in fibre or waveguide beamsplitters 80 and near-unit efficiency photon detectors.…”

Entanglement of distinguishable quantum memories

“…of detectors registers the photons [11] and φ 0 is a stable intermemory phase [18]. The probability of two photon collection and detection during a window T = 60 ns is ∼ 10 −5 resulting in an entanglement rate of order Hz [18].…”

“…Alternative approaches include applying aσ z interaction for an appropriate time to a single memory or recording ∆t and modifying the phase of subsequent rotations of the entangled state. For trapped ions, this feed forward operation can be completed many orders of magnitude faster than entanglement is generated [18] resulting in minimal additional overhead. However, any feedforward operation will require a temporal resolution π/∆ω in order to track the entangled state phase 2∆ωt and faithfully produce φ c .…”

Analysis of quantum network coding for realistic repeater networks

“…In order to solve this problem, quantum repeaters have been proposed [11] and many of the components have been experimentally demonstrated [12,13]. A quantum repeater has multiple important roles: to create and share physical en- * satoh@sfc.wide.ad.jp † kaori@sfc.wide.ad.jp; Current address: HAL Laboratories ‡ kurosagi@sfc.wide.ad.jp § rdv@sfc.wide.ad.jp tanglement pairs (Bell pairs) between nearest neighbors over short distances, to perform purification of Bell pairs, and to create one long Bell pair by connecting two entangled pairs using entanglement swapping [11,[14][15][16][17][18][19].…”