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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Multi-Qubit Registers of Individually Addressable Solid-State Defect Centers
摘要： Mesoscopic ensembles of qubits offer a platform for near-term applications in quantum technologies, as well as for studying many-body physics. Key in exploiting these systems is the ability to coherently control constituent qubits in a manner that leaves the quantum states of neighboring qubits unperturbed. Atom-like emitters in solids have emerged as a promising platform for computing, communications, and sensing. In particular, their long coherence times, coherent optical transitions, and the ability to couple to nearby long-lived nuclear spins make them excellent candidates for building medium-scale registers of coupled qubits. Here, we present progress towards producing and controlling such individually addressable ensembles. Our approach relies on the natural inhomogeneous distribution of optical transitions for solid-state color centers. This distribution allows us to excite individual centers even in tightly-grouped clusters. With this, we first demonstrate super-resolution localization and readout of individual nitrogen vacancy (NV) centers in diamond. We probe a system of three NV centers, demonstrating localization with a mean precision of 0.74 nm. While super-resolution imaging of NV centers has been achieved with other techniques, our approach uniquely allows for individual readout of single NVs in a cluster in a manner that maintains the states of nearby spins. We perform simultaneous control of two spin populations in a cluster of NVs, demonstrating preservation of coherence of one population during the optical readout of another. Lastly, we discuss recent work towards scalable creation of such clusters and techniques for producing systems of coupled spins with various defect centers.
关键词： spin-spin coupling,quantum technologies,super-resolution localization,qubits,solid-state color centers,nitrogen vacancy centers,many-body physics,atom-like emitters
更新于2025-09-16 10:30:52
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Nonpairwise Interactions Induced by Virtual Transitions in Four Coupled Artificial Atoms
摘要： Various protocols implementing a quantum computer are being pursued, one of which is the adiabatic quantum computer. Natural interactions in electromagnetic environments are only two-body local interactions, but the construction or simulation of higher-order couplers is indispensable for a universal adiabatic quantum computer using conventional flux qubits (no nonstoquastic interactions). Here we show that in a specific flux-qubit coupler design without ancilla qubits, four-body stoquastic interactions are induced by virtual coupler excitations. For specific parameter regimes they are the leading effect and can be tuned up to the gigahertz range.
关键词： adiabatic quantum computer,four-body interactions,quantum computer,virtual transitions,flux qubits
更新于2025-09-16 10:30:52
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Experimentally feasible scheme for a high-fidelity controlled-phase gate with general cat-state qubits
摘要： We propose an efficient scheme for implementing the universal controlled-phase gate with logical qubits encoded in the superpositions of 2d circularly distributed coherent states (d is a positive integer), which have the advantage of protecting quantum information against the (d ? 1)-photon loss errors. We first demonstrate explicitly how to engineer dispersive interaction between the microwave cavity modes and multilevel transmon ancilla in circuit quantum electro-dynamical (QED) system. With the cat-state qubits, we realize the two-qubit controlled phase gate, and its operation time scales as 1/d. This means that it will be less susceptible to environmental disturbances if we employ larger d to encode information. Finally, we employ the experimentally reachable parameters in circuit QED system to numerically study the influence of the photon loss, the qubit relaxation, and the Kerr nonlinearity. The average fidelity for a controlled-phase gate has reached 0.9955 for d = 8 when the system imperfections are considered. The results demonstrate the great potential of our scheme for quantum computation in circuit QED system.
关键词： circuit QED system,cat-state qubits,controlled-phase gate,quantum computation
更新于2025-09-16 10:30:52
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Entangling two separable qubits using an entangled field state
摘要： In this paper, entanglement transfer from parity pair coherent states to separable qubits is studied. Parity pair coherent states are defined as superposition of pair coherent states. The entanglement of these states is obtained studying their entropy. We observe that the entanglement of the parity pair coherent states is a decreasing function of the photon number differences between the two modes. The interaction between the latter and the qubit system is studied considering Jaynes-Cummings model. The entanglement of the qubit system after interaction is calculated using concurrence. Our results reveal that the maximum of entanglement is achieved if the two modes have equal photon number. In addition, we observe that the maximum entanglement of the qubit system is an increasing (decreasing) function of the photon number difference between the two modes contingent on its odd (even) value.
关键词： qubits,entanglement transfer,parity pair coherent state
更新于2025-09-16 10:30:52
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Radio-Frequency-Detected Fast Charge Sensing in Undoped Silicon Quantum Dots
摘要： Spin qubits in silicon quantum dots offer a promising platform for a quantum computer as they have a long coherence time and scalability. The charge sensing technique plays an essential role in reading out the spin qubit as well as tuning the device parameters, and therefore, its performance in terms of measurement bandwidth and sensitivity is an important factor in spin qubit experiments. Here we demonstrate fast and sensitive charge sensing by radio frequency reflectometry of an undoped, accumulation-mode Si/SiGe double quantum dot. We show that the large parasitic capacitance in typical accumulation-mode gate geometries impedes reflectometry measurements. We present a gate geometry that significantly reduces the parasitic capacitance and enables fast single-shot readout. The technique allows us to distinguish between the singly- and doubly occupied two-electron states under the Pauli spin blockade condition in an integration time of 0.8 μs, the shortest value ever reported in silicon, by the signal-to-noise ratio of 6. These results provide a guideline for designing silicon spin qubit devices suitable for the fast and high-fidelity readout.
关键词： spin qubits,radio frequency reflectometry,silicon,charge sensing,quantum dots
更新于2025-09-16 10:30:52
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Quantum Process Tomography of a Controlled-Phase Gate for Time-Bin Qubits
摘要： Time-bin qubits, where information is encoded in a single photon at different times, have been widely used in optical-fiber- and waveguide-based quantum communications. With the recent developments in distributed quantum computation, it is logical to ask whether time-bin encoded qubits may be useful in that context. We have recently realized a time-bin qubit controlled-phase (C-phase) gate using a 2 × 2 optical switch based on a lithium-niobate waveguide, with which we demonstrated the generation of an entangled state. However, the experiment was performed with only a pair of input states and thus the functionality of the C-phase gate was not fully verified. In this research, we use quantum process tomography to establish a process fidelity of 97.1%. Furthermore, we demonstrate the controlled-NOT gate operation with a process fidelity greater than 94%. This study confirms that typical two-qubit logic gates used in quantum computational circuits can be implemented with time-bin qubits and thus it is a significant step forward for the realization of distributed quantum computation based on time-bin qubits.
关键词： quantum computation,time-bin qubits,controlled-phase gate,controlled-NOT gate,quantum process tomography
更新于2025-09-16 10:30:52
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Adiabatic two-qubit gates in capacitively coupled quantum dot hybrid qubits
摘要： The ability to tune qubits to flat points in their energy dispersions (“sweet spots”) is an important tool for mitigating the effects of charge noise and dephasing in solid-state devices. However, the number of derivatives that must be simultaneously set to zero grows exponentially with the number of coupled qubits, making the task untenable for as few as two qubits. This is a particular problem for adiabatic gates, due to their slower speeds. Here, we propose an adiabatic two-qubit gate for quantum dot hybrid qubits, based on the tunable, electrostatic coupling between distinct charge configurations. We confirm the absence of a conventional sweet spot, but show that controlled-Z (CZ) gates can nonetheless be optimized to have fidelities of ~99% for a typical level of quasistatic charge noise (σε ? 1 μeV). We then develop the concept of a dynamical sweet spot (DSS), for which the time-averaged energy derivatives are set to zero, and identify a simple pulse sequence that achieves an approximate DSS for a CZ gate, with a 5× improvement in the fidelity. We observe that the results depend on the number of tunable parameters in the pulse sequence, and speculate that a more elaborate sequence could potentially attain a true DSS.
关键词： charge noise,adiabatic two-qubit gates,capacitive coupling,quantum dot hybrid qubits,dynamical sweet spot
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - High Efficiency Quantum Memory in Multiplexed Large-OD Cold Atomic Ensemble
摘要： Quantum memories enabling the storage of an input photonic qubit and its later retrieval with a ?delity beating any classical device constitute essential components in quantum communication networks and optical quantum information processing [1]. Our main goal is to develop a new generation of memories which have a near-unity ef?ciency for storage-and-retrieval operations, as well as high multiplexing capabilities. In the recent years, we demonstrated for instance the implementation for quantum bits encoded in the orbital angular momentum degree of freedom, which provides an essential capability for future networks with multimode capability [2]. We also realized multiple-degree-of-freedom memory, which can ?nd applications in classical data processing but also in quantum network scenarios where states structured in phase and polarization have been shown to provide promising attributes [3]. Recently, we realized a multiplexed quantum memory for polarization encoded qubits with high storage-and-retrieval ef?ciency [4]. We report on a quantum memory for polarization qubits that combines an average conditional ?delity above 99% and ef?ciency around 68%, thereby demonstrating a reversible qubit mapping where more information is retrieved than lost. The qubits are encoded with weak coherent states at the single-photon level and the memory is based on electromagnetically-induced transparency in an elongated laser-cooled ensemble of cesium atoms, spatially multiplexed for dual-rail storage. The reported ef?ciency approaches the maximal performance achievable on the D2 line transition used here, as shown by a comprehensive model that includes all the involved atomic transitions. In the alkali-metal atoms, hyper?ne interaction in the excited state indeed introduces several levels which can have a strong effect on the medium susceptibility via off resonant excitation, thereby decreasing the ef?ciency when the optical depth (OD) is large. As shown by this model, switching to the D1 line can increase the ef?ciency above 90%. Preliminary results with optical depth around 400 led us to a storage-and-retrieval ef?ciency of 85% at the single-photon level.
关键词： Large-OD,Quantum memory,Polarization qubits,Multiplexed,Cesium atoms,Cold atomic ensemble,Electromagnetically-induced transparency
更新于2025-09-11 14:15:04
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Entanglement versus cooling in the system of a driven pair of two-level qubits longitudinally coupled with a boson mode field
摘要： The relationship among the entanglement creation within coherently pumped and closely spaced two-level emitters longitudinally coupled with a single-mode boson field, and the subsequent quantum cooling of the boson mode is investigated. Even though the two-level qubits are resonantly driven, we have demonstrated an efficient cooling mechanism well below limits imposed by the thermal background. Furthermore, the cooling effect is accompanied by entanglement of the qubit pair components when the dipole-dipole frequency shift is close to the frequency of the boson mode. The maximum boson mode cooling efficiency realizes on the expense of the entanglement creation. Importantly, this occurs for rather weak external pumping fields protecting the sample from the deteriorations. Finally, the conditions to effectively optimize these effects are described as well.
关键词： entanglement,boson mode,dipole-dipole interaction,two-level qubits,cooling
更新于2025-09-11 14:15:04
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Optical remote control of a single charge qubit
摘要： Both the electron transport-based qubits, implemented through double quantum dots, and the sources of indistinguishable single photons like self-assembled quantum dots are strong candidates for the implementation of quantum technologies, such as quantum computers and quantum repeaters. Here, we demonstrate a reliable way of coupling these two types of qubits, uncovering the possibility of controlling and reading out the population of the double quantum dot via optical excitation. It is also shown that, in spite of the decoherence mechanisms affecting the qubits, the entanglement between them is achievable and, consequently, the implementation of the suggested system in quantum technologies is feasible.
关键词： quantum technologies,quantum dots,entanglement,optical excitation,qubits
更新于2025-09-11 14:15:04