76 research outputs found
Noise-free high-efficiency photon-number-resolving detectors
High-efficiency optical detectors that can determine the number of photons in
a pulse of monochromatic light have applications in a variety of physics
studies, including post-selection-based entanglement protocols for linear
optics quantum computing and experiments that simultaneously close the
detection and communication loopholes of Bell's inequalities. Here we report on
our demonstration of fiber-coupled, noise-free, photon-number-resolving
transition-edge sensors with 88% efficiency at 1550 nm. The efficiency of these
sensors could be made even higher at any wavelength in the visible and
near-infrared spectrum without resulting in a higher dark-count rate or
degraded photon-number resolution.Comment: 4 pages, 4 figures Published in Physical Review A, Rapid
Communications, 17 June 200
Long distance decoy state quantum key distribution in optical fiber
The theoretical existence of photon-number-splitting attacks creates a
security loophole for most quantum key distribution (QKD) demonstrations that
use a highly attenuated laser source. Using ultra-low-noise, high-efficiency
transition-edge sensor photodetectors, we have implemented the first version of
a decoy-state protocol that incorporates finite statistics without the use of
Gaussian approximations in a one-way QKD system, enabling the creation of
secure keys immune to photon-number-splitting attacks and highly resistant to
Trojan horse attacks over 107 km of optical fiber.Comment: 4 pages, 3 figure
Microwave Packaging for Superconducting Qubits
Over the past two decades, the performance of superconducting quantum
circuits has tremendously improved. The progress of superconducting qubits
enabled a new industry branch to emerge from global technology enterprises to
quantum computing startups. Here, an overview of superconducting quantum
circuit microwave control is presented. Furthermore, we discuss one of the
persistent engineering challenges in the field, how to control the
electromagnetic environment of increasingly complex superconducting circuits
such that they are simultaneously protected and efficiently controllable
Heralding efficiency and correlated-mode coupling of near-IR fiber-coupled photon pairs
We report on a systematic experimental study of the heralding efficiency and generation rate of telecom-band infrared photon pairs generated by spontaneous parametric down-conversion and coupled to single-mode optical fibers. We define the correlated-mode coupling efficiency, an inherent source efficiency, and explain its relation to heralding efficiency. For our experiment, we developed a reconfigurable computer-controlled pump-beam and collection-mode optical apparatus which we used to measure the generation rate and correlated-mode coupling efficiency. The use of low-noise, high-efficiency superconducting nanowire single-photon detectors in this setup allowed us to explore focus configurations with low overall photon flux. The measured data agree well with theory, and we demonstrated a correlated-mode coupling efficiency of 97% ± 2%, which is the highest efficiency yet achieved for this type of system. These results confirm theoretical treatments and demonstrate that very high overall heralding efficiencies can, in principle, be achieved in quantum optical systems. It is expected that these results and techniques will be widely incorporated into future systems that require, or benefit from, a high heralding efficiency.United States. Dept. of Defense. Assistant Secretary of Defense for Research & Engineering (Air Force Contract FA8721-05-C-0002
Coherent Coupled Qubits for Quantum Annealing
Quantum annealing is an optimization technique which potentially leverages quantum tunneling to enhance computational performance. Existing quantum annealers use superconducting flux qubits with short coherence times limited primarily by the use of large persistent currents I[subscript p]. Here, we examine an alternative approach using qubits with smaller I[subscript p] and longer coherence times. We demonstrate tunable coupling, a basic building block for quantum annealing, between two flux qubits with small (approximately 50-nA) persistent currents. Furthermore, we characterize qubit coherence as a function of coupler setting and investigate the effect of flux noise in the coupler loop on qubit coherence. Our results provide insight into the available design space for next-generation quantum annealers with improved coherence.United States. Office of the Director of National IntelligenceUnited States. Intelligence Advanced Research Projects ActivityUnited States. Dept. of Defense. Assistant Secretary of Defense for Research & Engineering (FA8721-05-C-0002
The flux qubit revisited to enhance coherence and reproducibility
The scalable application of quantum information science will stand on reproducible and controllable high-coherence quantum bits (qubits). Here, we revisit the design and fabrication of the superconducting flux qubit, achieving a planar device with broad-frequency tunability, strong anharmonicity, high reproducibility and relaxation times in excess of 40âÎŒs at its flux-insensitive point. Qubit relaxation times Tâ across 22 qubits are consistently matched with a single model involving resonator loss, ohmic charge noise and 1/f-flux noise, a noise source previously considered primarily in the context of dephasing. We furthermore demonstrate that qubit dephasing at the flux-insensitive point is dominated by residual thermal-photons in the readout resonator. The resulting photon shot noise is mitigated using a dynamical decoupling protocol, resulting in Tââ85âÎŒs, approximately the 2Tâ limit. In addition to realizing an improved flux qubit, our results uniquely identify photon shot noise as limiting Tâ in contemporary qubits based on transverse qubitâresonator interaction
Characterization of superconducting through-silicon vias as capacitive elements in quantum circuits
The large physical size of superconducting qubits and their associated
on-chip control structures presents a practical challenge towards building a
large-scale quantum computer. In particular, transmons require a
high-quality-factor shunting capacitance that is typically achieved by using a
large coplanar capacitor. Other components, such as superconducting microwave
resonators used for qubit state readout, are typically constructed from
coplanar waveguides which are millimeters in length. Here we use compact
superconducting through-silicon vias to realize lumped element capacitors in
both qubits and readout resonators to significantly reduce the on-chip
footprint of both of these circuit elements. We measure two types of devices to
show that TSVs are of sufficient quality to be used as capacitive circuit
elements and provide a significant reductions in size over existing approaches
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