| 1. |
- Bengtsson, Andreas, 1991, et al.
(författare)
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Improved Success Probability with Greater Circuit Depth for the Quantum Approximate Optimization Algorithm
- 2020
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Ingår i: Physical Review Applied. - 2331-7019. ; 14:3
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Tidskriftsartikel (refereegranskat)abstract
- Present-day, noisy, small or intermediate-scale quantum processors-although far from fault tolerant-support the execution of heuristic quantum algorithms, which might enable a quantum advantage, for example, when applied to combinatorial optimization problems. On small-scale quantum processors, validations of such algorithms serve as important technology demonstrators. We implement the quantum approximate optimization algorithm on our hardware platform, consisting of two superconducting transmon qubits and one parametrically modulated coupler. We solve small instances of the NP (nondeterministic polynomial time)-complete exact-cover problem, with 96.6% success probability, by iterating the algorithm up to level two.
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| 2. |
- Biznárová, Janka, 1995, et al.
(författare)
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Intermodulation spectroscopy and the nonlinear response of two-level systems in superconducting coplanar-waveguide resonators
- 2024
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Ingår i: Physical Review Applied. - : American Physical Society (APS). - 2331-7019. ; 22:1
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Tidskriftsartikel (refereegranskat)abstract
- Two-level system (TLS) loss typically limits the coherence of superconducting quantum circuits. The loss induced by TLS defects is nonlinear, resulting in quality factors with a strong dependence on the circulating microwave power. We observe frequency mixing due to this nonlinearity by applying a two-tone drive to a coplanar waveguide resonator and measuring the intermodulation products using a multifrequency lock-in technique. This intermodulation spectroscopy method provides an efficient approach to characterizing TLS loss in superconducting circuits. Using harmonic balance reconstruction, we recover the nonlinear parameters of the device-TLS interaction, which are in good agreement with the standard tunneling model for TLSs.
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| 3. |
- Bylander, Jonas, 1978
(författare)
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Z-Gate Operation on a Superconducting Flux Qubit via its Readout SQUID
- 2015
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Ingår i: Physical Review Applied. - 2331-7019. ; 3:3, s. 034004-
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Tidskriftsartikel (refereegranskat)abstract
- Detuning a superconducting qubit from its rotating frame is one means to implement a Z-gate operation. In this work, we implement a Z gate by pulsing a current through the qubit’s readout dc SQUID. While the dc SQUID acts as a magnetic flux sensor for qubit readout, we in turn may use it as a flux actuator with tunable strength to impose a qubit frequency shift. Using this approach, we demonstrate Ramsey-type free-induction experiments with time constants as long as 280 ns and rotation frequencies as high as 1.4 GHz. We experimentally demonstrate an inferred Z-gate fidelity of approximately 90%, limited largely by the bandwidth of our system. In the absence of this limitation, we argue that the inferred fidelity may be improved to as high as 99%.
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| 4. |
- Jayaraman, Aditya, 1993, et al.
(författare)
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Loss and decoherence in superconducting circuits on silicon: Insights from electron spin resonance
- 2024
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Ingår i: Physical Review Applied. - 2331-7019. ; 22:1
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Tidskriftsartikel (refereegranskat)abstract
- Solid-state devices used for quantum computation and quantum sensing applications are adversely affected by loss and noise caused by spurious, charged two-level systems (TLS) and stray paramagnetic spins. These two sources of noise are interconnected, exacerbating the impact on circuit performance. We use an on-chip electron spin resonance (ESR) technique, with niobium nitride (NbN) superconducting resonators, to study surface spins on silicon and the effect of postfabrication surface treatments. We identify two distinct spin species that are characterized by different spin-relaxation times and respond selectively to various surface treatments (annealing and hydrofluoric acid). Only one of the two spin species has a significant impact on the TLS-limited resonator quality factor at low-power (near-single-photon) excitation. We observe a three- to fivefold reduction in the total density of spins after surface treatments and demonstrate the efficacy of ESR spectroscopy in developing strategies to mitigate loss and decoherence in quantum systems.
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| 5. |
- Niepce, David, 1984, et al.
(författare)
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High Kinetic Inductance Nb N Nanowire Superinductors
- 2019
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Ingår i: Physical Review Applied. - 2331-7019. ; 11:4
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate that a high kinetic inductance disordered-superconductor nanowire can realize a circuit element - known as a superinductor - with a characteristic impedance greater than the quantum resistance (RQ=h/4e2≃6.5kΩ) and a quality factor of 25 000 at single-photon excitation. By examining loss rates, we demonstrate that the microwave dissipation can be fully understood in the framework of two-level-system dielectric loss. Superinductors can suppress the quantum fluctuations of charge in a circuit, which has applications, for example, in devices for quantum computing, photon detection, and other sensors based on mesoscopic circuits.
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