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Electron Turbulence at Nanoscale Junctions

Nano Letters

Bushong, Neil B.; Gamble, John K.; Di Ventra, Massimiliano D.

Electron transport through a nanostructure can be characterized in part using concepts from classical fluid dynamics. Hence, it is natural to ask how far the analogy can be taken and whether the electron liquid can exhibit nonlinear dynamical effects such as turbulence. Here we present an ab initio study of the electron dynamics in nanojunctions which reveals that the latter indeed exhibits behavior quite similar to that of a classical fluid. In particular, we find that a transition from laminar to turbulent flow occurs with increasing current, corresponding to increasing Reynolds numbers. These findings reveal unexpected features of electron dynamics and shed new light on our understanding of transport properties of nanoscale systems.

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TYPE Journal Article YEAR 2017
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Demonstration of qubit operations below a rigorous fault tolerance threshold with gate set tomography

Nature Communications

Blume-Kohout, Robin J.; Gamble, John K.; Nielsen, Erik N.; Rudinger, Kenneth M.; Mizrahi, Jonathan; Fortier, Kevin M.; Maunz, Peter

Quantum information processors promise fast algorithms for problems inaccessible to classical computers. But since qubits are noisy and error-prone, they will depend on fault-tolerant quantum error correction (FTQEC) to compute reliably. Quantum error correction can protect against general noise if - and only if - the error in each physical qubit operation is smaller than a certain threshold. The threshold for general errors is quantified by their diamond norm. Until now, qubits have been assessed primarily by randomized benchmarking, which reports a different error rate that is not sensitive to all errors, and cannot be compared directly to diamond norm thresholds. Here we use gate set tomography to completely characterize operations on a trapped-Yb+-ion qubit and demonstrate with greater than 95% confidence that they satisfy a rigorous threshold for FTQEC (diamond norm ≤6.7 × 10-4).

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TYPE Journal Article YEAR 2017
ScopusOSTIDOI

Valley splitting of single-electron Si MOS quantum dots

Applied Physics Letters

Gamble, John K.; Harvey-Collard, Patrick; Jacobson, Noah T.; Baczewski, Andrew D.; Nielsen, Erik N.; Maurer, Leon; Montano, Ines M.; Rudolph, Martin R.; Carroll, Malcolm; Yang, C.H.; Rossi, A.; Dzurak, A.S.; Muller, Richard P.

Silicon-based metal-oxide-semiconductor quantum dots are prominent candidates for high-fidelity, manufacturable qubits. Due to silicon's band structure, additional low-energy states persist in these devices, presenting both challenges and opportunities. Although the physics governing these valley states has been the subject of intense study, quantitative agreement between experiment and theory remains elusive. Here, we present data from an experiment probing the valley states of quantum dot devices and develop a theory that is in quantitative agreement with both this and a recently reported experiment. Through sampling millions of realistic cases of interface roughness, our method provides evidence that the valley physics between the two samples is essentially the same.

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TYPE Journal Article YEAR 2016
ScopusOSTIDOI

Optimization of a solid-state electron spin qubit using gate set tomography

New Journal of Physics

Dehollain, Juan P.; Muhonen, Juha T.; Blume-Kohout, Robin J.; Rudinger, Kenneth M.; Gamble, John K.; Nielsen, Erik N.; Laucht, Arne; Simmons, Stephanie; Kalra, Rachpon; Dzurak, Andrew S.; Morello, Andrea

State of the art qubit systems are reaching the gate fidelities required for scalable quantum computation architectures. Further improvements in the fidelity of quantum gates demands characterization and benchmarking protocols that are efficient, reliable and extremely accurate. Ideally, a benchmarking protocol should also provide information on how to rectify residual errors. Gate set tomography (GST) is one such protocol designed to give detailed characterization of as-built qubits. We implemented GST on a high-fidelity electron-spin qubit confined by a single 31P atom in 28Si. The results reveal systematic errors that a randomized benchmarking analysis could measure but not identify, whereas GST indicated the need for improved calibration of the length of the control pulses. After introducing this modification, we measured a new benchmark average gate fidelity of , an improvement on the previous value of . Furthermore, GST revealed high levels of non-Markovian noise in the system, which will need to be understood and addressed when the qubit is used within a fault-tolerant quantum computation scheme.

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TYPE Journal Article YEAR 2016
ScopusOSTIDOI

Fabrication of quantum dots in undoped Si/Si0.8Ge0.2 heterostructures using a single metal-gate layer

Applied Physics Letters

Lu, Tzu-Ming L.; Gamble, John K.; Muller, Richard P.; Nielsen, Erik N.; Bethke, D.; Ten Eyck, Gregory A.; Pluym, Tammy P.; Wendt, J.R.; Dominguez, Jason J.; Lilly, M.P.; Carroll, Malcolm; Wanke, M.C.

Enhancement-mode Si/SiGe electron quantum dots have been pursued extensively by many groups for their potential in quantum computing. Most of the reported dot designs utilize multiple metal-gate layers and use Si/SiGe heterostructures with Ge concentration close to 30%. Here, we report the fabrication and low-temperature characterization of quantum dots in the Si/Si0.8Ge0.2 heterostructures using only one metal-gate layer. We find that the threshold voltage of a channel narrower than 1 μm increases as the width decreases. The higher threshold can be attributed to the combination of quantum confinement and disorder. We also find that the lower Ge ratio used here leads to a narrower operational gate bias range. The higher threshold combined with the limited gate bias range constrains the device design of lithographic quantum dots. We incorporate such considerations in our device design and demonstrate a quantum dot that can be tuned from a single dot to a double dot. The device uses only a single metal-gate layer, greatly simplifying device design and fabrication.

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TYPE Journal Article YEAR 2016
ScopusOSTIDOI

Microwave-driven coherent operation of a semiconductor quantum dot charge qubit

Nature Nanotechnology

Kim, Dohun; Ward, D.R.; Simmons, C.B.; Gamble, John K.; Blume-Kohout, Robin J.; Nielsen, Erik N.; Savage, D.E.; Lagally, M.G.; Friesen, Mark; Coppersmith, S.N.; Eriksson, M.A.

An intuitive realization of a qubit is an electron charge at two well-defined positions of a double quantum dot. This qubit is simple and has the potential for high-speed operation because of its strong coupling to electric fields. However, charge noise also couples strongly to this qubit, resulting in rapid dephasing at all but one special operating point called the 'sweet spot'. In previous studies d.c. voltage pulses have been used to manipulate semiconductor charge qubits but did not achieve high-fidelity control, because d.c. gating requires excursions away from the sweet spot. Here, by using resonant a.c. microwave driving we achieve fast (greater than gigahertz) and universal single qubit rotations of a semiconductor charge qubit. The Z-axis rotations of the qubit are well protected at the sweet spot, and we demonstrate the same protection for rotations about arbitrary axes in the X-Y plane of the qubit Bloch sphere. We characterize the qubit operation using two tomographic approaches: standard process tomography and gate set tomography. Both methods consistently yield process fidelities greater than 86% with respect to a universal set of unitary single-qubit operations.

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TYPE Journal Article YEAR 2015
ScopusOSTIDOI

Turbocharging Quantum Tomography

Blume-Kohout, Robin J.; Gamble, John K.; Nielsen, Erik N.; Maunz, Peter L.; Scholten, Travis L.; Rudinger, Kenneth M.

Quantum tomography is used to characterize quantum operations implemented in quantum information processing (QIP) hardware. Traditionally, state tomography has been used to characterize the quantum state prepared in an initialization procedure, while quantum process tomography is used to characterize dynamical operations on a QIP system. As such, tomography is critical to the development of QIP hardware (since it is necessary both for debugging and validating as-built devices, and its results are used to influence the next generation of devices). But tomography suffers from several critical drawbacks. In this report, we present new research that resolves several of these flaws. We describe a new form of tomography called gate set tomography (GST), which unifies state and process tomography, avoids prior methods critical reliance on precalibrated operations that are not generally available, and can achieve unprecedented accuracies. We report on theory and experimental development of adaptive tomography protocols that achieve far higher fidelity in state reconstruction than non-adaptive methods. Finally, we present a new theoretical and experimental analysis of process tomography on multispin systems, and demonstrate how to more effectively detect and characterize quantum noise using carefully tailored ensembles of input states.

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TYPE SAND Report YEAR 2015
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Results 1–50 of 63
Results 1–50 of 63