Isolating electrons on superfluid helium

Electrons floating on the surface of superfluid helium have been suggested as promising mobile spin quantum bits (qubits). Transferring electrons extremely efficiently in a narrow channel structure with underlying gates has been demonstrated, showing no transfer error while clocking $10^9$ pixels in a 3-phase charge coupled device (CCD). While on average, one electron per channel was clocked, it is desirable to reliably obtain a single electron per channel. We have designed an electron turnstile consisting of a narrow (0.8$\mu$m) channel and narrow underlying gates (0.5$\mu$m) operating across seventy-eight parallel channels. Initially, we find that more than one electron can be held above the small gates. Underlying gates in the turnstile region allow us to repeatedly split these electron packets. Results show a plateau in the electron signal as a function of the applied gate voltages, indicating quantization of the number of electrons per pixel, simultaneously across the seventy-eight parallel channels

Extremely efficient clocked electron transfer on superfluid helium

Unprecedented transport efficiency is demonstrated for electrons on the surface of micron-scale superfluid helium filled channels by co-opting silicon processing technology to construct the equivalent of a charge-coupled device (CCD). Strong fringing fields lead to undetectably rare transfer failures after over a billion cycles in two dimensions. This extremely efficient transport is measured in 120 channels simultaneously with packets of up to 20 electrons, and down to singly occupied pixels. These results point the way towards the large scale transport of either computational qubits or electron spin qubits used for communications in a hybrid qubit system

Encoding a magic state with beyond break-even fidelity

We distill magic states to complete a universal set of fault-tolerant logic gates that is needed for large-scale quantum computing. By encoding better quality input states for our distillation procedure, we can reduce the considerable resource cost of producing magic states. We demonstrate an error-suppressed encoding scheme for a two-qubit input magic state, that we call the CZ state, on an array of superconducting qubits. Using a complete set of projective logical Pauli measurements, that are also tolerant to a single circuit error, we propose a circuit that demonstrates a magic state prepared with infidelity $(1.87 \pm 0.16) \times 10^{-2}$. Additionally, the yield of our scheme increases with the use of adaptive circuit elements that are conditioned in real time on mid-circuit measurement outcomes. We find our results are consistent with variations of the experiment, including where we use only post-selection in place of adaptive circuits, and where we interrogate our output state using quantum state tomography on the data qubits of the code. Remarkably, the error-suppressed preparation experiment demonstrates a fidelity exceeding that of the preparation of the same unencoded magic-state on any single pair of physical qubits on the same device.Comment: 10 pages, 7 figures, comments welcom

Electrons on superfluid helium: towards single electron control

Electrons floating on the surface of superfluid helium have been suggested as promising mobile spin qubits. While no experimental measurements have been made, theoretical calculations have led to the conclusion that these spins will have long spin decoherence and relaxation times. In this thesis we study how well electrons can be transported on the surface of helium using two types of devices consisting of micron-sized helium-filled channels. The first type of device is fabricated using the standard silicon CMOS processing, which has underlying gates along the channels for clocking electrons. The electrons are photoemitted above channels, which are filled with superfluid helium by capillary action. Electron packets in the parallel channels can be efficiently transported over a billion pixels without detectable errors using the gates connected as a 3-phase charge-coupled device (CCD). To reliably obtain a single electron per channel the electrons are clocked into a turnstile region, where the channel narrows and the gates in the region allow us to repeatedly split these electron packets. The results show a plateau in the electron signal as a function of the applied gate voltages, indicating quantization of the number of electrons per channel. The second type of device moves electrons between channels and a thin film of helium by creating a smooth transition between the two regions. Bringing electrons onto a film above a metallic layer will expedite thermalization, while also allowing for electrical measurements of electron densities when they are above the channels. The transport measurements suggest that the electrons can be transported from one channel, across a helium-coated metal layer, to the neighboring channel. The extreme efficiency in clocking electrons over pixels and isolating them proves the concept of scalable mobile qubit systems. The ability to move the electrons on and off the thin film region can be used for electron thermalization, as well as a basis for combining transport experiments and future experiments requiring more precise gate control such as controlling electrons over quantum dots