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arXiv:1108.4153 · Revista Mexicana de Física

Atoms Talking to SQUIDs

Jennifer E. Hoffman, Jeffrey A. Grover, Kim, Z., A. K. Wood, J. R. Anderson, Alex J. Dragt, Mohammad Hafezi, Christopher J. Lobb, Luis A. Orozco, S. L. Rolston, Jacob M. Taylor, Constantine P. Vlahacos, F. C. Wellstood

Published Aug 21, 2011• 9 citations
arXivHTMLPDF

Abstract

We present a scheme to couple trapped $^{87}$Rb atoms to a superconducting flux qubit through a magnetic dipole transition. We plan to trap atoms on the evanescent wave outside an ultrathin fiber to bring the atoms to less than 10 $μ$m above the surface of the superconductor. This hybrid setup lends itself to probing sources of decoherence in superconducting qubits. Our current plan has the intermediate goal of coupling the atoms to a superconducting LC resonator.

DOI
10.48550/arxiv.1108.4153

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We present a scheme to couple trapped 87^{87}87Rb atoms to a superconducting flux qubit through a magnetic dipole transition. We plan to trap atoms on the evanescent wave outside an ultrathin fiber to bring the atoms to less than 10 μμμm above the surface of the superconductor. This hybrid setup lends itself to probing sources of decoherence in superconducting qubits. Our current plan has the intermediate goal of coupling the atoms to a superconducting LC resonator.
Hybrid quantum systems can be formed that combine the strengths of multiple platforms while avoiding the weaknesses. Here we report on progress toward a hybrid quantum system of neutral atom spins coupled to superconducting qubits. We trap laser-cooled rubidium atoms in the evanescent field of an ultrathin optical fiber, which will be suspended a few microns above a superconducting circuit that resonates at the hyperfine frequency of the Rb atoms, allowing magnetic coupling between the atoms and superconductor. As this will be done in a dilution refrigerator environment, the technical demands on the optical fiber is severe. We have developed and optimized a tapered fiber fabrication system, achieving optical transmission in excess of 99.95% , and fibers that can sustain 400 mW of optical power in a UHV environment. We have also optimized tapered fibers that can support higher order optical modes with high transmission (> 97%), which may be useful for different optical potential geometries. We have developed an in-situ tunable high-Q superconducting microwave resonator that can be tuned to within the resonator linewidth of the 6.8 GHz frequency of the Rb hyperfine transition.
Abstract We propose a hybrid quantum system where an LC resonator inductively interacts with a flux qubit and is capacitively coupled to a Rydberg atom. Varying the external magnetic flux bias controls the flux qubit flipping and the flux qubit-resonator interface. The atomic spectrum is tuned via an electrostatic field, manipulating the qubit-state transition of atom and the atom-resonator coupling. Different types of entanglement of superconducting, photonic and atomic qubits can be prepared via simply tuning the flux bias and electrostatic field, leading to the implementation of three-qubit Toffoli logic gate.
We describe advancements towards coupling superconducting qubits to neutral atoms. To produce a measurably large coupling, the atoms will need to be on the order of a few micrometers away from the qubit. A consequence of combining superconducting qubits and atoms is addressing their operational constraints, such as the deleterious light effects on superconducting systems and the magnetic field sensitivity of superconducting qubits. Our group proposes the use of optical-nanofiber-based optical dipole traps to confine atoms near the superconductor. Optical nanofibers (ONFs) have high-intensity evanescent waves that require less power than equivalent standard dipole traps. This thesis focuses on the fabrication and analysis of the behavior of ONFs. First we present the construction of the pulling apparatus. We outline the necessary steps for a typical pull, detailing the cleaning and alignment process. Then we examine the quality of the fibers by measuring their transmission and comparing our results to other reported measurements, demonstrating a two-order of magnitude decrease in loss. Next we present the modal evolution in ONFs using simulations and spectrogram analysis. We identify crucial elements to improve the transmission and demonstrate understanding of the modal dynamics during the pull. Then we study higher-order modes (HOMs) with ONFs using the first excited TE01, TM01, and HE21 modes. We demonstrate transmissions greater than 97% for 780 nm light when we launch the first excited LP11 family of modes through fibers with a 350 nm waist. This setup enables us to launch these three modes with high purity at the output, where less than 1% of the light is coupled to the fundamental mode. We then focus on the identification of modes on the ONF waist. First we use Rayleigh scattering to identify the modal content of an ONF. Bulk optics can convert the modes in the ONF, and we observe the controllable conversion of superpositions of modes. Finally, we use an evanescently-coupled tapered optical fiber probe that allows for the identification of the fundamental mode beating with HOMs and compare the results to simulations.
A hybrid quantum system is proposed by coupling the internal hyperfine transitions of a trapped atomic Bose-Einstein condensate (BEC) and a superconducting quantum interference device (SQUID) via the macroscopic quantum field of the flux qubit. The presence of the condensate leads to a bosonic enhancement of the Rabi frequency over the otherwise small single-particle magnetic-dipole transition matrix elements. This enhancement makes it possible to rapidly transfer and store qubit states in the BEC that were originally prepared in the SQUID. The fidelity of this transfer for different states is calculated, and a direct experimental protocol to determine the transfer fidelity by quantum tomography of the BEC qubit is presented.
Superconducting qubits are fabricated "loss-free" electrical circuits on a chip with size features of tens of nanometers. If cooled to cryogenic temperatures below -273 °C they behave as quantum elements, similar to atoms and molecules. Such a qubit can be manipulated by fast-oscillating magnetic fields and its resulting quantum state can be measured using a proper magnetic detector. These qubits could be the basic elements for a quantum processor. To that order, multiple qubits have to controllably interact, and to that purpose very high quality resonators are fabricated on the same chip. Effectively the interaction results from the phase-coherent exchange of a single photon between qubit and resonator. In this way it is highly analogous to the so-called cavity QED in atomic physics where single atoms couple to a single trapped photon, all inside a cavity resonator. In this thesis, a superconducting flux qubit has been coupled to an LC resonator and coherent exchange of a single photon between qubit and resonator has been achieved. A striking option in these superconducting circuits, not attainable in "common" atomic cavity-QED, is that the interaction between qubit and photon can be made very large, up to the limit that one no longer can talk about the qubit and resonator as such. This limit requires a fully different description and it leads to novel effects, e.g. in the energy spectrum of the qubit-resonator composite. The Bloch-Siegert spectral shift is experimentally demonstrated in this thesis. As a small but critical technical aside a new type of control line filter has been developed, which allows strong suppression of excess electrical noise that forms a threat for qubit coherence. Having two modes of operation, it shorts all low-frequency noise if control is idle while being transparent if control is active, where short or open is determined by the control line signal level itself.
Hybrid quantum systems have the potential of mitigating current challenges in developing a scalable quantum computer. Of particular interest is the hybridization between atomic and superconducting qubits. We demonstrate a novel experimental setup for transferring and trapping ultracold atoms inside a millikelvin cryogenic environment, where interactions between atomic and superconducting qubits can be established, paving the way for hybrid quantum systems. 87Rb^{87}\text{Rb}87Rb atoms are prepared in a conventional magneto-optical trap and transported via a magnetic conveyor belt into a UHV compatible dilution refrigerator with optical access. We store 5×1085\times10^{8}5×108 atoms with a lifetime of 794 seconds in the vicinity of the millikelvin stage.