Session Z06: Hybrid Quantum System and Qudits

Variational approach to atom-membrane dynamics

A time-dependent description of the interaction of a cold atom with a vibrating membrane is formulated using the Dirac-Frenkel variational method.  The time-dependent variational state is taken to be a superposition of a free atom with a thermal distribution of phonons and an atom bound to the membrane accompanied by a generalized coherent state of phonons.  An effective Lagrangian follows from this variational ansatz.  The resulting coupled Euler-Lagrange equations are treated analytically to give a description of the system.  Distinct dynamical regimes are identified with the use of zero-pole maps and root-locus diagrams in the complex s-plane.   

Rydberg atoms in optical tweezers near a nanophotonic cavity

While Rydberg arrays have emerged as a promising platform for quantum computation, their integration with a photonic interface could enable fast readout for error correction and quantum networking. This is challenging due to the strong electric field polarizability of Rydberg atoms, making them extremely sensitive to surface charges. Here we place Rydberg atoms in optical tweezers near a nanophotonic cavity and measure their coherence properties as a function of their distance from the cavity. At distances shorter than 100um, the fluctuations of the electric field are prohibitively large. At larger distances, decoupling sequences can be used to preserve coherence. Such distances can easily be bridged by using AODs [1]. Furthermore, we have demonstrated entanglement transport away from the nanophotonic cavity [2]. This renders the nanophotonic cavity a promising candidate for a photonic link for Rydberg arrays, situated a few hundred micrometers away. 

[1] Bluvstein et. al., arxiv: 2112.03923.

[2] Dordevic et. al., Science 373, 1511 (2021).   

A hybrid system for interfacing mm-wave and optical photons: Part 1

In this talk, I will describe a hybrid quantum platform for interfacing single optical and mm-wave photons using Rydberg atoms as mediators. Our system features a superconducting mm-wave resonator crossed with an optical Fabry-Perot cavity in a 4K cryostat, with optical access for trapping Rubidium atoms at the intersection of the two resonators. High quality factors for the mm-wave cavity, large dipole moments and long lifetimes of Rydberg states enable high single-atom cooperativities at the 100 GHz mm-wave transition. These capabilities can enable spin squeezing using the mm-wave cavity and quantum transduction between mm-wave and optical frequency regimes. I will also share some observations from our setup including cavity Rydberg electromagnetically induced transparency (EIT), Autler-Townes splitting of the Rydberg states with a coherent mm-wave drive, and mean-field optical nonlinearities induced by the mm-wave cavity. This talk is the first of two parts.   

A hybrid system for interfacing mm-wave and optical photons: Part 2

In the second of the two talks, I will describe integrating a high power ultra-violet (UV) 297 nm laser into our hybrid experimental setup. The UV laser closes the loop for interconversion between the mm-wave and optical modes. It also enables fast coherent excitation to the rydberg state, where the high cooperativity mm-wave cavity can be used to generate spin-squeezing. We observe that the UV laser strongly affects the finesse and the effective length of the optical cavity, and I will describe how we overcome these effects. I will also describe how the optical cavity can be used to efficiently detect depletion of the atoms from ground state due to the UV laser, thus enabling spectroscopy of the direct rydberg transition as well as observation of rabi-flopping in a cloud of atoms. I will also give an update on quantum transduction in this system.   

Is a retarded electron transfer shorter lived?

The Barium-Rubidium Interaction Experiment observes short lifetimes of central barium cations at ultracold temperatures due to electron transfer from the surrounding Bose-Einstein condensate of neutral rubidium atoms.

The question arises whether the coherence of the environment would at all play a role in fundamental interactions if the central cation is that short-lived.

This contribution presents semi-relativistic quantumelectrodynamic reasoning on the net-effect of the ultracold rubidium environment on the lifetime of the central barium cation.

Noteworthy is this result because it provides a generalization of resonant energy transfer theory simultaneously with a roadmap to an experimental confirmation of contributing coherence effects from the surrounding condensate on central impurities in a Bose-Einstein condensate.

    

Implementing symmetric entangling gates using quantum optimal control on nuclear-spin qudits in 87Sr atoms

Qudits can be robustly encoded in nuclear spins of alkaline earth atoms and manipulated with magneto-optical fields.  In [1], we showed that how arbitrary SU(d) single-qudit unitary maps can be implemented in such systems using quantum optimal control. We applied this in the case of the I=9/2 nuclear spin in ⁸⁷Sr, a d=10 dimensional qudit, through a combination of nuclear spin-resonance and a tensor AC-Stark shift.  Augmenting our toolkit with Rydberg dressing allows us to create any symmetric entangling two-qudit gate such as CPHASE. Our techniques can be used to implement a qudit entangler for qudits from d=2 to d=10 encoded in the nuclear spin using partial isometries. We also studied how decoherence due to leakage affects the creation of qudit entanglers and one could achieve a fidelity of   0.9979, 0.9966,  0.9734, and 0.9638 for d=2, 3, 5, and 7 respectively.  This provides a powerful platform to explore the various applications of quantum information processing of qudits including metrological enhancement with qudits, quantum simulation, universal quantum computation, and quantum error correction.

[1] S. Omanakuttan, A. Mitra, M. J. Martin, and I. H. Deutsch,” Quantum optimal control of ten-level nuclear spin qudits in ⁸⁷Sr,” Phys. Rev. A 104, L060401 (2021).

    

High fidelity state preparataion and measurement of ion qubits with spin I > 1/2

We present a method of state preparation and measurement (SPAM) that is amenable to trapped ion hyperfine qubits with nuclear spins

higher than I = 1/2. The ground states of these higher nuclear spin isotopes do not afford a simple

frequency selective preparation scheme on a strong transition, this limitation is circumvented by stroboscopicaly driving

strong and weak transitions. We demonstrate this method achieving a SPAM infidelity of < 1e-4,

allowing future systems to leverage naturally occurring isotopes with favorable wavelengths and

masses such as the 137Ba+ qubit used in this work. State preparation is performed by blending

standard, fast optical pumping using E1 transitions and narrow microwave or optical E2 transitions

addressing the extra states in the I=3/2 isotope of 137Ba+. Measurement is performed with repeated

coherent shelving to multiple metastable states.

    

AKLT ground state in "Bosonic Qiskit"

A collaboration within the Co-design Center for Quantum Advantage (C2QA) is currently building an instruction set architecture for hybrid qubit/oscillator circuit QED hardware using Qiskit, the IBM open-source software development kit (SDK). Qiskit has been designed to operate on two-level systems. The aim is to create a branch of Qiskit called "Bosonic Qiskit" which can accommodate continuous variable systems with a view to using "Bosonic Qiskit" to represent circuit QED hardware in the future. As a proof of concept of "Bosonic Qiskit", we use it to implement a symmetry protected topological state - the seminal AKLT chain - using the native bosonic operations made available in the SDK.