Session X06: Sensors, Quantum Characterization, and Processing

Multi-loop atomic Sagnac interferometry

Inertial sensors based on matter-wave interference show great potential for navigation, geodesy, or fundamental physics. Similar to the Sagnac effect, their sensitivity to rotations increases with the area enclosed by the interferometer. In the case of light interferometers, the latter can be enlarged by forming multiple fibre loops. However, the equivalent for matter-wave interferometers remains an experimental challenge. This contribution presents a concept for a multi-loop atom interferometer with a scalable area formed by multiple light pulses. It exploits ultra-cold atomic ensembles combined with symmetric beam splitting and a relaunch mechanism. Due to its scalability it offers the perspective of reaching unprecedented sensitivities for rotations in compact sensor setups.   

Two-dimensional Electromagnetically induced grating in a microwave driven cascade-type closed-loop atomic system

Electromagnetically induced grating (EIG) is a phenomenon similar to electromagnetically induced transparency (EIT) but creates with a control standing wave. Initially, EIG was proposed in a three-level Λ-type system [1] in which a transparent and opaque regions appear alternately in the atomic medium along the control field. If a probe field is made incident perpendicular to the above medium, then it acts as an amplitude grating. The same probe field also changes in the refractive index of the medium and it leads to Phase Grating. In the present work, we explore the impact of microwave field in a two-dimensional EIG using a cascade-type closed-loop atomic system. Our numerical analysis shows that the presence of microwave field leads to a spatial dependent amplitude suppression in the atomic system. We also observe that the microwave field can also dramatically enhances the high order diffraction of the probe field from the zeroth-order. Therefore the phase grating becomes more dominant in the presence of microwave field. We believe that the proposed system with microwave field can be used for designing a novel microwave sensing devices in the optical networking and communication.

[1] H.Y. Ling, et al., Phys. Rev. A 57,1338 (1998)   

Non-Poissonian ultrashort nanoscale electron pulses

The statistical character of electron beams used in current technologies, as described by a stream of particles, is random in nature. Using coincidence measurements of femtosecond pulsed electron pairs, we report the observation of sub-Poissonian electron statistics that are non-random due to two-electron Coulomb interactions, and that exhibit an anti-bunching signal of 1 part in 4 [1]. This advancement is a fundamental step towards observing a strongly quantum degenerate electron beam needed for many applications, and in particular electron correlation spectroscopy. [1] Sam Keramati, Will Brunner, T. J. Gay, and Herman Batelaan, Phys. Rev. Lett. 127, 180602 (2021).   

Testing constraints on periodic quantum evolution as a probe of incoherent error using a trapped-ion quantum computer

As quantum computers and simulators begin to produce results which cannot be verified classically, it will become imperative to develop effective tools to diagnose experimental errors on these devices. While state or process tomography is a natural way to probe sources of experimental error, the intense measurement requirements make these strategies infeasible in all but the smallest of quantum systems. In this talk, I will discuss the ways in which long-term dynamics under periodic driving can act as a sensitive, low-cost probe of incoherent error. I will present a measurement scheme built on easily accessible observables measured at varying times in the evolution. These measurements are constrained by theoretical bounds that become exponentially tight with evolution time, providing progressively stronger checks for the presence of incoherent error. I will include results from experimental implementation of this scheme in the presence of varying levels of incoherent error on two quantum computers, including our own trapped-ion experiment, and demonstrate the conditions under which deviations of the formulated bounds constitute a practical detection of incoherent error.   

Learning Quantum Phases of Matter Using a Basis-Enhanced Born Machine

The interplay between charge, spin, and other degrees of freedom in a quantum system is responsible for the emergence of exotic quantum phases, which range from ferromagnetic to spin liquid. The generative Born Machine is a quantum inspired machine learning tool that aims to learn a joint probability distribution from classical or quantum data obtained from simulation or experiment.  However, the full potential of the Born Machine in learning from quantum data has thus far been unrealized. In this work, by assembling training data from two distinct bases, we create a basis-enhanced Born Machine that is well-suited for pure quantum state reconstruction. We use the basis-enhanced Born Machine to learn across the ground state phase diagram of a 1D chain of Rydberg atoms and that of a 1D XY spin chain, accurately predicting quantum correlations and other observables. It is demonstrated that the improved model is able to capture the quantum state in various ordered phases as well as at the critical point to a quantum fidelity as high as 99%.   

Microfabricated piezo-optomechanical switches for trapped ion quantum computing

Quantum information processors and atomic clocks based on trapped ions continue to push towards more complex systems with demanding I/O, size, and power requirements. These demands motivate the replacement of external optical conditioning elements, such as amplitude, phase, frequency modulators, with integrated versions on the same chip. Here, we test a promising solution, a microfabricated piezo-optomechanical Mach-Zehnder modulator that is compatible with monolithic integration of surface ion traps^[1-2].We demonstrate the degree of amplitude and phase control necessary for quantum operations, testing directly on a trapped ion apparatus.

 

[1] P. R. Stanfield, et al, Opt. Express 27, 28588 (2019)

[2] M. Dong, et al., Nature Photonics 16 (1), 59-65 (2021)   

Low-entropy preparation and fast single-qubit rotations for 171Yb nuclear spin qubits

In the past decade, neutral atoms in optical tweezer arrays have become a promising candidate for quantum information applications. Among systems being explored, alkaline-earth(-like) atoms have a number of desirable features, such as the presence of a nuclear-spin qubit resistant to many forms of decoherence, the possibility of trapping Rydberg states, and the existence of a long-lived clock transition for shelving coherent qubits. Here we demonstrate progress towards the realization of a high-performance quantum information system in ¹⁷¹Yb [1]. Through a novel scheme, a 10x10 tweezer array can be loaded with 92.73(8)% filling, greatly simplifying the task of realizing defect-free qubit arrays. With Raman sideband cooling, the atoms are prepared in the motional ground state with >80% fidelity in each of the three directions, which will be crucial to high-fidelity single- and two-qubit gate operations. Using a novel single-beam Raman technique, high-fidelity single-qubit rotations can be effected in several 100 ns, and using Ramsey spectroscopy, decoherence timescales are measured, T₂ = 8.1(7) s and T₂* = 3.7(4) s. These results demonstrate several of the powerful features of alkaline-earth tweezer systems for quantum information.

[1] Jenkins, A., Lis, J. W., Senoo, A., McGrew, W. F., & Kaufman, A. M. (2021). Ytterbium nuclear-spin qubits in an optical tweezer array. arXiV:2112.06732   

A novel non-Gaussianity measure based on the Wigner relative entropy

The enhanced phase-space characteristics of non-Gaussian states of light, albeit necessary for universal quantum computing, render their understanding and production challenging. In attempts to circumvent these difficulties, several works have introduced non-Gaussianity measures, i.e., quantities that assign a real number to states depending on their non-Gaussian content (Genoni et al., 2007, 2008). Based on the Wigner entropy (Van Herstraeten & Cerf, 2021), we introduce a new measure μ_W(\{hat{ρ}), which is the Wigner relative entropy between an arbitrary N-mode state \hat{ρ} and its Gaussian associate \{hat{ρ}_G defined as 

μ_W(\hat{ρ}) = ∫ d^Nq d^Np W(q, p) [ln W(q, p)  - W_G(q, p)].

Here, W(q, p) and W_G(q, p) are the Wigner functions of the state and its Gaussian associate respectively. Our measure can be complex-valued, and we interpret its imaginary part as the negative volume of the Wigner quasi-distribution, while its real part provides information on other intrinsic properties of the state. We prove that μ_W(\hat{ρ}) is a valid non-Gaussianity measure and demonstrate its usefulness in representing states more perceptibly. In our work we discuss its relevance to non-Gaussian state generation and its connection to the more general context of resource theories.   

Robust nuclear spin entanglement via dipolar interactions in polar molecules

We propose a general protocol for on-demand generation of robust entangled states of nuclear and/or electron spins of ultracold polar molecules using strong and tunable electric dipolar interactions.  By encoding the effective spin-1/2 degree of freedom into molecular spin-rotational states, we derive spin-spin interactions of the Ising type, and show how to use these interactions to create long-lived cluster states of the nuclear spin sublevels of KRb($^1\Sigma$) molecules in their ground rotational states.  We also show that by inducing an avoided crossing of molecular energy levels with an external dc magnetic field (for $^2\Sigma$ molecules like YO) one can engineer XXZ-type couplings between the spin-rotational states with magnetically tunable spin coupling constants $J_z$ and $J_\perp$, enabling efficient magnetic control over electric dipolar interactions.    

Towards an all-electronic microwave-enabled trapped electron quantum computer

We explore electrons trapped in Paul traps as an attractive alternative to trapped ions to process quantum information. The combination of their extremely light mass and simple two-level spin structure enables high-speed operation while allowing for high-fidelity operation, and they can be manipulated with well-established microwave technology, removing some of the optical engineering challenges required to build a large-scale trapped-ion quantum computer.

Towards this goal, we trap single to few electrons in a millimeter-sized quadrupole Paul trap driven at 1.6 GHz in a room-temperature ultra-high vacuum setup. Electrons with sub-5 meV energies are introduced into the trap by near-resonant photoionisation of an atomic calcium beam and confined by microwave and static electric fields for several tens of milliseconds. A fraction of these electrons remains trapped longer and show no measurable loss for measurement times up to a second. Electronic excitation of the motion reveals secular frequencies which can be tuned over a range of several tens to hundreds of MHz.

Operating an electron Paul trap in a cryogenic environment may provide a platform for all-electric quantum computing with trapped electron spin qubits. Our recent feasibility study and simulation of common two-qubit error sources shows that error rates of less than 1E-4 at clock speeds of 1 MHz for transport and quantum gates should be feasible.