Electrons on helium form a unique two-dimensional system on the interface of liquid helium and vacuum. A small number of trapped electrons on helium exhibits strong interactions in the absence of disorder, and can be used as a qubit. Trapped electrons typically have orbital frequencies in the microwave regime and can therefore be integrated with circuit quantum electrodynamics (cQED), which studies light-matter interactions using microwave photons. Here, we experimentally realize a cQED platform with the orbitals of single electrons on helium. We deterministically trap one to four electrons in a dot integrated with a microwave resonator, allowing us to study the electrons' response to microwaves. Furthermore, we find a single-electron-photon coupling strength of [Formula: see text] MHz, greatly exceeding the resonator linewidth [Formula: see text] MHz. These results pave the way towards microwave studies of Wigner molecules and coherent control of the orbital and spin state of a single electron on helium.
Electrons on helium form a unique two-dimensional system on the interface of liquid helium and vacuum. A small number of trapped electrons on helium exhibits strong interactions in the absence of disorder, and can be used as a qubit. Trapped electrons typically have orbital frequencies in the microwave regime and can therefore be integrated with circuit quantum electrodynamics (cQED), which studies light-matter interactions using microwave photons. Here, we experimentally realize a cQED platform with the orbitals of single electrons on helium. We deterministically trap one to four electrons in a dot integrated with a microwave resonator, allowing us to study the electrons' response to microwaves. Furthermore, we find a single-electron-photon coupling strength of [Formula: see text] MHz, greatly exceeding the resonator linewidth [Formula: see text] MHz. These results pave the way towards microwave studies of Wigner molecules and coherent control of the orbital and spin state of a single electron on helium.
Electrons on helium form a unique two-dimensional system on the interface of superfluid 4He and vacuum. At low temperatures and weak confinement, trapped electrons can arrange into strongly correlated states known as Wigner molecules, which can be used to study electron interactions in the absence of disorder, or as a promising resource for quantum computation. Wigner molecules have orbital frequencies in the microwave regime and can therefore be integrated with circuit quantum electrodynamics, which would allow rapid detection and manipulation of such a molecule's orbital state. In this thesis, we demonstrate deterministic preparation of one to four-electron Wigner molecules in an electron-on-helium quantum dot located at the tip of an on-chip microwave resonator. The Wigner molecule dipole coupling to the resonator allows us to measure spectral features of Wigner molecules in the microwave regime. We find that different-sized Wigner molecules have strikingly different spectra, and each spectrum serves as a fingerprint for the molecule's internal structure and its surroundings. By modeling the electron-cavity system, we extract each molecule's orbital frequency and electron configuration. For a single electron, the orbital frequency crosses the cavity resonance and we measure a large coherent electron-photon coupling rate of g/2π=4.8 MHz, which exceeds the incoherent cavity decay rate by ten times. These results provide a solid base for the development of an electron-on-helium qubit. Looking forward, our platform allows for new microwave studies of strongly correlated electron states and can be used to couple a single microwave photon to a single electron spin.
The quantized lateral motional states and the spin states of electrons trapped on the surface of superfluid helium have been proposed as basic building blocks of a scalable quantum computer.Circuit quantum electrodynamics allows strong dipole coupling between electrons and a high-Q superconducting microwave resonator, enabling such sensitive detection and manipulation of electron degrees of freedom.Here, we present the first realization of a hybrid circuit in which a large number of electrons are trapped on the surface of superfluid helium inside a coplanar waveguide resonator.The high finesse of the resonator allows us to observe large dispersive shifts that are many times the linewidth and make fast and sensitive measurements on the collective vibrational modes of the electron ensemble, as well as the superfluid helium film underneath.Furthermore, a large ensemble coupling is observed in the dispersive regime during experiment, and it shows excellent agreement with our numeric model.The coupling strength of the ensemble to the cavity is found to be ≈1 MHz per electron, indicating the feasibility of achieving single electron strong coupling.
The quantized lateral motional states and the spin states of electrons trapped on the surface of superfluid helium have been proposed as basic building blocks of a scalable quantum computer.Circuit quantum electrodynamics allows strong dipole coupling between electrons and a high-Q superconducting microwave resonator, enabling such sensitive detection and manipulation of electron degrees of freedom.Here, we present the first realization of a hybrid circuit in which a large number of electrons are trapped on the surface of superfluid helium inside a coplanar waveguide resonator.The high finesse of the resonator allows us to observe large dispersive shifts that are many times the linewidth and make fast and sensitive measurements on the collective vibrational modes of the electron ensemble, as well as the superfluid helium film underneath.Furthermore, a large ensemble coupling is observed in the dispersive regime during experiment, and it shows excellent agreement with our numeric model.The coupling strength of the ensemble to the cavity is found to be ≈1 MHz per electron, indicating the feasibility of achieving single electron strong coupling.
The quest towards precise measurement and control of quantum mechanical systems is behind many if not most of the advances in mesoscopic physics in the recent years. This thesis follows that tradition, and describes the first realization of a hybrid circuit integrating electrons on helium (EonHe) with the circuit quantum electrodynamics (cQED) architecture. In this hybrid architecture, the quantized lateral motion of the electrons strongly couples with the transverse electric field inside a high-Q superconducting resonator, enabling sensitive detection and manipulation of the electron's in-plane motion degree of freedom. Such architecture also provide a plausible road map towards using the spin degrees of freedom of the electrons as a quantum memory. Our hope is to establish a novel model system for quantum control and manipulation, where the uncanny similarity between the electron hamiltonian and the strong coulomb interactions could give rise to unparallelly rich quantum phenomena. In our first experiment towards this goal, we demonstrate strong dispersive coupling between ∼105 electrons and a superconducting co-planar waveguide resonator. The effort of the second experiment focuses on bringing down this number to just a few electrons in an isolated trap. The CPW resonator was replaced by a quarter-wavelength strip-line resonator with one side terminated by a short (``the tuning fork"). This design changes the microwave mode that is visible to the electron dipole interaction, improves the coupling strength, and opens up the geometry for more sophisticated DC controls isolating an individual electron. The third experiment implements such DC control scheme, and manages to trap just a single electron in a small trap. This small electron trap is defined by multiple DC electrodes near the open-end of the tuning fork. The overall circuit design resembles those seen in the quantum dot community. Much effort was spent on hitting the tight space between the dispersive constraints of the cQED measurement scheme and the increasing complication of the sample. Single or few electron spectra are measured, and we provided an upper estimate on the acoustically broadened electron line-width, together with design changes that reduce the line-width of the electron states.
One of the key elements in circuit quantum electrodynamics (cQED) is the superconducting microwave resonator, which can enable strong coupling to and between nanoscale quantum systems, and as a result has emerged as an essential tool for both quantum information and quantum sensing. In this thesis, I will discuss new applications of superconducting resonators in both of these fields, which illustrates how quantum information and quantum sensing can be strongly tied together. As the field of quantum information continues to grow, various platforms for realizing a quantum computer have been proposed. One attractive platform is to control and manipulate the spin of an electron to form spin qubits, which have demonstrated a long coherence time, weak coupling to the environment, and high fidelity single- and two-qubit gates. However, scaling beyond two qubits has proven challenging, as has incorporating long-range coupling, although both are crucial for making spin qubits a scalable quantum computing platform. The confinement of electromagnetic fields in superconducting resonators enables strong coupling to small systems, such as the single electrons of a spin qubit. In the first part of this thesis, I will describe how a singlet-triplet spin qubit coupled to a high-impedance superconducting resonator constitutes a promising architecture for mediating long-ranged interactions based on a longitudinal coupling between the resonator and qubit. I will discuss the technical developments needed to successfully implement this novel type of interaction in a hybrid spin qubit-resonator device. This type of interaction has several advantages over the more commonly studied transverse coupling scheme: it does not require careful tuning of the qubit frequency to match that of the resonator, which facilitates scaling to a large number of qubits, and also avoids the loss of coherence, which is inevitable in resonant coupling. This type of interaction therefore opens promising paths for generating high-fidelity long-ranged two-qubit coupling by employing superconducting microwave resonators. By adapting techniques used in quantum information, we have also developed a new technique in quantum sensing, where we employ superconducting resonators as a probe of quantum materials such as mesoscopic systems, two-dimensional materials, and heterostructures, which are otherwise challenging to experimentally access with conventional techniques. In the second part of my thesis, I will discuss how implementing cQED techniques from quantum information allows us to probe the symmetry of the order parameter in unconventional superconductors. Unconventional superconductors are not readily available, and we therefore study an engineered system that comprises of interfacing a ferromagnet and superconductor, in which unconventional pairing can be induced into the ferromagnet. This platform allows us to simultaneously excite spin waves, a collective excitation of spins, in the ferromagnet, which opens new directions for studying interactions between coherently coupled magnons (quanta of spin waves) and unconventional superconducting states. Finally, by applying the same technique to a two-dimensional (2D) superconductor, tungsten ditelluride, we leverage this platform in an attempt to study the order parameter of its intrinsic superconducting phase. The nature of the order parameter in this material remains an open question, as both topological and superconducting phases can exist, possibly simultaneously. This work thus demonstrates a unique use of superconducting resonators beyond quantum information applications.
We propose to couple an on-chip high finesse superconducting cavity to the
lateral-motion and spin state of a single electron trapped on the surface of
superfluid helium. We estimate the motional coherence times to exceed 15
microseconds, while energy will be coherently exchanged with the cavity photons
in less than 10 nanoseconds for charge states and faster than 1 microsecond for
spin states, making the system attractive for quantum information processing
and cavity quantum electrodynamics experiments. Strong interaction with cavity
photons will provide the means for both nondestructive readout and coupling of
distant electrons.