Microwave Photons Research Articles

Chiral SQUID-metamaterial waveguide for circuit-QED

Superconducting metamaterials, which are designed and fabricated with structured fundamental circuit elements, have motivated recent developments of exploring unconventional quantum phenomena in circuit quantum electrodynamics (circuit-QEDs). We propose a method to engineer 1D Josephson metamaterial as a chiral waveguide by considering a programmed spatiotemporal modulation on its effective impedance. The modulation currents are in the form of travelling waves which phase velocities are much slower than the propagation speed of microwave photons. Due to the Brillouin-scattering process, non-trivial spectrum regimes where photons can propagate unidirectionally emerge. Considering superconducting qubits coupling with this metamaterial waveguide, we analyze both Markovian and non-Markovian quantum dynamics, and find that superconducting qubits can dissipate photons unidirectionally. Moreover, we show that our proposal can be extended a cascaded quantum network with multiple nodes, where chiral photon transport between remote qubits can be realized. Our work might open the possibilities to exploit SQUID metamaterials for realizing unidirectional photon transport in circuit-QED platforms.

Full counting statistics of the photocurrent through a double quantum dot embedded in a driven microwave resonator

Detection of single, itinerant microwave photons is an important functionality for emerging quantum technology applications as well as of fundamental interest in quantum thermodynamics experiments on heat transport. In a recent experiment [W. Khan et al., Nat. Commun. 12, 5130 (2021)], it was demonstrated that a double quantum dot (DQD) coupled to a microwave resonator can act as an efficient and continuous photodetector by converting an incoming stream of photons to an electrical photocurrent. In the experiment, average photon and electron flows were analyzed. Here we theoretically investigate, in the same system, the fluctuations of the photocurrent through the DQD for a coherent microwave drive of the resonator. We consider both the low frequency full counting statistics as well as the finite-frequency noise (FFN) of the photocurrent. Numerical results and analytical expressions in limiting cases are complemented by a mean-field approach neglecting dot-resonator correlations, providing a compelling and physically transparent picture of the photocurrent statistics. We find that for ideal, unity efficiency detection, the fluctuations of the charge current reproduce the Poisson statistics of the incoming photons, while the statistics for non-ideal detection is sub-Poissonian. Moreover, the FFN provides information of the system parameter dependence of detector short-time properties. Our results give novel insight into microwave photon-electron interactions in hybrid dot-resonator systems and provide guidance for further experiments on continuous detection of single microwave photons.

Parametric amplification of an optomechanical quantum interconnect

Connecting superconducting qubits to optical fiber necessitates the conversion of microwave photons to optical photons. Modern experimental demonstrations exhibit strong coupling between a microwave resonator and an optical cavity mediated through phononic modes in a mechanical oscillator. This paradigmatic transduction experiment is bounded by a theoretical efficiency with constant driving amplitudes on the electromagnetic resonators. By adding a parametric drive to the microwave resonator and optical cavity we discover the converted signal through the quantum transducer is amplified, while maintaining a lower level of the added noise. We propose a theoretical framework for time-dependent control of the driving lasers based on the input-output formalism of quantum optics, and solve analytically the transduction efficiency and added noise when the control signals parametrically drive the system. Our results show better transduction efficiency and lower added noise in varying parameter regimes relevant to current transduction experiments.

Generation of microwave photon perfect W states of three coupled superconducting resonators

We propose an efficient method for the generation of perfect W states on three microwave superconducting resonators, of which the two nearest neighbors are coupled by a symmetric direct current superconducting quantum interference device (dc-SQUID). With suitable external magnetic fluxes applied to the dc-SQUID symmetry loops, on-chip tunable interactions between neighboring resonators can be realized, and different perfect W states can be deterministically created on-demand in one step. Numerical simulations show that high-fidelity target states can be generated and our scheme is robust against imperfect parameter tuning and environment-induced decoherence. The present work may have potential applications for implementing quantum computation and quantum information processing based on microwave photons.

On-chip distribution of quantum information using traveling phonons.

Distributing quantum entanglement on a chip is a crucial step toward realizing scalable quantum processors. Using traveling phonons-quantized guided mechanical wave packets-as a medium to transmit quantum states is now gaining substantial attention due to their small size and low propagation speed compared to other carriers, such as electrons or photons. Moreover, phonons are highly promising candidates to connect heterogeneous quantum systems on a chip, such as microwave and optical photons for long-distance transmission of quantum states via optical fibers. Here, we experimentally demonstrate the feasibility of distributing quantum information using phonons by realizing quantum entanglement between two traveling phonons and creating a time-bin-encoded traveling phononic qubit. The mechanical quantum state is generated in an optomechanical cavity and then launched into a phononic waveguide in which it propagates for around 200 micrometers. We further show how the phononic, together with a photonic qubit, can be used to violate a Bell-type inequality.

Microwave mode cooling and cavity quantum electrodynamics effects at room temperature with optically cooled nitrogen-vacancy center spins

Recent experimental and theoretical studies demonstrated microwave mode cooling and cavity quantum electrodynamics (C-QED) effects at room temperature by using optically cooled nitrogen-vacancy (NV) spins. In this article, we consider improvements of these effects by exploring parameters in recent diamond maser experiments with a high frequency microwave resonator. By accounting for the rich electronic and spin levels, we provide a more complete treatment of optical pumping and dissipation in NV centers, and study the dependence of system performance on laser power. We predict the reduction of microwave photon number down to 261 (equivalent to a temperature of 116 K), about five times lower than the values reported recently. We also predict the laser-power controlled C-QED effects across weak-to-strong coupling regimes, and observe saturation of these effects under strong laser pumping. Our model can be modified straightforwardly to investigate similar effects with other solid-state spins and possible C-QED effects in maser operation.

Optimal Broadband Frequency Conversion via a Magnetomechanical Transducer

Developing schemes for efficient and broad-band frequency conversion of quantum signals is an ongoing challenge in the field of modern quantum information. Especially the coherent conversion between microwave and optical signals is an important milestone towards long-distance quantum communication. In this work, we propose a two-stage conversion protocol, employing a resonant interaction between magnetic and mechanical excitations as a mediator between microwave and optical photons. Based on estimates for the coupling strengths under optimized conditions for yttrium iron garnet, we predict close to unity conversion efficiency without the requirement of matching cooperativities. We predict a conversion bandwidth in the regions of largest efficiency on the order of magnitude of the coupling strengths which can be further increased at the expense of reduced conversion efficiency.

Hole Spin Qubits in Thin Curved Quantum Wells

Hole spin qubits are frontrunner platforms for scalable quantum computers because of their large spin-orbit interaction which enables ultrafast all-electric qubit control at low power. The fastest spin qubits to date are defined in long quantum dots with two tight confinement directions, when the driving field is aligned to the smooth direction. However, in these systems the lifetime of the qubit is strongly limited by charge noise, a major issue in hole qubits. We propose here a different, scalable qubit design, compatible with planar CMOS technology, where the hole is confined in a curved germanium quantum well surrounded by silicon. This design takes full advantage of the strong spin-orbit interaction of holes, and at the same time suppresses charge noise in a wide range of configurations, enabling highly coherent, ultrafast qubit gates. While here we focus on a Si/Ge/Si curved quantum well, our design is also applicable to different semiconductors. Strikingly, these devices allow for ultrafast operations even in short quantum dots by a transversal electric field. This additional driving mechanism relaxes the demanding design constraints, and opens up a new way to reliably interface spin qubits in a single quantum dot to microwave photons. By considering state-of-the-art high-impedance resonators and realistic qubit designs, we estimate interaction strengths of a few hundreds of MHz, largely exceeding the decay rate of spins and photons. Reaching such a strong coupling regime in hole spin qubits will be a significant step towards high-fidelity entangling operations between distant qubits, as well as fast single-shot readout, and will pave the way towards the implementation of a large-scale semiconducting quantum processor.

Ringing spectroscopy in the magnomechanical system

The ringing phenomenon has been studied in optical whispering gallery mode (WGM) resonators and can be used to sense the ultrafast process in spectroscopy. Here we observe the ringing phenomenon in a magnomechanical system for the first time, which is induced by the interference between the microwave photons converted from the damped phonons and the probing microwave photons. This interference eventually appears as a transparency window even along with the ringing phenomenon in the measured microwave reflection spectrum, which is influenced by the scanning speed and the input power. Then, the ringing spectroscopy is used to measure the coupling strength between the magnon and phonon modes, and outline the displacement profile of S1,2,2 mechanical mode in a YIG microsphere, demonstrating the theoretical analysis. In addition, the ring-up spectroscopy is developed in our magnomechanical system, laying the foundation for fast sensing based on mechanical motion.

Deterministic Loading of Microwaves onto an Artificial Atom Using a Time-Reversed Waveform.

Loading quantum information deterministically onto a quantum node is an important step toward a quantum network. Here, we demonstrate that coherent-state microwave photons with an optimal temporal waveform can be efficiently loaded onto a single superconducting artificial atom in a semi-infinite one-dimensional (1D) transmission-line waveguide. Using a weak coherent state (the number of photons (N) contained in the pulse ≪1) with an exponentially rising waveform, whose time constant matches the decoherence time of the artificial atom, we demonstrate a loading efficiency of 94.2% ± 0.7% from 1D semifree space to the artificial atom. The high loading efficiency is due to time-reversal symmetry: the overlap between the incoming wave and the time-reversed emitted wave is up to 97.1% ± 0.4%. Our results open up promising applications in realizing quantum networks based on waveguide quantum electrodynamics.

IOT-Based Injection-Locked Microwave Photonic Frequency Division Signal Processing

When building an injection-locked microwave photonic frequency division signal processing model for the Internet of Things, the waveform and frequency of the microwave have an important impact on its performance. How to optimize and adjust the injection-locked microwave photonic frequency division signal processing effect needs more research and exploration. Taking the traditional mode architecture as a reference, this paper constructs an injection-locked microwave photonic frequency division signal processing model based on the Internet of Things. In this paper, the popular deep analysis method is used to optimize the model, and the photonic technology is matched with the microwave analysis. The purpose of this construction is to weaken the microwave integration error and improve the calculation accuracy to a higher level. In addition, aiming at the difficult problem of microwave signal generation, this paper uses the optical injection method to lock the microwave photons and generate waveform signals, which makes the model data more representative, so as to solve the problems of unstable microwave signals and high transmission costs. This paper also discusses the possibility of microwave photon filtering and frequency division signal processing of microwaves. The optimal solution is determined by analyzing the experimental results of various technical means, thus proving that the injection-locked microwave photonic frequency division signal processing means has better stability and a higher fitting degree.

Magnon squeezing enhanced entanglement in a cavity magnomechanical system

We investigate the generation of entanglement in a cavity magnomechanical system, which consists of magnons and cavity microwave photons and phonons; the magnon–photon and magnon–phonon couplings are achieved by the magnetic dipole interaction and the magnetostrictive interaction, respectively. By introducing magnon squeezing induced by magnon self-Kerr nonlinearity, the magnon–photon, magnon–phonon, and photon–phonon entanglements are enhanced compared with the case without inserting magnon squeezing. We find that an optimal parameter of the squeezing exists, which yields the maximum entanglement. This study provides a new idea for exploring the properties of quantum entanglement in cavity magnomechanical systems, and may have some potential applications in quantum state engineering.

Large-bandwidth Transduction Between an Optical Single Quantum Dot Molecule and a Superconducting Resonator

Quantum transduction between the microwave and optical domains is an outstanding challenge for long-distance quantum networks based on superconducting qubits. For all transducers realized to date, the generally weak light-matter coupling does not allow high transduction efficiency, large bandwidth, and low noise simultaneously. Here we show that a large electric dipole moment of an exciton in an optically active self-assembled quantum dot molecule (QDM) efficiently couples to a microwave resonator field at a single-photon level. This allows for transduction between microwave and optical photons without coherent optical pump fields to enhance the interaction. With an on-chip device, we demonstrate a sizeable single-photon coupling strength of 16 MHz. Thanks to the fast exciton decay rate in the QDM, the transduction bandwidth between an optical and microwave resonator photon reaches several 100s of MHz.

Entangled microwave photons generation using cryogenic low noise amplifier (transistor nonlinearity effects)

This article mainly focuses on important quantum phenomenon called entanglement arising the nonlinearity property. This study uses a unique approach in which transistor nonlinearity effect (third-order nonlinearity) entangled microwave photons are created in a cryogenic low-noise amplifier (LNA). For entanglement analysis, the Hamiltonian of the designed cryogenic LNA (containing two coupled oscillators) is derived, and then, using the dynamic equation of motion, the oscillator’s number of photons and the phase-sensitive cross-correlation factor are calculated in the Fourier domain to calculate the entanglement metric. The oscillators are coupled to each other through the gate–drain capacitor, and nonlinear transconductance is as an important factor strongly manipulating the entanglement. As a main conclusion, the study shows that the designed circuit using transistor third-order nonlinearity has the ability to generate the entangled microwave photons at very low intrinsic transconductance and more importantly when the noise figure (NF) is strongly minimized. As a complementary task, the printed circuit board of the cryogenic LNA is designed and simulated to verify the ability of the circuit to achieve an ultralow NF, by which the probability of the generation of entangled microwave photons is increased.

Dynamic strain-mediated coherence based microwave photon detection within the transparent windows

We propose a scheme for quantum nondemolition (QND) measurement of microwave photons within the transparency windows in a hybrid quantum system. The quantum interface between the microwave and optical fields is characterized by the nitrogen-vacancy (NV) centers. The prominent characterization of NV centers which are embedded in the doubly clamped diamond nanoresonator (DNR) is that the electric field induced by the deformation can drive the electric–dipole–forbidden transitions of the NV’s ground states. The applied microwave field and the electrical field induced by the deformation of the DNR resonantly drive the triplet ground states of the NV centers, and the transparent windows appear around the transition frequencies of the NV’s ground states. Within the transparent windows, the shifts of the dark states induced by the weak optical field produce the third-order (Kerr) nonlinearity with reduced linear absorption. In this case, the populations of the NV centers are trapped in the dark state and the degrees of freedom of the NV centers can be decoupled from the effective Hamiltonian of the hybrid system. Therefore, the measurements of the microwave photons can be efficiently realized.

Nonreciprocal transmission in a four-mode cavity magnonics system

We propose a scheme to create optical nonreciprocal transmission in a double-cavity magnonics system, where one yttrium iron garnet sphere is placed in one of the cavities and a mechanical oscillator is shared by the coupled double microwave cavities. By manipulating the magnon–microwave photon coupling, we reveal the nonreciprocal propagation of electromagnetic fields at microwave frequencies in red and blue-detuned regimes. Furthermore, the nonreciprocal isolation ratio and group delay are analyzed. This scheme can inspire methods for constructing nonreciprocal devices.

Chiral cavity quantum electrodynamics

Cavity quantum electrodynamics, which explores the granularity of light by coupling a resonator to a nonlinear emitter1, has played a foundational role in the development of modern quantum information science and technology. In parallel, the field of condensed matter physics has been revolutionized by the discovery of underlying topological2,3,4, often arising from the breaking of time-reversal symmetry, as in the case of the quantum Hall effect. In this work, we explore the cavity quantum electrodynamics of a transmon qubit in a topologically nontrivial Harper–Hofstadter lattice5. We assemble the lattice of niobium superconducting resonators6 and break time-reversal symmetry by introducing ferrimagnets7 before coupling the system to a transmon qubit. We spectroscopically resolve the individual bulk and edge modes of the lattice, detect Rabi oscillations between the excited transmon and each mode and measure the synthetic-vacuum-induced Lamb shift of the transmon. Finally, we demonstrate the ability to employ the transmon to count individual photons8 within each mode of the topological band structure. This work opens the field of experimental chiral quantum optics9, enabling topological many-body physics with microwave photons 10,11 and providing a route to backscatter-resilient quantum communication.

Non-Hermitian shortcut to adiabaticity in Floquet cavity electromagnonics

Cavity magnon-polaritons have recently attracted intensive attention as a promising platform for exploring quantum phenomena. In this paper, we propose a Floquet engineering strategy to realize the non-Hermitian shortcut to adiabaticity in cavity magnon-polaritons. The nonadiabatic loss is restrained and the fast population transfer from the microwave photon to the magnon is realized. This model has potentially important applications in quantum information processing.

TEM at millikelvin temperatures: Observing and utilizing superconducting qubits

We present a case for developing a millikelvin-temperature transmission electron microscope (TEM). We start by reviewing known reasons for such development, then present new possibilities that have been opened up by recent progress in superconducting quantum circuitry, and finally report on our ongoing experimental effort. Specifically, we first review possibilities to observe a quantum mechanically superposed electromagnetic field around a superconducting qubit. This is followed by a new idea on TEM observation of microwave photons in an unusual quantum state in a resonator. We then proceed to review potential applications of these phenomena, which include low dose electron microscopy beyond the standard quantum limit. Finally, anticipated engineering challenges, as well as the authors' current ongoing experimental effort towards building a millikelvin TEM are described. In addition, we provide a brief introduction to superconducting circuitry in the Appendix for the interested reader who is not familiar with the subject.

Approaching microwave photon sensitivity with Al Josephson junctions.

Here, we experimentally test the applicability of an aluminium Josephson junction of a few micrometers size as a single photon counter in the microwave frequency range. We have measured the switching from the superconducting to the resistive state through the absorption of 10 GHz photons. The dependence of the switching probability on the signal power suggests that the switching is initiated by the simultaneous absorption of three and more photons, with a dark count time above 0.01 s.