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Microwave Photons

Creation of Shaped Single Microwave Photons with Frequency Adjustment Using a Fixed-Frequency Superconducting Qubit

A Novel Technique for Superconducting Computers Simplifies Quantum Communication: Frequency-Tunable Photons Produced Without Complicated Hardware

Increasing the number of physical qubits is necessary to go closer to the realisation of fault-tolerant quantum computing with superconducting qubits. There are several technological obstacles to overcome when integrating several qubits on a single chip or in a single cryogenic system, such as controlling qubit frequencies and wiring heat. Research on distributed quantum computing architectures where several processing nodes are connected by quantum communication channels has been spurred by this.

In a transmission line, an itinerant microwave photons is a viable medium for quantum communication between distant superconducting chips. This method works with superconducting circuits and allows for long-distance information transfer. Controlling the microwave photons frequency and temporal structure is crucial for efficient quantum information transfer. Due to fabrication errors that cause frequency variances between devices, it can be challenging to match the photon’s frequency with the receiver’s interface mode for the absorption process at the receiver. Furthermore, for high-fidelity communication, the temporal form of the photon must be controlled.

Prior efforts to accomplish photon shape and frequency tuning have usually depended on extra control mechanisms, like tunable couplers or flux-biased circuits, which require more biassing lines. The system becomes more complex as a result of these extra parts, making system scaling difficult. The study described is driven by the demand for easier, more scalable methods for producing frequency-tunable photons.

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A fixed-frequency circuit including a superconducting qubit dispersively connected to a resonator has been used by researchers to demonstrate a hardware-efficient method of producing frequency-tunable shaped microwave photons. In contrast to traditional techniques that employ resonant driving between particular energy levels, this novel approach stimulates photon emission via an off-resonant drive signal.

Treating the microwave resonator as an efficient band-pass filter that mediates the interaction between the transmission-line modes and the superconducting qubit is the crucial realisation. According to this viewpoint, the resonator shapes the density of states, and the qubit efficiently relates to modes close to the resonator frequency. The researchers discovered that the photon frequency can be adjusted by merely altering the drive signal’s detuning by running the system at a frequency that differs from the conventional transition frequency. As a result, no additional frequency-tuning mechanism is required.

The drive signal’s frequency and amplitude can be modulated to adjust the photon’s temporal form. Characterising the frequency-dependent photon emission rate (Γf), which is essentially established by the Lorentzian spectrum of the resonator functioning as a band-pass filter on the qubit–transmission-line connection, is the basis for the design of this modulation. For a target photon waveform, the necessary time dependency of the emission rate (Γf(t)) can be computed. Appropriate drive pulses can be constructed by monitoring the dependence of Γf and the emitted photon frequency (ωph) on the drive amplitude (Vd) and frequency (ωd).

A fixed-frequency transmon qubit connected to a fixed-frequency resonator was used for the tests. An effective resonator linewidth of κ/2π = 53.3 MHz was achieved by implementing both an intrinsic Purcell filter and a band-pass Purcell filter to minimise qubit decay via the Purcell effect while permitting a wide resonator linewidth (important for frequency tuning).

The researchers were able to prove:

• 40 MHz of frequency tunability while preserving the photon’s desired time-symmetric hyperbolic-secant form. The drive amplitude and frequency were modulated to produce this tuning range.
• Obtaining the shaped photons’ excellent temporal symmetry. By applying a phase modulation to the control drive pulse based on the observed photon phase, an initial phase distortion in the emitted waveform was fixed. The symmetry metric (s) improved from below 84% to above 98% as a result of this phase correction.
• Using tomography, the quantum state and process fidelity are confirmed. Photons emitted into states such as |0⟩, |1⟩, and superpositions were proven to be single-photons by quantum state tomography, which also produced consistently high average state fidelities of approximately 95% across the tunable frequency range. High process fidelities of about 95% were also seen in process tomography evaluation of the qubit-to-photon state transition.

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These outcomes show how reliable the method is. Its hardware-efficient design provides a simpler architecture that is essential for scalable quantum communication by doing away with the need for extra flux lines that are usually needed for frequency tweaking.

For scalability, the 40 MHz tuning range that has been shown is important. This tuning capability maintains a high success probability (P > 0.5) for frequency matching while enabling dependable communication between networks of up to N = 10 device pairs, with typical manufacturing variations in resonator frequency being around 10 MHz. On the other hand, if frequency tuning were not used, device frequencies would need to match within about 0.48 MHz for high mode matching (more than 99%), which is far more stringent than what is currently possible in terms of manufacturing.

In contrast to traditional approaches that depend on possibly erroneous assumptions, the characterisation method provides better shaping accuracy by directly observing the photon waveform to map drive parameters to emitted-photon properties.

To possibly increase the tunable frequency range and provide frequency multiplexing for more intricate networks, future research might examine the usage of several resonator modes. Another approach is to increase drive intensity up to a threshold (geff ≤ κ/4) in order to optimise the photon generation rate, which is constrained by the maximum feasible emission rate (Γf). However, this requires careful consideration of potential undesirable effects like as transitions and heating. For real-world applications, it would also be advantageous to create calibration techniques that do not necessitate direct photon waveform monitoring, hence avoiding lossy elements like circulators.

Overall, by making it possible to generate frequency-tunable, shaped single microwave photons for quantum communication in a hardware-efficient manner, this study represents a major step towards the development of large-scale, distributed superconducting quantum computers.

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