Researchers at The University of Tokyo and RIKEN have experimentally generated four orthogonal temporal modes of microwave photons, a technique that could significantly boost the capacity of quantum networks. The work demonstrates a method for encoding more quantum information within individual photons, using a fixed-frequency transmon qubit to shape the photons as they propagate along a waveguide. This photon-shaping allows for mode-selective absorption, with efficiencies exceeding 0.89 for matched modes and remaining below 0.13 for orthogonal ones. These results highlight the feasibility of temporal-mode engineering for constructing a higher-dimensional orthogonal basis.

Transmon Qubit Control Enables Four Temporal Modes

Generating single microwave photons shaped in four distinct ways now demonstrates a pathway to increasing the capacity of quantum networks, as a fixed-frequency transmon qubit proved instrumental in achieving this control. This photon-shaping technique allows for the creation of orthogonal temporal modes, essentially imprinting different characteristics onto individual photons propagating along a waveguide, and represents a step toward encoding more quantum information within each photon.

The ability to precisely manipulate these temporal modes relies on controlling the qubit’s relaxation process, which accompanies photon emission, enabling the engineering of specific photon waveforms. The process uses a resonator-assisted Raman process where the transmon qubit’s decay rate is modulated by a sender’s drive, effectively sculpting the emitted photon’s temporal profile.

This control is not merely theoretical. The researchers demonstrated mode-selective absorption, a critical function for receiving information in a multiplexed quantum network, with efficiencies exceeding 0.89 for photons with matching temporal modes. Conversely, absorption remained below 0.13 for photons encoded in orthogonal modes, highlighting the system’s ability to discriminate between different temporal “shapes”. This selectivity is achieved by implementing a time-reversed qubit excitation at the receiver, precisely matching the emission process to absorb photons with the correct temporal profile.

The independent control over both sender and receiver modes is a key aspect of this approach, allowing for selective photon absorption based on the orthogonality of the temporal modes. The team calibrated the receiver’s drive to target the same photon waveforms used during emission, ensuring accurate mode matching and efficient absorption.

Some data points revealed negative absorption values, potentially stemming from instabilities in the Josephson parametric amplifier used in the experiment. But the overall results confirm the feasibility of temporal-mode multiplexing. “The temporal modes constitute a highly orthogonal basis,” the paper states, “suggesting that this scheme offers a feasible approach to expanding the Hilbert-space dimension of quantum network channels.” The research extends beyond simply generating and absorbing these shaped photons; it demonstrates the potential for building higher-dimensional quantum networks.

By encoding information in these temporal modes, the amount of quantum data that can be transmitted through a single photon is increased, potentially boosting the overall capacity and fault tolerance of future quantum communication systems. The ability to reject photons with incorrect temporal profiles, while preserving their orthogonality for subsequent receivers, is particularly significant for multi-node architectures.

This ensures that information isn’t lost or corrupted as it travels through a complex network. The work highlights a method for constructing a higher-dimensional orthogonal basis for multiplexed quantum networks, offering a viable path toward more efficient and secure quantum information processing.

Microwave Photons for Quantum Communication

The ability to discriminate between four distinct microwave photon waveforms now exists, a key step towards increasing the capacity of quantum networks. Efficiencies reached 0.89 for photons with matching waveforms and suppression to below 0.13 for those with differing temporal profiles. This precise waveform control isn’t simply about sending more data. It’s about maintaining data integrity as it travels.

This preservation of orthogonality is important, as waveform distortion during rejection could corrupt information and limit the reach of quantum communication. Previous theoretical work highlighted this potential issue, but the experimental results demonstrate a solution by maintaining the distinct characteristics of the rejected photons.

Mode-Selective Absorption Exceeds 0.89 Efficiency

The achieved absorption efficiency of 0.89 for matched temporal modes demonstrates a significant advancement in the precision of quantum state transfer between superconducting qubits. This level of efficiency, observed when the sender and receiver modes align, was determined by calculating the ratio of residual to baseline photon flux, a metric used to quantify the success of the absorption process. Analysis of the data revealed a clear correlation between the delay time of the drive pulse and the absorption efficiency, with peak performance occurring at an optimal delay.

When the receiver mode matched the sender mode, designated as R_mm where m ranges from 0 to 3, the system consistently exhibited high absorption rates. Conversely, when the modes were mismatched, absorption was effectively suppressed, with efficiencies remaining below 0.13, confirming the system’s ability to discriminate between temporal modes.

This selective absorption is directly linked to the degree of overlap between the emitted and absorbed photon modes, quantified by the squared overlap integral, I’_mn. The team estimates that addressing these instabilities, through improved temperature control of amplifiers and mixers, could push absorption efficiency beyond 0.96 for matched modes and below 0.01 for orthogonal modes. Increasing the pulse-width parameter, κ_ph, is also identified as a means to shorten measurement duration and further enhance stability.

The preservation of phase information throughout the mode-selective absorption process is notable, as it ensures the integrity of the quantum state during transfer. The ability to reject photons when the receiver mode does not match the incoming photon mode, a scenario where m ≠ n, is important for building scalable quantum networks.

By assigning receiver mode indices higher than those of the sender, waveform distortion during rejection can be minimized, maintaining orthogonality between the photons. The squared overlaps among these rejected modes were found to be less than or equal to 0.01, indicating that the orthogonality is well-preserved even during the rejection process. This preservation of orthogonality is essential for enabling selective absorption at subsequent receivers in future multi-node architectures. The precise temporal control of the sender’s drive was key to generating single photons in four orthogonal temporal modes, a foundational step for achieving mode-selective absorption.

The observed efficiencies, coupled with the preservation of photon orthogonality, highlight the potential of temporal-mode engineering for constructing higher-dimensional orthogonal bases for multiplexed quantum networks. “Based on the residual phase distortion after phase compensation, we estimate using Eq. (9) that further suppression of time-dependent instabilities would allow the absorption efficiency to exceed 0.96 for mode-matched cases and to fall below 0.01 for orthogonal modes,” the researchers report. This level of control over photon characteristics represents a step towards realizing more robust and efficient quantum communication systems.

Orthogonal Temporal Modes Defined by Gram-Schmidt Procedure

The creation of four distinct photon waveforms, each precisely defined in time, underpins a new approach to expanding the capacity of quantum communication channels. Researchers used the Gram-Schmidt orthonormalization procedure to build a set of orthogonal mode functions, enabling the generation of these itinerant microwave photons. This method allows for the construction of multiple, independent pathways for quantum information within a single communication channel, potentially overcoming limitations imposed by temporal window and frequency bandwidth.

The ability to generate and selectively absorb photons shaped in this way relies on a carefully calibrated process at both the sending and receiving ends of a quantum link. A receiver’s drive, targeting the desired photon waveform, is calibrated using the same procedure as the sender, ensuring optimal absorption of the intended signal. Measurements revealed that the squared overlap between emitted and absorbed photon modes, denoted as I’_mn, is critical for achieving high absorption efficiencies.

Analysis of mode-selective absorption efficiencies, represented as R_mn, revealed a strong correlation with the delay of the drive pulse. The team observed efficiencies exceeding 0.89 for mode-matched cases, meaning when the sender and receiver were tuned to the same temporal mode. The technique’s advantage lies in its efficient use of available resources. Temporal-mode multiplexing is particularly beneficial when both temporal and spectral resources are limited, as multiple orthogonal modes can be constructed within the same time-frequency space.

This contrasts with approaches that require disjoint temporal or spectral regions for each mode, potentially increasing bandwidth demands. The researchers note that the number of orthogonal modes achievable is ultimately constrained by finite resources in quantum communication channels. The work demonstrates the feasibility of using temporal modes as a degree of freedom for multiplexing quantum information.

Scalable Quantum Networks via Temporal-Mode Engineering

This achievement relies on precise control over microwave photons, enabling the encoding of more quantum information within each transmitted particle. The technique uses a fixed-frequency transmon qubit to sculpt these temporal shapes, a critical component for manipulating the photons’ wave packets. The ability to selectively absorb these photons based on their temporal mode is central to the research, offering a pathway toward more robust and efficient quantum communication.

Photons rejected by a receiver do not disrupt subsequent transmissions; they maintain their orthogonality, allowing for selective absorption at later nodes in a network. This preservation of orthogonality is a key advantage for building multi-node architectures, where photons may traverse several intermediate stations before reaching their final destination.

The underlying principle stems from the commutative nature of the pulse mode operators associated with orthogonal modes, meaning they behave as independent quantum channels. A microwave drive at the sender initiates a transition, shaping the emitted photon into a specific temporal mode. This process, detailed in the paper, demonstrates how the effective decay rate of the photon can be controlled, allowing for the generation of the desired temporal waveforms.

The technique’s efficiency is further enhanced by the use of a fixed-frequency transmon qubit, simplifying the control and calibration procedures. The work builds on existing methods of encoding quantum information, such as time-bin and frequency-bin encoding, by offering a potentially more scalable solution.

While these earlier techniques have proven effective, they often require complex spectral filtering or precise timing synchronization. The observed efficiencies, coupled with the preservation of photon orthogonality, highlight the potential of temporal-mode engineering for constructing higher-dimensional orthogonal bases for multiplexed quantum networks.

👉 More information
🗞 Temporal-mode engineering of microwave photons for mode-selective quantum state transfer
✍️ Keika Sunada, Takeaki Miyamura, Kohei Matsuura, Zhiling Wang, Jesper Ilves, Shingo Kono and Yasunobu Nakamura
🧠 DOI: http://link.aps.org/doi/10.1103/wdgy-txz7

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