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Rabi-Driven Qubit

Technion Breakthrough: Rabi-Driven Qubit Unlocks Record Squeezing for Next-Gen Quantum Computing

E. Blumenthal, N. Gutman, I. Kaminer, and their colleagues at the Technion, Israel Institute of Technology, have developed a novel method for producing high-level squeezed states of light that are essential for the advancement of bosonic quantum computing. A key and actively manipulated component of this method is a Rabi-driven qubit. In particular, this method uses the fine control provided by a Rabi-driven qubit to manipulate the quantum characteristics of light in a system.

Fundamentally, a qubit more precisely, a transmon qubit in this study operates as a two-level quantum system. In superconducting resonators, the controller is responsible for coordinating interactions with one or more harmonic oscillators, which physically depict the vibrational modes of light. “Rabi-driven” means that the qubit is exposed to a well-controlled oscillating microwave field, also known as a “Rabi drive” or “radio wave” drive.

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This drive regulates the state of the qubit and is identified by its Rabi frequency. According to the sources, this system is thought to be capable of achieving an experimental Rabi frequency of MHz. To ensure that the transmon acts as a two-level system, it is essential that the amplitude of the Rabi drive be modest enough in relation to the transmon’s anharmonicity to prevent the transmon from populating unwanted higher excited states.

The basic process that makes it possible to create squeezed states depends on the Rabi-driven qubit being dispersively linked to the harmonic oscillator or oscillators. Without actually exchanging photons, dispersive coupling suggests an interaction in which the state of the qubit affects the frequency of the resonator. The dispersive shift measures this interaction. Modulating this interaction between the harmonic modes and the Rabi-driven qubit is the novel aspect. The Rabi drive on the qubit itself and the application of particular sideband tones to the linked resonator mode(s) are precisely combined to produce this modulation.

These sideband drives are meticulously adjusted from the resonator frequency by where is the modulation frequency in order to perform a single-mode squeezing operation. The necessary squeezed states are successfully produced by this coordinated interaction, which is referred to as a modulated Jaynes-Cummings interaction. It causes a time-dependent displacement of the harmonic mode quadrature. The resulting squeezing effective Hamiltonian shows how the squeezing operation is directly controlled by the state of the qubit, denoted by. In order to achieve entangled squeezing over two light modes, the technique is modified for two-mode squeezing by applying the two bottom sidebands to one mode and the two upper sidebands to another.

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This kind of application of a Rabi-driven qubit provides a number of noteworthy benefits and innovations:

• Conditional Squeezing: Generating conditional squeezing is a significant advancement. This indicates that the qubit’s state directly affects the direction or characteristics of the compressed state. Because of its conditional nature, the qubit and oscillator can get entangled, creating a situation in which the state of the qubit is inextricably tied to the compressed direction. For instance, the squeezing direction is dependent on the post-selected qubit state, and the composite system becomes entangled if the qubit is initialized in a superposition state. This “provides an additional layer of control” over the quantum system.
• Universal Control Over Bosonic States: This research lays the groundwork for “universal control” over photonic (light) states, which may be its most significant significance. The ability to repeatedly apply a specific set of Hamiltonians to carry out any desired quantum operation within a system’s Hilbert space is implied by universal control. This new conditional-squeezing operation, made possible by the Rabi-driven qubit, allows for complete control of quantum harmonic oscillators, even though conventional displacement and squeezing operations are not enough for universal control. A richer collection of polynomial operators, including controlled-displacement operators and higher-order terms, can be produced by combining this squeezing operation with common qubit manipulations (such as rotations and displacements) and commutation relations. More complex quantum algorithms and protocols that were previously unattainable are made possible by this capacity.
• Intra-cavity Generation and Efficiency: This technology accomplishes “intra-cavity generation” in contrast to conventional methods that use external devices to create squeezed states. This has the major benefit of minimizing signal loss during transit since it creates the compressed states right in the system where they will be utilized. Reducing the travel distance of squeezed states significantly increases overall efficiency and resilience because they are extremely susceptible to photon loss.
• Dynamic Decoupling: Using a Rabi-driven qubit in this technique has the inherent advantage of dynamically decoupling the qubit from low-frequency noise With the Rabi drive itself. This essentially lengthens the dephasing time of the qubit, strengthening the system’s resistance to outside noise.
• No Kerr Nonlinearity Required: The simulations show remarkable squeezing intensities of up to 12 dB for two-mode squeezing and 13.5 dB for single-mode squeezing. Interestingly, this is accomplished without the need for Kerr nonlinearity, which can cause state distortion but is frequently employed in other squeezing techniques. This makes it possible to create squeezed states that are clearer and more accurate.

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The suggested plan is intended for implementation on current circuit QED Quantum Electrodynamics systems. Transmon qubits are usually dispersive coupled to superior superconducting resonators to construct these platforms. Realistic experimental parameters, such a dispersive shift of -50 kHz and a Rabi frequency of 40 MHz, were employed in simulations for single-mode squeezing. It was shown that the squeezing could be produced in about 20 µs, which is a lot less time than the lives of the most advanced transmon-cavity systems available today. The simulation revealed that a maximum of 12.1 dB of squeezing was accomplished in roughly 33 µs for two-mode squeezing.

Notwithstanding the encouraging forecasts, the authors note some restrictions that presently impact the squeezed superposition’s amplitude. The approximations utilized in the theoretical modelling, especially the Rotating Wave Approximation (RWA), are the main cause of these restrictions. Only under particular circumstances relating to the photon number (n) in the resonator does the RWA, which simplifies the Hamiltonian by ignoring quickly oscillating parts, hold true. As a result, the greatest number of photons that may be stored in the created squeezed state before “higher-order terms come into effect” and the approximations fail is essentially limited.

Further restrictions on the photon number are imposed by the Magnus expansion, which is used to estimate the effective squeezing Hamiltonian and requires the interaction strength to be less than the modulation frequency. Conditional squeezing may be hampered by these higher-order effects, which would restrict the amplitude of a superposition of entangled squeezed states to about 4 dB.

Mitigating these higher-order corrections to improve effective squeezing and expanding the technique to more intricate multi-mode systems for sophisticated quantum states engineering are two future research avenues that the team has identified. Although decoherence issues were not taken into account in the simulations, given the developments in transmon-cavity settings, it is thought to be difficult, but not impossible, to achieve the simulated findings in an experiment.

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In conclusion

Finally, this novel technique allows the intra-cavity production of high-level, conditional squeezed states, with the Rabi-driven qubit acting as the dynamic and accurate control element. In addition to producing impressive squeezing levels comparable to current benchmarks, its controlled interaction with harmonic oscillators made possible by specially designed microwave and sideband drives offers a tangible route towards universal control over bosonic quantum information, opening up new possibilities for future quantum computing architectures and sensing technologies.

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