A group of physicists at the University of Oxford has shown a ground-breaking technique for producing intricate, higher order interactions in a seminal work that was published in Nature Physics. This technique may open the door for the next generation of continuous-variable quantum computers. The researchers successfully performed squeezing, trisqueezing, and quadsqueezing achieving the latter more than 100 times faster than traditional methods by utilizing the intrinsic “spin” of a single trapped atom to mediate interactions.
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The Quest for Nonlinearity
Fundamental to physics, quantum harmonic oscillators simulate everything from electromagnetic fields to molecular vibrations. Bosons like photons and phonons are used to represent excitations in these systems. Although coherent states can be produced via well-understood linear interactions those that produce or annihilate a single boson their computational capacity is constrained.
Scientists need nonlinear interactions to uncover fuller quantum behavior. Squeezing, or second-order interactions, has long been employed to increase the sensitivity of instruments like gravitational-wave detectors by reducing uncertainty in one observable (such as position) at the price of another (momentum). Nonetheless, continuous-variable quantum computation and error correction require higher-order interactions, such as third-order (trisqueezing) and fourth-order (quadsqueezing).
Because these interactions are either very weak or need very specialized technology, they have been infamously difficult to manufacture up until now. Quadsqueezing is a “outstanding challenge” in the field because in platforms such as trapped ions, the contact strength usually decreases by an order of magnitude with each consecutive order.
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A “Hybrid” Solution
The Oxford group, under the direction of O. Băzăvan and R. Srinivas, used a hybrid oscillator–spin system to get beyond these basic scaling limitations. They blended two linear spin-dependent forces (SDFs) rather than attempting to induce a fourth-order interaction directly.
In this configuration, a radio-frequency Paul trap contains a single 88Sr+ ion. The ion’s intrinsic electronic states function as a quantum “spin” or qubit, and its mobility functions as the harmonic oscillator. The researchers employed the spin to “mediate” the intended nonlinear effect in the oscillator’s motion by simultaneously driving two linear interactions and carefully altering their frequencies (detuning).
“By being able to generate SDFs conditioned on any Pauli operator, the spin component of the nonlinear interaction can be arbitrarily chosen,”. By adjusting the relative detuning of the laser beams, they were able to choose the precise order of interaction squeezing, trisqueezing, or quadsqueezing.
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Breaking Speed Records
The outcome was striking. More significantly, the group showed the first-ever use of quadsqueezing over any physical platform, achieving 9.5 dB of squeezing. They could drive the quadsqueezing interaction more than 100 times quicker than conventional approaches based on spatial derivatives of the electromagnetic field because their approach depends on linear interactions, which are intrinsically stronger.
The researchers recreated the Wigner functions of the resulting quantum states to confirm their findings. A visual “map” of the quantum state in phase space is given by these functions. The trisqueezed and quadsqueezed states demonstrated distinct non-Gaussian profiles, a crucial prerequisite for outperforming classical computers, whereas standard squeezing yields a Gaussian “pancake” shape.
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Implications for the Future
A “hitherto unexplored regime” of quantum research is made possible by the capacity to produce powerful higher-order interactions. These interactions provide the “resource” required for real-time quantum simulation of interacting boson models, such those found in quantum field theories and condensed matter physics. They are not merely theoretical curiosities.
Moreover, the method is quite scalable. To create even more intricate interactions, like cross-Kerr couplings and two-mode squeezing, the researchers propose that it might be expanded to numerous motional modes of an ion crystal. These are crucial components for a universal gate set in scalable quantum computing. “Our method presents no fundamental limit in the interaction order n,” the researchers conclude, pointing out that the strategy works on any platform that allows spin-dependent linear interactions, from diamond color centers to superconducting qubits.
This discovery is a major step toward the development of useful quantum devices that can replicate the intricacies of the natural world with previously unheard-of speed and accuracy.
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