Weak Nonlinearity Leads to Record-Breaking 14.6 dB Quantum Squeezing: A Step Forward for Quantum Metrology
An unusual degree of quantum squeezing in a superconducting microwave cavity has been shown by researchers at the International Quantum Academy and the Shenzhen Institute for Quantum Science and Engineering, marking a development in quantum optics. The greatest value of 14.6 dB recorded to date for microwave photonic states within such a cavity was attained by the researchers through the successful building of a weak nonlinear Kerr oscillator. This development provides a hardware-efficient technique to improve quantum error correction protocols and increase the sensitivity of quantum sensors.
The Search for “Quiet” Light
The Heisenberg Uncertainty Principle states that in the quantum world, it is impossible to simultaneously eliminate the noise in two conjugate variables (such as location and momentum or the quadratures of a microwave field). Scientists can “squeeze” the noise by decreasing it in one variable while raising it in another. The capacity of these compressed states to increase measurement sensitivity above the conventional quantum limit makes them valuable for identifying elusive phenomena like dark matter particles and gravitational waves.
Strong nonlinearity has always been necessary to achieve large degrees of squeezing. Strong nonlinearity, regrettably, has two drawbacks: although it makes it easier to create nonclassical states, it also unavoidably adds more decoherence, which makes quantum states “collapse” and lose their performance advantages.
Solving the Nonlinearity Conundrum
The team from Shenzhen, headed by Yuan Xu, Yanyan Cai, and Xiaowei Deng, adopted an alternative strategy. They used a mild Kerr nonlinear oscillator inside a superconducting microwave cavity in place of a strong nonlinearity. To enable accurate characterization, this cavity, which has a single-photon lifetime of 395 microseconds, was dispersively connected to an auxiliary superconducting qubit.
Weak nonlinearity is problematic because it usually produces squeezing relatively slowly. The researchers created a subtly off-resonant microwave drive to get around this. They were able to see cyclic dynamics in the quantum squeezing evolution because of this drive. The scientists found that they could successfully increase the squeezing rate by turning the device into a displaced frame. The displacement-enhanced squeezing can be intuitively understood with the squeezing rate amplified by a factor of β2, where β stands for the displacement amplitude.
The Innovation of Trotterization
The researchers used a complex mathematical and experimental technique called Trotterization to achieve the record-breaking 14.6 dB level. Using this method, the Kerr oscillator was alternatively displaced in phase space in opposite directions. This “echoed” development preserved and amplified the desirable two-photon squeezing term while eliminating unwanted interactions, namely the photon-blockade term.
The outcomes were striking. The researchers found that squeezing increased linearly during the initial cycles of evolution, averaging 1.92 dB per cycle for vacuum states. The technique was sufficiently flexible to compress multiphoton Fock states (states with a certain number of photons) for N up to 6 in addition to vacuum states. In precise metrology, these squeezed Fock states may provide even greater sensitivities than squeezed vacuum states alone.
A Novel Approach to Computing and Sensing
This work has far-reaching consequences outside of the lab. Fisher Information (FI), a measure of the most information a quantum state may offer for a certain parameter, was high in the created states. The team demonstrated the usefulness of its record-breaking squeezing for next-generation sensors by observing a metrological gain of 12.8 dB over the usual quantum limit.
Large compressed states are also essential resources for quantum error correction. They help shield quantum information from environmental “noise” by producing Gottesman-Kitaev-Preskill (GKP) states and squeezed cat states. This method is an appealing substitute for earlier approaches that necessitated more intricate, decoherence-prone setups since it is hardware-efficient and uses a modest nonlinearity that is simpler to maintain and regulate.
In conclusion
The team has paved the way for quantum technologies by showing that weak nonlinearity may beat strong nonlinearity when paired with intelligent drive engineering and displacement. Techniques that handle the fine balance between nonlinearity and decoherence will be crucial as the research advances toward practical quantum advantage.
A future in which quantum-enhanced sensitivity is a common tool in the physical sciences is promised by the success of this experiment, which implies that comparable displacement-enhanced techniques might be modified for other platforms, such as mechanical resonators, acoustic phonons, or magnons.
