Instead of starting from a system that has been cooled to its ground state, hot Schrödinger cat states are quantum superposition states that are created from thermally stimulated initial states. In contrast, Schrödinger cat states are typically demonstrated experimentally as “cold” Schrödinger cat states, which begin with a pure vacuum Fock state produced by cooling the system to its ground state.
Hot Schrödinger cat states in theory
This is a more thorough explanation:
A cat, a macroscopic and out-of-equilibrium system at body temperature, was in a superposition of two mixed states dominated by classical fluctuations in Schrödinger’s original thought experiment. Since they are derived from mixed thermal states, the “hot” Schrödinger cat states produced in this study are closer to this original concept.
Motivation: The difficulty of attaining ground-state cooling for some continuous-variable quantum systems, like levitated particles and nanomechanical oscillators, is the driving force behind the creation of hot Schrödinger cat states. These systems struggle to attain their quantum ground state, but can achieve lengthy coherence durations. Thus, new opportunities for these systems are created by the ability to produce and monitor quantum phenomena such as superposition in thermally excited states.
Method of Preparation: Using solely unitary interactions with a transmon qubit, the researchers created these hot cat states in a microwave cavity mode. They modified the echoing conditional displacement (ECD) and qcMAP protocols, two well-known “cold” cat state protocols from the field of circuit quantum electrodynamics (cQED). The procedure entails:
- Using a heat bath to equilibrate the cavity mode and provide a thermal starting state. These initial states had a purity of as low as 0.06, which is much hotter than the physical environment and corresponds to a cavity mode temperature of up to 1.8 Kelvin.
- Using the transmon qubit to generate a quantum superposition of displaced heat states by applying the modified ECD and qcMAP protocols.
- Calculating the “hot” Schrödinger cat states’ Wigner functions.
Important Findings and Features:
- Despite being heavily mixed, the resultant hot Schrödinger cat states showed Wigner-negative interference patterns for all initial thermal states under investigation, indicating their nonclassical nature.
- In contrast to their equivalence for cold cats, the two methods (ECD and qcMAP) produced different results when applied to warm starting states.
- Like the displaced thermal states themselves, the ECD state’s Wigner function displayed an interference pattern with an envelope that increased in radius and shrank in amplitude as the thermal excitation number (nth) increased.
- The interference pattern in the qcMAP state’s Wigner function declined in amplitude more slowly than the displaced thermal states, but its envelope shortened as nth increased.
- A characteristic of quantum superposition is the formation of coherence through the unitary processes, as demonstrated by the additional peak in the coherence functions of the prepared states.
- According to theoretical study, the thermal occupation number should not affect the contrast of the interference fringes in the marginal distributions under optimal circumstances.
Quantum Technology Implications: There are various possible ramifications if hot Schrödinger cat states are successfully generated and characterised.
- Reduced cooling needs for creating quantum technology and investigating quantum phenomena in a larger variety of physical systems.
- Due to the observation of quantum characteristics in highly mixed states, there is potential for noise-tolerant quantum protocols.
- Bosonic qubit encoding developments, especially the qcMAP protocol, which exhibits saturated parity values even at finite mode temperatures.
- Saturated parity offers opportunities for quantum metrology in less demanding temperature conditions.
- Enhanced comprehension of how thermal noise affects quantum coherence.
- Motivation for novel quantum control methods appropriate for systems with higher temperatures.
Essentially, this study shows that quantum superposition is not limited to very low temperature settings. It is feasible to create and monitor quantum features in states with high thermal excitation by using particular unitary protocols, which could increase the range and use of quantum technologies.
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In brief
In this Science Advances study, “hot” Schrödinger cat states quantum superpositions arising from mixed thermal states instead of pure ground states are experimentally created and analysed. Using unitary interactions with a transmon qubit, the researchers were able to create these extremely mixed quantum states in a microwave cavity with initial purities as low as 0.06.
Even though these hot cat states were impure, they showed distinct quantum characteristics, such as Wigner-negative interference patterns, showing that very pure beginning states are not always required to observe quantum phenomena. The study also examined the possibility of their findings for continuous-variable quantum systems where ground-state cooling is difficult, comparing two unique preparation techniques, echoing conditional displacement (ECD) and qcMAP, and found differing results when applied to thermal states.
News source: Hot Schrödinger cat states
What is a quantum state?
A mathematical entity that represents the knowledge of a quantum system is called a quantum state in quantum physics. The creation, development, and measurement of a quantum state are outlined in quantum mechanics. The outcome is a forecast for the state-represented system. All that can be learnt about a quantum system is exhausted by knowledge of the quantum state and the laws governing the system’s temporal evolution.
The definition of quantum states might vary depending on the type of system or issue.