Zenith Grant Awardee
Gerard Milburn; Sally Shrapnel
Information as fuel for a quantum clock
Not all energy is useful: disordered energy, a.k.a. heat, ultimately limits the efficiency of machines. Access to a source of low disorder — low entropy — enables us to make machines by pushing components of the world away from thermal equilibrium. This can be done using a battery but also by extracting information, thereby decreasing disorder, through measurement. A clock is just such a machine: it can run by extracting information from disorder. We will demonstrate a superconducting quantum circuit that can transition from an information-driven clock to a work-driven clock in a single physical implementation, thus demonstrating the equivalence of information and fuel for a quantum clock. In so doing we will determine the theoretical limits to the accuracy of clocks arising from the quantum limits to information extraction.
On a practical level, we believe that our project will demonstrate the quantum thermodynamics limits to the accuracy of physical clocks. This may have important implications for quantum technologies more generally. But it may also have implications for our understanding of time in the physical world. One of the outstanding problems in modern physics is the difficulty of reconciling quantum theory and general relativity. Much of this problem turns on an understanding of time. A promising idea, due to Rovelli, suggests that physical time ultimately depends on the concept of thermal equilibrium. As thermodynamics is generally regarded as a desiderata for new physics this may well provide a path to the terra incognita between quantum and gravity. Our information driven clock is an operational statement of Rovelli's idea.
There are some intriguing philosophical implications of our experiment. Can we view the information/fuel duality as a physical consequence of the quantum measurements necessary to reveal the phenomenology of time keeping or does it presuppose the inclusion of intelligent agents to make observations? There has long been a tension between the time of the physicists and our first person experience of passing time. By grounding time keeping in the physics of irreversibility and measurement perhaps we can reconcile these two views. Our experiment will go some way to clearing the conceptual ground for such a claim. At a deeper level we hope to show that thermal, information-driven clocks can give important insights for the alignment of casual and temporal arrows by highlighting the importance of contingent measurement records in the curiously perspectival feel of causal relations and temporal direction.
A clock is a machine and as such is constrained by the laws of thermodynamics. It matters not if it operates by quantum or classical principles; thermodynamics provides the fundamental constraints on clocks and the accuracy of time keeping.
Clocks may be periodic, e.g. pendulum clocks, quartz clocks, atomic clocks; or they may be aperiodic e.g. water clocks, radio-carbon dating, Mach's thermal clock. In every case, a clock is driven by increasing its free energy and is necessarily a dissipative system. Usually the free energy is increased by doing work on the clock using a falling weight or a battery (quartz clock). In contrast, our project will consider how the free energy of the clock may be increased by reducing the entropy by a quantum limited measurement. We call this an information driven clock as the working system of the clock is driven away from thermal equilibrium by extracting information.
To realize this clock, we choose superconducting quantum circuits with an exquisite control of their quantum degrees of freedom and ability to characterize our system at every step by quantum state tomography. The specific scheme we propose will consist of a superconducting qubit coupled to a resonator which enable us to change the free energy either by increasing the internal energy though work or decreasing the entropy by extracting information through quantum measurement of a qubit state. This system will allow us to observe the transition from an information driven clock to a work driven clock in a single physical implementation with use of external driving, quantum limited amplification and feedback operations.
Using this clock, we will demonstrate the equivalence of information and fuel for a quantum clock, formulate the thermodynamic constraints on time keeping and demonstrate the theoretical limits to the accuracy of such clocks arising from the quantum limits to information extraction.