Thermodynamics with Correlations: Enhancing the Performance of Quantum Maxwell Demons

Adrian Chapman and Akimasa Miyake have recently published their paper in Physical Review E, demonstrating that a Maxwell demon can exploit correlations in its memory to enhance its thermodynamic performance over the demon which cannot. In this classic thought experiment, the demon uses information, rather than energy, to perform a thermodynamic task such as refrigeration. One example is realized by a demon who controls the door of a bipartite container filled with gas. Using a microscopic measuring device, the demon allows only faster molecules to one side while allowing only slower molecules to the other, apparently performing cooling for free. Only when one accounts for the inevitable energy cost of erasing the device’s memory does one see that this supposed perpetual motion machine is actually a refrigerator. This fact suggests that information has value in thermodynamics.

In their paper, the authors consider a simple concrete model of physical system, which acts as an autonomous Maxwell demon. In many previous models, the assumption has been that the demon’s information is uncorrelated: its actions at one time do not depend on what it has learned at earlier times. This convenient assumption may neglect some important physics however, especially in a quantum world, where information can be correlated in a “spooky” way. The authors construct a framework for their model in which quantum correlations can be incorporated in full generality and find that correlations can be thermodynamically useful in the same way as energy or uncorrelated information in the original thought experiment. Their framework relies on techniques from condensed matter physics, which are specialized for handling correlations.

Correlated states of knowledge are not the exception, but rather the rule for realistic thermodynamic systems. This research will thus likely cross-fertilize the fields of thermodynamics and condensed matter physics and ignite further such analyses that incorporate correlations.

The full article is available online at http://journals.aps.org/pre/abstract/10.1103/PhysRevE.92.062125.

QuantumThermodynamicsImage

Quantum information is passed along sequentially to the demon, D, determining whether it will exchange energy with a hot thermal bath or a cold thermal bath.

CircuitCompositionwEqns

The one-dimensional nature of the model allows for correlated information to be efficiently stored and updated according to the interaction sequence.

GHZPhase

Correlations allow for the emergence of a new operational phase, wherein the demon erases and refrigerates simultaneously as it consumes correlations.

 

 

 

 

Doing More with Quantum Phases of Spin Chains

Jacob Miller and Akimasa Miyake have recently published a paper in Physical Review Letters showing that a particular class of one-dimensional spin chains, belonging to a type of quantum phase, are uniformly useful for certain quantum information processing tasks. Our work takes place in the context of measurement-based quantum computation, a means of using entangled many-body systems to perform quantum computation requiring only measurements on individual spins. One advantage of this formalism is that it lets us make interesting connections between quantum information science, which deals with concepts like entanglement and quantum computation, and condensed matter physics, which is concerned with emergent phenomena and phases of matter.

Our result is that a particular phase of one-dimensional spin chains -- known as a symmetry-protected topological phase -- can be used to perform any single-qubit information processing task we would like. In particular, while standard measurement-based protocols require knowledge of all of the microscopic details of the many-body systems being worked with, our protocol works for any arbitrary state in this phase (so long as two minimal conditions on the state are met). Since 1D spin chains are limited to single-qubit operations in measurement-based quantum computation, this can be thought of as the best resource characterization we could hope for in a 1D symmetry-protected topological phase.

The full article is available online at http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.120506.

 

Coincidence? Nope!

The close relationship between gate fidelity and a string order parameter for a class of states within our phase. Gate fidelity here quantifies how useful a state is for quantum information processing, while string order parameters are standard quantities used to characterize symmetry-protected topological phases. Our protocol works by first purifying arbitrary states into a "fixed-point" form, with the different curves here representing different degrees of purification. As the purification increases (red → yellow → green), the gate fidelity and string order parameter behave identically, demonstrating the close relationship between the quantum information and condensed matter aspects of this phase.

“Quanta Magazine” Interviews Ivan Deutsch

Ivan Deutsch Q&A in "Quanta Magazine"

Ivan Deutsch Q&A in “Quanta Magazine”

 

The Ivan Deutsch group at CQuIC received a welcome surprise with the publication of an interview with Dr. Deutsch in “Quanta Magazine“. The Deutsch group’s research has focused on the control of a 16 dimensional quantum system, or “qudit” in the quantum information parlance. This system offers the opportunity to study robust quantum control techniques (where errors introduced in the control protocols do not introduce too many errors into the behavior of the system), as well as the opportunity to examine novel quantum effects.