Scientific American - USA (2020-05)

68 Scientific American, May 2020 Illustration by George Retseck

have to perform and proposed a scheme for quantifying the pre-
cision, using small-scale information theory. In other recent work,
my collaborators and I showed that fluctuation relations and new-
fangled entropy functions are two consistent approaches to small-
scale thermodynamics, and we used each approach to elucidate
the other. Quantum thermodynamicists in London, Cologne, and
elsewhere have extended and sharpened this research.

A NEW QUANTUM ENGINE
Just as traditional thermodynamics helped to describe the phys-
ics of steam engines, our efforts in quantum thermodynamics
can help us invent quantum engines. Experimentalists have now

created quantum engines with photons (particles of light), elec-

tronic systems and superconducting qubits (quantum circuits

in  which current can flow forever without dissipating).

Recently I designed a new quantum engine with Christopher D.

White, now at the University of Maryland, Sarang Gopalakrish-

nan, now at the City University of New York, and Gil Refael of the

California Institute of Technology. Being theorists, we initially

devised the engine as a thought experiment that existed in our

minds. But we are also envisioning how scientists could build a

real version of the engine using the quantum tools found in labo-

ratories today. For instance, by cooling atoms, then trapping and

manipulating them with lasers, one could bring our design to life.

Envisioning a Quantum Engine

The field of thermodynamics arose in the era of steam engines. “Quantum steampunk” physicists work to update this area of physics

for quantum technologies, as in quantum engines. One such engine, the many-body-localization (MBL)-mobile shown here, exists

as a thought experiment now, but it could be built in the near future. Just as a car engine goes through four steps in a cyclic process

that pushes a car forward, the MBL-mobile goes through a four-step quantum process to produce work.

Intake Stroke

Exhaust Compression

Power Stroke

Cold air in

Piston

Spark

MBL Phase

Transition Transition

Thermal Phase

Extremely hilly

environment

(large energy peaks

and valleys)

Environment is

less extreme

System

is chilled

Atom

Laser

Atoms can

jump position System is

heated

Car Engine

A car engine pulls in cold air and lets out hot air in a four-step cycle

that pushes a car forward. During the intake stroke, the engine

draws in cool air; gasoline is injected. During compression, a piston

moves upward inside a cylinder to compress this mixture. Then

during the power stroke, spark plugs ignite the air and gasoline.

The combustion pushes the piston down and turns the car’s wheels.

Finally, during exhaust, the hot combustion gases are expelled.

Many-Body-Localization-Mobile

The MBL-mobile goes through a four-step process that begins with atoms

in a phase of matter called many-body localization. The atoms are chilled

while in an environment with tall energy hills and low valleys, which keep

the atoms from moving around much. Next, we change the lasers’ settings

to flatten the hills and valleys in the atoms’ landscape. The engine enters

a ther mal phase where its atoms can move around, and the engine absorbs

heat. Finally, we transition back.