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Quantum Thermal Transistor

The control of electric currents in solids is at the origin of modern electronics which has revolutionised our daily life. The diode and the transistor introduced by Braun  and Bardeen and Brattain  are undoubtedly the corner stones of modern information technologies. Such devices allow for rectifying, switching, modulating, and even amplifying the electric current.


Researchers have experimented with similar devices which would control the  heat flow. An important step forward in this direction has been carried out in 2006 by Li et al. by introducing phononic counterpart of a field-effect transistor. In their device, composed as its electronic analogue, of three interconnected solid elements, the temperature bias plays the role of the voltage bias and the heat currents carried by phonons play the role of the electric currents.


Thermal Transistor

Electronic transistor made of three terminals, the source (S), the gate (G), and the drain (D). The gate is used to actively control (by applying a voltage bias V on it) the apparent electric conductivity of the channel between the source and the drain. The radiative analogue of an electronic transistor basically consists in a source and a drain, labelled by the indices S and D, which are maintained at temperatures TS and TD (which play an analogue role as the voltage) using thermostats where TS>TD so that a net heat flux is transferred from the source towards the drain. A thin layer of a metal-isulator transition material (MIT) labelled by G of width δ is placed between the source and the drain at a distance d from both media and operates as a gate.


Like an electronic transistor controls electrical current in a circuit, a thermal transistor controls heat current, amplifying it or switching it between values. Macroscale thermal transistors recycle waste heat in power systems; while microscale ones route heat away from delicate devices.


Macroscale thermal transistors could be used in applications that recycle waste heat that has been harvested from power stations and other energy systems. Currently, there are methods for transporting and guiding this heat, but not for controlling, amplifying, and switching the heat on and off, as the quantum thermal transistor can do.


Quantum Thermal transistor

In 2016, Karl Joulain et al., at the University of Poitiers and CNRS in France, published a paper on the quantum thermal transistor in a recent issue of Physical Review Letters. Researchers designed a quantum thermal transistor that can control heat currents, in analogy to the way in which an electronic transistor controls electric current.


“To manage electricity, one uses electronic diodes, transistor and amplifiers,” Joulain told Phys.org. “We would like to do the same thing with thermal currents. We would like to make logical thermal circuits in the same way electronic thermal circuits have been designed. In this way, wasted heat could be guided, switched on or off, amplified or modulated.” Although this is not the first thermal transistor, it is the first that is made of quantum objects. Other thermal transistors are made with macroscopic materials, such as solids or phase change materials.


The new quantum thermal transistor consists of three two-level systems, meaning they have two states. These systems can be implemented as spins, where each spin can be in either the up state or the down state. Any one of these spin systems can control the heat current flowing through the other two. The researchers theoretically demonstrated that the thermal current can be controlled, modulated, and amplified by a sufficiently large amount so that it can switch the spins between their two states, producing a transistor effect.


The transistor could be used to control thermal currents in a variety of nanostructures made of quantum objects. In the future, for instance, the device could in principle be fabricated with quantum dots embedded in nanoparticles.


A three-qubit transistor design offers a way to manipulate the system’s heat flow by hitting one of the qubits with a laser

A typical quantum thermal transistor consists of three qubits coupled to three thermal baths. Varying the temperature of any one of these baths allows the corresponding coupled qubit to be used to control the heat current through the other two qubits. The transistor effect appears when the current in the former qubit is strong enough to switch the flow to the other two qubits.


Instead of varying the bath temperature, the researchers propose applying a repetitive force to one of the qubits using a pulsed laser. Their model shows that by doing so, researchers could use the excited qubit to control whether a current flows through the other two and to determine the amplitude of that current. Predictions indicate this design could increase current amplitude by a factor of 150 or more.


The researchers say that their solution, unlike previous proposals, will work even if the bath temperature approaches zero. It also avoids the energy-intensive step of changing bath temperature. The team says that their device could be realized using ultrathin-film qubits excited by a laser, writes Rachel Berkowitz


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