Photons break records in heat transport

Nature Physics has just published our paper titled Quantum-limited heat conduction over macroscopic distances. Here, we report an improvement in the range of this kind of maximally efficient heat conduction by a factor of 10,000 taking it up to a meter. This brief video tells pretty much what is going on:

Artistic impression of quantum-limited heat conduction of photons over macroscopic distances. Credit: Heikka Valja.

Artistic impression of quantum-limited heat conduction of photons over macroscopic distances. Credit: Heikka Valja.

The quantum limit

What quantum-limited heat transport actually means? In brief, it corresponds to the maximum power that can flow in a single heat conduction channel. Thus it is an upper limit. You can transport less power if your channel is bad, but you cannot transport more.

A single heat conduction channel can be made in many ways, but basically it means that in the transverse direction of the channel, the profile of the heat carrier is fixed. In the longitudinal direction, a continuum of wavelengths is possible.

Previous experiments

Although in previous experiments quantum-limited heat transport has been observed for lattice vibrations, or phonons, electrons, and electromagnetic fluctuations, the achieved distances have been very short compared with our macroscopic world. The previous record in the distance was roughtly 50 micrometers achieved by the research group of Prof. Jukka Pekola (Aalto University) by connecting two resistors into a loop made out of superconducting aluminum wires.

Our device

In contrast, we used microwave photons flying in a superconducting transmission line as the heat carriers. Photons are generally know to be good heat carriers over long distances.

Here is a schematic image of our device and the measurement scheme:

Device structure and simplified measurement scheme. A superconducting meandering transmission line on a silicon chip is terminated at both ends by micron-sized resistors. A pair of tunnel junctions at each resistor is used to measure the temperatures of their conduction electrons. The temperature drop at the left resistor due to active cooling of the right resistor is used to extract the thermal conductance between the two resistors. This quantum-limited heat conduction arises from the exchange of microwave photons flying in the transmission line. Credit: Matti Partanen.

Device structure and simplified measurement scheme. A superconducting meandering transmission line on a silicon chip is terminated at both ends by micron-sized resistors. A pair of tunnel junctions at each resistor is used to measure the temperatures of their conduction electrons. The temperature drop at the left resistor due to active cooling of the right resistor is used to extract the thermal conductance between the two resistors. This quantum-limited heat conduction arises from the exchange of microwave photons flying in the transmission line. Credit: Matti Partanen.

In addition to minimizing the effect of accidentally absorbed microwave photons into the transmission line owing to the use of a superconductor, we also needed to carefully engineer the tiny resistors at the ends of the line. These resistors are the two bodies that exchange heat through the line. In particular, we matched the resistance of both resistors to the so-called characteristic impedance of the line, that is, to 50 Ω. Because of this matching, most of the incoming photons got absorbed at the resistors taking the heat conduction to the quantum limit. (See this paper for first theoretical considerations of the phenomenon.)

Here is a close image of the resistor:

Atomic-force microscope image of one of the resistors in the device used to demonstrate quantum-limited heat conduction over macroscopic distances. Note that there are excess inactive copies of some of the structures owing to the employed fabrication methods. Credit: Matti Partanen.

Atomic-force microscope image of one of the resistors in the device used to demonstrate quantum-limited heat conduction over macroscopic distances. Note that there are excess inactive copies of some of the structures owing to the employed fabrication methods. Credit: Matti Partanen.

What can this be useful for

I have been really excited about the development of the superconducting quantum computer and large recent efforts in building also other quantum technological devices. Almost all single-quantum devices need to be precisely initialized in the beginning of their operation. Potentially engineered resistors could be used to carry out such initialization and then decoupled from the device. Here are some thoughts on how one could implement this in practice in the superconducting quantum computer.

Finally here is an articstic image to inspire you about microwave heat transport:

Artistic impression of quantum-limited heat conduction of photons over macroscopic distances. Credit: Riikka Maria Partanen.

Artistic impression of quantum-limited heat conduction of photons over macroscopic distances. Credit: Riikka Maria Partanen.

Acknowledgements

We are really lucky that we are able to implement our ideas and make your dreams come true. There are many institutions and people to thank, but being breif, I would like thank the European Research Council (ERC) for the Starting Grant that I used to build up my laboratory. I think that ERC is the best thing that has happened to European science. It regards excellence as the sole evaluation criterion. I am also very thankful for many research grants from private foundations and from the Academy of Finland, including the COMP Centre of Excellence that my group is part of. A break point in my career was in 2007 when Academitian Risto Nieminen decided to promote me as one of the PIs in COMP. I also thank Prof. Jukka Pekola for introducing me to the facinating field of experimental low-temperature physics and mesoscopic heat transport.

Last but not least, I thank my awesome QCD group and especially M.Sc. Matti Partanen and Dr. Kuan Yen Tan for their years of hard work on these results!

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Mikko is the leader of Quantum Computing and Devices (QCD) Labs at Aalto University, Department of Applied Physics. His main research interests include quantum nanoelectronics and quantum gases.
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We tied quantum knots

As just reported by Nature Physics, our collaboration between Hall Labs (Amherst College, USA) and my Quantum Computing and Devices (QCD) Labs has succeeded in tying the first knot solitons in quantum matter. We created these quantum knots in the field  describing a dilute Bose-Einstein condensate (BEC), that is, a gas a atoms forming a superfluid.

In contrast to knotted ropes, our quantum knots exist in a field that assumes a certain direction at every point of space. Thus this so-called order parameter field is a vector field, as is a typical magnetic field and the velocity field of wind or water currents. In fact, knots have been observed before in water and light fields, but we were the first ones to take them to the quantum world in practice.

Let us first see how the knot looks like in theory:

Topological structure of a quantum-mechanical knot soliton. The white ring is the core of the soliton (field pointing down), and the surrounding colored bands define a set of nested tori that illustrate the linked structure of its field lines. The boundary of the knot lies near the dark grey lines (field pointing up). Credit: David Hall.

Topological structure of a quantum-mechanical knot soliton. The white ring is the core of the soliton (field pointing down), and the surrounding colored bands (field pointing sideways) define a set of nested tori that illustrate the linked structure of its field lines. The boundary of the knot lies near the dark grey lines (field pointing up). Credit: David Hall.

In the above image, you see 16 rings. Along each ring, the order parameter field is essentially pointing to a single fixed direction. However, the direction changes from a ring to ring. Thus these rings are a bit like the contour lines of constant altitude in a typical map, but more complicated.

Each ring goes through every other ring once, that is, all rings linked. Thus the resulting structure is topologically stable and it cannot be separated without breaking the rings. In other words, one cannot untie the knot within the superfluid unless one destroys the state of the quantum matter.

Now you are ready to watch our video. We first focus on the field  directions that point left and right. This gives an image two linked rings (green and purple in the above image). It is amazing how well visible these rings are in the experimental images. Enjoy:

Each time an image of the condensate is taken, it is first let to expand and the different magnetic sublevels of the atoms are separated spatially. Thus we needed to make a new condensate for every frame of the video. I would had never guessed about four years ago when we started our collaboration that some day the experimental apparatus is so stable and the experiment so reproducible that we are able to conveniently make videos of the creation processes. Wow! Prof. David Hall has really tuned his BEC machine to unimaginable performance.

In practice, we tied the knot by squeezing the structure into the condensate from its outskirts. This required us to initialize the quantum field to point in a particular direction, after which we suddenly changed the applied magnetic field to bring an isolated null point, at which the magnetic field vanishes, into the center of the cloud. Then we just waited for less than a millisecond for the magnetic field to do its trick and tie the knot as show in the above video.

Previously, we have made monopoles (see also here) in a rather similar way except that we were moving the null point slowly. The technical improvement enabling us to tie the knots was that we sped up the ramp by orders of magnitude. An image of the experimental set up is shown in my previous blog post.

Here is an artistic image of our quantum knot. It looks great, for example, as a background image of your mobile phone.

Artistic impression of a quantum-mechanical knot soliton. See instruction below to access the full-resolution image. Credit: Heikka Valja.

Artistic impression of a quantum-mechanical knot soliton. Credit: Heikka Valja.

Knots have been used and appreciated by human civilizations for thousands of years. For example, they have enabled great seafaring expeditions and inspired intricate designs and patterns. The ancient Inca civilization used a system of knots known as quipu to store information. In modern times, knots have been thought to play important roles in the quantum-mechanical foundations of nature, although they have thus far remained unseen in quantum dynamics.

In everyday life, knots are typically tied on ropes or strings with two ends. However, these kinds of knots are not what mathematicians call topologically stable since they can be untied without cutting the rope. In stable knots, the ends of the ropes are glued together. Such knots can be relocated within the rope but cannot be untied without scissors.

Mathematically speaking, the quantum knot we created realizes a mapping referred to as Hopf fibration that was discovered by Heinz Hopf in 1931. The Hopf fibration is still widely studied in physics and mathematics. Now it has been experimentally demonstrated for the first time in quantum matter.

Our results are the beginning of the story of realizing quantum knots. It would be great to see even more sophisticated quantum knots to appear such as those with knotted cores. Also it would be important to create these knots in conditions where the state of the quantum matter would be inherently stable. Such system would allow for detailed studies of the stability of the knot itself.

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Mikko is the leader of Quantum Computing and Devices (QCD) Labs at Aalto University, Department of Applied Physics. His main research interests include quantum nanoelectronics and quantum gases.
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Quantum-mechanical monopoles discovered

We did it! Our monopole collaboration (Aalto University, Finland, and Amherst College, USA) succeeded in observing an isolated point-like monopole in a quantum field itself for the first time. This discovery connects to important characteristics of the elusive magnetic monopole. The results will be published tomorrow in Science magazine.

artistic_figure_2_s

Artistic illustration of a quantum-mechanical monopole. Credit: Heikka Valja.

We performed an experiment in which we modified a gas of rubidium atoms prepared in a nonmagnetic state, so-called polar phase, near absolute zero temperature. Under these extreme conditions we were able to create a monopole in the quantum-mechanical field that describes the gas. This structure, know as a topological point defect, resembles, for example, that of the magnetic monopole particle as described in grand unified theories of particle physics.

exp_vs_theory_2

 Comparison of the experimentally obtained image of the Bose–Einstein condensate containing the monopole (left) with the theoretical prediction (right). Brighter area has higher particle density and the different colors denote the internal spin state of the atoms. Credit: monopole collaboration.

In an experiment published about a year ago, we used the gas to detect a monopole within a synthetic (a.k.a. artificial) magnetic field, but there was no isolated monopole in the quantum field describing the gas itself. Now we have finally witnessed the quantum-mechanical monopole!

In the nonmagnetic state of the gas, no quantum whirlpools or monopoles in the synthetic magnetic field are created. However, magnetic order prevails in the sample, and we were able to manipulate this with similar adjustments to an externally applied magnetic field.

david hall7

View towards the main experimental chamber of the apparatus at Amherst College, showing the magnetic field coils and optical components required to create the superfluid containing the isolated monopole. Credit: Marcus DeMaio/Amherst College April 2015.

The result is a remarkable step forward in quantum research. It is important to understand the structure of monopoles and other topological entities because they, for example, appear in the models of the early universe and affect the properties of many different materials, such as metals.

The discovery of a magnetic monopole particle is still in the future. However, our new result establishes that the structure of a quantum mechanical monopole does appear in nature, and therefore it further supports the possibility that magnetic monopoles exist.

And now enjoy our video telling how the monopoles were created and modelled:

Finally, I give thumbs up for the hard-working scientists at Large Hadron Collider (LHC) and IceCube Neutrino Observatory. They are looking for the magnetic monopole particle! I hope that they succeed.

 

Original article (available from May 1st, 2015):

M. W. Ray, E. Ruokokoski, K. Tiurev, M. Möttönen, and D. S. Hall, ”Observation of isolated monopoles in a quantum field”, Science, DOI: 10.1126/science.1258289 (2015).

Previous work:

M. W. Ray, E. Ruokokoski, S. Kandell, M. Möttönen, and D. S. Hall, “Observation of Dirac Monopoles in a Synthetic Magnetic Field”, Nature 505, 657 (2014).

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Mikko is the leader of Quantum Computing and Devices (QCD) Labs at Aalto University, Department of Applied Physics. His main research interests include quantum nanoelectronics and quantum gases.
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Quantum Technology Marches On

In my last column in Helsingin sannomat, I wrote on the importance and great potential of quantum technology. It is truly the way of the future.

Just yesterday we had the kick-off event of the Center for Quantum Engineering (CQE) here at Aalto. Inspiring talks were given, for example, by Prof. Charlie Marcus (an ex Harvard Professor who moved to Copenhagen to led in their Center for Quantum Devices) and Colin Williams (D-Wave Systems, Inc.). At Aalto, CQE provides an exciting opportunity to work together at the frontiers of quantum technology, providing in future practical applications or critical components for such application.

Now I am thinking about the topic of my next column that will be published on May 4th. Perhaps it will be on the D-Wave quantum solver that Colin was talking about. On the other hand, we will be having big news out from my QCD Labs very soon (stay tuned). It is a difficult choice!

 

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Mikko is the leader of Quantum Computing and Devices (QCD) Labs at Aalto University, Department of Applied Physics. His main research interests include quantum nanoelectronics and quantum gases.
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