Physicists use light waves to accelerate supercurrents, enable ultrafast quantum computing

Jigang Wang and his collaborators have demonstrated light-induced acceleration of supercurrents, which could enable practical applications of quantum mechanics such as computing, sensing and communicating. Larger image. Image courtesy of Jigang Wang.

https://www.news.iastate.edu/news/2019/07/01/supercurrents

AMES, Iowa – Jigang Wang patiently explained his latest discovery in quantum control that could lead to superfast computing based on quantum mechanics: He mentioned light-induced superconductivity without energy gap. He brought up forbidden supercurrent quantum beats. And he mentioned terahertz-speed symmetry breaking.

Then he backed up and clarified all that. After all, the quantum world of matter and energy at terahertz and nanometer scales – trillions of cycles per second and billionths of meters – is still a mystery to most of us.

“I like to study quantum control of superconductivity exceeding the gigahertz, or billions of cycles per second, bottleneck in current state-of-the-art quantum computation applications,” said Wang, a professor of physics and astronomy at Iowa State University whose research has been supported by the Army Research Office. “We’re using terahertz light as a control knob to accelerate supercurrents.”

Superconductivity is the movement of electricity through certain materials without resistance. It typically occurs at very, very cold temperatures. Think -400 Fahrenheit for “high-temperature” superconductors.

Terahertz light is light at very, very high frequencies. Think trillions of cycles per second. It’s essentially extremely strong and powerful microwave bursts firing at very short time frames.

Wang and a team of researchers demonstrated such light can be used to control some of the essential quantum properties of superconducting states, including macroscopic supercurrent flowing, broken symmetry and accessing certain very high frequency quantum oscillations thought to be forbidden by symmetry.

It all sounds esoteric and strange. But it could have very practical applications.

“Light-induced supercurrents chart a path forward for electromagnetic design of emergent materials properties and collective coherent oscillations for quantum engineering applications,” Wang and several co-authors wrote in a research paper just published online by the journal Nature Photonics.

In other words, the discovery could help physicists “create crazy-fast quantum computers by nudging supercurrents,” Wang wrote in a summary of the research team’s findings.

Finding ways to control, access and manipulate the special characteristics of the quantum world and connect them to real-world problems is a major scientific push these days. The National Science Foundation has included the “Quantum Leap” in its “10 big ideas” for future research and development.

“By exploiting interactions of these quantum systems, next-generation technologies for sensing, computing, modeling and communicating will be more accurate and efficient,” says a summary of the science foundation’s support of quantum studies. “To reach these capabilities, researchers need understanding of quantum mechanics to observe, manipulate and control the behavior of particles and energy at dimensions at least a million times smaller than the width of a human hair.”

Wang and his collaborators – Xu Yang, Chirag Vaswani and Liang Luo from Iowa State, responsible for terahertz instrumentation and experiments; Chris Sundahl, Jong-Hoon Kang and Chang-Beom Eom from the University of Wisconsin-Madison, responsible for high-quality superconducting materials and their characterizations; Martin Mootz and Ilias E. Perakis from the University of Alabama at Birmingham, responsible for model building and theoretical simulations – are advancing the quantum frontier by finding new macroscopic supercurrent flowing states and developing quantum controls for switching and modulating them.

A summary of the research team’s study says experimental data obtained from a terahertz spectroscopy instrument indicates terahertz light-wave tuning of supercurrents is a universal tool “and is key for pushing quantum functionalities to reach their ultimate limits in many cross-cutting disciplines” such as those mentioned by the science foundation.

And so, the researchers wrote, “We believe that it is fair to say that the present study opens a new arena of light-wave superconducting electronics via terahertz quantum control for many years to come.”

Extending nonlinear coherent quantum control with intense terahertz pulses

Christoph Lange, Thomas Maag, Andreas Bayer, Sebastian Baierl, Matthias Hohenleutner, Dieter Schuh, Dominique Bougeard, Rupert Huber, Martin Mootz, John Sipe, Stephan Koch and Mackillo Kira

Two-dimensional field-resolved spectroscopy of Landau electrons reveals population inversion and multiwave mixing that are explained by Coulomb interactions between electrons and the ionic background.

http://spie.org/newsroom/6634-extending-nonlinear-coherent-quantum-control-with-intense-terahertz-pulses

19 October 2016, SPIE Newsroom. DOI: 10.1117/2.1201608.006634

Dynamics in solid-state systems are governed by many-body interactions that are inherently tied to the high density of electrons and ions. For most elementary excitations, however, Coulomb (i.e., elastic) scattering leads to dephasing within a few to a few-hundreds of femtoseconds. Coherent quantum control (the precise manipulation of the phases of quantum states) is therefore usually considered to be a daunting challenge in many-body systems.

In 1961, however, Walter Kohn found that the cyclotron resonance (CR) of Landau electrons (i.e., the harmonic motion of electrons in a magnetic field) is immune to electron–electron Coulomb interactions. Kohn's theorem thus shows that the CR is one of the most robust manifestations of the quantum harmonic oscillator, and excludes the possibility of any nonlinear light–matter interactions.1 Although the CR has given rise to a number of sophisticated quantum phenomena, such as ultrastrong light–matter coupling,2 superradiance,3 coherent control,4 and superfluorescence,5 the complete absence of nonlinearities suggests that many intriguing possibilities (e.g., quantum logic operations) are excluded.

In this work,6 we show how strong terahertz (THz) pulses can be used to create non-perturbative THz excitations of a magnetically biased, 2D electron gas (2DEG). The pulses induce strong, coherent nonlinearities and facilitate coherent quantum control of multiple Landau levels, leading to population inversion. In our approach, the 2DEG is contained within two 30nm-wide gallium arsenide quantum wells (each n-doped at 1.6 × 1011cm−2) and is biased with a perpendicular magnetic field of 3.5T. These conditions provide a filling factor of 0.95 and a CR frequency (νc) of 1.45THz.

In our experiments, strong linearly polarized THz transients—which are incident perpendicular to the sample—cause the excitation of a coherent polarization. We monitor this polarization by examining the orthogonal polarization component of the reradiated field, as illustrated in Figure 1(a). For weak THz transients with an amplitude of 0.7kVcm−1 (black curve), we observe long-lived coherent oscillations. When we increase the field up to 8.7kVcm−1 (red curve), however, we find that the coherence is strongly diminished. By fitting an exponentially decaying sinusoidal function to the data, we extract the 1/e decay constant (τc) and find—see Figure 1(b)—that there is an abrupt drop in the coherence at about 3kVcm−1. We use a microscopic many-body theory to calculate the dynamics of the electrons. In this way, we retrieve the Landau-level population as a function of THz amplitude—blue and red bars in the inset of Figure 1(b)—which reveals that interactions with longitudinal optical phonons lead to the sharp drop of coherence. Furthermore, our theory shows that the population climbs up to the 6th Landau level and that population inversion is achieved.

 

Figure 1. (a) The transmitted terahertz (THz) field is used to monitor the coherent inter-Landau level polarization. Results are shown for different amplitudes of the driving field (between 0.7 and 8.7kVcm- 1), as a function of the electro-optic sampling (EOS) delay time (t). (b) The decay constant (τc) that is extracted from the data in (a), as a function of the initial field (). The Landau-level population, for fields of 4.3 and 8.7kVcm-1 (blue and red bars, respectively), is shown in the inset. f: Population density. ℏωLO: Longitudinal optical phonon energy.

To systematically test whether coherent nonlinear control of massively many-body Landau states is possible, we perform 2D, phase-resolved THz spectroscopy. To this end, we use a strong, single-cycle THz pulse (A) to create a highly excited coherent state, and a weak pulse (B) to probe the quantum state of the CR. We then measure the coherent polarization dynamics as a function of the electro-optic sampling time (t) of B, as well as the relative delay (τ) between the pulses. If Kohn's theorem applies, the correlated nonlinear polarization response ()—i.e., representing the interactions between A and B—should be zero, as verified for small amplitudes of the electric field ().6 However, strong coherent modulations in  occur at multiples of νc−1 along both time axes (i.e., τ and t) even for moderate values of  —see Figure 2(a) and (b)—of 1.4 and 4.3kVcm−1. These wavefronts in the time domain are linked to distinct features in the frequency domain that represent the pump-probe, as well as four- and six-wave mixing processes. Furthermore, the relative amplitudes of the off-diagonal pump-probe and four-wave mixing signals are evidence of the non-perturbative scaling exhibited by the surprisingly strong nonlinearities.6

 

Figure 2. (a) and (b) The nonlinear THz polarization response, , for two representative field amplitudes () of pulse A. Example wavefronts that represent a pump-probe (black line) and a four-wave mixing process (red line) are indicated in (a). (c) and (d) Fourier transformation of the data from (a) and (b), as a function of the frequencies νt and ντ. The pump-probe (PP), four-wave mixing (4WM), and six-wave mixing (6WM) signatures are circled.

Our microscopic theory allows us to assess the relative contributions of the nonlinearities by making a ‘switch-off’ analysis.6 The results of this analysis reveal that band-structure effects are initially dominant (at  of 1.4kVcm−1), but are overruled by much stronger, many-body interactions between electrons and dopant ions that occur as  increases. These Coulomb effects thus dynamically gain strength with increasing  and induce a range of fully coherent nonlinearities. It is such nonlinearities that are responsible for the non-perturbative four- and six-wave mixing signals in this setting (i.e., beyond the realm of Kohn's theorem).

In summary, we have demonstrated that strong terahertz pulses can be used to create strong, coherent nonlinearities in Landau systems that are suitable for THz quantum control. Our results show that the cyclotron resonance is a promising candidate for quantum control at THz clock rates as it provides both the long-lived coherence and nonlinearities that are required for switching operations. In more general terms, Coulomb correlations are used to examine the internal degrees of freedom of many-body quantum systems. Our work suggests that the role of massive many-body interactions (usually considered as detrimental to quantum control) will have to be reassessed. Currently, we are investigating coherent quantum control of Landau electrons in non-polar semiconductors, in which an even larger number of Landau levels may contribute to the nonlinearity before phonon interactions become relevant.

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Christoph Lange, Thomas Maag, Andreas Bayer, Sebastian Baierl, Matthias Hohenleutner, Dieter Schuh, Dominique Bougeard, Rupert Huber

Department of Physics
University of Regensburg

Regensburg, Germany

Martin Mootz, Stephan Koch, Mackillo Kira

Department of Physics
University of Marburg

Marburg, Germany

John Sipe

Department of Physics
University of Toronto

Toronto, Canada

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References:

1. W. Kohn, Cyclotron resonance and de Haas-van Alphen oscillations of an interacting electron gas, Phys. Rev. 123, p. 1242-1244, 1961.

2. G. Scalari, C. Maissen, D. Turčinková, D. Hagenmüller, S. De Liberato, C. Ciuti, C. Reichl, et al., Ultrastrong coupling of the cyclotron transition of a 2D electron gas to a THz metamaterial, Science 335, p. 1323-1326, 2012.

3. Q. Zhang, T. Arikawa, E. Kato, J. L. Reno, W. Pan, J. D. Watson, M. J. Manfra, et al., Superradiant decay of cyclotron resonance of two-dimensional electron gases, Phys. Rev. Lett. 113, p. 047601, 2014. doi:10.1103/PhysRevLett.113.047601

4. T. Arikawa, X. Wang, D. J. Hilton, J. L. Reno, W. Pan, J. Kono, Quantum control of a Landau-quantized two-dimensional electron gas in a GaAs quantum well using coherent terahertz pulses, Phys. Rev. B 84, p. 241307(R), 2011. doi:10.1103/PhysRevB.84.241307

5. G. T. Noe II, J.-H. Kim, J. Lee, Y. Wang, A. K. Wójcik, S. A. McGill, D. H. Reitze, A. A. Belyanin, J. Kono, Giant superfluorescent bursts from a semiconductor magneto-plasma,Nat. Phys. 8, p. 219-224, 2012.

6. T. Maag, A. Bayer, S. Baierl, M. Hohenleutner, T. Korn, C. Schüller, D. Schuh, et al., Coherent cyclotron motion beyond Kohn's theorem, Nat. Phys. 12, p. 119-123, 2016.