Artificial atoms become chiral

    Anton Frisk Kockum

    • Wallenberg Center for Quantum Technology, Chalmers University of Technology, Gothenburg, Sweden

Physics 16, 103

Selective interaction of devices with left and right propagation modes could pave the way for directional information flow in quantum computing based on superconducting circuits.

C.Joshi et al. [1]; adapted from APS/Carin Cain
Figure 1: An emitter (top), which could be a superconducting atom or circuit, couples to a 1D waveguide at two points separated by a quarter wavelength distance. Time-modulated coupling imparts a phase (𝜑) at the left and right points, and the difference, or relative phase, tunes the interference between the two radiation paths. The result is a chiral atom that couples either forward-mode (blue) or backward-mode (red), depending on the relative phase.An emitter (top), which could be a superconducting atom or circuit, couples to a 1D waveguide at two points separated by a quarter wavelength distance. Time-modulated coupling imparts a phase (𝜑) at the left and right points, and the difference, or… Show more

If a source emits a wave that spreads across an object and is then measured by a detector, the principle of reciprocity states that the measured signal will remain unchanged if the source and detector switch places. This symmetry is a prevalent feature in all physical systems, but for certain applications it constitutes an obstacle. For example, to create an isolator, a gadget that allows signals to pass in one direction but not the other, reciprocity must be broken. Such non-reciprocal devices – those defined by preferential direction or “chirality” in their emission or absorption – are valuable in many fields. Recently, non-reciprocal devices have been implemented in superconducting electrical circuits used in quantum computing, but all have had drawbacks. Now Chaitali Joshi and his colleagues at the California Institute of Technology have built a simpler non-reciprocal device: an “artificial atom” made of a superconducting circuit, which can be coupled exclusively to left- or right-moving signals in a waveguide. microwave wave. [1]. This chiral design could be used in quantum networks to enable control of the flow of information between multiple artificial atoms coupled to a waveguide.

Superconducting circuits are one of the most important platforms for quantum computing [2]. But they would benefit from having non-reciprocal components which could help them keep calm and route quantum information [2–5]. Previous work has demonstrated non-reciprocal devices that control the propagation of visible light using natural atoms and other single photon emitters [3]. In that work, light is confined to a planar waveguide which limits the polarization of light to specific orientations. An atom or other emitter coupled to the waveguide can then be made to emit and absorb only light that travels in one direction.

However, this visible light setup doesn’t work for superconducting circuits and the low-frequency microwaves they couple to. [6]. Because natural atoms are not very flexible microwave emitters, researchers typically use man-made atoms made of superconducting components arranged in a resonant circuit format. Like real atoms, these superconducting circuits have ground states and excited states, which can be set up for the desired application. The problem, however, is that the coupling between artificial atoms and microwave waveguides does not offer the same polarization dependence as the visible case [6]. Researchers have come up with other strategies, but existing chiral interfaces for superconducting circuits tend to be cumbersome, complex, or otherwise limited [2].

Some recently proposed and demonstrated schemes for chirality use a “giant molecule”, which is a pair of artificial atoms coupled together [7, 8]. Each atom is connected to a waveguide at a separate point. Interference effects alter the emission and absorption of each atom, thereby causing suppression or enhancement of transmission through the waveguide. Joshi and his colleagues have taken this idea and simplified it such that only one artificial atom is needed as an emitter. They designed an artificial atom that couples to a 1D waveguide at multiple points separated by single wavelength distances, making an extension of the giant molecule concept in the form of a “giant atom” [9, 10].

To achieve the required interference effects using a single emitter, the researchers not only had to set the distance between the coupling points, but also establish the phase of the coupling at each point. They achieved this by using additional superconducting artificial atoms as couplers between the emitter atom and the waveguide. Using a magnetic field, the team could tune the coupler atoms to effectively control the coupling between the emitter and the waveguide. The relative phase between the modulations of the two couplers resulted in the crucial phase difference that lets forward or backward propagating light pass through the waveguide (Fig. 1). The phase difference in the modulation was easy to tune, and thus the chirality of the interaction could easily be flipped from one direction to another.

The researchers demonstrated the properties of their device in a series of experiments. First, they measured the transmission of a weak photon signal in resonance with the atom. This measurement showed that the coupling to the forward or backward propagating photons decreased from strong to incredibly small as the relative phase of the modulation signals varied. Next, the researchers increased the strength of the probe’s signal enough to saturate the atom’s first transition. At that point, they observed the so-called Mollow triplet, a well-known quantum-optical phenomenon, thus demonstrating that the chirality of the interaction did not just work for a single photon. Finally, they probed the transition between the first and second excited states of the artificial atom, showing that the coupling between these states could also be made chiral. They also observed how the phase of the probe’s photons changed depending on the state of the atom. In doing so, they made a quantum logic gate between the atom and a photon.

A natural next step would be to show that the new chiral device can transmit more than just a stream of microwave photons. For example, the team could try to transfer a quantum state from one artificial atom to another and vice versa. Such a demonstration would be an important step towards building large quantum networks with superconducting artificial atoms. Implementing a large network will require further suppression of leak channels in the setup and increasing the coupling strength between the artificial atoms and the waveguide. These improvements should, however, be simple enough to implement.


  1. C.Joshi et al.Resonance fluorescence of a chiral artificial atom, Phys. Reverend X 13021039 (2023).
  2. X. Gu et al.Microwave photonics with superconducting quantum circuits, Phys. Representative. 718-7191 (2017).
  3. P.Lodahl et al.chiral quantum optics, Nature 541473 (2017).
  4. JI Cirac et al.Quantum state transfer and entanglement distribution between distant nodes in a quantum network, Phys. Rev. Lett. 783221 (1997).
  5. HJ Kimble, The quantum internet, Nature 4531023 (2008).
  6. M. Casariego et al.Quantum microwave propagation: towards applications in communication and sensing, Quantum skiing. Techn. 8023001 (2023).
  7. BIT. Gumond et al.A unidirectional on-chip photonics interface for superconducting circuits, NPJ Quantum Inf. 632 (2020).
  8. B. Kannan et al.On-demand directional microwave photon emission using waveguide quantum electrodynamics, Nat. Phys. 19394 (2023).
  9. AF Kockum, Quantum optics with giant atoms the first five years, International Symposium on Mathematics, Quantum Theory and Cryptography 125 (2020).
  10. B. Kannan et al.Waveguide quantum electrodynamics with superconducting artificial giant atoms, Nature 583775 (2020).

About the author

Image by Anton Frisk Kockum

Anton Frisk Kockum is a senior research scientist at the Wallenberg Center for Quantum Technology (WACQT) at Chalmers University of Technology in Gothenburg, Sweden. Lui received his PhD from Chalmers in 2014 and then was a postdoctoral researcher at RIKEN in Japan before joining WACQT. His research is in theoretical physics, dealing with quantum optics and quantum information, including superconducting quantum computing, giant artificial atoms, machine learning for the characterization of quantum systems, ultrastrong light matter coupling, and quantum acoustics.

Thematic areas

Atomic and molecular physics Quantum physics

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