In quantum computing based on superconducting circuits, selective interaction with left and right propagation modes can enable directional information flow.
The concept of reciprocity suggests that when a source emits a scattered wave from an object and is then detected by a detector, the measured signal will not change if the source and detector are changed. Although this symmetry is a fundamental property of all physical systems, it can be problematic in some cases. For example, to design an isolator, a device that allows signals to pass in one direction but not in the other direction, reciprocity must be broken. These non-reciprocal devices, which can be defined by "chirality" or the preferred direction of their emission or absorption, are useful in a variety of industries.
What is Chirality?
If an object or system can be distinguished from its mirror counterpart and cannot be superimposed on it, then it is chiral. On the other hand, it is not possible to distinguish the mirror image of an achiral object from the real object, for example a sphere. In the case of enantiomorphs or molecules, meaning "opposite forms" in Greek, enantiomers are chiral objects and their mirror images. It is possible to place an achiral substance on its mirror counterpart. This is sometimes called an amphichiral object.
Lord Kelvin used this expression for the first time in his second Robert Boyle Lecture, which he gave to the Young Science Club of Oxford University in 1893 and was later published in 1894.
His statement is as follows;
“I call any geometric object or group of points 'chiral' and claim that something is chiral if its idealized reflection in a plane mirror cannot be made to coincide with itself.”
If we go back to our news;
Superconducting electrical circuits used in quantum computing have recently been equipped with non-reciprocal devices, although each has its drawbacks. Now, Chaitali Joshi and his colleagues at the California Institute of Technology have created an "artificial atom" consisting of a superconducting circuit that can only be coupled to signals moving left or right in a microwave waveguide. This chiral architecture can be implemented in quantum networks to allow control of the information flow between multiple artificial atoms connected to a waveguide.
One of the most popular platforms for quantum computing is superconducting circuits. However, it would be advantageous to have non-reciprocal elements that could keep them calm and channel quantum information. Previous research has shown that natural atoms and other single photon emitters can be used as non-reciprocal devices to regulate the propagation of visible light. In this study, the light is contained within a planar waveguide that limits the polarization of the light in certain directions. Then an atom or other emitter attached to the waveguide can be designed to emit and absorb light in only one direction.
However, superconducting circuits and the low-frequency microwaves they pair with will not work with this visible light configuration. Because natural atoms are not very flexible microwave emitters, researchers often use artificial atoms made from superconducting components assembled in a resonant circuit arrangement. These superconducting circuits have ground states and excited states, just like real atoms, and can be tuned for a particular application. The problem is that, unlike the visual situation, the coupling between the generated atoms and the microwave waveguides does not provide the same polarization dependence. Although researchers have developed a variety of approaches, chiral interfaces for superconducting circuits currently in use are either large, complex, or have other disadvantages.
A "giant molecule", a pair of synthetic atoms bonded to each other, is used in some recently proposed and demonstrated chirality techniques. A waveguide is attached to each atom at a different location. The emission and absorption of each atom is affected by interference effects that suppress or increase transmission along the waveguide. This concept has been simplified by Joshi and colleagues such that only one synthetic atom is required as the emitter. To realize an extension of the giant molecule concept in the form of a "giant atom", they created an artificial atom that connects to a 1D waveguide in multiple locations separated by just one wavelength.
To produce the necessary interference effects with a single emitter, the researchers had to decide the spacing between the junctions and the phase of the coupling at each point. They used extra artificial superconducting atoms as couplers between the emitter atom and the waveguide to achieve this. Scientists have successfully tuned the coupler atoms to efficiently manage the connection between the emitter and waveguide by applying a magnetic field. The critical phase difference that allows the forward or backward emitted light to flow through the waveguide was produced by the relative phase of the modulations of the two couplers. Adjusting the phase difference of the modulation was simple, making it easy to change the chirality of the interaction from one direction to the other.
The researchers demonstrated the capabilities of their invention through a series of experiments. They started by measuring the propagation of the weak photonic signal at resonance with the atom. This measurement showed that as the relative phase of the modulation signals fluctuated, binding to forward or backward emitted photons decreased from strong to negligible. The strength of the probe signal was then increased by the researchers to the point where the first pass of the atom was saturated. Later, the Mollow triplet, a well-known quantum-optical phenomenon, was discovered and demonstrated that the chirality of the interaction is not limited to functioning for a single photon.
Finally, by looking at the transition between the first and second excited states of the artificial atom, they showed that chirality can also be added to the link between these states. They also noticed how the state of the atom affects the phase of their probe photons. As a result, they succeeded in realizing a quantum logical gate between an atom and a photon.
The next logical step would be to demonstrate that the new chiral device can transmit more than a simple stream of microwave photons. For example, the group might try to move a quantum state from one fictitious atom to another or vice versa. An important step towards creating gigantic quantum networks with artificial superconducting atoms will be taken with such a demonstration. In order to realize a large network, the loss channels of the assembly will need to be further reduced and the connection between the artificial atoms and the waveguide will need to be strengthened. However, implementing these enhancements will be fairly simple.
📩 26/06/2023 22:42