Representation for the qubits of the Bloch sphere. Credit: University of Konstanz
These laws of quantum physics offer many unique and far-reaching possibilities for advanced information processing, quantum computation and cryptography. The basic building blocks of quantum operations include superconducting resonances, photons and light. All quantum systems are not without their flaws and scientists continue to search for better alternatives.
Scientists from the University of Konstanz's Department of Physics have published their latest publication in Physical Review Research. They discovered a way to transform a free electron in vacuum into an "qubit", a quantum bit at two levels. These qubits are essential building blocks for quantum computer information processing. The researchers used the electron beam from a transmission electron microscope to generate the free-electron qubits. They also interacted it with the classical laser light's electric field. "The resulting matter wave interferences create a periodic modulation in the electron energy into discrete energy levels that are well-defined. This is what we use to form qubits," says Professor Peter Baum, who heads the research team.
The physical background
The researchers use the electron beam from a transmission electron microscope to generate their qubits. They also intersect it with an electric field created by classical laser light. The oscillations of light waves cause the beam electrons to be periodically accelerated or decelerated in rapid succession. Professor Peter Baum, who is the head of the research team, explains that the rapid interaction of the electron beam with the optical cycles of laser light results in periodic modulation and differentiation of electron energy into distinct, well-defined energy levels. This quantization can be detected using our instruments and is used as a resource to form qubits.
Microscopy of electrons attoseconds
The experiment's intersection of electron beam and laser beam does not just result in the mentioned phenomena in the energy realm, but also makes it relevant to qubit generation. Additional useful phenomena can be created in the time domain by choosing the correct laser parameters. The electron beam transforms into a series of very short electron pulses that have durations in the attosecond region.
Peter Baum illustrates these numbers by saying, "This is the millionth of one billionth part of an second and even light only covers the size of larger molecules in such a short time span." These extremely short electron pulses can be used for ultrafast electron microscopy (microscopy of complex light/matter interactions), where they allow maximum temporal resolution and a huge spatial resolution at the atomic level.
Qubits in mass production
The qubits and attosecond electro pulses of the experiment have another practical characteristic: they produce a high number of qubits (or electron pulses) per second. The high flux can be achieved using both a non-pulsed, continuous electron source and a non-pulsed laser beam. This allows almost all electrons in the electron beam to be modulated. Qubit production is limited only by the limitations of high-energy electron sources.
This is not the only reason laser-shaped, free electrons and qubits make great objects for further research. An electron, as an elementary particle, does not interact with any material in the vacuum of free space. Peter Baum adds that decoherence, which is the loss of information to an environment, is quite slow. "Moreover, the laser-optical control electron beams can be switched quickly and is flexible. Laser control of free-electron qubits could play an important part in the future, both for fundamental research and for applications in quantum information.
More details about the Physics of the Qubits
If you look closely, you will see that the electrons freed from the electron beam are not point particles. They are wave functions with a finite length coherence that covers multiple light oscillations. The same final energy can be generated coherently at multiple times by the optical field cycles. Matter-wave interferences result in a periodic modulation and generation of energy spectrum sidebands. These sidebands are used by researchers to create a two-level quantum system. Quantum operations can be performed using simple free-space propagation. Different sidebands are given nonlinear matter waves due to the rest mass. The beam then undergoes a second laser interaction, which generates sidebands a few centimeters later. The researchers are able to reach nearly any point on Bloch sphere in this manner, i.e. The "coordinate system", in which qubit states can be geometrically represented as points on a unit-sphere's surface.
Continue reading Researchers demonstrate a attosecond boost in electron microscopy
More information: M. V. Tsarev and al., Free-electron qubits & maximum-contrast pulses via temporal Talbot renewals, Physical Review Research (2021). M. V. Tsarev and colleagues, Maximum-contrast attosecond Pulses via Temporal Talbot Revivals. (2021). DOI: 10.1103/PhysRevResearch.3.043033
