Researchers at UNSW Sydney have made improved qubits using concepts from high school chemistry.
Above -A silicon qubit high-frequency measurement stage, which is positioned inside a dilution refrigerator to cool the chip to around 0.1 degrees above absolute zero. Picture: UNSW/Ken Leanfore
In a paper published today in Nature Communications, UNSW quantum computing researchers describe how they created artificial atoms in a silicon ‘quantum dot’, a tiny space in a quantum circuit where electrons are used as qubits (or quantum bits), the basic units of quantum information.
The results experimentally demonstrate that robust spin qubits can be implemented in multielectron quantum dots up to at least the third valence shell. Their utility indicates that it is not necessary to operate quantum dot qubits at single-electron occupancy, where disorder can degrade their reliability and performance. Furthermore, the larger size of multielectron wavefunctions combined with EDSR can enable higher control fidelities, and should also enhance exchange coupling between qubit.
Scientia Professor Andrew Dzurak explains that unlike a real atom, an artificial atom has no nucleus, but it still has shells of electrons whizzing around the centre of the device, rather than around the atom’s nucleus.
Researchers configured a quantum device in silicon to test the stability of electrons in artificial atoms.
They applied a voltage to the silicon via a metal surface ‘gate’ electrode to attract spare electrons from the silicon to form the quantum dot, an infinitesimally small space of only around 10 nanometres in diameter.
“As we slowly increased the voltage, we would draw in new electrons, one after another, to form an artificial atom in our quantum dot,” says Dr Saraiva, who led the theoretical analysis of the results.
“In a real atom, you have a positive charge in the middle, being the nucleus, and then the negatively charged electrons are held around it in three dimensional orbits. In our case, rather than the positive nucleus, the positive charge comes from the gate electrode which is separated from the silicon by an insulating barrier of silicon oxide, and then the electrons are suspended underneath it, each orbiting around the centre of the quantum dot. But rather than forming a sphere, they are arranged flat, in a disc.”
Mr Leon, who ran the experiments, says the researchers were interested in what happened when an extra electron began to populate a new outer shell. In the periodic table, the elements with just one electron in their outer shells include Hydrogen and the metals Lithium, Sodium and Potassium.
“When we create the equivalent of Hydrogen, Lithium and Sodium in the quantum dot, we are basically able to use that lone electron on the outer shell as a qubit,” Ross says.
“Up until now, imperfections in silicon devices at the atomic level have disrupted the way qubits behave, leading to unreliable operation and errors. But it seems that the extra electrons in the inner shells act like a ‘primer’ on the imperfect surface of the quantum dot, smoothing things out and giving stability to the electron in the outer shell.”
Watch the spin
Achieving stability and control of electrons is a crucial step towards silicon-based quantum computers becoming a reality. Where a classical computer uses ‘bits’ of information represented by either a 0 or a 1, the qubits in a quantum computer can store values of 0 and 1 simultaneously. This enables a quantum computer to carry out calculations in parallel, rather than one after another as a conventional computer would. The data processing power of a quantum computer then increases exponentially with the number of qubits it has available.
It is the spin of an electron that we use to encode the value of the qubit, explains Professor Dzurak.
“Spin is a quantum mechanical property. An electron acts like a tiny magnet and depending on which way it spins its north pole can either point up or down, corresponding to a 1 or a 0.
“When the electrons in either a real atom or our artificial atoms form a complete shell, they align their poles in opposite directions so that the total spin of the system is zero, making them useless as a qubit. But when we add one more electron to start a new shell, this extra electron has a spin that we can now use as a qubit again.
“Our new work shows that we can control the spin of electrons in the outer shells of these artificial atoms to give us reliable and stable qubits. This is really important because it means we can now work with much less fragile qubits. One electron is a very fragile thing. However an artificial atom with 5 electrons, or 13 electrons, is much more robust.”
Once the periodic properties of elements were unveiled, chemical behavior could be understood in terms of the valence of atoms. Ideally, this rationale would extend to quantum dots, and quantum computation could be performed by merely controlling the outer-shell electrons of dot-based qubits. Imperfections in semiconductor materials disrupt this analogy, so real devices seldom display a systematic many-electron arrangement. We demonstrate here an electrostatically confined quantum dot that reveals a well-defined shell structure. We observe four shells (31 electrons) with multiplicities given by spin and valley degrees of freedom. Various fillings containing a single valence electron—namely 1, 5, 13 and 25 electrons—are found to be potential qubits. An integrated micromagnet allows us to perform electrically-driven spin resonance (EDSR), leading to faster Rabi rotations and higher fidelity single-qubit gates at higher shell states. We investigate the impact of orbital excitations on single qubits as a function of the dot deformation and exploit it for faster qubit control.
Qubit architectures based on electron spins in gate-defined silicon quantum dots benefit from a high level of controllability, where single and multi-qubit coherent operations are realised solely with electrical and magnetic manipulation. Furthermore, their direct compatibility with silicon microelectronics fabrication offers unique scale-up opportunities1. However, fabrication reproducibility and disorder pose challenges for single-electron quantum dots. Even when the single-electron regime is achievable, the last electron often is confined in a very small region, limiting the effectiveness of electrical control and interdot tunnel coupling. Many-electron quantum dots were proposed as a qubit platform decades ago2, with the potential of resilience to charge noise3,4 and a higher tunable tunnel coupling strength to other qubits5. In the multielectron regime, the operation of a quantum dot qubit is more sensitive to its shape.