Physicists in the US are the first to use a combination of light and sound to control the state of a diamond-based quantum bit (qubit) of information. The team used a laser pulse and a sound wave to modify the energy state of an electron in a nitrogen vacancy (NV) centre in diamond. According to the researchers, the technique could be further developed for controlling qubits in a chip-based network of NV centres.
A nitrogen vacancy (NV) centre occurs when two neighbouring carbon atoms in diamond are replaced by a nitrogen atom and an empty lattice site. For anyone trying to build a quantum computer, NVs are useful as qubits because they have an electron that is extremely well isolated from the surrounding lattice – information can be stored in an NV by placing it in a certain electronic state, which can then be maintained for a long time, even at room temperature. What is more, an NV qubit can be entangled with the polarization state of a photon, and such spin–photon entanglement could form the basis of future quantum computers.
However, this isolation also has its downside for quantum-computer designers because it makes it very difficult to connect and control NV-based qubits in a quantum computer. One possible way forward is to use sound waves, which could propagate through a chip containing many NVs and modify how individual NVs interact with light.
Now, Andrew Golter, Hailin Wang and colleagues at the University of Oregon and Oregon State University have shown how sound waves can change the photon energy required to put the NV into an excited state. They have also shown that sound can be used to control the NV centres in a way that preserves their quantum coherence – something that is important for quantum-computing applications.
The team began by coating a 500 μm-thick diamond wafer with a film of zinc oxide just 400 nm thick. Two antenna-like electrodes were placed at opposite ends of the wafer. Zinc oxide is a piezoelectric material, so when an oscillating electronic signal is applied to one of the electrodes, it creates a mechanical vibration on the surface of the wafer. This creates a surface acoustic wave (SAW) with a frequency of about 900 MHz that propagates across the wafer to the second electrode, where it is detected (see figure).
The wafer contains an NV centre that is several microns below the zinc-oxide-coated surface. This is close enough that the surrounding diamond lattice expands and contracts as the SAW passes overhead. When the SAW is switched off, the NV will absorb and emit photons of frequency ωo – which corresponds to the excitation energy of the NV. When the SAW is switched on, however, the vibrational energy of the lattice can also contribute to the excitation of the NV. This results in two new frequencies – called “sidebands” – at which photons can also be absorbed and emitted. These sidebands occur at ωo ± ωm, where ωm is the frequency of the SAW.
The team detected these sidebands by shining light on the NV to put it into an excited state, and then observing the fluorescent light that is given off when the NV drops down to its lowest energy state. As expected, the team observed light at three frequencies, ωo and ωo ± ωm.
The physicists were also able to measure quantum interference within the primary transition (ωo) and the sideband transitions. This is important because it shows that the opto-acoustic and optical transitions of the NV centre are coherent, and therefore both light and sound can be used to control its quantum state.
In particular, sound waves could offer a way of controlling the quantum states of large numbers of NV centres that are integrated within a chip. In such schemes, NV-based qubits could be designed to emit and absorb quanta of sound, or phonons. These phonons could be used to transport quantum information between qubits in an integrated quantum computer.
“You can imagine a 2D array of NV centres networked together by sound waves travelling back and forth across the surface of the diamond,” explains Golter. He also points out that the opto-acoustical control of qubits has already been successfully demonstrated using trapped-ion qubits. “Combining optical and acoustic control the way that we did follows on from techniques used to great success in trapped-ion systems, where lasers and the vibrations of the ions within the trap were used for quantum control and to couple multiple ions together.”
The research is described in Physical Review Letters.