Advisory Board

Dr. Jeremy L. O’Brien

The PhysOrg article Physicists Demonstrate Qubit-Qutrit Entanglement said

For the first time, physicists have entangled a qubit with a “qutrit” — the 3D version of the 2D qubit. Qubit-qutrit entanglement could lead to advantages in quantum computing, such as increased security and more efficient quantum gates, as well as enable novel tests of quantum mechanics.
 
The research team, composed of physicists from the University of Queensland, the University of Bristol, and the University of Waterloo, has published its results in a recent issue of Physical Review Letters. The researchers made qutrits with biphotons (two correlated photons), resulting in “biphotonic qutrits”. Then, they entangled these qutrits with photonic qubits (made with one photon) using a combination of linear optic elements and measurements.
 
A qutrit, just as it sounds, is the quantum information analogue of the classical trit. Due to its quantum mechanical nature, a qutrit can exist in superpositions of its three basis states. This is similar to how a qubit can exist in superpositions of its two states. Because of the qutrit? 3D nature, though, it can carry much more information than the qubit. (A string of n classical bits holds 1n states, a string of n qubits holds 2n states, and a string of n qutrits holds 3n states.)

Jeremy L. O’Brien, Ph.D. was one of these physicists and is Reader, Centre for Quantum Photonics, University of Bristol.
 
Jeremy’s research interests are centered on the fundamental quantum physics at the heart of quantum information and quantum computation, ranging from prototype systems for scalable quantum computing to generalized quantum measurements, quantum control, and quantum metrology. The experimental systems in which he has explored this physics include single photon quantum optics and correlated and confined electrons in the solid state.
 
A major focus of his current research is to design, experimentally demonstrate, and optimize the components for photonic quantum technologies. This includes single photon sources, detectors and circuits, as well as their integration in nanoscale devices. Highlights include the demonstration of a 2-photon CNOT gate; complete characterization of such a gate via quantum process tomography; invention of a simple entangling logic gate; an optical phase measurement below the standard quantum limit with four photons; the demonstration of an all optical fibre CNOT gate; and a silica-on-silicon waveguide CNOT gate on an optical chip.
 
A parallel focus is to use multi-photon quantum optics techniques for fundamental tests of quantum mechanics. This research has included the design and implementation quantum nondemolition (QND) measurements on photons, and their use in tests of quantum complementarity; the exploration of higher dimensional (qudit) entangled photon states; an experimental test of quantum weak values; implementation of generalized measurements, including nonlocal ones; and the development of ancilla-assisted quantum process tomography and experimental quantum process discrimination.
 
His work in solid state physics has included an effort to fabricate a phosphorus in silicon quantum computer and related studies of confined and correlated electron systems. A major highlight was the first demonstration of the controlled placement of single phosphorus atoms on a silicon surface using a scanning tunnelling microscope (STM) and hydrogen resist technique — an important step towards nuclear spin qubits. Underpinning this work, nanostructures fabricated in ultra-high mobility GaAs 2DEGs, showed that the anomalous “0.7 feature”, attributed to correlated electron effects, led to a phenomenological model to explain this feature. Related studies of quantum oscillations in quasi-one-dimensional organic conductors (TMTSF)2X, shed light on the non-monotonic temperature dependence. Studies of superconducting materials provided an insight into the unusual superconducting state of alpha-uranium, through ac magnetic susceptibility measurements, and an understanding of the paramagnetic limiting in the high-Tc cuprate YBCO, through mapping of the magnetic field-temperature phase diagram to 150 T, in capacitive GHz measurements.
 
Read his LinkedIn profile.