Quantum Information Group, Entangled-Light-Emitting Diode
For some important applications, quantum computers have potentially massive processing
power, due to the way data is encoded upon quantum bits (qubits). One of the resources
required to operate an optical quantum computer, is entangled light. At Toshiba,
our research on entangled light sources has resulted in many important achievements.
These include realisation of the first semiconductor source of triggered entangled
photons, creation of time-evolving entangled light states, and recently the first
electrically driven source of entangled light.
Entangled light possesses the unusual feature that its constituent particles (photons)
have inter-related properties, in this case polarisation. Measurement of one photon
affects the polarisation of the other, even if they are separated by huge distances.
This curious phenomenon was famously declared by Einstein to be “spukhafte
Fernwirkung” or “spooky action at a distance”. These properties
of entangled light derive from the fact that according to quantum mechanics, the
photon pair exist in a superposition state, and the polarisation of the pair is
uncertain until measurement of one photon.
We create photon pairs using nanometer-scale regions of semiconductor known as quantum
dots. Their small size means quantum dots can capture a maximum of two negative
and positive charges (electrons and holes respectively). The electrons and holes
recombine to emit a pair of photons.
However, photon pairs emitted by conventional quantum dots are not entangled, as
the energies of the emitted photons are polarisation dependent. This means the polarisation
of a photon can be determined by measurement of it's energy, providing the dreaded
‘which-path‘ information that is well know to destroy entanglement.
We have solved this problem by pioneering a technique to optimise the size and shape
of the quantum dot so that the energies of the emitted photons are equal, and entangled
light can be emitted. This led to realisation of the first semiconductor source
of triggered entangled photon pairs, which we achieved by driving a single quantum
dot with a laser.
We have subsequently made many advances in the performance and operation of the
device, which include enhanced resolution quantum interferometry, creation of time-evolving
entangled states, and improvement of the fidelity, or purity, of the entangled light
to 91%. However, entangled light produced previously by us and others requires
a laser beam as a power source. For applications such as optical quantum computing
that require many entangled photons, the practical advantages of creating entangled
light by electrical current are very significant. In collaboration with the University
of Cambridge, we now report in the journal Nature, realisation of the first
electrically driven source of entangled photons.
Our device is based on a conventional light-emitting-diode (LED) structure, but
additionally contains a specially optimised quantum dot. A voltage applied to the
LED causes a current to flow, and the quantum dot captures the charge required to
emit a pair of photon. In addition, the thickness of the semiconductor material
surrounding the quantum dot was optimised to regulate the rate charge is transferred
to the dot. Without this feature, entanglement is destroyed by extra charge. We
demonstrate that the device works well in both d.c. and a.c. mode, with fidelities
up to 82%.
An additional fundamental advantage of the entangled LED is that it has the potential
to operate on demand, supplying one entangled pair nearly every cycle. When combined
with the practical advantage offered by electrical excitation, the entangled LED
will allow simultaneous operation of many entangled light sources on a single chip,
opening the path to ultra-powerful semiconductor processors based on quantum computation.
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