The efficient synchronization of unique and independently created photons
(i.e., light particles) has long been a problem in quantum physics.
Understanding this would have significant ramifications for the processing
of quantum information that depends on interactions between many
photons.
At ambient temperature, an atomic quantum memory was used by Weizmann
Institute of Science researchers to successfully synchronize a single,
independently produced photon. Their article, which was released in Physical
Review Letters, may pave the way for further research into multi-photon
states and their application to quantum information processing.
"The project idea came about several years ago, when our group and the
group of Ian Walmsley demonstrated an atomic quantum memory with an inverted
atomic-level scheme compared to the typical memories—the ladder memory,
named fast ladder memory (FLAME)," said Omri Davidson, one of the
researchers who conducted the study, to Phys.org. These memories are quick
and devoid of noise, making them effective for synchronizing single
photons.
Multi-photon states must be successfully created in order for photonic
quantum computing and other quantum information protocols to work. Since the
majority of quantum sources used in research up to this point are
probabilistic, they are unsuitable for reliably producing multi-photon
states.
Davidson and his coworkers have investigated the prospect of creating these
states using an atomic quantum memory, which are systems that can store
photons' quantum states while preserving the quantum information they
convey. They assumed that their atomic quantum memory would be able to hold
photons produced through probabilistic processes and release them when
needed to produce a multi-photon state.
The goal of the current study was to show single photon synchronization for
the first time utilizing an independent room-temperature atomic quantum
memory, according to Davidson. "To do this, we rebuilt the memory with a
number of enhancements and created a single-photon source that produces
photons that can effectively communicate with the memory. The real photon
synchronization, which connected the photon source and memory modules with
the appropriate experiment control circuits, was finally ready for
demonstration.
The researchers' quantum memory FLAME, which was created as part of earlier
research, is based on an inverted atomic-level structure known as a ladder
scheme. FLAME is both fast and noise-free compared to traditional
ground-state memories, which are often sluggish and susceptible to noise,
but can only retain data for a limited amount of time. They hoped that it
would enable them to produce multi-photon quantum states since speed and
absence of noise are necessary characteristics for the synchronization of
single photons.
The little wavelength mismatch of the signal and control light-field
transitions is the second benefit of the particular ladder scheme used with
rubidium atoms, according to Davidson. "Due to the reduced two-photon
Doppler broadening, this permits a reasonably extended memory lifetime
compared to other ladder methods with a bigger wavelength mismatch. The
photons may effectively couple with the memory because we produced them
using the same atomic-level structure as our memory.
The success of the team's experiment was largely due to the FLAME memory
scheme's several benefits, which allowed them to quickly synchronize
individual photons. They achieved a throughput of more than 1,000
synchronized photon pairs per second using their atomic quantum memory to
store and retrieve single photons with an end-to-end efficiency of e2e=25%
and final antibunching of g(2)h=0.023.
The amount of photon antibunching, or G (2) h, indicates how "single" the
individual photons are. In contrast to classical light, which has g(2)h=1,
perfect single photons have g(2)h=0. Due to the memory's noise-free
functioning, the researchers' synchronized photons continue to be virtually
flawless single-photons at g(2)h=0.023.
"We were able to synchronize photons that are compatible with atomic
systems at high rate," stated Davidson. "Many photonic quantum information
protocols, such as a deterministic two-qubit entangling gate, depend on
photons that may interact with atoms. Previous examples of photon
synchronization either employed broadband photons that are incompatible with
atomic systems or extremely slow photons that are compatible with atomic
systems.
In comparison to earlier demonstrations employing photons that are
compatible with atomic systems, the photon synchronization rate that
Davidson and his colleagues achieved in their tests is more than 1,000 times
better. Their finding opens up new research directions for the investigation
of atom-multiphoton interactions, including so-called deterministic
two-photon entangling gates. It could have important ramifications for the
development of quantum information processing and quantum optics systems in
the future.
We are investigating two study avenues right now," Davidson continued. The
first is to create robust photon-photon interactions with rubidium atoms in
a synchronization-like environment. We will be able to show a deterministic
entangling gate between the synchronized single-photons if we are successful
in our endeavor.
These gates are a crucial part of photonic quantum computation because they
allow for a reduction in the resource overhead compared to the methods
currently being used (called linear-optic quantum computation). The
scalability of these systems is limited since, up to this point, only cold
atom setups, not hot atoms, have been used to show these gates.
The FLAME memory will also be improved in Davidson and his coworkers'
upcoming research so that it can store photonic qubits as opposed to only
single photons in one polarization state, or a photon in a quantum
superposition of two polarization states. They could eventually be able to
use photons for quantum calculations thanks to this.
