Quantum Systems Linked with Near-Perfect Data Transfer

Scientists are continually striving to improve the efficiency of quantum teleportation, a process vital for secure quantum communication and computation. Ravi Kamal Pandey from the Department of Physics, Institute of Science, Banaras Hindu University, and Shraddha Singh from Nehru Gram Bharti (Deemed to be University), working with Dhiraj Yadav from IILM University and Devendra Kumar Mishra from Banaras Hindu University, have demonstrated a significant advance in this field. Their research details a method for achieving near-perfect quantum teleportation between distinct types of quantum encoding, discrete and continuous variables, utilising a hybrid entangled resource. This is particularly noteworthy as teleportation from discrete to continuous variables has historically been less efficient than the reverse process, and this new approach, employing cross-Kerr nonlinearity and linear optical components, overcomes this limitation, potentially paving the way for more robust and versatile quantum networks.

For decades, fully realising the potential of quantum communication has been hampered by the difficulty of transferring information between different types of quantum systems. Now, a method achieving near-perfect teleportation between distinct quantum encodings offers a major step forward, potentially unlocking more flexible and powerful quantum networks.

Scientists are increasingly focused on the reliable transmission of quantum information, a field with implications for secure communication and advanced computation. Quantum teleportation, a process of transferring quantum states, offers a potential solution, yet achieving perfect state transfer remains a significant challenge. A qubit, the basic unit of quantum information, can be encoded in the polarization of a single photon (discrete-variable or DV) or in the superposition of phase-opposite coherent states of an optical field (continuous-variable or CV).

DV systems, while convenient, are more susceptible to signal loss than their CV counterparts. This new scheme utilizes a hybrid approach, combining the strengths of both DV and CV encodings to enhance the fidelity of quantum state transfer. Conventional limitations of teleporting from DV to CV arise from difficulties in distinguishing between certain quantum states and the need for complex operations on the receiving end.

A team has demonstrated a method employing cross-Kerr nonlinearity, a process where the refractive index of one light beam is altered by another. Along with standard linear optical components such as beam splitters and phase shifters. By carefully manipulating these elements, almost perfect teleportation from a DV qubit to a CV qubit is attainable. Alice, the sender, combines the unknown DV qubit with one of these entangled modes using a beam splitter.

Then, photon counting measurements are performed on the output modes, allowing for the reconstruction of the original quantum state at Bob’s location. The success of this method hinges on the ability to perform specific unitary transformations on the received CV qubit, effectively correcting for any distortions introduced during the teleportation process.

Cross-Kerr mediation and linear optics enable discrete-continuous variable quantum state transfer

Initially, cross-Kerr nonlinearity was employed to mediate the transfer of quantum state between discrete-variable (DV) and continuous-variable (CV) systems. This nonlinearity, a non-linear optical effect, allows for interactions between photons without changing their number, proving essential for achieving high-fidelity teleportation. Alongside this, passive linear optical components, polarizing beam splitters, standard beam splitters. Phase shifters, were integrated into the experimental setup to manipulate the polarization and phase of photons.

These components are favoured due to their simplicity and minimal impact on quantum coherence. Polarization-encoded single photons representing the DV quantum system were prepared alongside phase-opposite coherent states constituting the CV system, requiring careful control of laser parameters and non-linear crystals to ensure the desired quantum properties.

These states were then combined with a hybrid entangled resource, linking the polarization of the DV photon with the phase of the CV coherent state. Establishing the necessary correlations for state transfer. The project focused on overcoming limitations inherent in reversing the direction of quantum teleportation — previous attempts at teleporting from CV to DV achieved near-unit success. But the reverse process typically plateaued at a maximum probability of 1/2.

To address this, the methodology incorporated a specific arrangement of optical elements designed to improve Bell state discrimination, vital for successful teleportation — the experimental design deliberately avoided requiring the receiver to perform non-unitary operations on the coherent state qubit. Here, this can introduce errors and reduce fidelity. Instead, the scheme relies on manipulating the entangled resource and linear optics to achieve the desired state transfer without these problematic steps. Enhancing the practicality and reliability of hybrid quantum teleportation.

Achieving near-perfect discrete-to-continuous and continuous-to-discrete variable quantum teleportation via cross-Kerr nonlinearity

Success probabilities of 1/2 were previously considered the upper limit for quantum teleportation from discrete-variable (DV) to continuous-variable (CV) encodings. In turn, this almost perfect teleportation between DV polarization qubits and CV qubits, encoded in phase-opposite coherent states, is achievable by integrating cross-Kerr nonlinearity with passive linear optical components, including polarizing beam splitters, beam splitters, and phase shifters.

For teleportation from CV to DV, perfect quantum teleportation can be attained for all non-zero photon count cases, assuming appreciable coherent amplitudes. Probability of failure becomes negligibly small under these conditions, and analysis of the teleported state shows that, for cases involving photon counts, the received state is identical to the original information state encoded in the DV basis.

Previous attempts, such as those by Park et al.. Were limited to a maximum success probability of 1/2 due to the inability to fully resolve Bell states and the non-unitary operation needed at the receiver. With a hybrid entangled resource and the described optical setup, The project overcomes these obstacles. Time evolution within the cross-Kerr interaction, governed by the Hamiltonian Hck = ħχ Nx Ny, allows for phase modulation of the coherent state without affecting the Fock state. Meanwhile, a key element in achieving high-fidelity transfer. At the same time, the dynamics of the cross-Kerr nonlinearity, described by the unitary evolution Uck = exp[−itχ Nx Ny], enable a phase shift of θ = χt on the coherent state. Across almost perfect teleportation in both directions.

Reversible quantum state transfer between discrete and continuous variables realised using cross-Kerr nonlinearity

Scientists have long sought to bridge the divide between distinct types of quantum information, specifically discrete and continuous variables. Across years, transferring quantum states from one to the other proved remarkably uneven. With teleportation readily achieved in one direction but hampered in the reverse. Now, a new approach utilising cross-Kerr nonlinearity alongside standard optical components appears to have overcome this asymmetry, demonstrating near-perfect state transfer.

Here, this isn’t merely a technical refinement; it addresses a fundamental bottleneck in building more flexible quantum networks — previous attempts struggled to maintain fidelity when moving from the discrete, qubit-like systems to the continuous, wave-like states. Limiting the potential for hybrid quantum processing. By cleverly manipulating the interaction between light beams, researchers have effectively levelled the playing field, and opening up possibilities for combining the strengths of both approaches.

At the same time, the practical implications extend beyond fundamental science, potentially enabling more adaptable quantum communication protocols and more powerful quantum sensors. However, it remains important to acknowledge that this effort represents a step forward within a controlled laboratory setting. By scaling up such a system to accommodate multiple qubits or over extended distances will certainly introduce new difficulties.

Unlike earlier demonstrations, this method relies on specific nonlinear materials — this may present limitations in terms of availability and performance. The broader field needs to address the issue of error correction, as even small imperfections can quickly degrade quantum information, and once these hurdles are addressed, we can anticipate a surge in research exploring hybrid quantum architectures. The long-term vision is a quantum internet where different types of quantum devices can seamlessly communicate and collaborate, while this effort brings that vision a little closer to reality.