Quantum Computing is an imminent technology that would not take much overtaking the classical computers. The new computing techniques are expected to revolutionize the ongoing processes that are currently being accomplished through the available classical computers. Quantum Computers are expected to appear with immense computing speed which on one hand will be a great facility but on the other hand, malicious users may utilize the power of quantum computers to compromise systems, databases and networks. The current article has investigated the possible threat to cyber security when quantum computers will be available publicly. The article has tried to elaborate on cyber security in the post-quantum era in terms of security challenges and possible countermeasures. The emergence of quantum computing poses notable obstacles for the field of quantum cryptography in the quickly changing cyber security scene. A major worry is that existing cryptographic algorithms are susceptible to quantum assaults, especially those based on RSA and ECC, which could be effectively cracked by quantum algorithms such as Shor’s algorithm. Furthermore, the creation and application of quantum-resistant algorithms are still in their infancy, necessitating a great deal of investigation and verification to guarantee their resilience against quantum attacks. The administration and distribution of keys in quantum cryptography also presents a challenge because the infrastructure for quantum key distribution (QKD) is still in its infancy and has some real-world drawbacks, such as the requirement for sophisticated quantum hardware and distance restrictions. An additional layer of complexity arises from guaranteeing interoperability between classical and quantum systems. The worldwide cybersecurity community must work together to develop, design, and implement efficient post-quantum cryptography technologies in order to meet these difficulties.