Medium Large Transport
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 2
Time Pressure
Aircraft Equipment Problem Critical; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Took Evasive Action; General Declared Emergency
Aircraft
Aircraft X was handed off from the sector below the sector that I was working and climbed into my airspace; to FL350; without incident.  After a few minutes of level flight; the pilot of Aircraft X asked to descend.  I replied that I would have lower in just a moment clearing traffic; and my D side and I began to coordinate with the sector below for lower.  The pilot came back to inform me that he had lost cabin pressure; needed to declare an emergency; and needed to descend immediately.  I called the traffic that was directly below him at FL340.  The aircraft were diverging; and would soon be tail to tail in a moment.  The pilot did not report the passing aircraft in sight; but once the aircraft were tail to tail (but less than 5 miles) Aircraft X began his emergency descent.  We had cleared traffic down to FL240; and I let the pilot know to descend and maintain FL240.  He said he was going down to 100.  I tried to explain that I understood; and was simply trying to let him know that I was clearing traffic and had done so at that point through FL240.We continued to coordinate with lower sectors; which resulted in the low altitude sector turning an aerial refueling group in an AR route.  I then informed the pilot to descend and maintain 100; and gave him the altimeter that I was given by the low altitude sector.  Aircraft X was then switched to the low altitude sector  and then to the next Center.  It appeared he was continuing on his route at the lower altitude for the time being.  Neither aircraft received a RA. This was an emergency situation; and there is really nothing that can be done to prevent the reoccurrence of the event.  It was unfortunate that there was traffic directly below.  I could have asked the pilot to report the aircraft in sight; but I feel he would have descended down anyway due to the emergency either using visual separation or TCAS to know the traffics position.  I would like more information on how to communicate from the pilot in this situation.  I feel that there was a misunderstanding regarding the altitude information that I was trying to provide.  I did not expect the pilot to stop his descent at FL240; and understood the time critical nature of the emergency; and his need to immediately descend to 100.  Next time I would word the information differently; to let them know it was not an 'assigned altitude' but information regarding traffic.
Cruising at FL350 I felt a pressurization change in my ears.  I looked at the gauges and noticed the cabin was losing pressure rapidly.  A Cabin Alt Caution posted followed by a Cabin Alt Warning message shortly after.  The oxygen masks also deployed in the cabin. We donned our oxygen masks; established communication; and started a descent down to 10;000 declaring an emergency.  In the descent; we ran the QRH checklist and was able to gain control of the cabin pressure in manual mode.  We spoke with the flight attendants and one passenger was crying and a few ears were popping. We leveled off at 10;000 with the pressurization valve working in manual mode.  We then spoke with our dispatcher and made the decision to return back to ZZZ.  The rest of the flight was uneventful. We ended up swapping planes and headed back to intended destination.  All of the passengers seemed unharmed. Aircraft losing pressure rapidly. Pressurization valve went completely open in flight in Auto mode. Emergency Descent procedures.
ZZZ Center Controller and pilot of aircraft report of a loss of pressurization and an emergency descent resulting on a loss of separation.